@article{tomko_runnerstrom_wang_chu_nolen_olson_kelley_cleri_nordlander_caldwell_et al._2021, title={Long-lived modulation of plasmonic absorption by ballistic thermal injection}, volume={16}, ISSN={["1748-3395"]}, DOI={10.1038/s41565-020-00794-z}, abstractNote={Light–matter interactions that induce charge and energy transfer across interfaces form the foundation for photocatalysis1,2, energy harvesting3 and photodetection4, among other technologies. One of the most common mechanisms associated with these processes relies on carrier injection. However, the exact role of the energy transport associated with this hot-electron injection remains unclear. Plasmon-assisted photocatalytic efficiencies can improve when intermediate insulation layers are used to inhibit the charge transfer5,6 or when off-resonance excitations are employed7, which suggests that additional energy transport and thermal effects could play an explicit role even if the charge transfer is inhibited8. This provides an additional interfacial mechanism for the catalytic and plasmonic enhancement at interfaces that moves beyond the traditionally assumed physical charge injection9–12. In this work, we report on a series of ultrafast plasmonic measurements that provide a direct measure of electronic distributions, both spatially and temporally, after the optical excitation of a metal/semiconductor heterostructure. We explicitly demonstrate that in cases of strong non-equilibrium, a novel energy transduction mechanism arises at the metal/semiconductor interface. We find that hot electrons in the metal contact transfer their energy to pre-existing free electrons in the semiconductor, without an equivalent spatiotemporal transfer of charge. Further, we demonstrate that this ballistic thermal injection mechanism can be utilized as a unique means to modulate plasmonic interactions. These experimental results are well-supported by both rigorous multilayer optical modelling and first-principle ab initio calculations. An energy transduction mechanism across metal/semiconductor interfaces, which relies on electron–electron energy transfer rather than the transport of charge, is demonstrated through ultrafast infrared spectroscopy. This ballistic thermal injection process allows for extended modulation of plasmonic absorption in epsilon-near-zero media.}, number={1}, journal={NATURE NANOTECHNOLOGY}, author={Tomko, John A. and Runnerstrom, Evan L. and Wang, Yi-Siang and Chu, Weibin and Nolen, Joshua R. and Olson, David H. and Kelley, Kyle P. and Cleri, Angela and Nordlander, Josh and Caldwell, Joshua D. and et al.}, year={2021}, month={Jan}, pages={47–51} }
 @article{nolen_runnerstrom_kelley_luk_folland_cleri_maria_caldwell_2020, title={Ultraviolet to far-infrared dielectric function of n-doped cadmium oxide thin films}, volume={4}, ISSN={["2475-9953"]}, DOI={10.1103/PhysRevMaterials.4.025202}, abstractNote={Spectroscopic ellipsometry and Fourier transform infrared spectroscopy were applied to extract the ultraviolet to far-infrared $(150--33333\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1})$ complex dielectric functions of high-quality, sputtered indium-doped cadmium oxide (In:CdO) thin crystalline films on MgO substrates possessing carrier densities $({N}_{d})$ ranging from $1.1\ifmmode\times\else\texttimes\fi{}{10}^{19}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}3}$ to $4.1\ifmmode\times\else\texttimes\fi{}{10}^{20}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}3}$. A multiple oscillator fit model was used to identify and analyze the three major contributors to the dielectric function and their dependence on doping density: interband transitions in the visible, free-carrier excitations (Drude response) in the near- to far-infrared, and IR-active optic phonons in the far-infrared. More specifically, values pertinent to the complex dielectric function such as the optical band gap $({E}_{g})$, are shown here to be dependent upon carrier density, increasing from approximately 2.5--3 eV, while the high-frequency permittivity (${\ensuremath{\varepsilon}}_{\ensuremath{\infty}}$) decreases from 5.6 to 5.1 with increasing carrier density. The plasma frequency (${\ensuremath{\omega}}_{p}$) scales as $\sqrt{{N}_{d}}$, resulting in ${\ensuremath{\omega}}_{p}$ values occurring within the mid- to near-IR, and the effective mass (${m}^{*}$) was also observed to exhibit doping density-dependent changes, reaching a minimum of $0.11{m}_{o}$ in unintentionally doped films ($1.1\ifmmode\times\else\texttimes\fi{}{10}^{19}\phantom{\rule{4pt}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}3}$). Good quantitative agreement with prior work on polycrystalline, higher-doped CdO films is also demonstrated, illustrating the generality of the results. The analysis presented here will aid in predictive calculations for CdO-based next-generation nanophotonic and optoelectronic devices, while also providing an underlying physical description of the key properties dictating the dielectric response in this atypical semiconductor system.}, number={2}, journal={PHYSICAL REVIEW MATERIALS}, author={Nolen, J. Ryan and Runnerstrom, Evan L. and Kelley, Kyle P. and Luk, Ting S. and Folland, Thomas G. and Cleri, Angela and Maria, Jon-Paul and Caldwell, Joshua D.}, year={2020}, month={Feb} }
 @article{radue_runnerstrom_kelley_rost_donovan_grimley_lebeau_maria_hopkins_2019, title={Charge confinement and thermal transport processes in modulation-doped epitaxial crystals lacking lattice interfaces}, volume={3}, ISSN={["2475-9953"]}, DOI={10.1103/PhysRevMaterials.3.032201}, abstractNote={Heterogeneous nanosystems offer a robust potential for manipulating various functional material properties, beyond those possible from their individual constituent materials. We demonstrate the formation of a class of materials with a homogeneous lattice but spatially heterogeneous electrical functionality; specifically, we develop epitaxial modulation-doped thin films in which the spatial separation of electronic charge densities is achieved without perturbing the parent crystal's compositional or structural homogeneity. Unlike the previous realizations of modulation doping in crystals, our materials demonstrate periodic layering of spatially segregated, varying electronically donor-doped regions in a single compositionally and structurally homogenous single-crystalline lattice. We demonstrate the formation of ``modulation-doped epitaxial crystals'' (MoDECs) using alternating layers of doped cadmium oxide, and the ability to spatially confine regions of variable carrier concentration via low potential-energy barriers in a spatially homogeneous, epitaxial crystal with a chemically and structurally homogenous lattice (i.e., no chemical or structural lattice interfaces). The low potential energy that confines electrons within the doped layers coupled with the crystalline nature of the MoDECs and lack of lattice interfaces presents a platform to study the electron thermal boundary resistances at low-energy electronic barriers. We find that the electron interfacial density does not impede thermal conductivity, despite evidence that the doped layers retain their carrier concentrations. Thus, the negligible thermal boundary resistances at the electronic interfaces result in the thermal conductivities of the MoDECs being related to only a series resistance sum of the thermal resistances of each of the individual layers, with no thermal resistances from the electronic boundaries that maintain charge separation. This is in stark contrast with other nanoscale multilayer materials, where thermal boundary resistances at the internal material interfaces reduce the thermal conductivity of the multilayer compared to that of the parent materials. The ability to modulation dope epitaxially grown films with no structural heterogeneity in the lattice will further enable unique platforms for mid-IR photonics, such as hyperbolic metamaterials, optical filters with spatially discrete optical absorption, or energy harvesting based on charge injection across modulation-doped interfaces.}, number={3}, journal={PHYSICAL REVIEW MATERIALS}, author={Radue, Elizabeth and Runnerstrom, Evan L. and Kelley, Kyle P. and Rost, Christina M. and Donovan, Brian F. and Grimley, Everett D. and LeBeau, James M. and Maria, Jon-Paul and Hopkins, Patrick E.}, year={2019}, month={Mar} }
 @article{yang_lu_manjavacas_luk_liu_kelley_maria_runnerstrom_sinclair_ghimire_et al._2019, title={High-harmonic generation from an epsilon-near-zero material}, volume={15}, ISSN={["1745-2481"]}, DOI={10.1038/s41567-019-0584-7}, abstractNote={High-harmonic generation (HHG) from a compact, solid-state medium is highly desirable for applications such as coherent attosecond pulse generation and extreme ultra-violet (EUV) spectroscopy, yet the typically weak conversion of pump light to HHG can largely hinder its applications. Here, we use a material operating in its epsilon-near-zero (ENZ) region, where the real part of its permittivity vanishes, to greatly boost the efficiency of the HHG process at the microscopic level. In experiments, we report high-harmonic emission up to the 9th order directly from a low-loss, solid-state ENZ medium: indium-doped cadmium oxide, with an excitation intensity at the GW cm-2 level. Furthermore, the observed HHG signal exhibits a pronounced spectral red-shift as well as linewidth broadening, resulting from the photo-induced electron heating and the consequent time-dependent resonant frequency of the ENZ film. Our results provide a novel nanophotonic platform for strong field physics, reveal new degrees of freedom for spectral and temporal control of HHG, and open up possibilities of compact solid-state attosecond light sources.}, number={10}, journal={NATURE PHYSICS}, author={Yang, Yuanmu and Lu, Jian and Manjavacas, Alejandro and Luk, Ting S. and Liu, Hanzhe and Kelley, Kyle and Maria, Jon-Paul and Runnerstrom, Evan L. and Sinclair, Michael B. and Ghimire, Shambhu and et al.}, year={2019}, month={Oct}, pages={1022-+} }
 @article{kelley_runnerstrom_sachet_shelton_grimley_klump_lebeau_sitar_suen_padilla_et al._2019, title={Multiple Epsilon-Near-Zero Resonances in Multilayered Cadmium Oxide: Designing Metamaterial-Like Optical Properties in Monolithic Materials}, volume={6}, ISSN={["2330-4022"]}, DOI={10.1021/acsphotonics.9b00367}, abstractNote={In this Letter, we demonstrate a new class of infrared nanophotonic materials based on monolithic, multilayered doped cadmium oxide (CdO) thin films, where each CdO layer is individually tuned to s...}, number={5}, journal={ACS PHOTONICS}, author={Kelley, Kyle P. and Runnerstrom, Evan L. and Sachet, Edward and Shelton, Christopher T. and Grimley, Everett D. and Klump, Andrew and LeBeau, James M. and Sitar, Zlatko and Suen, Jonathan Y. and Padilla, Willie J. and et al.}, year={2019}, month={May}, pages={1139–1145} }
 @article{runnerstrom_kelley_folland_nolen_engheta_caldwell_maria_2019, title={Polaritonic Hybrid-Epsilon-near-Zero Modes: Beating the Plasmonic Confinement vs Propagation-Length Trade-Off with Doped Cadmium Oxide Bilayers}, volume={19}, ISSN={["1530-6992"]}, DOI={10.1021/acs.nanolett.8b04182}, abstractNote={Polaritonic materials that support epsilon-near-zero (ENZ) modes offer the opportunity to design light-matter interactions at the nanoscale through extreme subwavelength light confinement, producing phenomena like resonant perfect absorption. However, the utility of ENZ modes in nanophotonic applications has been limited by a flat spectral dispersion, which leads to small group velocities and extremely short propagation lengths. Here, we overcome this constraint by hybridizing ENZ and surface plasmon polariton (SPP) modes in doped cadmium oxide epitaxial bilayers. This results in strongly coupled hybrid modes that are characterized by an anticrossing in the polariton dispersion and a large spectral splitting on the order of 1/3 of the mode frequency. These hybrid modes simultaneously achieve modal propagation and ENZ mode-like interior field confinement, adding propagation character to ENZ mode properties. We subsequently tune the resonant frequencies, dispersion, and coupling of these polaritonic-hybrid-epsilon-near-zero (PH-ENZ) modes by tailoring the modal oscillator strength and the ENZ-SPP spectral overlap. PH-ENZ modes ultimately leverage the most desirable characteristics of both ENZ and SPP modes, allowing us to overcome the canonical plasmonic trade-off between confinement and propagation length.}, number={2}, journal={NANO LETTERS}, author={Runnerstrom, Evan L. and Kelley, Kyle P. and Folland, Thomas G. and Nolen, J. Ryan and Engheta, Nader and Caldwell, Joshua D. and Maria, Jon-Paul}, year={2019}, month={Feb}, pages={948–957} }
 @article{grimley_wynn_kelley_sachet_dean_freeman_maria_lebeau_2018, title={Complexities of atomic structure at CdO/MgO and CdO/Al2O3 interfaces}, volume={124}, ISSN={["1089-7550"]}, DOI={10.1063/1.5053752}, abstractNote={We report the interface structures of CdO thin films on (001)-MgO and (0001)-Al2O3 substrates. Using aberration corrected scanning transmission electron microscopy, we show that epitaxial growth of (001)-CdO∥(001)-MgO occurs with a lattice misfit greater than 10%. A high density of interface misfit dislocations is found to form. In combination with molecular dynamics simulations, we show that dislocation strain fields form and overlap in very thin heterostructures of CdO and MgO (<3 nm). On the c-Al2O3 substrate, we find that CdO grows with a surface normal of [025]. We show that three rotation variants form due to the symmetry of the sapphire surface. These results contribute insights into the epitaxial growth of these rock-salt oxides.}, number={20}, journal={JOURNAL OF APPLIED PHYSICS}, author={Grimley, Everett D. and Wynn, Alex P. and Kelley, Kyle P. and Sachet, Edward and Dean, Julian S. and Freeman, Colin L. and Maria, Jon-Paul and LeBeau, James M.}, year={2018}, month={Nov} }
 @article{ceglia_scalora_vincenti_campione_kelley_runnerstrom_maria_keeler_luk_2018, title={Viscoelastic optical nonlocality of low-loss epsilon-near-zero nanofilms}, volume={8}, ISSN={["2045-2322"]}, DOI={10.1038/s41598-018-27655-z}, abstractNote={Abstract Optical nonlocalities are elusive and hardly observable in traditional plasmonic materials like noble and alkali metals. Here we report experimental observation of viscoelastic nonlocalities in the infrared optical response of epsilon-near-zero nanofilms made of low-loss doped cadmium-oxide. The nonlocality is detectable thanks to the low damping rate of conduction electrons and the virtual absence of interband transitions at infrared wavelengths. We describe the motion of conduction electrons using a hydrodynamic model for a viscoelastic fluid, and find excellent agreement with experimental results. The electrons’ elasticity blue-shifts the infrared plasmonic resonance associated with the main epsilon-near-zero mode, and triggers the onset of higher-order resonances due to the excitation of electron-pressure modes above the bulk plasma frequency. We also provide evidence of the existence of nonlocal damping, i.e., viscosity , in the motion of optically-excited conduction electrons using a combination of spectroscopic ellipsometry data and predictions based on the viscoelastic hydrodynamic model.}, journal={SCIENTIFIC REPORTS}, author={Ceglia, Domenico and Scalora, Michael and Vincenti, Maria A. and Campione, Salvatore and Kelley, Kyle and Runnerstrom, Evan L. and Maria, Jon-Paul and Keeler, Gordon A. and Luk, Ting S.}, year={2018}, month={Jun} }
 @article{runnerstrom_kelley_sachet_shelton_maria_2017, title={Epsilon-near-Zero Modes and Surface Plasmon Resonance in Fluorine-Doped Cadmium Oxide Thin Films}, volume={4}, ISSN={["2330-4022"]}, DOI={10.1021/acsphotonics.7b00429}, abstractNote={In this report we demonstrate fluorine-doped CdO as a model infrared plasmonic material by virtue of its tunable carrier density, high mobility, and intense extreme-subwavelength plasmon–polariton coupling. Carrier concentrations ranging from 1019 to 1020 cm–3, with electron mobility values as high as 473 cm2/V·s, are readily achieved in epitaxial CdO films over a thickness range spanning 50 to 500 nm. Carrier concentration is achieved by reactive sputtering in an Ar/O2 atmosphere with trace quantities of CF4. Infrared reflectometry measurements demonstrate the possibility of near-perfect plasmonic absorption through the entire mid-IR spectral range. A companion set of reflectivity simulations are used to predict, understand, and optimize the epsilon-near-zero plasmonic modes. In the context of other transparent conductors, CdO exhibits substantially higher electron mobility values and thus sharp and tunable absorption features. This highlights the utility of high-mobility transparent conducting oxides as...}, number={8}, journal={ACS PHOTONICS}, author={Runnerstrom, Evan L. and Kelley, Kyle P. and Sachet, Edward and Shelton, Christopher T. and Maria, Jon-Paul}, year={2017}, month={Aug}, pages={1885–1892} }
 @article{yang_kelley_sachet_campione_luk_maria_sinclair_brener_2017, title={Femtosecond optical polarization switching using a cadmium oxide-based perfect absorber}, volume={11}, ISSN={1749-4885 1749-4893}, url={http://dx.doi.org/10.1038/NPHOTON.2017.64}, DOI={10.1038/nphoton.2017.64}, abstractNote={Ultrafast control of the polarization state of light may enable a plethora of applications in optics, chemistry and biology. However, conventional polarizing elements, such as polarizers and waveplates, are either static or possess only gigahertz switching speeds. Here, with the aid of high-mobility indium-doped cadmium oxide (CdO) as the gateway plasmonic material, we realize a high-quality factor Berreman-type perfect absorber at a wavelength of 2.08 μm. On sub-bandgap optical pumping, the perfect absorption resonance strongly redshifts because of the transient increase of the ensemble-averaged effective electron mass of CdO, which leads to an absolute change in the p-polarized reflectance from 1.0 to 86.3%. By combining the exceedingly high modulation depth with the polarization selectivity of the perfect absorber, we experimentally demonstrate a reflective polarizer with a polarization extinction ratio of 91 that can be switched on and off within 800 fs. Indium-doped cadmium oxide performs polarization switching on a subpicosecond timescale.}, number={6}, journal={Nature Photonics}, publisher={Springer Science and Business Media LLC}, author={Yang, Yuanmu and Kelley, Kyle and Sachet, Edward and Campione, Salvatore and Luk, Ting S. and Maria, Jon-Paul and Sinclair, Michael B. and Brener, Igal}, year={2017}, month={May}, pages={390–395} }
 @article{kelley_sachet_shelton_maria_2017, title={High mobility yttrium doped cadmium oxide thin films}, volume={5}, ISSN={["2166-532X"]}, DOI={10.1063/1.4993799}, abstractNote={Donor doped CdO thin films on c-plane sapphire are prepared by reactive co-sputtering from Cd-metal and Y-metal targets which are driven using pulsed-dc and RF power respectively. Intrinsic CdO exhibits a carrier density of 1.8 × 1019 cm−3 and a mobility of 330 cm2 V−1 s−1. By increasing the Y-flux, carrier density values can be increased smoothly and reproducibly to a maximum value of 3.3 × 1020 cm−3. Mobility increases with Y flux, and exhibits a broad plateau between approximately 5 × 1019 cm−3 and 2 × 1020 cm−3. Higher carrier concentrations produce a sharp drop in mobility. The increase in mobility is attributed to a reduction of intrinsic donors (i.e., oxygen vacancies) with increasing carrier density while the ultimate decrease in mobility results from a combination of factors including cadmium vacancies, reduced crystal quality, and smaller crystallite sizes, all of which accompany carrier density values greater than the mid 1020 cm−3 range. This work demonstrates that CdO thin films can be prepared by magnetron sputtering with transport properties and crystal quality that are comparable to those grown using molecular beam epitaxy.}, number={7}, journal={APL MATERIALS}, author={Kelley, Kyle P. and Sachet, Edward and Shelton, Christopher T. and Maria, Jon-Paul}, year={2017}, month={Jul} }
 @inproceedings{ceglia_scalora_vincenti_campione_kelley_maria_keeler_luk_2017, title={Observation of nonlocal optical response in doped-cadmium-oxide epsilon-near-zero thin films}, DOI={10.1109/iceaa.2017.8065557}, abstractNote={We study optically-excited nonlocalities in thin films of doped cadmium oxide. Although these effects are usually weak and hardly observable in the optical response of noble metals, the free-electron nonlocality is significantly increased in doped-cadmium-oside thin films. This increase is due mainly to: (i) low electron scattering rates; and (ii) interband transitions due to valence-band and inner-core electrons that occur far from the epsilon-near-zero frequency. The optical nonlocality manifests itself in the blueshift of the epsilon-near-zero mode, an associated reflectance dip, and the onset of higher-order modes. We model the structure using a generalized hydrodynamic theory that treats the free electrons in the film as a viscoelastic fluid. We demonstrate that both elasticity and viscosity play a significant role in the optical response of the film. The elasticity induces optical resonances associated with the longitudinal pressure modes of the free-electrons fluid, leading to a thickness-dependent permittivity. The viscosity introduces nonlocal damping and additional losses. In our view, this demonstration furthers our understanding of the dynamics of light-matter interactions, and adds a significant stepping stone toward the ability to effectively manipulate linear and nonlinear optical properties at the nanoscale.}, booktitle={2017 International Conference on Electromagnetics in Advanced Applications (ICEAA)}, author={Ceglia, D. and Scalora, M. and Vincenti, M. A. and Campione, S. and Kelley, K. and Maria, J. P. and Keeler, G. A. and Luk, T. S.}, year={2017}, pages={1462–1464} }