@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={Energy and charge transfer across metal-semiconductor interfaces are the fundamental driving forces for a broad range of applications, such as computing, energy harvesting, and photodetection. However, the exact roles and physical separation of these two phenomena remains unclear, particularly in plasmonically-excited systems or cases of strong nonequilibrium. We report on a series of ultrafast plasmonic measurements that provide a direct measure of electronic distributions, both spatially and temporally, following optical excitation of a metal-semiconductor heterostructure. For the first time, we explicitly show 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 electrons in the semiconductor, without transfer of charge. These experimental results findings are well-supported by both rigorous multilayer optical modeling and first-principle, ab initio calculations.}, 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{radue_tomko_giri_braun_zhou_prezhdo_runnerstrom_maria_hopkins_2018, title={Hot Electron Thermoreflectance Coefficient of Gold during Electron-Phonon Nonequilibrium}, volume={5}, ISSN={["2330-4022"]}, DOI={10.1021/acsphotonics.8b01045}, abstractNote={The temperature-dependent reflectivity of metals is quantified by the thermoreflectance coefficient, which is a material-dependent parameter that depends on the metallic band structure, electron scattering dynamics, and photon wavelength. After short-pulse laser heating, the electronic subsystem in a metal can be driven to temperatures much higher than that of the lattice, which gives rise to unique nonequilibrium electron and phonon scattering dynamics, leading to a “hot electron” thermoreflectance that is different from the traditionally measured equilibrium coefficient. In this work, we analytically quantify and experimentally measure this hot electron thermoreflectance coefficient through ultrafast pump–probe measurements of thin gold films on silica glass and sapphire substrates. We demonstrate the ability to not only quantify the thermoreflectance during electron–phonon nonequilibrium but also validate this coefficient’s predicted dependence on the absolute temperature of the electronic subsystem. T...}, number={12}, journal={ACS PHOTONICS}, author={Radue, Elizabeth L. and Tomko, John A. and Giri, Ashutosh and Braun, Jeffrey L. and Zhou, Xin and Prezhdo, Oleg V. and Runnerstrom, Evan L. and Maria, Jon-Paul and Hopkins, Patrick E.}, year={2018}, month={Dec}, pages={4880–4887} }
 @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} }