@article{peters_ashby_hallen_2023, title={Barrier/seed system for electroless metallization on complex surfaces using (aminomethylaminoethyl)phenethyltrimethoxysilane self-assembled films}, volume={41}, ISSN={["2166-2754"]}, DOI={10.1116/6.0002519}, abstractNote={High frequency signals propagate along the edges of conductors. If the conductors are electroplated, then a conducting seed layer is needed at least on one edge, so care must be taken to ensure the electrical quality of these layers. A poor, high resistance seed layer may carry all the current at 10 GHz due to reduced skin depth. In this work, we study the initial quality of self-assembled monolayer (SAM)-based seed layers that are compatible with complex surfaces including through-silicon vias (TSVs), as are used in via-last three-dimensional semiconductor device packaging. In particular, morphology, adhesion, and resistivity are found to vary with the electroless catalyst and electroless metal deposition parameters; inductance-induced losses are also influenced by edge resistivity and metal choice. The seed layer must be fabricated on a barrier that will withstand diffusion, yet be thin enough to provide a conformal surface that allows for continuous seed layer deposition. Standard barrier and seed layer deposition methods such as evaporation or sputtering require either a line of sight from the source or aspect ratios large enough to provide scattering from the background gas within the structure to coat all surfaces. Such via holes are difficult to reliably fabricate and rely on tight parameter control. We propose a barrier layer based on an aromatic self-assembled monolayer (SAM) that also aids catalyst and high-quality electroless copper seed-layer attachment. The viability of the SAM barrier layer is determined by the quality of the deposited copper seed film, judged quantitatively by thin film resistivity and qualitatively by surface adhesion and morphological properties such as cracks and bubbles. Insights to the origins of problems are described and an optimal scheme identified. Atomic force microscopy (AFM) is used to verify results at each fabrication step. Extensions for use as a photolithographic resist layer are suggested. Our SAM approach for TSV applications yields a “smart” seed layer that can be used with a “simple,” scalloped, easy to fabricate, via hole.}, number={4}, journal={JOURNAL OF VACUUM SCIENCE & TECHNOLOGY B}, author={Peters, J. K. L. and Ashby, G. D. and Hallen, H. D.}, year={2023}, month={Jul} }
 @article{walker_emanuel_hallen_2023, title={Self-alignment of whole wafers using patterning for capillary forces}, volume={41}, ISSN={["2166-2754"]}, DOI={10.1116/6.0002518}, abstractNote={Three dimensional packaging schemes take advantage of multiple substrate materials, functionality, and reduced area constraints. Alignment of stacks of wafers becomes difficult as the number increases. We investigate full-wafer self-alignment as a means for solving this problem. To date, capillary self-alignment has only been accomplished with tiny, millimeter-sale, objects. Here, wafer-level self-alignment is demonstrated with capillary alignment forces, and we describe several needed, nontrivial advances and considerations compared to the chip alignment. The patterning scheme and the alignment force character are found to be crucial to ensure alignment at the wafer scale. Avoidance of alignment at local minima with the use of multiple length scales, prevention of upper wafer dragging by balancing the wafer and using engineered flow channels, and increased pattern features at small misalignments to combat the decreased alignment force are all discussed. A capture range of a few millimeters in position and several degrees in rotation for the self-alignment is achieved by patterning a hydrophobic self-assembled monolayer. These advances for large structure self-alignment offer a path forward for self-assembly of wafer stacks or other complex, large structures useful for mmWave, 5G antennas, for example. The scheme is compatible with a bonding scheme using the bonding precursor as the alignment fluid.}, number={3}, journal={JOURNAL OF VACUUM SCIENCE & TECHNOLOGY B}, author={Walker, Ernest M. and Emanuel, Ako and Hallen, Hans D.}, year={2023}, month={May} }
 @article{fallah dizche_duel-hallen_hallen_2022, title={Early Warning of mmWave Signal Blockage Using Diffraction Properties and Machine Learning}, volume={26}, ISSN={["1558-2558"]}, url={http://dx.doi.org/10.1109/lcomm.2022.3204636}, DOI={10.1109/lcomm.2022.3204636}, abstractNote={Sensitivity to blockage challenges performance of millimeter-wave (mmWave) communication systems. We apply the MiniRocket machine learning (ML) method to provide reliable early warning of mobile mmWave signal blockage hundreds of milliseconds ahead, thus facilitates a proactive response. MmWave signal datasets for training and testing the ML method are created using our low-complexity physics-based simulation tool, which models diffraction accurately. Our insights and numerical results illustrate that the proposed early warning method is facilitated by the diffraction-induced pre-blockage signal patterns and is robust to diverse environmental and mobility conditions.}, number={12}, journal={IEEE COMMUNICATIONS LETTERS}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Fallah Dizche, Amirhassan and Duel-Hallen, Alexandra and Hallen, Hans}, year={2022}, month={Dec}, pages={2944–2948} }
 @article{huang_green_huang_hallen_han_lim_2022, title={Room-temperature upconverted superfluorescence}, volume={8}, ISSN={["1749-4893"]}, url={http://dx.doi.org/10.1038/s41566-022-01060-5}, DOI={10.1038/s41566-022-01060-5}, journal={NATURE PHOTONICS}, publisher={Springer Science and Business Media LLC}, author={Huang, Kai and Green, Kory Kevin and Huang, Ling and Hallen, Hans and Han, Gang and Lim, Shuang Fang}, year={2022}, month={Aug} }
 @article{nakakubo_yoshioka_morita_ishimatsu_kiani_hallen_dickey_oki_2021, title={Dynamic control of reflective/diffusive optical surfaces on EGaIn liquid metal}, volume={11}, ISSN={["2159-3930"]}, DOI={10.1364/OME.425432}, abstractNote={We report a method to dynamically control the surface of gallium-based liquid metal to switch between reflective/diffusive states by removing/depositing oxide films via electrochemical redox reactions. Electrochemical oxidation deposits rough surface oxides that are optically diffusive. Electrochemical reduction returns the metal to a pristine, smooth, and reflective state. This switching is achieved using only ±1.4V. In addition, a meniscus of liquid metal can be pumped to switch between concave/convex shapes. This work expands the range of optical applications of dynamic liquid metal surfaces.}, number={7}, journal={OPTICAL MATERIALS EXPRESS}, author={Nakakubo, Keisuke and Yoshioka, Hiroaki and Morita, Kinichi and Ishimatsu, Ryoichi and Kiani, Abolfazl and Hallen, Hans and Dickey, Michael D. and Oki, Yuji}, year={2021}, month={Jul}, pages={2099–2108} }
 @inproceedings{hallen_hernandez_murray_liu_2021, title={Fluorescence nonlocality in two-dimensional Materials}, url={http://dx.doi.org/10.1117/12.2595933}, DOI={10.1117/12.2595933}, abstractNote={The combination of far-field and near-field scanning optical microscopy (NSOM) with a position (line) and wavelength dependent detection allow us to observe the fluorescence form points away from the excitation spot. This contrasts with confocal illumination/detection and with fluorescent imaging of uniform illumination, the two commonly used fluorescence measurement modes. We discuss the origins of the nonlocal emission and argue that the results can be used to measure the exciton diffusion length in these two-dimensional materials. In particular, we study transition metal dichalcogenides. We use near-field second harmonic generation (SHG) with NSOM detection of the mode-locked femtosecond laser pulse generated signals.}, booktitle={Ultrafast Nonlinear Imaging and Spectroscopy IX}, publisher={SPIE}, author={Hallen, Hans D. and Hernandez, Landon and Murray, William and Liu, Zhiwen}, editor={Liu, Zhiwen and Psaltis, Demetri and Shi, KebinEditors}, year={2021}, month={Aug} }
 @article{sunda-meya_hallen_2021, title={Surface plasmon resonance is possible in an optical fiber tapered to a point}, volume={39}, ISSN={["1520-8559"]}, DOI={10.1116/6.0000965}, abstractNote={Surface plasmon resonance sensing using a flat surface or weakly narrowed fiber has been shown to be a sensitive indicator of probe environment. We report that surface plasmon sensing is possible at the tip of a sharpened optical fiber, tapered to a nanosized point, via a proof-of-principle study and modeling. The size of the region of interaction of the light in the tapered fiber with the outer environment is limited by the last ∼ 10  μm of the taper, so sampling is accomplished in an ultrasmall volume of liquid. Changes of light retroreflected through the fiber were measured as the chemical environment of the thin-metal coated, etched fiber optic tip was changed. We present a model for the sensor that shows how surface plasmon sensing, usually an angle-sensitive measurement, can work when implemented in backreflection from a tapered fiber.}, number={4}, journal={JOURNAL OF VACUUM SCIENCE & TECHNOLOGY A}, author={Sunda-Meya, Doryne and Hallen, Hans D.}, year={2021}, month={Jul} }
 @inproceedings{green_long_huang_han_hallen_lim_2021, title={Unidirectional optical mechano-sensing with upconversion nanoparticles}, url={http://dx.doi.org/10.1117/12.2597041}, DOI={10.1117/12.2597041}, abstractNote={Cells respond to forces, and their quantification can potentially inform on the role of mechanics in cell development, differentiation, tissue repair and homeostasis. Other force sensitive processes include cancer cell metastasis, heart development in embryos driven by fluid forces, and individual cell response to tension by enhancing microtubule growth and connections. Development of current mechano-sensing approaches has not yielded many options, especially in directional force measurement. We present a sharpened fiber-based approach for uniaxial forces. An upconversion nanoparticle (UCNP) is mounted on the tip of the fiber and optically accessed through the fiber, which is manipulated as a probe. In UCNPs, the modification of the crystal field via mechanical forces result in changes in emission intensity, spectral shifts, upconversion luminescence (UCL) lifetime and ratiometric UCL response. We report on a discernably large peak shift of between 5-10 nm, and an apparent phase transition, with increasing amount of applied force in the micro Newton regime, in a single direction. Moreover, the peak shift is linear to the applied compression force. We investigate the influence of the UCNP force sensing process using Raman spectroscopy.}, booktitle={Ultrafast Nonlinear Imaging and Spectroscopy IX}, publisher={SPIE}, author={Green, Kory and Long, Brandon and Huang, Kai and Han, Gang and Hallen, Hans D. and Lim, Shuang Fang}, editor={Liu, Zhiwen and Psaltis, Demetri and Shi, KebinEditors}, year={2021}, month={Aug} }
 @article{ali_duel-hallen_hallen_2020, title={Early Warning of mmWave Signal Blockage and AoA Transition Using sub-6 GHz Observations}, volume={24}, ISSN={["1558-2558"]}, DOI={10.1109/LCOMM.2019.2952602}, abstractNote={The susceptibility of millimeter-wave (mmWave) signals to physical blockage and abrupt signal strength variations presents a challenge to reliable 5G communication. This work proposes and examines the feasibility of utilizing lower-frequency signals as early-warning indicators of mobile mmWave signal blockage or recovery. A physics-based channel simulation tool incorporating Fresnel diffraction and image sources is employed to demonstrate that sub-6 GHz signals “lead” mmWave signals in reaching a specific signal-strength threshold by several to tens of milliseconds at mobile speeds, suggesting early-warning systems are viable. This predictive approach stems from frequency-dependent properties of diffraction and does not assume a specific topology or mobile and obstacle speeds. Realistic simulations that include transitions from line of sight (LoS) to non-line of sight (NLoS) and reflection scenarios are employed to verify the proposed prediction capabilities. Moreover, prediction of the strongest multipath component and its angle of arrival (AoA) using sub-6 GHz observations is investigated.}, number={1}, journal={IEEE COMMUNICATIONS LETTERS}, author={Ali, Ziad and Duel-Hallen, Alexandra and Hallen, Hans}, year={2020}, month={Jan}, pages={207–211} }
 @article{emanuel_walker_hallen_2020, title={Modified SAMs and templates for achieving self-alignment of full wafers}, volume={24}, ISSN={["1613-4990"]}, DOI={10.1007/s10404-020-02352-4}, number={7}, journal={MICROFLUIDICS AND NANOFLUIDICS}, author={Emanuel, Ako and Walker, Ernest M., III and Hallen, Hans D.}, year={2020}, month={Jun} }
 @article{lim_wirth_bravo_schuck_hallen_2019, title={Large Available Volume Particles for Enhanced Deep UV Local Raman Sensing}, volume={11086}, ISSN={["1996-756X"]}, DOI={10.1117/12.2529434}, abstractNote={The combination of resonance Raman with deep UV excitation, DUVRR, gives greater selectivity and eliminates background fluorescence, enabling sensitive detection of UV absorbing nucleotide bases and amino acids. We demonstrate this combination with our 3D nanopore structure design. Resonance Raman is specific to a molecule absorbing at the excitation, while plasmon resonance of a small, shape-, index- and size- tuned metal dramatically increases the electric field strength in the active region. The 3D nanostructure exploits nanopores that retain the advantages of small-gap antennas but increases the ease of fabrication, availability, and detection volume compared to conventional plasmon-based designs, such as gaps between two particles, by being inherently single particle, with edge enhancement open to diffusion, and by possessing a large number of pores per particle. We show the large local field enhancement (hot spots) of the pores. Comparisons with an Al and silica coated/uncoated microsphere template with/without nanopores clearly show a significant blue shift of the 280 nm peak to (the more useful) 265 nm, in the presence of a hollow sphere with nanopores. Raman measurement of Tryptophan on an aluminum nanopore structure with excitation from our tunable OPO system in the visible and deep UV region indicate visible excitation causes more fluorescence and is less specific for the tryptophan, even displaying a Raman peak at the silicon substrate, while the deep-UV Raman spectra, at an energy close to the nanopore resonance, shows no substrate signal and peaks with close correlation to the known tryptophan vibrations.}, journal={UV AND HIGHER ENERGY PHOTONICS: FROM MATERIALS TO APPLICATIONS 2019}, author={Lim, Shuang Fang and Wirth, Janina and Bravo, Angel Fernandez and Schuck, Peter James and Hallen, Hans D.}, year={2019} }
 @inproceedings{hallen_long_2019, title={Resonance Raman techniques for complex biological systems}, volume={11122}, url={https://www.spiedigitallibrary.org/conference-proceedings-of-spie/11122/111220N/Resonance-Raman-techniques-for-complex-biological-systems/10.1117/12.2529642.short}, DOI={10.1117/12.2529642}, abstractNote={Resonance Raman offers a significant increase in Raman signal levels. We show how this can be used to select a specific molecule within a complex biosystem to study, in our case to determine if hemoglobin survives in ancient fossils. Key to this ability is the fact that the vibration must be on the same molecule as the absorption. Further, we show that the Raman fingerprint, or changes to it, can provide further selectivity or identify changes in that molecule based upon the particular sample. In our case, we find that the iron in the hemoglobin has oxidized into FeOOH, but still attached to both its porphyrin-like heme group and the protein network that gives the hemoglobin absorption. Very narrow Raman resonances are found in molecules with symmetry-forbidden, phonon-allowed absorptions. We show several in biologically relevant materials including that methylated-DNA (m-DNA) can be distinguished from non-methylated (n-DNA) with nano-bowtie- and resonance-enhanced Raman spectra. These efiects are retained when plasmon resonances are used to enhance a local region of the sample, but find that the overall signal from a uniformly distributed specimen is not increased significantly by the enhancement of a small region, so is not recommended unless the sample can be concentrated into that region.}, booktitle={Ultrafast Nonlinear Imaging and Spectroscopy VII}, publisher={International Society for Optics and Photonics}, author={Hallen, Hans D. and Long, Brandon J. N.}, year={2019}, month={Sep}, pages={111220N} }
 @misc{supplementary material for "early warning of mmwave signal blockage and aoa transition using sub-6 ghz observations"_2019, DOI={10.21227/3yjg-ra84}, journal={IEEE DataPort}, year={2019}, month={Nov} }
 @misc{supplementary material for "early warning of mmwave signal blockage and aoa transition using sub-6 ghz observations"_2019, DOI={10.21227/xyrw-mv20}, journal={IEEE DataPort}, year={2019}, month={Nov} }
 @article{a self-assembled covalent nanoglue_2018, url={http://dx.doi.org/10.1088/2053-1613/aac640}, DOI={10.1088/2053-1613/aac640}, abstractNote={Nano-glues rely on surface chemistry to intimately bond two surfaces together. The bond can be removable, which means it has to weaken, usually with temperature, or it can be a permanent bond, valued for strength, with a covalent bond yielding the highest strength. A practical covalent nanoglue based upon self-assembled monolayers (SAM) with amine and carboxyl termination is demonstrated, and applicable to any surface that bonds these SAM layers. For bonding flat or deformable layers, the SAM layers on each surface bond directly to each other with a peptide or nylon-like bond. For a removable bond or longer covalent structure, nylon chains are grown between the layers to bridge the gap for non-flat and non-flexible substrates. Bond strength and reliability are measured for several preparation schemes for the intermediate layer. A crystallization process is developed to pre-align the intermediate layer precursors and drive off the solvent to improve bond reliability and insure covalent bonding from wafer to wafer (W2W). Both covalent and removable bonds are created. Temperature dependence of the removable bond strength is measured, while the covalent bonds are stronger than our measurement process. The nanoglue does not bond until activated by (modest) heating, so alignment is enabled, and it is directly compatible with a wafer level fluid-self-alignment (FSA) process described elsewhere.}, journal={Translational Materials Research}, year={2018}, month={Jun} }
 @article{emanuel_hallen_2018, title={A self-assembled covalent nanoglue}, volume={5}, number={2}, journal={Translational Materials Research}, author={Emanuel, A. and Hallen, H. D.}, year={2018} }
 @article{speller_nusz_hallen_2018, title={Cell nucleus manipulation: hydrophobic probe and electric field driven motion}, volume={4}, ISSN={["2057-1976"]}, DOI={10.1088/2057-1976/aac9b0}, abstractNote={The manipulation of a cell nucleus is useful for biotechnology, and is often done with mechanical suction on a small capillary. Here, a nanoprobe with a hydrophobic coating is found to attract a nucleus strongly when brought into contact with it. The nucleus wraps around the probe securely enough that it remains attached as the probe moved and even as the probe is removed from the cell. If the tapered fiber probe is metal-coated prior to the hydrophobic monolayer attachment, the nucleus can be removed from the probe by applying a voltage bias, of either sign, to the metal layer. The removal process requires the presence of at least some cellular matter near the nucleus. A model is proposed to explain these interactions between the nucleus and probe. The transfer of the nucleus from one cell into another with the probe demonstrates the utility.}, number={4}, journal={BIOMEDICAL PHYSICS & ENGINEERING EXPRESS}, author={Speller, Danielle and Nusz, Greg and Hallen, Hans D.}, year={2018}, month={Jul} }
 @article{li_lim_puretzky_riehn_hallen_2018, title={DNA Methylation Detection Using Resonance and Nanobowtie-Antenna-Enhanced Raman Spectroscopy}, volume={114}, ISSN={["1542-0086"]}, DOI={10.1016/j.bpj.2018.04.021}, abstractNote={We show that DNA carrying 5-methylcytosine modifications or methylated DNA (m-DNA) can be distinguished from DNA with unmodified cytosine by Raman spectroscopy enhanced by both a bowtie nanoantenna and excitation resonance. In particular, m-DNA can be identified by a peak near 1000 cm−1 and changes in the Raman peaks in the 1200–1700 cm−1 band that are enhanced by the ring-absorption resonance. The identification is robust to the use of resonance Raman and nanoantenna excitation used to obtain significant signal improvement. The primary differences are three additional Raman peaks with methylation at 1014, 1239, and 1639 cm−1 and spectral intensity inversion at 1324 (C5=C6) and 1473 cm−1 (C4=N3) in m-DNA compared to that of DNA with unmodified cytosine. We attribute this to the proximity of the methyl group to the antenna, which brings the (C5=C6) mode closer to experiencing a stronger near-field enhancement. We also show distinct Raman spectral features attributed to the transition of DNA from a hydrated state, when dissolved, to a dried/denatured state. We observe a general broadening of the larger lines and a transfer of spectral weight from the ∼1470 cm−1 vibration to the two higher-energy lines of the dried m-DNA solution. We attribute the new spectral characteristics to DNA softening under high salt conditions and find that the m-DNA is still distinguishable via the ∼1000 cm−1 peak and distribution of the signal in the 1200–1700 cm−1 band. The nanoantenna gain exceeds 20,000, whereas the real signal ratio is much less because of a low average enhanced region occupancy even with these relatively high DNA concentrations. It is improved when fixed DNA in a salt crystal lies near the nanoantenna. The Raman resonance gain profile is consistent with A-term expectations, and the resonance is found at ∼259 nm excitation wavelength.}, number={11}, journal={BIOPHYSICAL JOURNAL}, author={Li, Ling and Lim, Shuang Fang and Puretzky, Alexander and Riehn, Robert and Hallen, Hans D.}, year={2018}, month={Jun}, pages={2498–2506} }
 @article{li_lim_puretzky_long_riehn_hallen_2018, title={DNA methylation detection using UV nano bowtie antenna enhanced Raman spectroscopy}, volume={10727}, ISSN={["1996-756X"]}, DOI={10.1117/12.2321283}, abstractNote={Methylation in DNA is a controlling factor in gene expression, embryonic development, and has been found to be important in infections and cancer. From a basic biology point of view, great heterogeneity has been found in methylation levels within tissues, so questions arises as to how and why. We show that methylated-DNA (m-DNA) can be distinguished from non-methylated (n-DNA) with nano-bowtie- and resonance- enhanced Raman spectra. By tuning the bowtie antenna to the resonance wavelength, both gains can be realized. Two additional Raman peaks in the 1200 – 1700 cm-1 band appear with methylation: one at 1239 cm-1 and the other at 1639 cm-1; a weak peak near 1000 cm-1 also appears with methylation. We also find that the two spectral features, although the latter with slight modification, can be used to distinguish the methylation state even when the DNA is denatured, as we show when we induce crystallization of the salts in the solution with increased excitation power, or allow it to happen naturally via solvent evaporation, and the DNA is trapped within the salt crystals. A comparison between liquid/solution to dried/denatured state m-DNA shows a general broadening of the larger lines and a transfer of spectral weight from the ~1470 cm-1 vibration to two higher energy lines. The applicability of the resonance-Raman in these spectra is shown by demonstrating that the Raman spectral characteristics hardly change as the Raman resonance in excitation wavelength is approached. Finally, we comment on real signal gain in this double-resonance system.}, journal={UV AND HIGHER ENERGY PHOTONICS: FROM MATERIALS TO APPLICATIONS 2018}, author={Li, Ling and Lim, Shuang Fang and Puretzky, Alexander and Long, Brandon J. N. and Riehn, Robert and Hallen, Hans}, year={2018} }
 @article{hallen_philbrick_2018, title={Lidar detection of small aerosol size distribution}, volume={10636}, ISSN={["1996-756X"]}, DOI={10.1117/12.2304890}, abstractNote={The major unknown in the global climate radiation balance calculations is the effect of aerosols. The extinction of aerosols depends upon the wavelength, size, concentration, composition, and to a lesser extent, shape of the aerosols. Thus, methods are needed to determine and model these quantities. The size distribution of larger aerosols can be monitored with multistatic lidar, at least in the spherical approximation. We can use this approximation in humid environments, and for old desert dusts in which the aspect ratio is typically below two. Aerosols that are small compared to the incident wavelength present a Rayleigh-like scattering dependence, and the size cannot be determined using multistatic lidar techniques. We discuss the analysis of true extinction from Raman lidar measurements at several wavelengths for determining the size distribution of aerosols. The Angstrom ratio, which is the natural log of the extinction ratio divided by the natural log of the wavelength ratio, has been used in column-integrated measurements to classify aerosols. Lidar backscatter Angstrom ratio measurements have also been used to classify aerosols as a function of range. However, the use for aerosol size distribution has not been investigated in detail before this work. We find, from Raman lidar measurements, Mie models of extinction and backscatter Angstrom ratios, that small aerosols make a significant contribution to optical scattering, and find that size information can be extracted from the lidar data.}, journal={LASER RADAR TECHNOLOGY AND APPLICATIONS XXIII}, author={Hallen, Hans D. and Philbrick, C. Russell}, year={2018} }
 @article{long_zheng_schweitzer_hallen_2018, title={Resonance Raman Imagery of Semi-Fossilized Soft Tissues}, volume={10753}, ISSN={["1996-756X"]}, DOI={10.1117/12.2321298}, abstractNote={The discovery of soft structures in dinosaur bone with the morphological and molecular characteristics of blood vessels in extant vertebrates was both surprising and controversial. Mounting evidence suggests that these soft tissues are blood vessels, their preservation driven in part by reactive oxygen species derived from hemoglobin degradation. More data are needed to support this hypothesis. Raman spectroscopy, and resonance Raman in particular, can provide detailed information as to the chemical makeup of these samples. We used two different excitation wavelengths in microscale Raman measurements to look for lines characteristic of degraded heme molecules, both in ancient vessels and modern analogues taken from semi-fossilized, hemoglobin-soaked ostrich bones. In both samples, we observed two regimes: dark colored, stiff regions and more transparent, elastic regions. We discovered that the two apparent regimes in the samples had different strengths of Raman returns, and that resonance effects greatly affected the Raman intensity. In all cases, there was some evidence of degraded heme spectra, though the increased returns indicated that the dark regimes had reacted more strongly with the heme specie. The modern vessels displayed a resonance Raman intensity consistent with hemoglobin molecular structures, which indicated resonance spectra would provide understanding of the ancient heme molecule. To investigate the two regimes more thoroughly, we acquired Raman spectra over areas where the sample transitioned from one regime to another. Variable wavelength resonance Raman measurements over the whole sample were used to give more information about the heme species present, in both ancient and modern samples.}, journal={ULTRAFAST NONLINEAR IMAGING AND SPECTROSCOPY VI}, author={Long, Brandon and Zheng, Wenxia and Schweitzer, Mary and Hallen, Hans}, year={2018} }
 @article{wirth_hallen_lim_2017, title={Influence of Gold Metallodielectric Partial-Shell Geometrical Irregularities on Dark Plasmon Resonances}, volume={121}, ISSN={["1932-7447"]}, url={http://dx.doi.org/10.1021/acs.jpcc.7b05060}, DOI={10.1021/acs.jpcc.7b05060}, abstractNote={The geometric asymmetry of real, fabricated gold partial shells leads to orientation and gold partial coverage dependent local fields and scattering. We illustrate this with single-particle measurements and finite element calculations. In particular, we show that the position and number of well-defined protrusions on the edge of a partial shell qualitatively change the spectra. The metallic protrusions result in geometrical asymmetry, which leads to excitation of the optically dark quadrupole mode as a function of incident light excitation and polarization. The far-field scattering peaks result from the bright dipole resonance with contribution from the dark resonance in the presence of the partial-shell surface protrusions. With more dark modes, the overall scattered intensity decreases, reflecting the energy trapped in the local electric fields of the dark modes, until it ultimately dissipates in the metal.}, number={29}, journal={JOURNAL OF PHYSICAL CHEMISTRY C}, publisher={American Chemical Society (ACS)}, author={Wirth, Janina and Hallen, Hans and Lim, Shuang Fang}, year={2017}, month={Jul}, pages={15937–15942} }
 @inproceedings{philbrick_hallen_2017, title={Signatures of dynamical processes in Raman lidar profiles of the atmosphere}, volume={10191}, url={https://www.spiedigitallibrary.org/conference-proceedings-of-spie/10191/101910E/Signatures-of-dynamical-processes-in-Raman-lidar-profiles-of-the/10.1117/12.2262583.short}, DOI={10.1117/12.2262583}, abstractNote={Raman lidar measurements provide profiles of several different tracers of spatial and temporal variations, which are excellent signatures for studies of dynamical processes in the atmosphere. An examination of Raman lidar data collected during the last four decades clearly show signatures of atmospheric planetary waves, gravity waves, low-level jets, weather fronts, turbulence from wind shear at surfaces and at the interface of the boundary layer with the free troposphere. Water vapor profiles are found to be important as a tracer of the sources of turbulence eddies associated with thermal convection, pressure waves, and wind shears, which result from surface heating, winds, weather systems, orographic forcing, and regions of reduced atmospheric stability. Examples of these processes are selected to show the influence of turbulence on profiles of atmospheric properties. Turbulence eddies generated in the wind shear region near the top of the boundary layer are found to mix into the atmospheric boundary layer. Results from several prior research projects are examined to gain a better understanding of processes impacting optical propagation through the many sources of turbulence observed in the lower atmosphere. Advances in lasers, detectors, and particularly in high-speed electronics now available are expected to provide important opportunities to improve our understanding of the formation processes, as well as for tracking of the sources and dissipation of turbulence eddies.}, booktitle={Laser Radar Technology and Applications XXII}, publisher={International Society for Optics and Photonics}, author={Philbrick, C. Russell and Hallen, Hans D.}, year={2017}, month={May}, pages={101910E} }
 @inproceedings{hallen_willitsford_neely_chadwick_philbrick_2016, title={Atmospheric absorption versus deep ultraviolet (pre-)resonance in Raman lidar measurements}, volume={9832}, url={http://dx.doi.org/10.1117/12.2224149}, DOI={10.1117/12.2224149}, abstractNote={The Raman scattering of several liquids and solid materials has been investigated near the deep ultraviolet absorption features corresponding to the electron energy states of the chemical species present. It is found to provide significant enhancement, but is always accompanied by absorption due to that or other species along the path. We investigate this trade-off for water vapor, although the results for liquid water and ice will be quantitatively very similar. An optical parametric oscillator (OPO) was pumped by the third harmonic of a Nd:YAG laser, and the output frequency doubled to generate a tunable excitation beam in the 215-600 nm range. We use the tunable laser excitation beam to investigate pre-resonance and resonance Raman spectroscopy near an absorption band of ice. A significant enhancement in the Raman signal was observed. The A-term of the Raman scattering tensor, which describes the pre-resonant enhancement of the spectra, is also used to find the primary observed intensities as a function of incident beam energy, although a wide resonance structure near the final-state-effect related absorption in ice is also found. The results suggest that use of pre-resonant or resonant Raman LIDAR could increase the sensitivity to improve spatial and temporal resolution of atmospheric water vapor measurements. However, these shorter wavelengths also exhibit higher ozone absorption. These opposing effects are modeled using MODTRAN for several configurations relevant for studies of boundary layer water and in the vicinity of clouds. Such data could be used in studies of the measurement of energy flow at the water-air and cloud-air interface, and may help with understanding some of the major uncertainties in current global climate models.}, note={– An invited paper}, booktitle={Proc. SPIE}, author={Hallen, Hans and Willitsford, A.H. and Neely, R.R. and Chadwick, C.T. and Philbrick, C.R.}, year={2016}, pages={983210 (11 pp.) -} }
 @article{willitsford_chadwick_kurtz_philbrick_hallen_2016, title={Resonance-Enhanced Raman Scattering of Ring-Involved Vibrational Modes in the B-1(2u) Absorption Band of Benzene, Including the Kekule Vibrational Modes nu(9) and nu(10)}, volume={120}, ISSN={["1520-5215"]}, url={http://dx.doi.org/10.1021/acs.jpca.5b08159}, DOI={10.1021/acs.jpca.5b08159}, abstractNote={Resonance Raman spectroscopy provides much stronger Raman signal levels than its off-resonant counterpart and adds selectivity by excitation tuning. Raman preresonance of benzene has been well studied. On-resonance studies, especially at phonon-allowed absorptions, have received less attention. In this case, we observe resonance of many of the vibration modes associated motion of the carbons in the ring while tuning over the (1)B2u absorption, including the related ν9 (CC stretch Herzberg notation, ν14 Wilson notation) and ν10 (CH-parallel bend Herzberg notation, ν15 Wilson notation) vibrational modes along with the ν2 (CC-stretch or ring-breathing Herzberg notation, ν1 Wilson notation) mode and multiples of the ν18 (CCC-parallel bend Herzberg notation, ν6 Wilson notation) vibrational mode. The ring-breathing mode is found to mix with the b2u modes creating higher frequency composites. Through the use of an optical parametric oscillator (OPO) to tune through the (1)B2u absorption band of liquid benzene, a stiffening (increase in energy) of the vibrational modes is observed as the excitation wavelength nears the (1)B2u absorption peak of the isolated molecule (vapor) phase. The strongest resonance amplitude observed is in the 2 × ν18 (e2g) mode, with nearly twice the intensity of the ring-breathing mode, ν2. Several overtones and combination modes, especially with ν2 (a1g), are also observed to resonate. Raman resonances on phonon-allowed excitations are narrow and permit the measurement of vibrations not Raman-active in the ground state.}, number={4}, journal={JOURNAL OF PHYSICAL CHEMISTRY A}, author={Willitsford, Adam H. and Chadwick, C. Todd and Kurtz, Stewart and Philbrick, C. Russell and Hallen, Hans}, year={2016}, month={Feb}, pages={503–506} }
 @article{chadwick_willitsford_philbrick_hallen_2015, title={Deep ultraviolet Raman spectroscopy: A resonance-absorption trade-off illustrated by diluted liquid benzene}, volume={118}, url={http://dx.doi.org/10.1063/1.4938531}, DOI={10.1063/1.4938531}, abstractNote={The magnitude of resonance Raman intensity, in terms of the real signal level measured on-resonance compared to the signal level measured off-resonance for the same sample, is investigated using a tunable laser source. Resonance Raman enhancements, occurring as the excitation energy is tuned through ultraviolet absorption lines, are used to examine the 1332 cm−1 vibrational mode of diamond and the 992 cm−1 ring-breathing mode of benzene. Competition between the wavelength dependent optical absorption and the magnitude of the resonance enhancement is studied using measured signal levels as a function of wavelength. Two system applications are identified where the resonance Raman significantly increases the real signal levels despite the presence of strong absorption: characterization of trace species in laser remote sensing and spectroscopy of the few molecules in the tiny working volumes of near-field optical microscopy.}, number={24}, journal={J. Appl. Phys.}, author={Chadwick, C.T. and Willitsford, A.H. and Philbrick, C.R. and Hallen, Hans}, year={2015}, month={Dec}, pages={243101 (6 pp.) -} }
 @inproceedings{philbrick_hallen_2015, title={Lidar investigations of atmospheric dynamics}, volume={9612}, url={http://dx.doi.org/10.1117/12.2188641}, DOI={10.1117/12.2188641}, abstractNote={Ground based lidar techniques using Raleigh and Raman scattering, differential absorption (DIAL), and supercontinuum sources are capable of providing unique signatures to study dynamical processes in the lower atmosphere. The most useful profile signatures of dynamics in the lower atmosphere are available in profiles of time sequences of water vapor and aerosol optical extinction obtained with Raman and DIAL lidars. Water vapor profiles are used to study the scales and motions of daytime convection cells, residual layer bursts into the planetary boundary layer (PBL), variations in height of the PBL layer, cloud formation and dissipation, scale sizes of gravity waves, turbulent eddies, as well as to study the seldom observed phenomena of Brunt–Väisälä oscillations and undular bore waves. Aerosol optical extinction profiles from Raman lidar provide another tracer of dynamics and motion using sequential profiles atmospheric aerosol extinction, where the aerosol distribution is controlled by dynamic, thermodynamic, and photochemical processes. Raman lidar profiles of temperature describe the stability of the lower atmosphere and measure structure features. Rayleigh lidar can provide backscatter profiles of aerosols in the troposphere, and temperature profiles in the stratosphere and mesosphere, where large gravity waves, stratospheric clouds, and noctilucent clouds are observed. Examples of several dynamical features are selected to illustrate interesting processes observed with Raman lidar. Lidar experiments add to our understanding of physical processes that modify atmospheric structure, initiate turbulence and waves, and describe the relationships between energy sources, atmospheric stability parameters, and the observed dynamics.}, note={– An invited paper}, booktitle={Proc. SPIE}, author={Philbrick, C.R. and Hallen, Hans}, year={2015}, pages={96120C (16 pp.) -} }
 @inproceedings{hallen_willitsford_weeks_philbrick_2015, title={UV resonance Raman signatures of phonon-allowed absorptions and phonon-driven bubble formation}, volume={9584}, url={http://dx.doi.org/10.1117/12.2188528}, DOI={10.1117/12.2188528}, abstractNote={We have measured UV resonance Raman near and at the resonance phonon-allowed absorption lines of several liquid species. Resonance absorption with excitation on the symmetry-forbidden but strongly phonon coupled bands in the 230- 290 nm spectral band present enhancement corresponding to the vapor phase absorptions rather than those of the liquid phase. This effect is related to the coherence forced by the internal molecular resonance required to absorb light at this energy. Large resonance gains (~3500x) reflect the narrower vapor phase lines. At the low laser fluence used, bubble formation is observed when the excitation energy corresponds to the maximum in Raman signal generation, not at the wavelength of maximum absorption in the liquid sample, which is several nanometers away.}, note={– An invited paper}, booktitle={Proc. SPIE}, author={Hallen, Hans and Willitsford, A. and Weeks, R. and Philbrick, C.R.}, year={2015}, pages={95840P (8 pp.) -} }
 @article{li_green_hallen_lim_2014, title={Enhancement of single particle rare earth doped NaYF4: Yb, Er emission with a gold shell}, volume={26}, ISSN={0957-4484 1361-6528}, url={http://dx.doi.org/10.1088/0957-4484/26/2/025101}, DOI={10.1088/0957-4484/26/2/025101}, abstractNote={Upconversion of infrared light to visible light has important implications for bioimaging. However, the small absorption cross-section of rare earth dopants has limited the efficiency of these anti-Stokes nanomaterials. We present enhanced excitation absorption and single particle fluorescent emission of sodium yttrium fluoride, NaYF4: Yb, Er based upconverting nanoparticles coated with a gold nanoshell through surface plasmon resonance. The single gold-shell coated nanoparticles show enhanced absorption in the near infrared, enhanced total emission intensity, and increased green relative to red emission. We also show differences in enhancement between single and aggregated gold shell nanoparticles. The surface plasmon resonance of the gold-shell coated nanoparticle is shown to be dependent on the shell thickness. In contrast to other reported results, our single particle experimental observations are corroborated by finite element calculations that show where the green/red emission enhancement occurs, and what portion of the enhancement is due to electromagnetic effects. We find that the excitation enhancement and green/red emission ratio enhancement occurs at the corners and edges of the doped emissive core.}, number={2}, journal={Nanotechnology}, publisher={IOP Publishing}, author={Li, Ling and Green, Kory and Hallen, Hans and Lim, Shuang Fang}, year={2014}, month={Dec}, pages={025101} }
 @inproceedings{philbrick_hallen_2014, title={Laser remote sensing of species concentrations and dynamical processes}, volume={9080}, url={http://dx.doi.org/10.1117/12.2050696}, DOI={10.1117/12.2050696}, abstractNote={A review of current lidar techniques summarizes present capabilities to: (1) measure atmospheric concentrations of most major and several minor molecular species using Raman scattering and DIAL techniques, (2) detect and measure concentrations of certain trace level species, (3) characterize active dynamical processes in the troposphere based upon using water vapor as a tracer, and (4) describe interesting thermodynamic properties based upon rotational Raman temperature profiles, multi-wavelength aerosol distributions, and changes in the phase states of water. Advances in lasers and detectors have extended the range of wavelengths available through the ultraviolet, visible, and infrared spectrum by using tunable laser techniques and supercontinuum broad spectrum lasers. Prior studies are reviewed, several applications for the technology are suggested which extend the techniques proposed to future investigations. In particular, the extension of tunable laser sources into the ultraviolet region has opened opportunities to use resonance Raman techniques, which provide greatly increased sensitivity for certain molecular species, such as hydrocarbons. The developments of supercontinuum lasers and tunable OPO lasers has enabled long-path trace concentration measurements of molecular spectra lines to detect and measure the concentrations of many species, as well as to distinguish any interfering species.}, booktitle={Proc. SPIE, Int. Soc. Opt. Eng.}, author={Philbrick, C.R. and Hallen, H.D.}, year={2014}, pages={90800Z (15 pp.) -} }
 @inproceedings{niu_philbrick_hallen_2014, title={Remote aerosol species-identification using IR scattering spectroscopy}, volume={9080}, url={http://dx.doi.org/10.1117/12.2050689}, DOI={10.1117/12.2050689}, abstractNote={Identification of atmospheric aerosol species and their chemical composition may help to trace their source and better estimate their impact on climate and environment. Optical scattering of aerosols depends primarily on aerosol chemical composition, size distribution, particle shape and the wavelength used. Extraction of features due to the aerosol complex refractive index from scattering spectroscopy at a single angle of observation allows composition identification via the spectral fingerprint, as shown computationally with Mie calculations of the optical scattering. Size-dependent scattering effects are eliminated by using near-forward scattering, such as in the scattering aureole. The only features of the aerosol aureole scattering spectra that very rapidly with wavelength are associated with the composition, so the aureole can give a reliable identification of aerosol composition.}, booktitle={Proc. SPIE, Int. Soc. Opt. Eng.}, author={Niu, Shupeng and Philbrick, C.R. and Hallen, H.D.}, year={2014}, pages={908010 (12 pp.) -} }
 @inproceedings{hallen_niu_li_2014, title={Time and neighbor interaction in resonance Raman spectroscopy}, volume={9198}, url={http://dx.doi.org/10.1117/12.2062001}, DOI={10.1117/12.2062001}, abstractNote={We study systems in which the resonance Raman process is fast due to the requirement for phonon involvement in the absorption. The resonance enhancement is found to track the isolated molecule, or vapor phase, absorption since the molecule does not have time to exchange energy with its neighbors. This corroborates with studies of pre-resonance, where Heisenberg’s uncertainty principle enforces a rapid process, but differs from resonance on electronically allowed transitions, where the resonance allows a relatively prolonged interaction. High resolution excitation spectroscopy reveals large gains and narrow features usually associated with the isolated molecule. Vibration energies shift as the resonance is approached and the excited state vibration levels are probed. Several multiplets and overtone modes are enhanced along with the strongly coupled ring-breathing mode in aromatic molecules.}, note={– An invited paper}, booktitle={Proc. SPIE}, author={Hallen, Hans and Niu, Shupeng and Li, Ling}, year={2014}, pages={91980F (8 pp.) -} }
 @inproceedings{hallen_neely_willitsford_chadwick_philbrick_2013, title={Coherence in UV resonance Raman spectroscopy of liquid benzene and toluene, but not ice}, volume={8845}, url={http://dx.doi.org/10.1117/12.2024313}, DOI={10.1117/12.2024313}, abstractNote={We have measured UV resonance Raman scattering at and near the resonance absorption lines of liquid benzene and toluene. Resonance occurs for excitation on the symmetry-forbidden but strongly phonon coupled states in the 1B2u band, ~230-270 nm, resulting in enhancements corresponding to the vapor phase absorptions rather than those of the liquid phase. This effect is related to the coherence forced by the internal molecular resonance required to absorb light at this energy. The resonance gains (~1000x) are larger than expected due to the narrower vapor phase lines. Several multiplet and overtone modes are enhanced along with the strongly coupled ring-breathing mode. A contrasting case of resonance Raman of ice is also discussed; in this case resonance is observed for excitation energy corresponding to absorptions that depend upon the final state shielding by the neighbors, and corresponds with the solid phase absorption. This typifies the more common, slow, time dependence of the resonance Raman process.}, note={– An invited paper}, booktitle={Proc. SPIE, Int. Soc. Opt. Eng.}, author={Hallen, Hans and Neely, R.R. and Willitsford, A.H. and Chadwick, C.T. and Philbrick, C.R.}, year={2013}, pages={884511 (9 pp.) -} }
 @article{jia_duel-hallen_hallen_2013, place={USA}, title={Data-Aided Noise Reduction for Long-Range Fading Prediction in Adaptive Modulation Systems}, volume={62}, ISSN={["1939-9359"]}, url={http://dx.doi.org/10.1109/TVT.2013.2241090}, DOI={10.1109/tvt.2013.2241090}, abstractNote={The long-range prediction (LRP) of fading signals enables adaptive transmission methods for rapidly varying mobile radio channels encountered in vehicular communications, but its performance is severely degraded by the additive noise and interference. A data-aided noise reduction (DANR) method is proposed to enhance the accuracy of fading prediction and to improve the spectral efficiency of adaptive modulation systems enabled by the LRP. The DANR includes an adaptive pilot transmission mechanism, robust noise reduction (NR), and decision-directed channel estimation. Due to improved prediction accuracy and low pilot rates, the DANR results in higher spectral efficiency than previously proposed NR techniques, which rely on oversampled pilots. It is also demonstrated that DANR-aided LRP increases the coding gain of adaptive trellis-coded modulation (ATCM). Finally, for low-to-medium signal-to-noise ratio (SNR) values, we show that LRP-enabled adaptive modulation performs better for realistic reflector configurations than for the conventional Jakes model (JM) data set.}, number={5}, journal={IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY}, author={Jia, Tao and Duel-Hallen, Alexandra and Hallen, Hans}, year={2013}, month={Jun}, pages={2358–2362} }
 @inproceedings{hallen_long_hook_pangle_philbrick_2013, title={Multistatic lidar measurements of non-spherical aerosols}, volume={8731}, url={http://dx.doi.org/10.1117/12.2017710}, DOI={10.1117/12.2017710}, abstractNote={Lidar is a powerful tool for measuring the vertical profiles of aerosols in the atmosphere using Rayleigh and Raman lidar techniques. Bistatic lidar can be used to obtain the angular structure of the scattered light. When the aerosols are uniformly distributed, this information can be analyzed to provide particle size distribution information. However, dusts tend to be irregularly shaped particles with varied composition. We investigate the impact of the irregular shape using optical scattering at several wavelengths, scanning electron microscopy, and T-matrix calculations. In particular, we study the rapid loss of Mie scattering resonances as the particle shape departs from spherical. Different size distributions produced by different size-cuts of Arizona Road Dust (ARD) are studied.}, booktitle={Proc. SPIE, Int. Soc. Opt. Eng.}, author={Hallen, Hans and Long, B.J.N. and Hook, D.A. and Pangle, G.E. and Philbrick, C.R.}, year={2013}, pages={87310P (13 pp.) -} }
 @inproceedings{pangle_hook_long_philbrick_hallen_2013, title={Optical extinction dependence on wavelength and size distribution of airborne dust}, volume={8731}, url={http://dx.doi.org/10.1117/12.2017850}, DOI={10.1117/12.2017850}, abstractNote={The optical scattering from laser beams propagating through atmospheric aerosols has been shown to be very useful in describing air pollution aerosol properties. This research explores and extends that capability to particulate matter. The optical properties of Arizona Road Dust (ARD) samples are measured in a chamber that simulates the particle dispersal of dust aerosols in the atmospheric environment. Visible, near infrared, and long wave infrared lasers are used. Optical scattering measurements show the expected dependence of laser wavelength and particle size on the extinction of laser beams. The extinction at long wavelengths demonstrates reduced scattering, but chemical absorption of dust species must be considered. The extinction and depolarization of laser wavelengths interacting with several size cuts of ARD are examined. The measurements include studies of different size distributions, and their evolution over time is recorded by an Aerodynamic Particle Sizer. We analyze the size-dependent extinction and depolarization of ARD. We present a method of predicting extinction for an arbitrary ARD size distribution. These studies provide new insights for understanding the optical propagation of laser beams through airborne particulate matter.}, booktitle={Proc. SPIE, Int. Soc. Opt. Eng.}, author={Pangle, G.E. and Hook, D.A. and Long, B.J.N. and Philbrick, C.R. and Hallen, Hans}, year={2013}, pages={87310N (14 pp.) -} }
 @article{willitsford_chadwick_hallen_kurtz_philbrick_2013, title={Resonance enhanced Raman scatter in liquid benzene at vapor-phase absorption peaks}, volume={21}, url={http://dx.doi.org/10.1364/OE.21.026150}, DOI={10.1364/OE.21.026150}, abstractNote={The resonance enhanced Raman spectra in the 1B2u mode of the forbidden benzene electronic transition band, ~230-270 nm, has been investigated. Resonance enhanced Raman scattering in both liquid benzene and liquid toluene exhibit the greatest enhancement when the wavelength of excitation is tuned to the vapor-phase absorption peaks; even though the sample volume is in a liquid state. Raman signals for the symmetric breathing mode of the carbon ring are found to be resonantly enhanced by several orders of magnitude (>500X) with deep UV excitation compared to non-resonant visible excitation. Since the benzene absorbs near this resonant wavelength, its effect on the sampled volume cannot be neglected in determining the resonance gain, as we discuss in detail. Large resonant gains correspond with excitation at the 247, 253, and 259 nm absorption peaks in the benzene vapor spectrum. The narrow region of resonance gain is investigated in detail around the absorption peak located at 259 nm using 0.25 nm steps in the excitation wavelength. We observe the resonance gain tracking the vapor phase absorption peaks and valleys within this narrow range. Results are interpreted in terms of the coherence forced by the use of a forbidden transition for resonance excitation.}, number={22}, journal={Optics Express}, author={Willitsford, Adam and Chadwick, C. Todd and Hallen, Hans and Kurtz, Stewart and Philbrick, C. Russell}, year={2013}, pages={26150–26161} }
 @inproceedings{hook_pangle_long_philbrick_hallen_2013, title={Understanding lidar returns from complex dust mixtures}, volume={8731}, url={http://dx.doi.org/10.1117/12.2017698}, DOI={10.1117/12.2017698}, abstractNote={Lidar is a powerful tool for measuring the vertical profiles of aerosols. Dusts are irregularly-shaped particles with varied composition and strong index of refraction variations in the LWIR. We measure dust indices using ellipsometry and transmission through KBr pellets. Milling makes the ellipsometry data less dependent on incidence angle, and the results of measurements on milled materials agree with those from transmission measurements. Measurements show that the spectrum of a milled Arizona Road Dust (ARD) approaches that of pure quartz, indicating a decrease of absorption efficiency for particles larger than the absorption length. These indices of refraction will be used in the future to simulate extinction for the beam of a LWIR lidar.}, booktitle={Proc. SPIE, Int. Soc. Opt. Eng.}, author={Hook, D.A. and Pangle, G.E. and Long, B.J.N. and Philbrick, C.R. and Hallen, Hans}, year={2013}, pages={87310M (12 pp.) -} }
 @inproceedings{long_hook_pangle_hallen_philbrick_2013, title={Using a laser aureole to study aerosols}, volume={8731}, url={http://dx.doi.org/10.1117/12.2017707}, DOI={10.1117/12.2017707}, abstractNote={Aerosol optical scattering experiments are often large, expensive, and provide poor control of dust uniformity and size distribution. The size distribution of such suspended atmospheric aerosols varies rapidly in time, since larger particles settle quickly. Even in large chambers, 10 micron particles settle in tens of seconds. We describe lab-scale experiments with stable particle distributions. A viscous colloidal solution can stabilize the particles for sufficient time to measure optical scattering properties. Colloids with different concentrations or size distributions enable nearly time independent studies of prepared distributions. We perform laser aureole scattering from a colloid containing a few percent by volume of Arizona Road Dust (ARD) in mineral oil and glycerin, and 1-micron polystyrene spheres in water. We discuss aureole analysis, the differences expected in scattering properties due to the index of refraction of the mineral oil medium versus air, and the impact of non-spherical shape on the scattering. This research demonstrates that particles suspended in a viscous medium can be used to simulate aerosol optical scattering in air, while enabling signal averaging, offering reproducibility, and easing problems resulting from parameter variations in studies of dust properties.}, booktitle={Proc. SPIE, Int. Soc. Opt. Eng.}, author={Long, B.J.N. and Hook, D.A. and Pangle, G.E. and Hallen, Hans and Philbrick, C.R.}, year={2013}, pages={87310O (12 pp.) -} }
 @article{wu_jia_calderbank_duel-hallen_hallen_2012, place={USA}, title={Enabling Code Diversity for Mobile Radio Channels using Long-Range Fading Prediction}, volume={11}, ISSN={["1558-2248"]}, url={http://dx.doi.org/10.1109/TWC.2012.101712.111884}, DOI={10.1109/twc.2012.101712.111884}, abstractNote={Code diversity integrates space-time coding with beamforming by using a small number of feedback bits to select from a family of space-time codes. Different codes lead to different induced channels at the receiver, where Channel State Information (CSI) is used to instruct the transmitter how to choose the code. Feedback can be combined with sub-optimal low complexity decoding of the component codes to match Maximum-Likelihood (ML) decoding performance of any individual code in the family. It can also be combined with ML decoding of the component codes to improve performance beyond ML decoding performance of any individual code. Prior analysis of code diversity did not take into account the effect of the mobile speed and the delay in the feedback channel. This paper demonstrates the practicality of code diversity in space-time coded systems by showing that performance gains based on instantaneous feedback are largely preserved when long-range prediction of time-varying correlated fading channels is employed to compensate for the effect of the feedback delay. To maintain prediction accuracy for realistic SNR, noise reduction that employs oversampled pilots is used prior to fading prediction. We also propose a robust low pilot rate method that utilizes interleaving to improve the spectral efficiency. Simulations are presented for two channel models: the conventional Jakes model and a realistic physical channel model where the parameters associated with the reflectors vary in time and the arrival rays have different strengths and asymmetric arrival angles.}, number={12}, journal={IEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS}, author={Wu, Yiyue and Jia, Tao and Calderbank, Robert and Duel-Hallen, Alexandra and Hallen, Hans}, year={2012}, month={Dec}, pages={4362–4371} }
 @article{li_fang lim_puretzky_riehn_hallen_2012, title={Near-field enhanced ultraviolet resonance Raman spectroscopy using aluminum bow-tie nano-antenna}, volume={101}, url={http://dx.doi.org/10.1063/1.4746747}, DOI={10.1063/1.4746747}, abstractNote={An aluminum bow-tie nano-antenna is combined with the resonance Raman effect in the deep ultraviolet to dramatically increase the sensitivity of Raman spectra to a small volume of material, such as benzene used here. We further demonstrate gradient-field Raman peaks for several strong infrared modes. We achieve a gain of [Formula: see text] in signal intensity from the near field enhancement due to the surface plasmon resonance in the aluminum nanostructure. The on-line resonance enhancement contributes another factor of several thousands, limited by the laser line width. Thus, an overall gain of hundreds of million is achieved.}, number={11}, journal={Applied Physics Letters}, author={Li, Ling and Fang Lim, Shuang and Puretzky, Alexander A. and Riehn, Robert and Hallen, H.D.}, year={2012} }
 @article{li_lim_puretzky_riehn_hallen_2012, title={Near-field enhanced ultraviolet resonance Raman spectroscopy using aluminum bow-tie nano-antenna}, volume={101}, number={11}, journal={Applied Physics Letters}, author={Li, L. and Lim, S. F. and Puretzky, A. A. and Riehn, R. and Hallen, H. D.}, year={2012} }
 @inproceedings{philbrick_hallen_snyder_wyant_2011, title={Remote Sensing of Atmospheric Aerosol Properties}, url={https://ams.confex.com/ams/91Annual/webprogram/Paper186827.html}, booktitle={Proceedings of the 91th Annual AMS Meeting, Third Symposium on Aerosol-Cloud-Climate Interactions, Aerosol and Cloud Properties, Seattle, WA, 23--27 January 2011}, author={Philbrick, Russell and Hallen, Hans and Snyder, Michelle and Wyant, Andrea M.}, year={2011} }
 @article{clark_taylor_hallen_2010, title={Novel split-tip proximal probe for fabrication of nanometer-textured, in-plane oriented polymer films}, volume={28}, ISSN={["2166-2746"]}, url={http://dx.doi.org/10.1116/1.3437473}, DOI={10.1116/1.3437473}, abstractNote={Novel fabrication schemes are required to deposit nanoscale materials that contain molecules oriented in the plane of the surface. The breakage of in-plane symmetry allows devices to be fabricated in this plane, enabling molecular electronics to follow this successful paradigm of semiconductor devices. The authors discuss here the fabrication of a unique split-tip optical nanoprobe that can be used to both orient molecules on a surface with a strong, localized electric field and deposit them with nanoscale resolution. Ultraviolet light injected through the probe into the region of aligned molecules causes the deposition. The production of the split-tip probe is significantly different than that of the related near-field scanning optical microscope (NSOM) probe, since the stresses in the metal layer must be held by the metal film–silica interface rather than within the film as it encircles the silica of a NSOM probe. Mounting of the probe to ensure reliable electrical contacts is also described.}, number={4}, journal={JOURNAL OF VACUUM SCIENCE & TECHNOLOGY B}, author={Clark, Beverly, III and Taylor, M. P. and Hallen, H. D.}, year={2010}, month={Jul}, pages={687–692} }
 @inproceedings{philbrick_hallen_wyant_wright_snyder_2010, title={Optical Remote Sensing Techniques Characterize the Properties of Atmospheric Aerosols}, volume={7684}, url={http://dx.doi.org/10.1117/12.850453}, DOI={10.1117/12.850453}, abstractNote={Several laser remote sensing techniques are used to characterize the properties of aerosols. The various techniques include: backscatter, optical extinction using Raman scatter, and bistatic/multistatic scattering using the polarization ratio of the scattering phase function. The number density, size, and size distribution are obtained under the assumption of spherical scatterers. Other measurements can be used to describe additional properties, such as aerosol type based upon approximate refractive index and detected departure from spherical, when simultaneous measurements at several wavelengths and several angles are analyzed. Examples are shown to demonstrate our present capability to characterize aerosol particles using recently developed techniques.}, booktitle={Proc. SPIE - Int. Soc. Opt. Eng.}, author={Philbrick, C. R. and Hallen, Hans and Wyant, A. and Wright, T. and Snyder, M.}, year={2010}, pages={76840J (11 pp.) -} }
 @inproceedings{philbrick_wright_snyder_hallen_brown_verghese_edwards_2009, place={Phoenix, Arizona}, title={Characteristics of atmospheric aerosols using optical remote sensing}, url={http://ams.confex.com/ams/pdfpapers/163441.pdf}, booktitle={Proc. of Fourth Symposium on Lidar Atmospheric Applications, as part of the 89th AMS Annual Meeting}, author={Philbrick, C. Russell and Wright, Timothy and Snyder, Michelle and Hallen, Hans and Brown, Andrea M. and Verghese, Sachin and Edwards, Perry S.}, year={2009}, pages={8 pp.} }
 @article{connor_hallen_lalush_sumner_zhong_2009, place={UK}, title={Comparison of diffraction-enhanced computed tomography and monochromatic synchrotron radiation computed tomography of human trabecular bone}, volume={54}, ISSN={["1361-6560"]}, url={http://dx.doi.org/10.1088/0031-9155/54/20/006}, DOI={10.1088/0031-9155/54/20/006}, abstractNote={Diffraction-enhanced imaging (DEI) is an x-ray-based medical imaging modality that, when used in tomography mode (DECT), can generate a three-dimensional map of both the apparent absorption coefficient and the out-of-plane gradient of the index of refraction of the sample. DECT is known to have contrast gains over monochromatic synchrotron radiation CT (SRCT) for soft tissue structures. The goal of this experiment was to compare contrast-to-noise ratio (CNR) and resolution in images of human trabecular bone acquired using SRCT with images acquired using DECT. All images were acquired at the National Synchrotron Light Source (Upton, NY, USA) at beamline X15 A at an x-ray energy of 40 keV and the silicon [3 3 3] reflection. SRCT, apparent absorption DECT and refraction DECT slice images of the trabecular bone were created. The apparent absorption DECT images have significantly higher spatial resolution and CNR than the corresponding SRCT images. Thus, DECT will prove to be a useful tool for imaging applications in which high contrast and high spatial resolution are required for both soft tissue features and bone.}, number={20}, journal={PHYSICS IN MEDICINE AND BIOLOGY}, author={Connor, D. M. and Hallen, H. D. and Lalush, D. S. and Sumner, D. R. and Zhong, Z.}, year={2009}, month={Oct}, pages={6123–6133} }
 @inproceedings{wu_jia_calderbank_duel-hallen_hallen_2009, place={Torino, Italy}, title={Integration of code diversity and long-range channel prediction in wireless communication}, url={http://dx.doi.org/10.1109/ICEAA.2009.5297458}, DOI={10.1109/ICEAA.2009.5297458}, abstractNote={Code diversity integrates space-time coding with beamforming by using a small number of feedback bits to select from a family of space-time codes. Different codes lead to different induced channels at the receiver, where Channel State Information (CSI) is used to instruct the transmitter how to choose the code. Feedback can be combined with sub-optimal low complexity decoding of the component codes to match Maximum-Likelihood (ML) decoding performance of any individual code in the family. It can also be combined with ML decoding of the component codes to improve performance beyond ML decoding performance of any individual code. Prior analysis of code diversity did not take into account the effect of the mobile speed and the delay in the feedback channel. This paper demonstrates the practicality of code diversity in space-time coded systems by showing that predicted performance gains based on instantaneous feedback are largely preserved when the feedback is based on long-range prediction of rapidly time-varying correlated fading channels. Simulations are presented for two channel models; the first is the Jakes model where angles of arrival are uniformly distributed and the arrival rays have equal strengths, and the second is a model derived from a physical scattering environment where the parameters associated with the reflectors vary in time and the arrival rays have different strengths and non-symmetric arrival angles.}, booktitle={Proceedings of the 2009 International Conference on Electromagnetics in Advanced Applications, ICEAA '09}, author={Wu, Yiyue and Jia, Tao and Calderbank, Robert and Duel-Hallen, Alexandra and Hallen, Hans}, year={2009}, pages={241–244} }
 @inproceedings{philbrick_hallen_2009, place={Noordwijk, Netherlands}, title={Measurements of contributors to atmospheric climate change}, url={http://www.spaceflight.esa.int/pac-symposium2009/proceedings/papers/s2_40philb.pdf}, booktitle={Proceedings of the 19th ESA Symposium on European Rocket and Balloon Programmes and Related Research}, author={Philbrick, C.R. and Hallen, H.}, year={2009}, pages={141–6} }
 @inproceedings{philbrick_brown_willitsford_edwards_wyant_liu_chadwick_hallen_2009, title={Remote Sensing of Chemical Species in the Atmosphere}, url={http://ams.confex.com/ams/pdfpapers/150051.pdf}, note={– An invited paper}, booktitle={Proc. of Fourth Symposium on Lidar Atmospheric Applications, as part of the 89th AMS Annual Meeting in Phoenix, Arizona, 11--15 January 2009}, author={Philbrick, C. Russell and Brown, David M. and Willitsford, Adam H. and Edwards, Perry S. and Wyant, Andrea M. and Liu, Zhiwen Z. and Chadwick, C. Todd and Hallen, Hans}, year={2009}, pages={8 pp.} }
 @inproceedings{mehta_duel-hallen_hallen_2009, place={Boston, MA, United states}, title={Template design and propagation gain for multipath UWB channels with per-path frequency-dependent distortion}, url={http://dx.doi.org/10.1109/MILCOM.2009.5380076}, DOI={10.1109/MILCOM.2009.5380076}, abstractNote={Due to the large bandwidth allocation, Ultra-Wideband (UWB) channels exhibit frequency-dependent distortion of individual multipath components. This per-path distortion is particularly significant in outdoor UWB applications, where line-of-sight (LOS) or non-distorted reflected signals might not be available at the receiver, and the dominant propagation mechanisms involve shadowing (diffraction) or reflection by small objects (e.g. signs or lamp-posts). In this paper, a physical model is employed in the design of robust correlation receiver templates for outdoor single and multipath impulse radio channels characterized by per-path distortion. It is demonstrated that receivers which employ a set of partial derivatives templates are near-optimal in terms of energy capture while the simple transmit pulse template provides excellent complexity-performance trade-offs for most practical scenarios. Moreover, iterative receiver structures that maintain the energy capture in the presence of overlapping multipath components are investigated. Finally, a large gap between the propagation gains of the transmit pulses in the lower and upper bands of the FCC spectrum is characterized for several propagation mechanisms.*}, booktitle={Proceedings - IEEE Military Communications Conference MILCOM}, author={Mehta, Neil and Duel-Hallen, Alexandra and Hallen, Hans}, year={2009}, pages={7 pp. -} }
 @inproceedings{jia_duel-hallen_hallen_2008, place={Princeton, NJ, United states}, title={Improved long-range prediction with data-aided noise reduction for adaptive modulation systems}, url={http://dx.doi.org/10.1109/CISS.2008.4558694}, DOI={10.1109/CISS.2008.4558694}, abstractNote={A novel data-aided noise reduction (DANR) method is proposed to enhance the accuracy of long-range prediction (LRP) for wireless fading channels, thereby improving the spectral efficiency (SE) of adaptive modulation (AM) system enabled by the LRP. This method includes an adaptive pilot transmission mechanism, robust noise reduction and decision-directed channel estimation. An improved practical AM scheme is used to test the proposed DANR method. Since this method maintains low pilot rates, it results in higher SE than previously proposed noise reduction (NR) techniques, which rely on oversampled pilots. These conclusions are confirmed for practical prediction ranges using the standard Jakes model and our realistic physical model.}, booktitle={CISS 2008, The 42nd Annual Conference on Information Sciences and Systems}, author={Jia, Tao and Duel-Hallen, Alexandra and Hallen, Hans}, year={2008}, pages={1161–1166} }
 @inproceedings{jia_duel-hallen_hallen_2008, place={Piscataway, NJ, USA}, title={Performance of adaptive coded modulation enabled by long-range fading prediction with data-aided noise reduction}, url={http://dx.doi.org/10.1109/MILCOM.2008.4753263}, DOI={10.1109/MILCOM.2008.4753263}, abstractNote={Performance of uncoded adaptive modulation (UAM) and adaptive coded modulation (ACM) enabled by the long-range prediction (LRP) that utilizes data-aided noise reduction (DANR) is investigated for rapidly varying mobile radio channels. Due to improved prediction accuracy and low pilot rate, the DANR-aided LRP outperforms previously proposed prediction methods that rely on oversampled pilots to achieve noise reduction (NR). While ACM is more sensitive to prediction errors than UAM, utilization of DANR substantially increases its spectral efficiency (SE) relative to previously proposed methods. The set of SNR values and prediction ranges where positive coding gain is achieved by ACM enabled by DANR-aided LRP is determined. It is also demonstrated that adaptive modulation (AM) aided by LRP has better performance for the realistic physical model than for the Jakes model in the practical SNR range.}, booktitle={MILCOM 2008 - 2008 IEEE Military Communications Conference}, author={Jia, Tao and Duel-Hallen, A. and Hallen, H.}, year={2008}, pages={7 pp. -} }
 @article{lei_due-hallen_hallen_2008, title={Reliable adaptive modulation and interference mitigation for mobile radio slow frequency hopping channels}, volume={56}, ISSN={["1558-0857"]}, url={http://dx.doi.org/10.1109/TCOMM.2008.040662}, DOI={10.1109/TCOMM.2008.040662}, abstractNote={The long range fading prediction algorithm for Slow Frequency Hopping (SFH) systems is proposed and demonstrated to enable combined adaptive modulation and adaptive frequency diversity to mitigate the effects of fading and partial-band interference. Significant performance gains are demonstrated relative to non-adaptive methods in realistic mobile radio SFH channels where the total bandwidth does not exceed approximately 15 times the coherence bandwidth.}, number={3}, journal={IEEE TRANSACTIONS ON COMMUNICATIONS}, author={Lei, Ming and Due-Hallen, Alexandra and Hallen, Hans}, year={2008}, month={Mar}, pages={352–355} }
 @inproceedings{willitsford_chadwick_hallen_philbrick_2008, title={Resonance Raman measurements utilizing a deep UV source}, volume={6950}, url={http://dx.doi.org/10.1117/12.778253}, DOI={10.1117/12.778253}, abstractNote={Raman scattering techniques have long been used as unique identifiers for spectral fingerprints of chemical and biological species. Raman lidar has been utilized on a routine basis to remotely measure several constituents in the atmosphere. While Raman scattering is very reliable in uniquely identifying molecules, it suffers from very small scattering cross sections that diminish its usefulness at increased ranges and decreased concentrations of the species of interest. By utilizing a resonance Raman technique, where the laser excitation is tuned near an electronic absorption band, it is possible to increase the Raman scattering cross section. An optical parametric oscillator (OPO) with a UV tuning range of ~220 nm - 355 nm has been utilized to explore the wavelength dependence of Raman scattering for diamond, water, benzene, and toluene. Resonance enhancements of the Raman spectra have been studied.}, booktitle={Proc. SPIE - Int. Soc. Opt. Eng.}, author={Willitsford, A. and Chadwick, C.T. and Hallen, H. and Philbrick, C.R.}, year={2008}, month={Apr}, pages={69500–1} }
 @article{hallen_hallen_2008, title={Synthesis of carboxylic acid monolayers by ozonolysis of 10-undecenyltrichlorosilane SAMs}, volume={112}, ISSN={["1932-7455"]}, url={http://dx.doi.org/10.1021/jp709664r}, DOI={10.1021/jp709664r}, abstractNote={A high-yield method has been developed for the production of carboxyl-terminated alkylsiloxane monolayers on silicon using the ozonolysis and hydrolysis of 10-undecenyltrichlorosilane self-assembled monolayers (SAMs). Contact angles with water, a common measure of hydrophilicity and thus an effective measure of carboxyl termination, were brought to 0° on receding and approximately 16° on advancing, compared to 98° and 105° respectively before ozonation. Ellipsometry showed the presence of a full monolayer, 1 nm thick before ozonation, that decreases by about 0.2 nm during ozone treatment, largely due to the removal of a carbon atom in the process. This removed material coalesces as nanoparticles, observed with atomic force microscopy (AFM) imaging. The process has a surprisingly narrow ozone dose window, with excess ozonation resulting in complete layer removal. The carboxylate moieties allow further chemical modification of the surface in addition to the hydrophilic surface that can be produced by exposi...}, number={6}, journal={JOURNAL OF PHYSICAL CHEMISTRY C}, author={Hallen, Mark A. and Hallen, Hans D.}, year={2008}, month={Feb}, pages={2086–2090} }
 @article{hallen_2007, title={Nano-Raman Spectroscopy: Surface Plasmon Emission, Field Gradients, and Fundamentally Near Field Propagation Effects}, volume={3}, url={https://link.springer.com/article/10.1007/s12030-008-9013-1}, DOI={10.1007/s12030-008-9013-1}, abstractNote={Nano-Raman spectra differ from far-field Raman spectra. The differences result from a strong electric field gradient near the metal tip, propagation, and polarization, but the dependence upon probe-sample distance can only be explained by the inclusion of surface plasmons and the near-field, non-propagating terms of the dipole emission. A simple model based upon these components accurately describes distance-dependent data measured with a near-field scanning optical microscope. Our essentially near-field model will apply generally to Raman spectroscopy near a nanoscale conductor.}, note={– An invited paper}, number={3-4}, journal={NanoBiotechnology}, author={Hallen, H. D.}, year={2007}, pages={197–202} }
 @article{nanoscale optical imaging of pigment particles in paint with near-field scanning optical microscopy_2007, volume={25}, url={http://dx.doi.org/10.1116/1.2404686}, DOI={10.1116/1.2404686}, abstractNote={The distribution of pigment at the nano to micron scale illuminates the length scale of failure in paint samples. The authors use optical and topographical images from near-field scanning optical microscopy (NSOM) to compare a high quality paint sample with one that fails a standard quality control test based on visual inspection. NSOM provides the required nanometer to micrometer mesoscopic regime resolution and range, combined with simultaneous topographic and optical information. Features such as pigment clumping and pigment density fluctuations are simultaneously analyzed. The two types of samples are distinguished by fluctuations at different length scales. The authors observed individual pigment particles near the polymer surface of both samples.}, number={1}, journal={Journal of Vacuum Science and Technology B: Microelectronics and Nanometer Structures}, year={2007}, pages={54–57} }
 @article{clark_gurguis_hallen_2007, title={Nanoscale optical imaging of pigment particles in paint with near-field scanning optical microscopy}, volume={25}, number={1}, journal={Journal of Vacuum Science & Technology. B, Microelectronics and Nanometer Structures}, author={Clark, B. and Gurguis, G. and Hallen, H. D.}, year={2007}, pages={54–57} }
 @inproceedings{ma_duel-hallen_hallen_2007, place={Piscataway, NJ, USA}, title={Physical modeling and template design for UWB channels with per-path distortion}, url={https://doi.org/10.1109/MILCOM.2006.302545}, DOI={10.1109/MILCOM.2006.302545}, abstractNote={Recent experimental and physical modeling studies demonstrate that, as opposed to systems with smaller bandwidth, the ultra-wideband (UWB) channel exhibits frequency-dependent distortion of individual multipath components. This per-path distortion is particularly significant in outdoor UWB applications, where line-of-sight (LOS) or non-distorted reflected signals might not be available at the receiver (for example, in a canyon-like street). In these cases, the dominant propagation mechanisms involve shadowing (diffraction) and reflection by small objects (e.g. signs or a lamp-posts). In this paper, a physical model is developed to investigate the position-dependent distortion of the UWB pulse. The results indicate that both the shadowed pulse and the reflected pulse (by small objects with dimensions bounded by the wavelengths present in the signal) are distorted. Design of optimal and suboptimal templates for the correlation receiver are investigated. The UWB pulses that accommodate robust template choice given by the transmit pulse shape for all propagation conditions and satisfy the FCC spectral mask for outdoor channels are identified. Finally, we analyze the frequency-dependent propagation gain of the UWB channels in various outdoor conditions. This knowledge quantifies the potential benefits of adapting the transmitted signal to the dominant propagation mechanism}, note={– An invited paper}, booktitle={MILCOM 2006}, author={Ma, L. and Duel-Hallen, A. and Hallen, H.}, year={2007}, pages={7 pp. -} }
 @inbook{hallen_2007, title={Special Issue of the Journal NanoBiotechnology: Physical Methods of Nanophotonics}, volume={3}, url={https://link.springer.com/article/10.1007/s12030-009-9019-3}, DOI={10.1007/s12030-009-9019-3}, abstractNote={A sharp metal tip, or conducting nanoparticles, can concentrate light and electric field. This is a primary method by which photonics systems can be adapted for characterization and fabrication at the nanoscale: nanophotonics. In the process of confining the light, new processes have been identified, and novel schemes for enhancing the effects have been invented. Our understanding and ability to manipulate light and therefore materials at the nanoscale is now blossoming and is of great interest to a variety of disciplines. Our goal with this special issue is to present the current state of understanding, key insights that permit a qualitative grasp of the phenomena, and recent ideas to improve the localization and signal level. As the number of molecules sampled decreases, as it must for increased resolution, the signal level is also reduced. One of the most important realizations in this field to date is that it is possible to obtain sufficient enhancement that measurements are still possible. The articles in this issue can be broadly categorized as theoretical modeling, experimental characterization, or fabrication, although there is, and must be, an interplay between these in all the papers. In the theoretical paper by Tsukerman, Cajko, and Dai, the power of state of the art calculation methods for designing nanoscale optical concentrators is illustrated, and the plasmon that underlies much of the novel capabilities makes its appearance. Such advances are key to attaining measurable signal levels from small volumes. The paper by Dhawan, Gerhold, and VoDinh describes the fabrication (and modeling) of nanotip arrays, in this case for surface-enhanced Raman use. Raman spectroscopy is one of the most powerful optical techniques for identifying species, for measuring stress or confinement, and for quantitative analysis. It is therefore not surprising that it is used as a demonstration vehicle for nanoscale studies. The paper by Neacsu, Berweger, and Raschke show how it can be taken to the limit of single molecule Raman, by the use of tip-enhanced illumination. The paper by Hallen describes the plasmon effects in the detection (rather than illumination) part of the process. This caps our understanding of the various differences between nanoRaman and far-field Raman, mostly due to the presence of the tip, which have been measured over the past 15 or so years. The other commonly used optical technique is fluorescence. The paper by Cade, Culfaz, Eligal, RitmanMeer, Huang, Festy, and Richards describes the current capabilities available for this technique at the nanoscale, again tied to modeling studies so that an understanding of the processes is possible. The other two papers describe some of the most interesting efforts currently underway to fabricate nanoscale materials in a controlled fashion. Myhra uses tip-induced local anodic oxidation as a means to modify or etch nanoscale patterns into a surface, and Leggett describes how self-assembled monolayers can be used or modified with the optical fields of a sharp tip to produce patterns for their own use or as templates for further growth. The breadth and capabilities described in this special issue show the vitality of activities in nanophotonics. The amazing achievements to date are harbingers of the applications that will be developed over the next several years. Nanobiotechnol (2007) 3:147 DOI 10.1007/s12030-009-9019-3}, number={3-4}, booktitle={NanoBiotechnology}, publisher={Elsevier}, author={Hallen, Hans D.}, year={2007}, pages={147} }
 @inproceedings{duel-hallen_lei_yang_hallen_2006, place={Anaheim, CA, USA}, title={Feasibility of adaptive transmission for multicarrier and frequency hopping mobile radio systems}, url={http://137.226.34.227/dblp/db/conf/woc/woc2006.html}, note={– An invited paper}, booktitle={Proceedings of the Sixth IASTED International Multi-Conference on Wireless and Optical Communications}, author={Duel-Hallen, A. and Lei, Ming and Yang, Tung-Sheng and Hallen, H.}, year={2006}, pages={52–7} }
 @inproceedings{philbrick_liu_hallen_brown_willitsford_2006, title={Lidar Techniques Applied To Remote Detection of Chemical Species in the Atmosphere}, booktitle={Proceedings of the International Symposium on Spectral Sensing Research (ISSSR)}, author={Philbrick, C.R. and Liu, Z. and Hallen, H. and Brown, D. and Willitsford, A.}, year={2006} }
 @article{duel-hallen_hallen_yang_2006, place={USA}, title={Long range prediction and reduced feedback for mobil radio adaptive OFDM systems}, volume={5}, ISSN={["1558-2248"]}, url={http://dx.doi.org/10.1109/TWC.2006.04219}, DOI={10.1109/TWC.2006.04219}, abstractNote={Adaptive orthogonal frequency division multiplexing (AOFDM) modulation is a promising technique for achieving high data rates required for wireless multimedia services. To accomplish efficient adaptive channel loading, the channel state information (CSI) needs to be fed back to the transmitter. Since the fading channel varies rapidly for fast vehicle speeds, long range fading prediction (LRP) is required for mobile radio AOFDM to insure reliable adaptation. We use past channel observations to predict future CST and perform adaptive bit and power allocation for the OFDM system. We derive the minimum mean-square-error (MMSE) long-range channel prediction that utilizes the time and frequency domain correlation functions of the Rayleigh fading channel. Since the channel statistics are usually unknown, robust prediction methods that do not require the knowledge of the correlation functions are developed. Statistical model of the prediction error is created and used in the design of reliable adaptive modulation. In addition, several methods that significantly reduce the feedback load for mobile radio AOFDM systems are developed and compared. We use a standard sum-of-sinusoids model and our realistic physical model to validate performance of proposed methods. Simulation results demonstrate reliable performance and robustness of the proposed techniques, thus validating feasibility of AOFDM for rapidly varying mobile radio channels}, number={10}, journal={IEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS}, author={Duel-Hallen, Alexandra and Hallen, Hans and Yang, Tung-Sheng}, year={2006}, month={Oct}, pages={2723–2733} }
 @article{nusz_johannes_allen_hallen_2006, title={Self-assembled monolayer coating of biological probes to avoid protein adhesion}, volume={2}, url={https://link.springer.com/article/10.1007/s12030-006-0008-5}, DOI={10.1007/s12030-006-0008-5}, abstractNote={Probes designed to locally illuminate structures within plant cells are described. The probes studied are etch-sharpened single mode optical fibers, coated with aluminum, similar to probes used for near-field scanning optical microscopy. We find that cellular material adheres to the probes that are not coated with a self-assembled monolayer octadecyltrichlorosilane. The hydrophobic monolayer coating enabled these probes to be inserted into and removed from plant cells with no protein adhesion to the probes. This allows probe reinsertion and it causes less damage to the target cell, greatly facilitating in vivo optical study of cells.}, number={1-2}, journal={NanoBiotechnology}, author={Nusz, Greg and Johannes, Eva and Allen, Nina Strömgren and Hallen, Hans D.}, year={2006}, pages={61–65} }
 @inproceedings{lei_duel-hallen_hallen_2005, place={New York, NY, United states}, title={Enabling adaptive modulation and interference mitigation for slow frequency hopping communications}, volume={2005}, url={http://dx.doi.org/10.1109/SPAWC.2005.1506045}, DOI={10.1109/SPAWC.2005.1506045}, abstractNote={We investigate the optimal minimum mean square error (MMSE) long range prediction (LRP) algorithm for slow frequency hopping (SFH) systems that employ coherent detection. Statistical model of the prediction accuracy is developed and used in the design of reliable adaptive modulation techniques. Moreover, adaptive modulation is combined with adaptive transmitter frequency diversity to mitigate the effect of fading and partial-band interference. Both standard Jakes model and a realistic physical model are employed to test the performance. Analysis and simulation results show that significant performance gains can be achieved relative to non-adaptive methods.}, booktitle={IEEE Workshop on Signal Processing Advances in Wireless Communications, SPAWC}, author={Lei, Ming and Duel-Hallen, Alexandra and Hallen, Hans}, year={2005}, pages={348–352} }
 @article{jahncke_brandt_fellows_hallen_2004, title={Choosing a preamplifier for tuning fork signal detection in scanning force microscopy}, volume={75}, ISSN={["0034-6748"]}, url={http://dx.doi.org/10.1063/1.1777386}, DOI={10.1063/1.1777386}, abstractNote={In scanning probe microscopy, it is critical to maintain small probe sample separations for high resolution imaging. Quartz crystal tuning forks are typically used for detecting shear forces in near-field scanning optical microscopy and normal forces in other atomic force-related microscopies. In this article we compare several tuning fork based detection schemes to determine which solution gives the best signal to noise ratio. The high impedance and low signals produced by the tuning fork necessitate care in selection of an appropriate preamplifier. We find that a carefully guarded voltage preamplifier sensing a mechanically driven tuning fork performs the best, but an electrically driven fork with a current preamplifier offers simpler construction with only 25% lower signal to noise ratio on average.}, note={Also appeared in the Virtual Journal of Nanoscale Science and Technology 10 (9), Aug. 30 (2004).}, number={8}, journal={REVIEW OF SCIENTIFIC INSTRUMENTS}, author={Jahncke, CL and Brandt, O and Fellows, KE and Hallen, HD}, year={2004}, month={Aug}, pages={2759–2761} }
 @inproceedings{yang_duel-hallen_hallen_2004, place={Monterey, CA, United states}, title={Enabling adaptive OFDM for mobile radio channels}, volume={2}, url={https://doi.org/10.1109/MILCOM.2004.1494882}, DOI={10.1109/MILCOM.2004.1494882}, abstractNote={Adaptive orthogonal frequency division multiplexing (AOFDM) modulation is a promising technique for achieving high data rates required for wireless multimedia services. To accomplish efficient adaptive channel loading, the channel state information (CSI) needs to be fed back to the transmitter. Since the fading channel varies rapidly for fast vehicle speeds, long range fading prediction (LRP) is required for mobile radio AOFDM to insure reliable adaptation. We use past channel observations to predict future CSI and perform adaptive bit and power allocation for the OFDM system. We derive the minimum mean-square-error (MMSE) long-range channel prediction that utilizes the time and frequency domain correlation functions of the Rayleigh fading channel. Since the channel statistics are usually unknown, robust prediction methods that do not require the knowledge of the correlation functions are developed. A statistical model of the prediction error is created and used in the design of reliable adaptive modulation. In addition, several methods that significantly reduce the feedback load for mobile radio AOFDM systems are developed and compared. We use a standard sum-of-sinusoids model and a novel physical model to test our algorithm. Simulation results demonstrate reliable performance and robustness of the proposed techniques, thus validating the feasibility of AOFDM for rapidly varying mobile radio channels.}, booktitle={Proceedings - IEEE Military Communications Conference MILCOM}, author={Yang, Tung-Sheng and Duel-Hallen, Alexandra and Hallen, Hans}, year={2004}, pages={704–710} }
 @article{yang_duel-hallen_hallen_2004, place={USA}, title={Reliable adaptive modulation aided by observations of another fading channel}, volume={52}, ISSN={["1558-0857"]}, url={http://dx.doi.org/10.1109/TCOMM.2004.826369}, DOI={10.1109/TCOMM.2004.826369}, abstractNote={Adaptive transmission techniques, such as adaptive modulation and coding, adaptive power control, adaptive transmitter antenna diversity, etc., generally require precise channel estimation and feedback of channel state information (CSI). For fast vehicle speeds, reliable adaptive transmission also requires long-range prediction of future CSI, since the channel conditions are rapidly time variant. In this paper, we propose using past channel observations of one carrier to predict future CSI and perform adaptive modulation without feedback for another correlated carrier. We derive the minimum mean-square error (MMSE) long-range channel prediction that uses the time- and frequency-domain correlation function of the Rayleigh fading channel. An adaptive MMSE prediction method is also proposed. A statistical model of the prediction error that depends on the frequency and time correlation is developed and is used in the design of reliable adaptive modulation methods. We use a standard stationary fading channel model (Jakes model) and a novel physical channel model to test our algorithm. Significant gains relative to nonadaptive techniques are demonstrated for sufficiently correlated channels and realistic prediction range.}, number={4}, journal={IEEE TRANSACTIONS ON COMMUNICATIONS}, author={Yang, TS and Duel-Hallen, A and Hallen, H}, year={2004}, month={Apr}, pages={605–611} }
 @inproceedings{hallen_taylor_2004, place={Dallas, TX}, title={The Electric Field Near a Split-Tip Proximal Probe for Molecular Orientation}, booktitle={Proceedings of the 2004 NSF DMII Grantee's Conference}, author={Hallen, H.D. and Taylor, Michael}, year={2004} }
 @inproceedings{yang_duel-hallen_hallen_2004, place={Boulder, CO}, title={Validation of Adaptive Transmission for Realistic Single- and Multi-Carrier Mobile Radio Channels}, note={– An invited paper}, booktitle={Proceedings of URSI'04, Special Session on Propagation Effects in Wireless Communications Channels}, author={Yang, T. S. and Duel-Hallen, A. and Hallen, H.}, year={2004}, pages={75} }
 @inproceedings{hallen_taylor_2003, place={Birmingham, AL}, title={A Split-Tip Proximal Probe for Nanoscale Deposition of Molecules with Controlled In-Plane Orientation}, booktitle={Proceedings of the 2003 NSF DMII Grantee's Conference}, author={Hallen, H.D. and Taylor, Michael}, year={2003} }
 @inproceedings{yang_duel-hallen_hallen_2003, place={Piscataway, NJ, USA}, title={Long range fading prediction to enable adaptive transmission at another carrier}, url={https://doi.org/10.1109/SPAWC.2003.1318949}, DOI={10.1109/SPAWC.2003.1318949}, abstractNote={Adaptive transmission techniques, such as adaptive modulation and coding, adaptive power control, adaptive transmitter antenna diversity, etc., usually require accurate channel estimation and feedback of channel state information (CSI). For fast vehicle speeds, reliable adaptive transmission also requires long range prediction (LRP) of future CSI since the channel conditions are rapidly time-variant. In this paper, we propose to use past channel observations of one carrier to predict future CSI and perform adaptive modulation without feedback for another correlated carrier. We develop the minimum mean-square-error (MMSE) long range channel prediction algorithm that utilizes the time and frequency domain correlation function of the Rayleigh fading channel. An adaptive MMSE prediction method is also proposed. A statistical model of the prediction error that depends on the frequency and time correlation is developed and is used in the design of reliable adaptive modulation methods. We use a standard stationary fading channel model (Jakes model) and a novel physical channel model to test our algorithm. Significant gains relative to non-adaptive techniques are demonstrated for sufficiently correlated channels and realistic prediction range.}, booktitle={2003 4th IEEE Workshop on Signal Processing Advances in Wireless Communications - SPAWC 2003 (IEEE Cat. No.03EX689)}, author={Yang, Tung-Sheng and Duel-Hallen, A. and Hallen, H.}, year={2003}, pages={195–9} }
 @article{huerth_hallen_2003, title={Quantitative method of image analysis when drift is present in a scanning probe microscope}, volume={21}, url={http://dx.doi.org/10.1116/1.1553972}, DOI={10.1116/1.1553972}, abstractNote={We present a quantitative method of image analysis that automatically corrects for system drift in comparisons of images before and after an event under study. The method utilizes two-dimensional correlations followed by automatic z scaling, shifting, and the calculation of a difference image from the before and after images. Optical reflectance near-field scanning optical microscope (NSOM) images that show the effects of oxygen electromigration are used as an example system. The tunnel current that induces the atomic motion is provided by the metallic NSOM aperture in a scanning tunneling microscope mode. The analyzed (difference) image clearly shows the effects that are otherwise obscured by native oxygen concentration variations, demonstrating the utility of the method. The related issues of false positive/negative probabilities for given noise levels and thresholds are also discussed.}, number={2}, journal={Journal of Vacuum Science and Technology B: Microelectronics and Nanometer Structures}, author={Huerth, S.H. and Hallen, Hans}, year={2003}, pages={714–718} }
 @article{huerth_hallen_moeckly_2003, title={Spectroscopy of voltage dependence of oxygen movement in YBa2Cu3O7-δ}, volume={67}, url={http://dx.doi.org/10.1103/PhysRevB.67.180506}, DOI={10.1103/PhysRevB.67.180506}, abstractNote={Injection of few-volt electrons at room temperature can displace oxygen atoms in the lattice of yttrium barium cuprate (YBCO). The metal cladding of a near-field scanning optical microscope (NSOM) probe tip is used as a tunnel electrode for locally injecting the electrons with controlled energies. The NSOM optical signal is used to detect changes in the local oxygen concentration. The data support bond breaking in a Franck-Condon-like effect causing enhanced diffusion of oxygen atoms in the lattice. The voltage dependence is consistent with the band structure of YBCO.}, number={18}, journal={Phys. Rev., B, Condens, Matter Mater. Phys.}, author={Huerth, S.H. and Hallen, Hans and Moeckly, B.}, year={2003}, month={May}, pages={180506–1} }
 @article{jahncke_hallen_2003, title={Stabilizing wide bandwidth, tuning fork detected force feedback with nonlinear interactions}, volume={93}, ISSN={["1089-7550"]}, url={http://dx.doi.org/10.1063/1.1528307}, DOI={10.1063/1.1528307}, abstractNote={Near-field scanning optical microscope force feedback can be destabilized by the anisotropy in response times engendered by nonlinear tip sample interactions. This nonlinear interaction, the tapping of the tip on layers adsorbed on the sample, is important when the intrinsic damping of the system is low. We present strong evidence of tapping on adlayers rather than the sample surface at operational distances, and numerically solve a model to find the dynamics of tip motion. These results illuminate the origins of feedback problems when using tuning fork detection of oscillation amplitude, and show an optimal technique that uses the rapid response of the tip-adlayer nonlinearity to circumvent the slow damping response and enable wide bandwidth, stable distance regulation for these systems.}, number={2}, journal={JOURNAL OF APPLIED PHYSICS}, author={Jahncke, CL and Hallen, HD}, year={2003}, month={Jan}, pages={1274–1282} }
 @article{hallen_ayars_jahncke_2003, place={UK}, title={The effects of probe boundary conditions and propagation on nano-Raman spectroscopy}, volume={210}, ISSN={["0022-2720"]}, url={http://dx.doi.org/10.1046/j.1365-2818.2003.01138.x}, DOI={10.1046/j.1365-2818.2003.01138.x}, abstractNote={Summary}, number={3}, journal={JOURNAL OF MICROSCOPY-OXFORD}, author={Hallen, HD and Ayars, EJ and Jahncke, CL}, year={2003}, month={Jun}, pages={252–254} }
 @article{hallen_jahncke_2003, place={UK}, title={The electric field at the apex of a near-field probe: implications for nano-Raman spectroscopy}, volume={34}, ISSN={["0377-0486"]}, url={http://dx.doi.org/10.1002/jrs.1048}, DOI={10.1002/jrs.1048}, abstractNote={Abstract}, note={– An invited paper}, number={9}, journal={JOURNAL OF RAMAN SPECTROSCOPY}, author={Hallen, HD and Jahncke, CL}, year={2003}, month={Sep}, pages={655–662} }
 @inproceedings{hallen_duel-hallen_hu_yang_lei_2002, place={Anaheim, CA, United states}, title={A physical model for wireless channels to provide insights for long range prediction}, volume={1}, url={http://dx.doi.org/10.1109/MILCOM.2002.1180517}, DOI={10.1109/MILCOM.2002.1180517}, abstractNote={Algorithms that predict the wireless channel for up to a few wavelengths cannot be adequately tested with stationary models. Ray-tracing or FDTD methods do not provide insights into the relationship between reflector configurations and the performance of long-range prediction. Therefore, we present a novel model that: (1) creates non-stationary datasets to test our previously proposed adaptive long range prediction algorithm, which enables practical realization of adaptive transmission techniques; (2) classifies the reflector geometries that have typical or most severe parameter variations, so that the reflector configurations for test datasets can be appropriately chosen; (3) provides limits on the speed of adaptation needed for an algorithm to predict the channel significantly into the future, and thereby reveal the timing of future deep fades, etc.; (4) illuminates the origins of the temporal and statistical properties of measured data. The algorithm performs similarly on channels given by the physical model or actual measured data, but differently on a channel simulated by the stationary Jakes model. The insights of the model accurately describe the performance of the algorithm in several scattering environments when prediction is employed with adaptive power control and adaptive modulation. Moreover, we study limits of the long-range prediction at frequencies other than the observed frequency, of importance in correlated uplink and downlink transmission, orthogonal frequency division multiplexing (OFDM) and frequency-hopping systems.}, booktitle={Proceedings - IEEE Military Communications Conference MILCOM}, author={Hallen, Hans and Duel-Hallen, Alexandra and Hu, Shengquan and Yang, Tung-Shen and Lei, Ming}, year={2002}, pages={627–631} }
 @inproceedings{hu_duel-hallen_hallen_2002, place={Piscataway, NJ, USA}, title={Adaptive modulation for transmitter antenna diversity mobile radio systems}, url={http://dx.doi.org/10.1109/ISIT.2002.1023377}, DOI={10.1109/ISIT.2002.1023377}, abstractNote={Adaptive transmission techniques such as adaptive modulation and transmitter antenna diversity have been proposed for mobile wireless systems to satisfy high data-rate service requirements. In rapidly time variant channels, these methods need the knowledge of future fading conditions and therefore require accurate long range fading prediction. We investigate three combined adaptive modulation and transmitter diversity schemes in conjunction with our previously proposed long-range fading channel prediction (LRP) algorithm. It is demonstrated that the novel combined schemes achieve significantly higher data rates than conventional adaptive modulation methods when aided by the LRP. Performance-complexity trade-off for several combined adaptive methods is examined. In addition to utilizing the Jakes fading model to test the proposed methods, we validate the LRP for antenna diversity systems using a novel realistic fading channel model.}, booktitle={Proceedings IEEE International Symposium on Information Theory,}, author={Hu, Shengquan and Duel-Hallen, A. and Hallen, H.}, year={2002}, pages={105} }
 @inproceedings{jahncke_hallen_2002, place={Rochester, NY}, title={Bandwidth enhancement for shear-force feedback by exploiting the nonlinear probe-sample interaction}, booktitle={Proc. of the 7th International Conference on Near-field Optics, NFO-7}, author={Jahncke, C.L. and Hallen, H.D.}, year={2002}, pages={41} }
 @article{rosa_hallen_2002, title={Compact method for optical induction of proximal probe heating and elongation}, volume={41}, url={http://dx.doi.org/10.1364/AO.41.002015}, DOI={10.1364/AO.41.002015}, abstractNote={A tapered, metal-coated, optical fiber probe will elongate when heated by light input through a fiber. The induced motion can be used for data storage or nanostructuring of a surface. The elongation produced by this alignment-free system is measured with force feedback in a near-field scanning optical microscope (NSOM). The input light intensity controls the elongation magnitude, which ranges from a few nanometers to more than 100 nm. A 0.5-mW input energy yields ∼20 nm of probe elongation. The elongation quantified here can create artifacts in any experiment using pulsed laser light with a NSOM or an atomic force microscope.}, number={10}, journal={Appl. Opt.}, author={Rosa, A.H. La and Hallen, Hans}, year={2002}, month={Apr}, pages={2015–19} }
 @article{jahncke_huerth_clark_hallen_2002, title={Dynamics of the tip-sample interaction in near-field scanning optical microscopy and the implications for shear force as an accurate distance measure}, volume={81}, ISSN={["1077-3118"]}, url={http://dx.doi.org/10.1063/1.1520711}, DOI={10.1063/1.1520711}, abstractNote={Near-field scanning optical microscopy uses shear-force feedback as the primary method to control the probe–sample distance. We describe the nonlinear interaction between the tip and sample with a simple truncated driven harmonic oscillator model. The model accurately describes the measured dynamics of this system. Insights are gained into the mechanism behind this interaction, and we give strong evidence that the probe taps on sample surface adlayers in normal operation, but will tap the underlying sample surface when the oscillation is nearly quenched.}, note={Also appeared in the Virtual Journal of Nanoscience and Technology 6 (22), (2002).}, number={21}, journal={APPLIED PHYSICS LETTERS}, author={Jahncke, CL and Huerth, SH and Clark, B and Hallen, HD}, year={2002}, month={Nov}, pages={4055–4057} }
 @inproceedings{jahncke_ayars_hallen_2002, place={Rochester, NY}, title={Gradient-Field Raman: Selection Rules in the Near Field}, booktitle={Proc. of the 7th International Conference on Near-field Optics, NFO-7}, author={Jahncke, C.L. and Ayars, E.J. and Hallen, H.D.}, year={2002}, pages={142} }
 @inproceedings{la rosa_biehler_sinharay_hallen_2002, place={Seattle, WA, United states}, title={Interferometric method to characterize thermal elongation of near-field optics probes}, volume={4777}, url={http://dx.doi.org/10.1117/12.472244}, DOI={10.1117/12.472244}, abstractNote={This paper presents a new method that exploits the interference and polarization properties of light to monitor, in real time, the rapid thermal elongation of near-field optical probes. The typically flat (nanometer in size) morphology of the probe apex serves as one mirror of a Fabry-Perot type cavity; a flat semitransparent metal coated surface constitutes the other mirror. The optical-interferometry set-up permits distance acquisition with a high frequency bandwidth (compared to other methods based on electronic feedback) while control of the light polarization allows an increase of the signal to noise ratio of the measurements.}, booktitle={Proceedings of SPIE - The International Society for Optical Engineering}, author={La Rosa, A. and Biehler, B. and Sinharay, A. and Hallen, H.}, year={2002}, pages={420–423} }
 @article{jahncke_ayars_hallen_2002, title={Raman selection rules in the presence of an electric field gradient}, volume={8}, url={http://dx.doi.org/10.1017/S1431927602104223}, DOI={10.1017/S1431927602104223}, abstractNote={Most of the Raman work (except NSOM) performed near metal surfaces has been concerned with SERS. We do not comment on SERS models here, but note which peaks not normally seen in Raman are expressed. We discuss elsewhere [4] the SERS work on benzene, several cyanides, and C 60 . Here we concentrate on pyrazine, which is a benzene ring with two opposing carbons replaced with N. It has been studied extensively since it is complicated enough to have several vibration modes (with D 2h symmetry), but simple enough that the bulk vibrations are either Raman or IR active, but not both. It has been studied on Ni, Cu, Au, and Ag. [5-7] When shifts of the usual Raman lines between bulk or solution and surface Raman are used as a gauge of substrate interaction, the pyrazine reacts most with Ni, then Au and Cu, and least with Ag. [5] We therefore concentrate on the Ag results. The strong IR line at1484 cm -1 is the strongest new line in the various papers, but with intensity comparable to the normally allowed modes in some studies [6, 7], and much reduced from them in others. [5] Another strong IR line at 1418 cm -1 is often nearly as strong as the 1484 cm -1 line, [5, 7] but absent in other studies [6] and for studies involving the strongly interacting metals. [5] The strong IR – strong new Raman lines analogy for GFR holds, but the irreproducibility is troublesome. Perhaps the orientation of the molecules, which matters in GFR, depends strongly on sample preparation. The intensity of these lines is sensitive to electrochemical potential, [7] and the orientation of pyrazine on Ni does change with concentration, lying flat at low concentrations and on-end at higher concentrations. [5] This could explain why these in-plane (so not GFR affected for flat orientation) vibrations are not completely reproducible. In NSOM, a sharpened optical fiber is coated with aluminum to form an aperture. The probe is positioned near the surface under lateral force feedback. The NSOM is used in illumination mode, with 514 nm Ar ion laser light coupled into the fiber probe. Reflected light is collimated with a 0.50 NA lens, passed through a holographic filter, focused into a Czerny-Turner spectrometer, and finally collected onto a cooled (-45 C) CCD camera. A near field spectra of KTP is shown in Fig. 1. The general shape is typical of the spectra at various distances from the sample. This region of the Raman spectrum contains vibrations primarily from TiO 6 stretching modes in the KTP. The near-field peak shown in green in figure 1 is the strong totally symmetric, A1, vibration mode, which has been observed before in both near- and far-field measurements. [3] The changes observed as the probe approaches the surface are rather small, so we resort to comparative spectra. Several spectra taken far from the surface were averaged and subtracted from single spectra acquired closer to the surface. Two peaks at different energies than the original peaks are observed: the B1 peak (not allowed in the}, number={S02}, journal={Microscopy and Microanalysis}, publisher={Cambridge University Press}, author={Jahncke, C. L. and Ayars, E. J. and Hallen, H. D.}, year={2002}, pages={1518–1519} }
 @inproceedings{yang_duel-hallen_hallen_2002, place={Piscataway, NJ, USA}, title={Reliable adaptive modulation using long-range prediction at a different carrier frequency}, url={http://dx.doi.org/10.1109/ISIT.2002.1023631}, DOI={10.1109/ISIT.2002.1023631}, abstractNote={For fast vehicle speeds, reliable adaptive transmission requires prediction of future channel state information (CSI) since the channel conditions are rapidly time-variant. We propose to use past channel observations of one carrier to predict future CSI and perform adaptive modulation for another correlated carrier. A statistical model of the prediction error that depends on the frequency and time correlation is developed and is used in the design of reliable adaptive modulation methods. Significant gains relative to nonadaptive techniques are demonstrated for sufficiently correlated channels and realistic prediction range. Both Jakes (1974) fading model and a novel realistic physical model of frequency selective fading are used to validate performance of the proposed method.}, booktitle={Proceedings IEEE International Symposium on Information Theory,}, author={Yang, Tung-Sheng and Duel-Hallen, A. and Hallen, H.}, year={2002}, pages={359-} }
 @inproceedings{huerth_hallen_2002, place={Rochester, NY}, title={Tunnel-Electron-Induced Oxygen Movement in YBa2Cu3O7-δ Measured with Near-Field Optical Microscopy}, booktitle={Proc. of the 7th International Conference on Near-field Optics, NFO-7}, author={Huerth, S. H. and Hallen, H.D.}, year={2002}, pages={208} }
 @inproceedings{hallen_hu_lei_duel-hallen_2001, place={Calgary, Alta., Canada}, title={A physical model for wireless channels to understand and test long range prediction of flat fading}, volume={1}, booktitle={Wireless 2001. 13th International Conference on Wireless Communications. Proceedings}, author={Hallen, H. and Hu, Shengquan and Lei, Ming and Duel-Hallen, A.}, year={2001}, pages={99–107} }
 @article{tanner_loebach_cook_hallen_2001, title={A pulsed jumping ring apparatus for demonstration of Lenz's law}, volume={69}, ISSN={["0002-9505"]}, url={http://dx.doi.org/10.1119/1.1376377}, DOI={10.1119/1.1376377}, abstractNote={Lenz’s law is often demonstrated in classrooms by the use of Elihu Thomson’s jumping ring. However, it is ironic that a thorough analysis of the physics of the ac jumping ring reveals that the operation is due mainly to a phase difference, not Lenz’s law. A complete analysis of the physics behind the ac jumping ring is difficult for the introductory student. We present a design for a pulsed jumping ring which can be fully described by the application of Lenz’s law. Other advantages of this system are that it lends itself to a rigorous analysis of the force balances and energy flow. The simple jumping ring apparatus closely resembles Thomson’s, but is powered by a capacitor bank. The jump heights were measured for several rings as a function of energy stored in the capacitors. A simple model describes the data well. Currents in both the drive coil and ring are measured and that of the drive coil modeled to illuminate some properties of the capacitors. An analysis of the energy flow in the system explains the higher jump heights, to 2 m, when the ring is cooled.}, number={8}, journal={AMERICAN JOURNAL OF PHYSICS}, author={Tanner, P and Loebach, J and Cook, J and Hallen, HD}, year={2001}, month={Aug}, pages={911–916} }
 @article{ayars_jahncke_paesler_hallen_2001, place={UK}, title={Fundamental differences between micro- and nano-Raman spectroscopy}, volume={202}, ISSN={["0022-2720"]}, url={http://dx.doi.org/10.1046/j.1365-2818.2001.00878.x}, DOI={10.1046/j.1365-2818.2001.00878.x}, abstractNote={Electric field polarization orientations and gradients close to near‐field scanning optical microscope (NSOM) probes render nano‐Raman fundamentally different from micro‐Raman spectroscopy. With x‐polarized light incident through an NSOM aperture, transmitted light has x, y and z components allowing nano‐Raman investigators to probe a variety of polarization configurations. In addition, the strong field gradients in the near‐field of a NSOM probe lead to a breakdown of the assumption of micro‐Raman spectroscopy that the field is constant over molecular dimensions. Thus, for nano‐Raman spectroscopy with an NSOM, selection rules allow for the detection of active modes with intensity dependent on the field gradient. These modes can have similar activity as infra‐red absorption modes. The mechanism can also explain the origin and intensity of some Raman modes observed in surface enhanced Raman spectroscopy.}, number={1}, journal={JOURNAL OF MICROSCOPY-OXFORD}, author={Ayars, EJ and Jahncke, CL and Paesler, MA and Hallen, HD}, year={2001}, month={Apr}, pages={142–147} }
 @inproceedings{hu_duel-hallen_hallen_2001, place={Banff, Canada}, title={Selective Transmitter Diversity and Adaptive Power Control for Realistic Mobile Radio Channels}, booktitle={Proceedings of WOC'2001}, author={Hu, S. and Duel-Hallen, A. and Hallen, H.}, year={2001}, pages={178–182} }
 @inproceedings{hu_duel-hallen_hallen_2000, place={Serrento, Italy}, title={Adaptive modulation using long range prediction for flat rayleigh fading channels}, url={https://doi.org/10.1109/ISIT.2000.866451}, DOI={10.1109/ISIT.2000.866451}, abstractNote={We theoretically analyze the statistical behavior of prediction errors generated by our previously proposed (Duel-Hallen et al. 2000) long range prediction algorithm, and investigate adaptive modulation design using predicted channel state information (CSI). Both numerical and simulation results show that accurate prediction of the fading channel far ahead makes adaptive transmission feasible for rapidly time-varying mobile radio channels.}, booktitle={IEEE International Symposium on Information Theory - Proceedings}, author={Hu, S. and Duel-Hallen, A. and Hallen, H.}, year={2000}, pages={159 -} }
 @article{ayars_hallen_jahncke_2000, title={Electric field gradient effects in Raman spectroscopy}, volume={85}, ISSN={["0031-9007"]}, url={http://dx.doi.org/10.1103/PhysRevLett.85.4180}, DOI={10.1103/PhysRevLett.85.4180}, abstractNote={Raman spectra of materials subject to strong electric field gradients, such as those present near a metal surface, can show significantly altered selection rules. We describe a new mechanism by which the field gradients can produce Raman-like lines. We develop a theoretical model for this "gradient-field Raman" effect, discuss selection rules, and compare to other mechanisms that produce Raman-like lines in the presence of strong field gradients. The mechanism can explain the origin and intensity of some Raman modes observed in SERS and through a near-field optical microscope (NSOM-Raman).}, number={19}, journal={PHYSICAL REVIEW LETTERS}, author={Ayars, EJ and Hallen, HD and Jahncke, CL}, year={2000}, month={Nov}, pages={4180–4183} }
 @article{huerth_taylor_hallen_moeckly_2000, title={Electromigration in YBa2Cu3O7-delta using a metal clad near-field scanning optical microscope probe}, volume={77}, ISSN={["0003-6951"]}, url={http://dx.doi.org/10.1063/1.1314296}, DOI={10.1063/1.1314296}, abstractNote={Electromigration or electron-induced-migration (EIM) of oxygen in the hightemperature superconductor YBa2Cu3O7−δ alters the superconducting properties through variations in the oxygen concentration. We study this process with unprecedented spatial resolution and find that the transport of oxygen through a grain boundary into a neighboring grain is unlikely, and that hot electron effects dominate the mechanism for EIM in this system. The extent of the EIM effects implies that grain boundary scattering is strong for these electrons. EIM is induced with the tunnel current from the metal cladding on a near-field optical microscope (NSOM). Variations in the oxygen concentration due to fabrication, aging, and electromigration are imaged optically and corroborated to the grain structure.}, number={14}, journal={APPLIED PHYSICS LETTERS}, author={Huerth, SH and Taylor, MP and Hallen, HD and Moeckly, BH}, year={2000}, month={Oct}, pages={2127–2129} }
 @inproceedings{hu_duel-hallen_hallen_2000, title={Long-Range Prediction Makes Adaptive Modulation Feasible for Realistic Mobile Radio Channels}, booktitle={Proc. of 34rd Annual Conference on Information Sciences and Systems CISS'2000}, author={Hu, S. and Duel-Hallen, A. and Hallen, H.}, year={2000}, pages={WP4–7 - WP4–13} }
 @article{duel-hallen_hu_hallen_2000, title={Long-range prediction of fading signals - Enabling adapting transmission for mobile radio channels}, volume={17}, ISSN={["1558-0792"]}, url={http://dx.doi.org/10.1109/79.841729}, DOI={10.1109/79.841729}, abstractNote={It was previously proposed to adapt several transmission methods, including modulation, power control, channel coding, and antenna diversity to rapidly time variant fading channel conditions. Prediction of the channel coefficients several tens-to-hundreds of symbols ahead is essential to realize these methods in practice. We describe a novel adaptive long-range fading channel prediction algorithm (LRP) and its utilization with adaptive transmission methods. The LRP is validated for standard stationary fading models and tested with measured data and with data produced by our novel realistic physical channel model. Both numerical and simulation results show that long-range prediction makes adaptive transmission techniques feasible for mobile radio channels.}, note={– An invited paper}, number={3}, journal={IEEE SIGNAL PROCESSING MAGAZINE}, author={Duel-Hallen, A and Hu, SQ and Hallen, H}, year={2000}, month={May}, pages={62–75} }
 @inproceedings{hallen_rosa_2000, place={Research Triangle Park, NC}, title={Near-Field Scanning Optical Microscopy as an Imaging Tool for Silicon Carrier Processes}, booktitle={Proc. of International Conference on Silicon Dielectric Interfaces}, author={Hallen, H. D. and Rosa, A. H. La}, year={2000} }
 @inbook{ayars_paesler_hallen_2000, title={Near-field Raman spectroscopy: electric field gradient effects}, volume={165}, ISBN={0750306858}, number={2000}, booktitle={Microbeam Analysis 2000: proceedings of the Second Conference of the International Union of Microbeam Analysis Societies held in Kailua-Kona, Hawaii, 9-14 July 2000}, publisher={Bristol: Institute of Physics Publishing}, author={Ayars, E. J. and Paesler, M. A. and Hallen, H. D.}, editor={Williams, D. B. and Shimizu, R.Editors}, year={2000}, pages={115–116} }
 @inproceedings{ayars_paesler_hallen_2000, place={Kailua-Kona, Hawaii}, title={Near-field Raman spectroscopy: electric field gradient effects}, volume={165:1}, url={https://books.google.com/books?id=Ay6KGRmX-3gC&q=ayars&f=false#v=snippet&q=ayars&f=false}, note={– An invited paper}, booktitle={Proceedings of IUMAS 2000, The 2000 Meeting of the International Union of Microbeam Analysis Societies}, author={Ayars, E. J. and Paesler, M. A. and Hallen, H. D.}, year={2000}, pages={115} }
 @inproceedings{huerth_hallen_2000, place={Charlotte, NC}, title={Optical characterization of YBCO using a near-field scanning optical Microscope}, booktitle={Proceedings of the Southeast Regional Meeting on Optoelectronics, Photonics, and Imaging}, author={Huerth, S. H. and Hallen, H. D.}, year={2000} }
 @article{ayars_hallen_2000, title={Surface enhancement in near-field Raman spectroscopy}, volume={76}, ISSN={["0003-6951"]}, url={http://dx.doi.org/10.1063/1.126818}, DOI={10.1063/1.126818}, abstractNote={The intensity and selection rules of Raman spectra change as a metal surface approaches the sample. We study the distance dependence of the new Raman modes with a near-field scanning optical microscope (NSOM). The metal-coated NSOM probe provides localized illumination of a metal surface with good distance control. Spectra are measured as the probe approaches the surface, and the changes elucidated with difference spectra. Comparisons to a theoretical model for Raman excitation by evanescent light near the probe tip indicate that while the general trends are well described, the data show oscillations about the model.}, number={26}, journal={APPLIED PHYSICS LETTERS}, author={Ayars, EJ and Hallen, HD}, year={2000}, month={Jun}, pages={3911–3913} }
 @inproceedings{hu_hallen_duel-hallen_1999, title={Adaptive Power Control Using Long Range Prediction for Realistic Fast Fading Channel Models and Measured Data}, url={https://pdfs.semanticscholar.org/c77a/2149dd49861fd5f3c1c54db7a23fa549f0d6.pdf}, booktitle={5th International Symposium on Communication Theory and Application ISCTA'99}, author={Hu, S. and Hallen, H. and Duel-Hallen, A.}, year={1999}, pages={118–120} }
 @inproceedings{eyceoz_hu_duee-hallen_hallen_1999, place={Piscataway, NJ, USA}, title={Adaptive prediction, tracking and power adjustment for frequency non-selective fast fading channels}, url={http://dx.doi.org/10.1109/CTMC.1999.790226}, DOI={10.1109/CTMC.1999.790226}, abstractNote={Previously, the authors introduced a novel algorithm for long range prediction of fading channels. This algorithm finds the linear minimum mean squared error (MMSE) estimate of the future fading coefficients given a fixed number of previous observations. In this paper, we show that the superior performance of this algorithm is due to its lower sampling rate relative to the conventional (data rate) methods of fading prediction. We also enhance the algorithm by an adaptive prediction and tracking method that increases accuracy and maintains the robustness of long-term prediction as the physical channel parameters vary. Finally, we show that large improvements in the bit error rate (BER) are possible when the proposed prediction method is used. In particular, analysis of the channel inversion with threshold technique is presented to demonstrate that the BER can be reduced to the level of and beyond the additive white Gaussian noise (AWGN) channel.}, booktitle={1999 IEEE Communications Theory Mini-Conference (Cat. No.99EX352)}, author={Eyceoz, T. and Hu, S. and Duee-Hallen, A. and Hallen, H.}, year={1999}, pages={1–5} }
 @inproceedings{huerth_taylor_paesler_hallen_1999, place={Beijing, China}, title={Near-Field Scanning Optical Microscopy of Electromigration in YBCO}, booktitle={Proceedings of the Second Asia-Pacific Workshop on Near-field Optics}, author={Huerth, Suzanne and Taylor, Michael and Paesler, Michael and Hallen, Hans}, year={1999} }
 @inproceedings{hu_hallen_duel-hallen_1999, place={Piscataway, NJ, USA}, title={Physical channel modeling, adaptive prediction and transmitter diversity for flat fading mobile channel}, url={http://dx.doi.org/10.1109/SPAWC.1999.783099}, DOI={10.1109/SPAWC.1999.783099}, abstractNote={Deterministic modeling of flat fading wireless channels using the method of images is presented. The models identify typical and worst-case channel parameter variation rates and yield datasets of channel strength vs. position, which are used to test a long-range adaptive prediction algorithm previously presented by the authors. Our physical insights and unique long-range prediction capability for the fast fading can be used in conjunction with space diversity-including novel transmitter diversity schemes for a single receiving antenna-or adaptive modulation to significantly reduce or eliminate the effects of deep fades in wireless communications.}, booktitle={1999 2nd IEEE Workshop on Signal Processing Advances in Wireless Communications (Cat. No.99EX304)}, author={Hu, S. and Hallen, H. and Duel-Hallen, A.}, year={1999}, pages={387–90} }
 @inproceedings{hu_eyceoz_duel-hallen_hallen_1999, place={New Orleans, LA, United states}, title={Transmitter antenna diversity and adaptive signaling using long range prediction for fast fading DS/CDMA mobile radio channels}, volume={2}, url={http://dx.doi.org/10.1109/WCNC.1999.796779}, DOI={10.1109/WCNC.1999.796779}, abstractNote={Previously, the authors introduced a novel algorithm for long range prediction of flat fading channels that can reliably predict future fading coefficients far beyond the coherence time. This prediction capability provides enabling technology for power and bandwidth efficient adaptive modulation and coding methods. In this paper, we extend our results to the direct sequence code division multiple access (DS/CDMA) channels. It is demonstrated that frequency selective channels encountered in CDMA can also be predicted far ahead, and that the proposed prediction methods can be combined with RAKE receivers. In addition, we utilize prediction to improve performance of the selective transmitter diversity system proposed for the third generation CDMA.}, note={– An invited paper}, booktitle={IEEE Wireless Communications and Networking Conference, WCNC}, author={Hu, Shengquan and Eyceoz, Tugay and Duel-Hallen, Alexandra and Hallen, Hans}, year={1999}, pages={824–828} }
 @article{eyceoz_duel-hallen_hallen_1998, title={Deterministic channel modeling and long range prediction of fast fading mobile radio channels}, volume={2}, url={http://dx.doi.org/10.1109/4234.718494}, DOI={10.1109/4234.718494}, abstractNote={In wireless communication systems, the direct signal and the reflected signals form an interference pattern resulting in a received signal given by the sum of these components. They are distinguished by their Doppler shifts at the mobile. Since the parameters associated with these components are slowly varying, the fading coefficients can be accurately predicted far ahead. This novel approach to fading channel prediction is combined with transmitter signal optimization to mitigate the effects of "deep fades", which severely limit the performance of mobile radio systems. This capability will potentially help to reduce power requirements for wireless channels and improve the system performance.}, number={9}, journal={IEEE Communications Letters}, author={Eyceoz, Tugay and Duel-Hallen, Alexandra and Hallen, Hans}, year={1998}, pages={254–256} }
 @inproceedings{hallen_paesler_jahncke_1998, place={San Diego, CA, United states}, title={Raman spectroscopy: Probing the border between near-field and far-field spectroscopy}, volume={3467}, url={http://dx.doi.org/10.1117/12.326819}, DOI={10.1117/12.326819}, abstractNote={We have performed Raman spectroscopy using a near-field scanning optical microscope. The small sample volume coupled with the light-starved nature of the Raman effect makes nano-Raman studies difficult. We present results showing near-field effects in an investigation of Rb-doped KTP. These effects include a change in selection rules due to the presence of a z-polarization component in the near-field, a surface-enhancement effect in near-field Raman data, a reduced Rayleigh tail, and simultaneous topography with the near-field probe. An image taken within a Raman feature demonstrates that nano-Raman imaging is indeed possible if the near-field instrument has considerable long-term stability.}, note={– An invited paper}, booktitle={Proceedings of SPIE - The International Society for Optical Engineering}, author={Hallen, H.D. and Paesler, M.A. and Jahncke, C.L.}, year={1998}, pages={199–201} }
 @inproceedings{eyceoz_duel-hallen_hallen_1998, place={Cambridge, MA, United states}, title={Using the physics of the fast fading to improve performance for mobile radio channels}, url={http://dx.doi.org/10.1109/ISIT.1998.708753}, DOI={10.1109/ISIT.1998.708753}, abstractNote={The fading of wireless signals is not a random process, it is deterministic since it is the result of interference between signals following various paths to the receiver. We exploit this deterministic nature of the fading signals and propose a novel approach to communication over the fast fading channels. The key step in this communication system is the prediction of a future block of samples of the fading coefficient from previous observations. This prediction algorithm is closely tied to other important components: feedback from the receiver to the transmitter, transmitter signal optimization, and tracking at the receiver. The objective of this work is to anticipate "deep fades", which severely limit the performance of mobile radio systems. This capability will potentially result in reduced power requirements for wireless channels.}, booktitle={IEEE International Symposium on Information Theory - Proceedings}, author={Eyceoz, T. and Duel-Hallen, A. and Hallen, H.}, year={1998}, pages={159 -} }
 @article{hsu_mcdaniel_hallen_1997, title={A shear force feedback control system for near-field scanning optical microscopes without lock-in detection}, volume={68}, ISSN={["0034-6748"]}, url={http://dx.doi.org/10.1063/1.1148247}, DOI={10.1063/1.1148247}, abstractNote={An improvement to the currently used ac impedance detection method for tip-sample distance control in near-field scanning optical microscopes is described and demonstrated. The output signal of the electronic bridge is increased by a factor of 5000 so that a root-mean-square chip can be used in place of sensitive lock-in detection. It is shown that the signal-to-noise ratio of this new method is high enough to detect 0.07 nm changes in topography. In addition, this modification makes the electronics for the shear force feedback compact and inexpensive.}, number={8}, journal={REVIEW OF SCIENTIFIC INSTRUMENTS}, author={Hsu, JWP and McDaniel, AA and Hallen, HD}, year={1997}, month={Aug}, pages={3093–3095} }
 @article{jahncke_hallen_1997, title={A versatile stable scanning proximal probe microscope}, volume={68}, ISSN={["0034-6748"]}, url={http://dx.doi.org/10.1063/1.1147988}, DOI={10.1063/1.1147988}, abstractNote={We present a novel scanning proximal probe microscope design utilizing a piezoelectric driven coarse positioning mechanism in x, y, and z, while maintaining relatively small lateral dimensions. The instrument is suitable for insertion into a Dewar. The primary purpose of this work is to develop a stable yet versatile instrument in order to meet the signal averaging limitations imposed by low signal level measurements. We have implemented a near field scanning optical microscope with this system, whose key features include simultaneous detection of reflected and transmitted signals, unique “center of mass” tip oscillator for shear force feedback, and overall microscope stability.}, number={4}, journal={REVIEW OF SCIENTIFIC INSTRUMENTS}, author={Jahncke, CL and Hallen, HD}, year={1997}, month={Apr}, pages={1759–1763} }
 @inbook{hallen_paesler_1997, place={New York}, title={Lens}, booktitle={The Macmillan Encyclopedia of Physics}, publisher={Macmillan}, author={Hallen, H.D. and Paesler, M. A.}, editor={Rigden, John S.Editor}, year={1997} }
 @article{paesler_hallen_yakobson_jahncke_boykin_meixner_1997, title={Near-field optical spectroscopy: enhancing the light budget}, volume={3}, journal={Microscopy and Microanalysis}, author={Paesler, M.A. and Hallen, H.D. and Yakobson, B.I. and Jahncke, C.J. and Boykin, P.O. and Meixner, A.}, year={1997}, pages={815} }
 @article{la rosa_yakobson_hallen_1997, title={Optical imaging of carrier dynamics in silicon with subwavelength resolution}, volume={70}, url={http://dx.doi.org/10.1063/1.118661}, DOI={10.1063/1.118661}, abstractNote={Characteristic rate variations of carrier processes are imaged using near-field scanning optical microscopy. We couple both a visible pump and an infrared probe light through a subwavelength aperture to investigate the interband recombination and intraband diffusion of excess carriers in oxidized silicon. Typical values of the locally measured life time constants agree well with those obtained by conventional space-averaged techniques. Moreover, the images locate defects, reveal variations, and can map the regions in which a recombination process is active.}, number={13}, journal={Applied Physics Letters}, author={La Rosa, A.H. and Yakobson, B.I. and Hallen, H.D.}, year={1997}, pages={1656–1658} }
 @inproceedings{eyceoz_duel-hallen_hallen_1997, title={Prediction of fast fading parameters by resolving the interference pattern}, volume={1}, url={http://dx.doi.org/10.1109/ACSSC.1997.680049}, DOI={10.1109/ACSSC.1997.680049}, abstractNote={We investigated a new deterministic approach to the estimation and prediction of the fading channel by exploiting the physical nature of the flat fading signal. Since the direct signal and the reflected signals form an interference pattern, the received signal is given by a sum of several scattered components. These are distinguished by their Doppler shifts at the mobile. The slowly varying parameters associated with these components are determined and tracked, and the composite fading signal is predicted. This approach will potentially result in the ability to anticipate and avoid "deep fades" which severely limit the performance of wireless communications systems and will aid in the development of low power mobile radio systems.}, booktitle={Conference Record of the Thirty-First Asilomar Conference on Signals, Systems and Computers (Cat. No.97CB36136)}, author={Eyceoz, T. and Duel-Hallen, A. and Hallen, H.}, year={1997}, pages={167–171} }
 @inproceedings{la rosa_yakobson_hallen_1996, place={Boston, MA, USA}, title={Imaging of silicon carrier dynamics with near-field scanning optical microscopy}, volume={406}, url={https://doi.org/10.1557/PROC-406-189}, DOI={10.1557/PROC-406-189}, abstractNote={A contactless high spatial resolution technique has been developed to characterize semiconductor materials using the Near-Field Scanning Optical Microscope. The technique can be used to non-invasively measure: surface topography, defect content, and carrier lifetime variations in silicon. The success of the technique relies on the sensitive detection of changes in infrared transmission induced by local generation of free carriers using pulsed visible radiation. Here we extend the application of this technique to characterize silicon on insulator materials. We also include computer simulation results to address the role played by diffusion in the ultimate lateral resolution that can be achieved using this technique.}, booktitle={Materials Research Society Symposium - Proceedings}, author={La Rosa, A.H. and Yakobson, B.I. and Hallen, H.D.}, year={1996}, pages={189–194} }
 @article{jahncke_hallen_paesler_1996, place={UK}, title={Nano-Raman spectroscopy and imaging with a near-field scanning optical microscope}, volume={27}, DOI={10.1002/(SICI)1097-4555(199608)27:8<579::AID-JRS13>3.0.CO;2-4}, abstractNote={Raman spectroscopy was performed using a near-field scanning optical microscope. This avoids the diffraction limit inherent in conventional optical microscopy techniques involving far-field optical components, and allows volumes significantly smaller than the cube of the wavelength to be investigated. The small sample volume coupled with the light-starved nature of the Raman effect itself makes such nano-Raman studies difficult. A near-field Raman microscope is described and results showing near-field effects in an investigation of Rb-doped KTP are presented. An image taken within a Raman feature demonstrates that nano-Raman imaging is indeed possible if the near-field instrument has considerable long-term stability, and that several unique aspects of the near-field data recommend this approach.}, note={– An invited paper}, number={8}, journal={J. Raman Spectrosc.}, author={Jahncke, C.L. and Hallen, H.D. and Paesler, M.A.}, year={1996}, month={Aug}, pages={579–86} }
 @inproceedings{jahncke_hallen_1996, place={Boston, MA, USA}, title={Near-field Raman spectra: Surface enhancement, z-polarization, fiber Raman background and Rayleigh scattering}, volume={1}, url={https://doi.org/10.1109/LEOS.1996.565182}, DOI={10.1109/LEOS.1996.565182}, abstractNote={Near-field optical microscopy (NSOM) is a technique that offers several advantages over conventional optical microscopy. We present evidence for a z-polarization component and a surface-enhancement effect in near-field Raman data. Observation of Raman scattering from the fiber probe and Rayleigh scattering may be used to determine probe quality. The sample studied is KTiOPO/sub 4/ (KTP) a nonlinear optical material, for which a comparison of near-field and far-field spectra is shown.}, booktitle={Conference Proceedings - Lasers and Electro-Optics Society Annual Meeting-LEOS}, author={Jahncke, C.L. and Hallen, H.D.}, year={1996}, pages={176–177} }
 @article{hallen_chang_miller_pfeiffer_west_hess_1996, title={Penetration of laterally quantized flux lamina into a superconducting wire network}, volume={99}, url={http://dx.doi.org/10.1016/0038-1098(96)00123-8}, DOI={10.1016/0038-1098(96)00123-8}, abstractNote={Abstract We report direct, high resolution images of the magnetic field above a superconducting wire grid as applied flux penetrates into an initially empty state or into a state with half the plaquettes containing a magnetic quantum. We find two qualitatively different initial approaches to equilibrium: in wide structures for the former, or with long, linear structures for the latter case. In the approach to an empty state, we observe the formation of residual quasi-bound magnetic vortex/anti-vortex pairs.}, number={9}, journal={Solid State Communications}, author={Hallen, H.D. and Chang, A.M. and Miller, R. and Pfeiffer, L.N. and West, K. and Hess, H.F.}, year={1996}, pages={651–654} }
 @article{hallen_la rosa_jahncke_1995, title={Near-field scanning optical microscopy and spectroscopy for semiconductor characterization}, volume={152}, url={http://dx.doi.org/10.1002/pssa.2211520126}, DOI={10.1002/pssa.2211520126}, abstractNote={The applicability of near-field scanning optical microscopy (NSOM) for optical characterization of semiconductors is discussed. The NSOM technique and some of its properties relevant to real-time in-situ measurements are reviewed. Several optical characterization methods widely used in the far-field, including reflectance, reflectance-difference spectroscopy, Raman spectroscopy, ellipsometry, and carrier lifetime, are evaluated for their use with NSOM. Experimental data are included for some of these methods. It is concluded that several, but not all, of the standard optical characterization methods can be coupled with NSOM to provide higher spatial resolution. The applicability of NSOM as a real-time in-situ probe shares some of the problems of other proximal probe methods, but offers enough new capabilities to warrant its application.}, number={1}, journal={Physica Status Solidi (A) Applied Research}, author={Hallen, H.D. and La Rosa, A.H. and Jahncke, C.L.}, year={1995}, pages={257–268} }
 @article{la rosa_yakobson_hallen_1995, title={Origins and effects of thermal processes on near-field optical probes}, volume={67}, url={http://dx.doi.org/10.1063/1.115143}, DOI={10.1063/1.115143}, abstractNote={An aluminum‐coated tapered fiber probe, as used in near‐field scanning optical microscopy (NSOM), is heated by the light coupled into it. This can destroy the probe or may modify the sample, which can be problematic or used as a tool. To study these thermal effects, we couple modulated visible light of various power through probes. Simultaneously coupled infrared light senses the thermal effects. We report their magnitude, their spatial and temporal scales, and real‐time probe damage observations. A model describes the experimental data, the mechanisms for induced IR variation, and their relative importance.}, number={18}, journal={Applied Physics Letters}, author={La Rosa, A.H. and Yakobson, B.I. and Hallen, H.D.}, year={1995}, pages={2597–2599} }
 @article{jahncke_paesler_hallen_1995, title={Raman imaging with near-field scanning optical microscopy}, volume={67}, url={http://dx.doi.org/10.1063/1.114615}, DOI={10.1063/1.114615}, abstractNote={Raman spectroscopy in conjunction with near‐field scanning optical microscopy is used to image Rb‐doped KTiOPO4 within a spectral feature with high spatial resolution. We present Raman spectra as well as the first Raman images obtained in the near field. Differences between near‐field and far‐field Raman measurements are discovered and discussed.}, number={17}, journal={Appl. Phys. Lett.}, author={Jahncke, C.L. and Paesler, M.A. and Hallen, H.D.}, year={1995}, month={Oct}, pages={2483–5} }
 @article{yakobson_larosa_hallen_paesler_1995, title={Thermal/optical effects in NSOM probes}, volume={61}, url={http://dx.doi.org/10.1016/0304-3991(95)00121-2}, DOI={10.1016/0304-3991(95)00121-2}, abstractNote={The coupling of thermal and optical effects in near-field scanning optical microscope (NSOM) fitted with a coated tapered fiber probe is considered. Heating of the probe occurs due to the absorption of light in the metal coating in the tapered region of the probe at relatively large (tens of microns) distances from the probe apex. Conduction of heat that occurs with a time constant of tens of milliseconds results in an elevated probe temperature over a rather large region of the taper. This in turn, affects the optical throughput of the NSOM. Experimental measurement of thermal/optical coupling and a discussion of coupling mechanisms is included.}, number={1-4}, journal={Ultramicroscopy}, author={Yakobson, B.I. and Larosa, A. and Hallen, H.D. and Paesler, M.A.}, year={1995}, pages={179–185} }
 @inproceedings{hallen_yakobson_larosa_paesler_1995, title={Thermal/temporal response of the NSOM probe/sample system}, volume={2535}, url={http://dx.doi.org/10.1117/12.218693}, DOI={10.1117/12.218693}, abstractNote={In measurements of sample temporal response with a near-field scanning optical microscope, or NSOM, one must account for the temporal response of the probe. The coupling of thermal and temporal effects in an NSOM fitted with a coated tapered fiber probe is considered. Study of the perturbation of cw infrared light by a pulse of visible light simultaneously sent through an illumination mode NSOM allows one to separate the relatively slow thermal response of the probe from the appreciably faster response of a silicon sample imaged with the probe. Temporal and thermal contrast in NSOM imaging are discussed in terms of the results.}, booktitle={Proc. SPIE - Int. Soc. Opt. Eng.}, author={Hallen, H.D. and Yakobson, B.I. and LaRosa, A. and Paesler, M.A.}, year={1995}, pages={34–7} }
 @article{larosa_jahncke_hallen_1995, place={Raleigh, NC, USA}, title={Time as a contrast mechanism in near-field imaging}, volume={57}, url={http://dx.doi.org/10.1016/0304-3991(94)00157-I}, DOI={10.1016/0304-3991(94)00157-I}, abstractNote={Abstract Any modulation of a detected signal can be used as a contrast mechanism in imaging applications. Insofar as the time dependence of optical properties of imaged structures can be used to elucidate material properties, such time dependences can provide a modulation which can then be used as a contrast mechanism in imaging. We introduce a system in which time can be used as a contrast mechanism in imaging. We introduce a system in which time can be used as a contrast mechanism to study material nanostructures. Single-crystal silicon wafers are imaged in the infrared using a HeNe laser while the wafer is simultaneously pulsed with visible radiation. By studying the time dependence of the infrared transmittance, defect distribution on the nanometer scale can be imaged, and sample nanostructure can be studied.}, number={2-3}, journal={Ultramicroscopy}, author={LaRosa, A. and Jahncke, C.L. and Hallen, H.D.}, year={1995}, pages={303–308} }
 @inproceedings{la rosa_jahncke_hallen_1995, title={Time-resolved contrast in near-field scanning optical microscopy of semiconductors}, volume={2384}, url={http://dx.doi.org/10.1117/12.205917}, DOI={10.1117/12.205917}, abstractNote={We demonstrate the ability of near-field scanning optical microscopy (NSOM) technique to detect inhomogeneities of the dynamics of excess carriers in oxidized silicon wafers. NSOM is used to improve the spatial resolution of a standard IR-scattering optical technique, which is carried out in a noncontact fashion. Continuous wave infrared light is used as a detector of the time dependent carrier population produced by a pulsed visible laser. We will show high resolution images of carrier lifetime, and discuss some aspects of the NSOM measurement that differentiate it from its far field counterpart.}, note={– An invited paper}, author={La Rosa, Andres H. and Jahncke, Catherine L. and Hallen, Hans D.}, year={1995}, pages={101–108} }
 @inbook{hallen_buhrman_1993, title={BEEM: A Probe of Nanoscale Modification}, url={http://link.springer.com/chapter/10.1007/978-94-011-2024-1_15}, DOI={DOI: 10.1007/978-94-011-2024-1\_15}, number={239}, booktitle={Atomic and Nanometer-Scale Modification of Materials: Fundamentals and Applications}, publisher={Springer Netherlands}, author={Hallen, H. D. and Buhrman, R. A.}, editor={Avouris, PhaedonEditor}, year={1993}, pages={153–164} }
 @inbook{hallen_1993, place={Bellingham, WA}, title={Ballistic electron emission microscopy: from electron transport studies to nanoscale materials science}, volume={IS10}, booktitle={The Technology of Proximal Probe Lithography}, publisher={SPIE}, author={Hallen, H.D.}, editor={Marrian, ChristieEditor}, year={1993} }
 @article{hallen_seshadri_chang_miller_pfeiffer_west_murray_hess_1993, title={Direct spatial imaging of vortices in a superconducting wire network}, volume={71}, url={http://dx.doi.org/10.1103/PhysRevLett.71.3007}, DOI={10.1103/PhysRevLett.71.3007}, abstractNote={We report direct observations of vortices in a square superconducting wire grid imaged using scanning Hall probe microscopy. Real space images of vortex configurations are obtained as a function of the flux per unit cell f by measuring the local magnetic field just above the sample. At f=1/2 we observe domains of the checkerboard ground state. As f is reduced from 1/2 to 1/3 vacancies first penetrate the grain boundaries and then the checkerboard domains. Near f=-1/3 we observe domains of the 1/3 staircase ground state. Heating the sample close to T c produces correlated vortex hopping}, number={18}, journal={Physical Review Letters}, author={Hallen, H.D. and Seshadri, R. and Chang, A.M. and Miller, R.E. and Pfeiffer, L.N. and West, K.W. and Murray, C.A. and Hess, H.F.}, year={1993}, pages={3007–3007} }
 @inproceedings{hallen_hess_chang_pfeiffer_west_mitzi_1993, title={High-resolution scanning Hall probe microscopy}, volume={1855}, url={http://dx.doi.org/10.1117/12.146371}, DOI={10.1117/12.146371}, abstractNote={A high resolution scanning Hall probe microscope is used to spatially resolve vortices in high temperature superconducting Bi2Sr2CaCu2O8+(delta) crystals. We observe a partially ordered vortex lattice at several different applied magnetic fields and temperatures. At higher temperatures, a limited amount of vortex re-arrangement is observed, but most vortices remain fixed for periods long compared to the imaging time of several hours even at temperatures as high as 75 degree(s)K (the superconducting transition temperature for these crystals is approximately 84 degree(s)K). A measure of these local magnetic penetration depth can be obtained from a fit to the surface field of several neighboring vortices, and has been measured as a function of temperature. In particular, we have measured the zero temperature penetration depth and found it to be 275 +/- 40 nm.}, booktitle={Proc. SPIE - Int. Soc. Opt. Eng.}, author={Hallen, H.D. and Hess, H.F. and Chang, A.M. and Pfeiffer, L.N. and West, K.W. and Mitzi, D.B.}, year={1993}, pages={152–7} }
 @article{hallen_huang_fernandez_silcox_buhrman_1992, title={Hot electron interactions at the passivated gold-silicon interface}, volume={69}, url={https://doi.org/10.1103/PhysRevLett.69.2931}, DOI={10.1103/PhysRevLett.69.2931}, abstractNote={A few-eV hot electron is found to stimulate the motion of a gold atom, leading to adatom generation at the surfaces of a Au film. This is detected through ballistic electron emission microscopy (BEEM) observations of the resulting growth of an atomic layer terrace on the interfacial Au surface of Au/passivation-layer/Si samples. We deduce the presence of concurrently produced vacancies in the film from their (time-dependent) erect on the BEEM current. The phenomenon has similarities to electron-stimulated desorption}, number={20}, journal={Physical Review Letters}, author={Hallen, H.D. and Huang, T. and Fernandez, A. and Silcox, J. and Buhrman, R.A.}, year={1992}, pages={2931–2934} }
 @article{chang_hallen_harriott_hess_kao_kwo_miller_wolfe_ziel_chang_1992, title={Scanning Hall probe microscopy}, volume={61}, url={http://dx.doi.org/10.1063/1.108334}, DOI={10.1063/1.108334}, abstractNote={We describe the implementation of a scanning Hall probe microscope of outstanding magnetic field sensitivity (∼0.1 G) and unprecedented spatial resolution (∼0.35 μm) to detect surface magnetic fields at close proximity to a sample. Our microscope combines the advantages of a submicron Hall probe fabricated on a GaAs/Al0.3Ga0.7As heterostructure chip and the scanning tunneling microscopy technique for precise positioning. We demonstrate its usefulness by imaging individual vortices in high Tc La1.85Sr0.15CuO4 films and superconducting networks, and magnetic bubble domains.}, number={16}, journal={Appl. Phys. Lett.}, author={Chang, A.M. and Hallen, H.D. and Harriott, L. and Hess, H.F. and Kao, H.L. and Kwo, J. and Miller, R.E. and Wolfe, R. and Ziel, J. and Chang, T.Y.}, year={1992}, month={Oct}, pages={1974–6} }
 @article{chang_hallen_hess_kao_kwo_sudbo_chang_1992, place={Switzerland}, title={Scanning Hall-probe microscopy of a vortex and field fluctuations in La1.85Sr0.15CuO4 films}, volume={20}, url={http://dx.doi.org/10.1209/0295-5075/20/7/012}, DOI={10.1209/0295-5075/20/7/012}, abstractNote={A high-resolution scanning Hall-probe microscope is used to spatially resolve vortices in high-temperature superconducting La1.85Sr0.15CuO4 films. At low magnetic fields, a disordered vortex arrangement is observed. A fit to the surface field of an individual vortex is consistent with one flux quantum, and is used to determine the local penetration depth and its temperature dependence. At higher fields, magnetic fluctuations are observed and compared to a collective pinning model. For films grown with the c-axis tilted from the surface normal, oval vortices are observed.}, number={7}, journal={Europhys. Lett. (Switzerland)}, author={Chang, A.M. and Hallen, H.D. and Hess, H.F. and Kao, H.L. and Kwo, J. and Sudbo, A. and Chang, T.Y.}, year={1992}, month={Dec}, pages={645–50} }
 @article{hallen_fernandez_huang_silcox_buhrman_1992, title={Scattering and spectral shape in ballistic-electron-emission microscopy of NiSi2-Si(111) and Au-Si samples}, volume={46}, url={http://dx.doi.org/10.1103/PhysRevB.46.7256}, DOI={10.1103/PhysRevB.46.7256}, abstractNote={The ballistic-electron-emission microscopy (BEEM) tip voltage versus collector current spectral shape is used as a tool to study Au-Si and ${\mathrm{NiSi}}_{2}$-Si samples on a nanometer scale. A method is outlined for the extraction of an energy-dependent transmission factor from the BEEM spectra. Results for Au-Si samples compare favorably with theoretical interface transmission calculations. Data from both systems suggest that the shape of the spectra is influenced by scattering. The influence of scattering induced broadening of the injected electron transverse momentum distribution is considered.}, number={11}, journal={Phys. Rev. B, Condens. Matter}, author={Hallen, H.D. and Fernandez, A. and Huang, T. and Silcox, J. and Buhrman, R.A.}, year={1992}, pages={7256–9} }
 @article{fernandez_hallen_huang_buhrman_silcox_1991, title={Ballistic electron emission microscopy studies of the NiSi2/Si(111) interface}, volume={9}, url={http://dx.doi.org/10.1116/1.585465}, DOI={10.1116/1.585465}, abstractNote={We have performed ballistic electron emission microscopy measurements on the NiSi2/Si(111) system under UHV conditions. Schottky barrier heights have been measured for types A and B interfaces and are found to be in agreement with other techniques. Through correlations between structural defects in the silicide and local increases in collector currents, we find that elastic scattering in the film can increase ballistic electron transmission rates across the interface.}, number={2}, journal={J. Vac. Sci. Technol. B, Microelectron. Process. Phenom.}, author={Fernandez, A. and Hallen, H.D. and Huang, T. and Buhrman, R.A. and Silcox, J.}, year={1991}, month={Mar}, pages={590–3} }
 @article{fernandez_hallen_huang_buhrman_silcox_1991, title={Elastic scattering in ballistic-electron-emission-microscopy studies of the epitaxial NiSi2/Si(111) interface}, volume={44}, url={http://dx.doi.org/10.1103/PhysRevB.44.3428}, DOI={10.1103/PhysRevB.44.3428}, abstractNote={We have fabricated epitaxial ${\mathrm{NiSi}}_{2}$ films on Si(111) under ultrahigh-vacuum conditions and conducted in situ ballistic-electron-emission-microscopy (BEEM) measurements on this coherent interface system. We demonstrate the sensitivity of BEEM to mechanisms which broaden the angular distribution of the injected electrons for transport through a Si(111) interface. Spatially localized increases in ballistic transmission rates are observed, attributable to the elastic scattering of the injected electrons by structural defects present on the surface or buried in the silicide film.}, number={7}, journal={Phys. Rev. B, Condens. Matter}, author={Fernandez, A. and Hallen, H.D. and Huang, T. and Buhrman, R.A. and Silcox, J.}, year={1991}, month={Aug}, pages={3428–31} }
 @article{hallen_fernandez_huang_buhrman_silcox_1991, title={Gold-silicon interface modification studies}, volume={9}, url={http://dx.doi.org/10.1116/1.585464}, DOI={10.1116/1.585464}, abstractNote={We have performed ballistic electron emission microscopy measurements on the Au–Si system with and without controlled monolayer impurities at the interface. At moderate sample to scanning tunneling microscopy tip biases ( 3 V) modified the local ballistic transmittance (BT), i.e. the scaling factor of the collector current versus voltage spectra, of the interface. Spatially, the modification typically consists of a region of decreased BT a few hundred A in diameter surrounded by a ring of increased BT. No change in Schottky barrier height is found. A model is presented which describes the decrease in terms of Au–Si interdiffusion, and the enhancement in terms of a thinning of an impurity layer between the Au and Si; connections are made to observations of the unstressed system.}, number={2}, journal={J. Vac. Sci. Technol. B, Microelectron. Process. Phenom.}, author={Hallen, H.D. and Fernandez, A. and Huang, T. and Buhrman, R.A. and Silcox, J.}, year={1991}, month={Mar}, pages={585–9} }
 @article{fernandez_hallen_huang_buhrman_silcox_1990, title={Ballistic electron studies and modification of the Au/Si interface}, volume={57}, url={http://dx.doi.org/10.1063/1.103754}, DOI={10.1063/1.103754}, abstractNote={The Au/Si(111) interface has been investigated with ballistic electron emission microscopy. The Schottky barrier (SB) height and ballistic transmittance have been measured on interfaces which have been prepared with different types of monolayer‐level dopants. Transmission rates but not the SB are found to depend strongly on the resulting degree of interdiffusion of the Au and Si at the interface. An irreversible modification in the transport properties of the buried interface can occur when the system is stressed with electrons injected at several volts above the Schottky barrier.}, number={26}, journal={Appl. Phys. Lett.}, author={Fernandez, A. and Hallen, H.D. and Huang, T. and Buhrman, R.A. and Silcox, J.}, year={1990}, month={Dec}, pages={2826–8} }
 @article{nelson_hallen_buhrman_1988, title={A structural and electrical comparison of thin SiO2 films grown on silicon by plasma anodization and rapid thermal processing to furnace oxidation}, volume={63}, url={http://dx.doi.org/10.1063/1.340450}, DOI={10.1063/1.340450}, abstractNote={We have used capacitance‐voltage (C‐V) techniques and x‐ray photoelectron spectroscopy (XPS) to study for the first time the electrical and structural properties of thin SiO2 films grown on silicon by plasma anodization and rapid thermal processes (RTO) and then compared them to furnace oxides. We have compared the SiO4 tetrahedral ring structure and the suboxide content of the ∼3‐nm‐thick interfacial region of these oxides and have found significant structural differences. By correlating these differences with measured electrical differences, we have identified the structural causes of some of the electrical characteristics of the plasma and RTO oxides. In plasma oxides we see larger amounts of silicon dangling bonds, Pb centers, at the Si‐SiO2 interface and have identified these dangling bonds as the source of a localized peak of interface states found at 0.3 eV above the silicon valence band. Low‐temperature rapid thermal annealing of the plasma oxides relieves localized compressive interfacial strain,...}, number={10}, journal={J. Appl. Phys.}, author={Nelson, S.A. and Hallen, H.D. and Buhrman, R.A.}, year={1988}, month={May}, pages={5027–35} }
 @inproceedings{robertazzi_hallen_buhrman_1988, title={Coupled Josephson local oscillator and detector experiments in the tetrahertz regime}, volume={879}, url={http://dx.doi.org/10.1117/12.943978}, DOI={10.1117/12.943978}, abstractNote={Recent coupled Josephson junction experiments in our laboratory have demonstrated that high critical current density tunnel junctions can serve as effective local oscillators at frequencies up to and in excess of the gap sum frequency of the junction, i.e. well above 1 Terahertz for a niobium or niobium compound tunnel junction. While the details of the behavior of such a THz. oscillator were found not to be in accord with the predictions of the accepted theory of the A.C. Josephson effect in the gap region significant radiation could be capacitively coupled from the oscillator junction to an adjacent junction, sufficient for SIS mixer experiments at Terahertz frequencies. Research efforts are now under way to further extend and ex-pand these studies. A high critical current density all NbN tunnel junction system is now under development for Terahertz applications and a new set of coupled Josephson oscillator - SIS detector experiments is being initiated using NbN tunnel junc-tions. In this paper we will review the original coupled junction high frequency experiments and report on the recent progress of the current NbN tunnel junction experiments.}, booktitle={Proc. SPIE - Int. Soc. Opt. Eng.}, author={Robertazzi, R.P. and Hallen, H.D. and Buhrman, R.A.}, year={1988}, pages={65–8} }
 @article{sulewski_sievers_russek_hallen_lathrop_buhrman_1987, title={Measurement of the superconducting energy gap in La-Ba-Cu oxide and La-Sr-Cu oxide}, volume={35}, url={http://dx.doi.org/10.1103/PhysRevB.35.5330}, DOI={10.1103/PhysRevB.35.5330}, abstractNote={We report on far-infrared (FIR) reflectivity measurements on sintered pressed powder samples of La-Ba-Cu oxide and La-Sr-Cu oxide. The best samples show fairly sharp transitions to zero measurable resistance with a transition midpoint of 28 K for the Ba alloy and 36 K for the Sr alloy. The FIR measurements reveal an energy gap below T/sub c/ with, depending on the measurement criterion used, values of 2..delta../k/sub B/T/sub c/ of 1.3--1.95 for La-Ba-Cu oxide and 1.6--2.7 for La-Sr-Cu oxide, which are lower than expected for a BCS superconductor.}, number={10}, journal={Phys. Rev. B, Condens. Matter}, author={Sulewski, P.E. and Sievers, A.J. and Russek, S.E. and Hallen, H.D. and Lathrop, D.K. and Buhrman, R.A.}, year={1987}, month={Apr}, pages={5330–3} }