Multiple Epsilon-Near-Zero Resonances in Multilayered Cadmium

Letter Cite This: ACS Photonics XXXX, XXX, XXX−XXX

pubs.acs.org/journal/apchd5

Multiple Epsilon-Near-Zero Resonances in Multilayered Cadmium Oxide: Designing Metamaterial-Like Optical Properties in Monolithic Materials

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Kyle P. Kelley,† Evan L. Runnerstrom,*,† Edward Sachet,† Christopher T. Shelton,† Everett D. Grimley,† Andrew Klump,† James M. LeBeau,† Zlatko Sitar,† Jonathan Y. Suen,‡ Willie J. Padilla,‡ and Jon-Paul Maria*,†,§ †

Department of Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina 27695, United States Department of Electrical and Computer Engineering, Duke University, Durham, North Carolina 27708, United States § Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, United States ‡

S Supporting Information *

ABSTRACT: 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 support a separate epsilonnear-zero (ENZ) resonance. Infrared reflectivity measurements reveal that the optical response of the multilayered stack combines multiple discrete absorption events, each associated with an individual ENZ plasmonic polaritonic mode. Structural and chemical characterization confirm that the multilayers are homoepitaxial and monolithic, with internal interfaces defined by discrete steps in dopant density and carrier concentration. Structurally, the layers are indistinguishable as they differ from their neighbors by only ∼1 in 10000 constituent atoms. The optoelectronic property contrast, however, is pronounced, as each layer maintains an independent electron concentration, as corroborated by secondary ion mass spectroscopy and numerical solutions to Poisson’s equation. It is this electron confinement that imbues each individual layer with the ability to independently resonate at separate mid-infrared frequencies. We additionally demonstrate simultaneous thermal emission of infrared light from each individual layer at its respective ENZ frequency, pursuant to Kirchhoff’s law of radiation. The highly localized property contrast intrinsic to these monoliths offers great potential in nanophotonics, plasmonics, and physics thanks to the ability to engineer infrared response and achieve metamaterial-like optical properties without the need for lithography or micro/nanofabrication. New possibilities arising from this work include strongly tunable and multimodal perfect absorbers as well as spectrally engineered and narrow-band light emitters. KEYWORDS: nanophotonics, CdO, infrared, thin film, epsilon-near-zero, plasmonics

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and nanofabrication, particularly over larger areas. In contrast to metals, conductive metal oxides naturally combine tunable plasma frequencies with high electronic mobility, allowing them to access high-quality IR resonances.17,21 The nanophotonics community now uses thin films of these materials, with controlled thickness, to achieve tunable surface plasmon polariton (SPP) resonances,22−24 and, more recently, tunable epsilon-near-zero (ENZ) modes.24−27 Plasmonic ENZ modes represent a novel approach to photon compression and manipulation in the IR. They offer extreme subwavelength confinement of electric fields and can be realized in a thin conducting slab bounded by a dielectric medium on both sides.25,28 In such a geometry, the internal

he ability to engineer light−matter interactions across the infrared (IR) is fundamentally important to the materials science, photonics, physics, and (bio)chemistry communities, with opportunities for advanced communication platforms,1−4 chemical sensing,5,6 energy harvesting,7,8 and catalysis.9,10 The central thread of this diverse application space is the core ability to concentrate electromagnetic fields into localized, subwavelength physical geometries using plasmonic resonance.1,11−14 Noble metals patterned into nanoscale shapes are the most common exhibits of this behavior in the visible spectrum,5,6,15,16 but metals are quite lossy at mid- to far-IR frequencies.17,18 Furthermore, because carrier concentration is fixed in metals, geometry must be manipulated to engineer an IR plasmonic response: composite metal-dielectric metamaterials and metasurfaces are perhaps the most common embodiments enabling IR nanophotonic behavior.19,20 This presents additional practical challenges related to patterning © XXXX American Chemical Society

Received: March 8, 2019

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DOI: 10.1021/acsphotonics.9b00367 ACS Photonics XXXX, XXX, XXX−XXX

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Table 1. Designed ENZ Layer Parameters ENZ layer 1 ENZ layer 2 ENZ layer 3

νENZ (cm−1/μm)

ne (cm−3)

d, range (nm, angle-dependent)

d, target (nm)

2600/3.85 3600/2.8 4600/2.2

8.8 × 1019 1.7 × 1020 2.7 × 1020

55−120 30−61 20−41

90 50 40

Figure 1. (a, inset) Schematic illustration of light coupling into CdO ENZ layers using the Kretschmann-Raether configuration: in this case, light is introduced to the CdO through the backside of the substrate via a prism; (a−c) Reflectivity maps of individual CdO ENZ layers showing a sharp minimum at energy-angle combinations that couple to the polariton mode. In each, a legend indicates the actual film thickness and carrier density.

evanescent fields associated with each metal−dielectric interface couple, splitting the dispersion relationship into a highenergy symmetric mode and a low-energy asymmetric mode.12,28,29 The high-energy mode asymptotically approaches the bulk plasmon energy and becomes an ENZ mode, with strong internal electric field confinement, when the conductor layer thickness is on the order of 50× smaller than the plasma frequency wavelength (λp/50).28 The ENZ mode is so-named because its dispersion relationship is flat and pinned along the bulk plasmon frequency, which is where the real part of the permittivity crosses zero. ENZ mode behavior offers a lithography-free avenue to create structures for extreme subwavelength confinement, but the high carrier density of elemental metals unfortunately requires that such layers be impractically thin, for example, 2 nm.28 Importantly, because the carrier density/plasma frequency in conductive metal oxides is tunable by doping, larger optical skin depths are accessible, allowing experimentally tractable ENZ layer thicknesses.24,28 While recent work has quantified this mode evolution and categorized different material families in terms of their suitability,12,26,28−30 combinations of multiple highquality ENZ modes with designed IR optical properties, in a manner akin to plasmonic metamaterials, have not been demonstrated. Here, we address this shortcoming and develop new ENZ mode functionality by using homoepitaxial, indium-doped cadmium oxide (In:CdO) multilayers, where each layer is individually tuned to support a separate and distinct ENZ mode in the mid-IR. The ability to prepare epitaxial homojunctions, where abrupt changes in carrier density can be grown-in, provides a pathway toward novel monolithic structures that display pronounced property interfaces but invisible physical interfaces. Here, metamaterial-like contrast arises within the multilayers from abrupt permittivity changes about the spectral range where the real part of the dielectric function crosses zero. Thus, changes in carrier density across an interface induce dramatic transitions between metallic and

dielectric behavior at specific frequencies. Importantly, the large optical skin depth of In:CdO at mid-IR frequencies affords us the ability to access individual ENZ modes within buried plasmonic layers. Ultimately, we identify opportunities to fabricate lithography-free thin film multilayers with metamaterial-like optical responses; such responses can be engineered for customized interactions with the IR electromagnetic spectrum. To demonstrate our proof of concept, we designed a stack of three In:CdO ENZ layers, where each is tuned for the resonant perfect absorption condition. The carrier density values required to target specific frequencies were estimated by modeling CdO with a complex dielectric function implementing a lossy Drude electron gas:23,24 ε(ω) = ε1 + iε2 = ε∞ −

ωp2 ω 2 + iγω

(1)

where ε1 and ε2 are the real and imaginary parts of the complex dielectric function, ε∞ is the high frequency dielectric constant, ωp = ne 2 /me*ε0 is the plasma frequency, ω is the angular

driving frequency, and γ is the angular damping frequency (inversely proportional to the electron mobility). We estimate the required film thickness d for perfect absorption in our ENZ layers using the following equation:27,31 λ ji ε2 yzz zz d = jjjj 3/2 z k εm { 2π sin θ tan θ

(2)

where ε2 is the value of the imaginary part of the dielectric function at the ENZ frequency, εm is the dielectric constant of the surrounding dielectric medium (for simplicity, taken as 2.1, which is the average dielectric constant of the top air dielectric and bottom sapphire dielectric), λ is the ENZ wavelength, and θ is the incident angle. We also used simulations based on Fresnel’s equations (using the transfer matrix method, or TMM) to confirm this estimate and pick a target thickness B


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value. The required CdO thickness for total absorption is on the order of 10−120 nm for coupling to the near to mid-IR energy range. Our demonstration vehicle for this report is a three-ENZ system with target absorption frequencies, νENZ‑1,‑2,‑3, of 2600 (3.85 μm), 3600 (2.8 μm), and 4600 cm−1 (2.2 μm). To prepare it, we need only calculate the necessary carrier density and thickness using the relationships above and calibrate CdO deposition to achieve the correct dopant profile. As a reference, we first prepared three individual single-layer samples tuned to support the three identified ENZ resonant frequencies. Table 1 gives the salient predicted parameters for each. To probe the plasmonic response, we measured the infrared reflectivity spectra as a function of incident angle and energy using the Kretschmann-Raether configuration, where light is prism-coupled into the plasmonic film, as illustrated in the inset of Figure 1a. Since an ENZ mode, like all surface plasmon polaritons, is a transverse-magnetic (TM) mode, only ppolarized light will resonate.12,28 Therefore, these data are presented as normalized intensity maps of Rp/Rs (p-polarized and s-polarized reflected light), which visualizes the plasmon polariton dispersion using s-polarized reflectivity as a natural, self-referenced background. Figure 1 shows such normalized reflectivity maps of wavenumber versus incident angle for single-layer ENZ thin films. In each, a strong reflectivity minimum is observed with minimal energy dispersion over the measured angle/wavevector range, a distinguishing characteristic of the ENZ mode.28 The experimentally observed thicknesses and carrier densities are provided in the legends, with close agreement to our design values. Achieving the proper experimental thickness for a given carrier concentration yields maximum absorption of p-polarized light, with the potential for resonant perfect absorption if the thickness is exactly correct. Our individual ENZ layers achieve 90−97% peak absorption with peak widths as narrow as 307 cm−1 and quality factors (ratio of peak energy to peak width) as high as 14.9. The sharp absorption features can be attributed to the small damping frequency γ (eq 1), which is enabled by high electron mobility in In:CdO for a broad range of carrier concentration (Figure S1). We now consider the three-layer monolith (Figure 2a), which we prepared using the identical deposition parameters as the individual epilayers. Since imperfections, such as point defects, additional orientations, and excessive dislocation densities can spoil high electronic mobility,32−34 we took care to characterize structure and surface quality using X-ray diffraction (XRD), scanning transmission electron microscopy (STEM) and atomic force microscopy (AFM). Figure 2b shows an XRD pattern for the three ENZ layer grown on rplane sapphire, with a clear and single [001] crystallographic growth direction. Additional XRD scans, including rocking curves and reciprocal space maps (Figures S2 and S3a) show that the multilayered stack shows no observable changes to epitaxial quality; by diffraction, this three-layer stack is a monolith. High-resolution STEM (Figure S3) confirms homoepitaxy between adjacent layers, and AFM (Figure S4) shows that all layers are smooth with RMS roughness values ≤2 nm. For the three ENZ layers to resonate independently, it is critical that donor dopant atoms and the associated free electrons are confined to their respective/intended layers. While our deposition and annealing temperatures are modest, we must still rule out long-range diffusion of In throughout the

Figure 2. (a) Schematic illustration of the monolithic, three-ENZ layered stack. (b) X-ray diffraction 2θ−ω scan of a three-layer CdO homostructure on r-plane sapphire, and (c) ToF-SIMS analysis for the CdO three-layer stack showing plateaus of In concentration that coincide with each spatially discrete ENZ layer. Concentration values are relative. The right axis corresponds to calculated electron concentrations (the light brown trace).

stack. To do so, we qualified the dopant concentration/ thickness profile with time-of-flight-secondary ion mass spectroscopy (ToF-SIMS, Figure 2c), which provides clear evidence of spatial dopant contrast and plateau concentrations in each layer. Knock-in effects and roughening during the depth profiling sputter process prevent exact quantification of interface sharpness, but the presence of the plateau regions, along with constant Cd concentration, strongly suggests that dopant atoms are confined to their intended layers. Localization of free electrons within each ENZ layer must also be confirmed. To explore electronic segregation and interfacial depletion effects, we used TCAD (Silvaco) to numerically solve Poisson’s equation for our layers. The simulations were developed using experimental material parameters for mobility, band gap, and electron affinity, while assuming a uniform distribution of indium (ND) within each layer. The light brown trace in Figure 2c shows the calculated result. The absolute magnitude of this trace with respect to the SIMS data is arbitrary, but it is noteworthy that the ratio of carrier concentration from layer to layer follows the same trend as the dopant concentration. Most importantly, we predict that depletion widths between layers (defined as the junction depth where n falls to ND/2) are less than 2 nm. The small depletion widths and segregated layers suggest that the stack maintains discretized carrier concentrations and thus discrete ENZ frequencies. Figure 3a,b show simulated and measured reflectivity maps of the three-ENZ layered stack. Both maps reveal three distinct absorption events related in energy to the individual layers. Our TMM and finite-difference time-domain (FDTD) simulations (Figure S5) accurately capture the unique features in the measured reflectivity map. See Figure S6 for the dielectric functions of each individual ENZ layer simulated using the Drude model and measured values for n and μ, which show how doping in In:CdO strongly tunes dielectric properties. (We attribute the small discrepancies in peak energy between the individual and stacked ENZ layers mainly to sputtering process drift.) We further used the FDTD technique to simulate the nearfield mode profiles of the three-layer stack (Figure S7). These simulations confirm ENZ mode behavior for all three of the C


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Figure 3. (a) Experimental reflectivity map of the three-layer CdO stack; (b) Transfer matrix method-simulated reflectivity map of the three-layer CdO stack; and (c) Reflectivity line scan taken at 50° for the three individual ENZ layers (top) and the three-layer stack (bottom), along with TMM- and FDTD-simulated spectra (dashed and faint purple lines, respectively).

Figure 4. (a) Experimental thermal emission profiles (T = 400 °C) for three individual ENZ layers (top panel) and the composite stack (bottom panel), along with the FDTD-simulated thermal emission spectrum for the composite stack (bottom panel, dashed line); (b) Experimental angular dependence of thermal emission from the three-ENZ stack at the three different peak emission energies as labeled in part (a); and (c) FDTDsimulated angular dependence of thermal emission from the three-ENZ stack.

three individual layers; we have thus demonstrated the ability to bury multiple ENZ mode resonators within in a monolithic structure. A noteworthy dissimilarity between the single layers and the three-layer stack is the ENZ absorption present at ∼2800 cm−1: in the stack, the relatively flat dispersion is attenuated at higher angles. We attribute this attenuation at high angle/wavevector to the evolution of a surface plasmon polariton (SPP) mode resulting from the average dielectric function (i.e., effective medium) across the full thickness of the three-layer stack. This is supported by the appearance of an additional low-energy dispersive feature at ∼50° in the threelayer stack. It has been shown, both theoretically and experimentally, that ENZ modes transition into SPP modes as the plasmon layer thickness increases past the optical skin depth.24,28 Aside from this modification of the lowest energy

absorption peaks in our three-layer stack; at the three resonant frequencies, the electric field is almost entirely confined within the individual ENZ layer that is tuned to resonate at that same frequency. One-dimensional near-field intensity profiles through the full stack (Figure S7d) show that extreme subwavelength confinement concentrates light by 30−65× within individual layers at each ENZ frequency. Importantly, these one-dimensional field profiles reveal that, even in buried layers, the ENZ mode is supported by a small, evanescently decaying field at the top surface. As previously discussed, we attribute the ability of these buried layers to sustain ENZ modes to the large optical skin depth of In:CdO at these frequencies.24 Consideration of 50° line cuts comparing the individual and stacked ENZ layers (Figure 3c) reveals that the stacked system’s optical response is essentially a superposition of the D


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devices using broadband plasmonic emitters38−40 and absorbers45−47 (Figure S8), multispectral notch/band-pass filters and arrays thereof,48−51 and layered hyperbolic metamaterials.52,53 Ultimately, because these monolithic stacks display metamaterial-like properties, require no lithography/ pattering, and contain no physical interfaces, they resemble something akin to a “bulk metamaterial”, despite the seeming self-contradiction in the term. This discovery will hopefully enable simple and scalable routes to engineer metamaterial-like optical responses in monolithic materials, leading to mid-IR absorption- and emission-by-design. Experimental Methods. Additional experimental detail is available in the Supporting Information. Briefly, In:CdO thin films and multilayers were prepared by pulsed DC magnetron reactive sputtering from a Cd metal target. Doping was achieved by RF cosputtering from an In metal target. All films were sputtered at 10 mTorr while flowing 20 sccm Ar and 14 sccm O2. The pulsed DC power supply applied 400 V to the Cd target for 80 μs at a rate of 800 Hz (1250 μs period and 6.4% duty cycle) for an average DC power of 32 W and a deposition rate of ∼23 nm/min. Films were typically grown on double sided epitaxial-polished r-plane sapphire substrates at a deposition temperature of 370 °C and further annealed at 700 °C for 30 min in pure oxygen. Reflectivity data were obtained using using a 90° calcium fluoride (CaF2) prism in the Kretschmann-Raether configuration. Data were collected using a Woollam infrared variableangle spectroscopic ellipsometer (IR-VASE), and all reflectivity data were recorded with p- and s-polarized light and plotted as R = Rp/Rs. Simulated reflectivity spectra were generated using either a home-built code employing a multilayer transfer-matrix method (TMM, MATLAB) or the finite-difference timedomain method (Lumerical FDTD Solutions). In both cases, the Drude model was used to generate the IR dielectric function of the In:CdO layers, based on Hall effect-measured electron concentration and mobility (m* = 0.21, ε∞ = 5.3). FDTD was additionally used with a refined finite element mesh to simulate local electric field profiles. Emission measurements were taken using a Bruker Vertex 80v Fourier transform infrared (FTIR) spectrometer in combination with an external rotating heating stage.

branch, the other ENZ layers’ resonant energy values closely match those measured for single layers. To complement the observed ENZ absorption properties, we investigate the thermal emission properties of stacked ENZ layers. Kirchhoff’s law of thermal radiation states the emissivity of a material body is equal to the absorptivity at thermal equilibrium.35 We thus expect stacked ENZ layers to thermally emit radiation at specific frequencies (i.e., νENZ‑1, νENZ‑2, and νENZ‑3) with a specific angular dependence. To measure thermal emission spectra, we again fabricated single- and threelayer ENZ structures (same design targets as in Table 1) on platinum-coated c-plane sapphire substrates. We employed this Pt mirror layer to facilitate forward emission. The emission spectra at 400 °C are plotted in Figure 4. It is immediately apparent that our three-layer ENZ stack exhibits metamaterial-like properties in emission as well as absorption. The emission spectrum of the three-layer stack exhibits several important characteristics: (i) The emissivity peak locations and intensities are very similar in the single layer and multilayer emitters, confirming the same additive behavior seen in the absorption spectra; (ii) Thermal emission from ENZ modes has a polar angle dependence, with maximum emissivity centered around 60−65°. Uncoincidentally, this is the Brewster angle of CdO (i.e., the incident angle at which no p-polarized light is reflected at an interface), where the highest intensity of thermal emission is predicted;36 and (iii) Our FDTD simulations of thermal emission and angular dependence capture the superposition of emission wavelengths and the effects of CdO’s Brewster angle. Although thermal emission from ENZ modes has been previously demonstrated,37 it is unprecedented for a monolithic structure with multiple tailored infrared wavelengths to exhibit the same effect. This discovery may lead to strategies toward engineering new multispectral and/or broadband infrared light sources.38−41

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CONCLUSION We have demonstrated a new class of ENZ-based nanophotonic materials formed from epitaxial In:CdO multilayers. Our three-ENZ stack displays metamaterial-like optical properties, with three distinct absorption events, each associated with an independent epsilon-near-zero plasmonic polaritonic mode. Structural and chemical characterization reveal a high degree of crystalline perfection and minimal dopant diffusion between layers, while TCAD simulations predict that accumulation/depletion widths between layers are on the order of 2 nm. These two results physically corroborate electron confinement and explain the stack’s ability to support distinct and independent ENZ modes. Strikingly, each layer is optically distinct, despite the structural similarity and the fact that the layers differ from their neighbors by only ∼1 in 10000 constituent atoms. The marked property contrast results from the epsilon-near-zero condition, where the relative optical properties, by definition, are extremely sensitive to the dielectric function. We also demonstrate light emission at each ENZ frequency, following Kirchoff’s law of radiation. This new ability to embed highly localized property contrast within a monolithic epitaxial structure, using simple and scalable techniques like sputtering, will open the door for new nanophotonic behaviors, like tunable coupling between polaritons42 and hyperbolic dispersion behavior.43,44 More specifically, we envision the multilayered ENZ materials explored here being used to design new mid-IR nanophotonic

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsphotonics.9b00367. Electronic properties of In:CdO films, additional characterization of In:CdO multilayers (high-resolution XRD, STEM, AFM), FDTD simulations of In:CdO multilayers (reflectivity map, electric field profiles), simulated In:CdO dielectric functions, reflectivity map of 7-layer In:CdO broadband absorber, and additional experimental details (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Evan L. Runnerstrom: 0000-0002-9054-708X E


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(17) Naik, G. V.; Shalaev, V. M.; Boltasseva, A. Alternative Plasmonic Materials: Beyond Gold and Silver. Adv. Mater. 2013, 25, 3264−3294. (18) Ordal, M. A.; Long, L. L.; Bell, R. J.; Bell, S. E.; Bell, R. R.; Alexander, R. W.; Ward, C. A. Optical Properties of the Metals Al, Co, Cu, Au, Fe, Pb, Ni, Pd, Pt, Ag, Ti, and W in the Infrared and Far Infrared. Appl. Opt. 1983, 22, 1099−1119. (19) Watts, C. M.; Liu, X.; Padilla, W. J. Metamaterial Electromagnetic Wave Absorbers. Adv. Mater. 2012, 24, OP98−OP120. (20) Yu, N.; Capasso, F. Flat Optics with Designer Metasurfaces. Nat. Mater. 2014, 13, 139−150. (21) Lounis, S. D.; Runnerstrom, E. L.; Llordés, A.; Milliron, D. J. Defect Chemistry and Plasmon Physics of Colloidal Metal Oxide Nanocrystals. J. Phys. Chem. Lett. 2014, 5, 1564−1574. (22) Sachet, E.; Losego, M. D.; Guske, J.; Franzen, S.; Maria, J.-P. Mid-Infrared Surface Plasmon Resonance in Zinc Oxide Semiconductor Thin Films. Appl. Phys. Lett. 2013, 102, 051111. (23) Sachet, E.; Shelton, C. T.; Harris, J. S.; Gaddy, B. E.; Irving, D. L.; Curtarolo, S.; Donovan, B. F.; Hopkins, P. E.; Sharma, P. A.; Sharma, A. L.; Ihlefeld, J.; Franzen, S.; Maria, J.-P. Dysprosium-Doped Cadmium Oxide as a Gateway Material for Mid-Infrared Plasmonics. Nat. Mater. 2015, 14, 414−420. (24) Runnerstrom, E. L.; Kelley, K. P.; Sachet, E.; Shelton, C. T.; Maria, J.-P. Epsilon-Near-Zero Modes and Surface Plasmon Resonance in Fluorine-Doped Cadmium Oxide Thin Films. ACS Photonics 2017, 4, 1885−1892. (25) Campione, S.; Liu, S.; Benz, A.; Klem, J. F.; Sinclair, M. B.; Brener, I. Epsilon-Near-Zero Modes for Tailored Light-Matter Interaction. Phys. Rev. Appl. 2015, 4, 044011. (26) Vassant, S.; Archambault, A.; Marquier, F.; Pardo, F.; Gennser, U.; Cavanna, A.; Pelouard, J. L.; Greffet, J. J. Epsilon-Near-Zero Mode for Active Optoelectronic Devices. Phys. Rev. Lett. 2012, 109, 237401. (27) Luk, T. S.; Campione, S.; Kim, I.; Feng, S.; Jun, Y. C.; Liu, S.; Wright, J. B.; Brener, I.; Catrysse, P. B.; Fan, S.; Sinclair, M. B. Directional Perfect Absorption Using Deep Subwavelength LowPermittivity Films. Phys. Rev. B: Condens. Matter Mater. Phys. 2014, 90, 085411. (28) Campione, S.; Brener, I.; Marquier, F. Theory of Epsilon-NearZero Modes in Ultrathin Films. Phys. Rev. B: Condens. Matter Mater. Phys. 2015, 91, 121408. (29) Berini, P. Long-Range Surface Plasmon Polaritons. Adv. Opt. Photonics 2009, 1, 484−588. (30) Vassant, S.; Hugonin, J.-P.; Marquier, F.; Greffet, J.-J. Berreman Mode and Epsilon Near Zero Mode. Opt. Express 2012, 20, 23971− 23977. (31) Feng, S.; Halterman, K. Coherent Perfect Absorption in Epsilon-Near-Zero Metamaterials. Phys. Rev. B: Condens. Matter Mater. Phys. 2012, 86, 165103. (32) Fonstad, C. G.; Rediker, R. H. Electrical Properties of HighQuality Stannic Oxide Crystals. J. Appl. Phys. 1971, 42, 2911−2918. (33) Janotti, A.; Van de Walle, C. G. Fundamentals of Zinc Oxide as a Semiconductor. Rep. Prog. Phys. 2009, 72, 126501. (34) Jain, S. C.; Willander, M.; Narayan, J.; Van Overstraeten, R. IIINitrides: Growth, Characterization, and Properties. J. Appl. Phys. 2000, 87, 965−1006. (35) Kirchhoff, G. Ueber Das Verhältniss Zwischen Dem Emissionsvermögen Und Dem Absorptionsvermögen Der Körper Für Wärme Und Licht. Ann. Phys. 1860, 185, 275−301. (36) Argyropoulos, C.; Le, K. Q.; Mattiucci, N.; D’Aguanno, G.; Alu, A. Broadband Absorbers and Selective Emitters Based on Plasmonic Brewster Metasurfaces. Phys. Rev. B: Condens. Matter Mater. Phys. 2013, 87, 205112. (37) Jun, Y. C.; Luk, T. S.; Robert Ellis, A.; Klem, J. F.; Brener, I. Doping-Tunable Thermal Emission From Plasmon Polaritons in Semiconductor Epsilon-Near-Zero Thin Films. Appl. Phys. Lett. 2014, 105, 131109. (38) Greffet, J.-J.; Carminati, R.; Joulain, K.; Mulet, J.-P.; Mainguy, S.; Chen, Y. Coherent Emission of Light by Thermal Sources. Nature 2002, 416, 61−64.

James M. LeBeau: 0000-0002-7726-3533 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge support for this work by NSF Grant CHE-1507947, by Army Research Office Grants W911NF161-0406 and W911NF-16-1-0037, and by Office of Naval Research Grant N00014-18-12107. We also thank the Efimenko and Genzer groups (NCSU, CBE) for providing access to the IR-VASE. EDG and JML gratefully acknowledge support from the National Science Foundation (DMR1350273). EDG acknowledges support for this work through a National Science Foundation Graduate Research Fellowship (DGE-1252376). The SIMS and STEM work was performed in part at the Analytical Instrumentation Facility (AIF), which is supported by the State of North Carolina and the National Science Foundation (Award Number ECCS-1542015). The AIF is a member of the North Carolina Research Triangle Nanotechnology Network (RTNN), a site in the National Nanotechnology Coordinated Infrastructure (NNCI).

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REFERENCES

(1) Gramotnev, D. K.; Bozhevolnyi, S. I. Plasmonics Beyond the Diffraction Limit. Nat. Photonics 2010, 4, 83−91. (2) Jacob, Z.; Shalaev, V. M. Plasmonics Goes Quantum. Science 2011, 334, 463−464. (3) Noginov, M. A.; Gu, L.; Livenere, J.; Zhu, G.; Pradhan, A. K.; Mundle, R.; Bahoura, M.; Barnakov, Y. A.; Podolskiy, V. A. Transparent Conductive Oxides: Plasmonic Materials for Telecom Wavelengths. Appl. Phys. Lett. 2011, 99, 021101. (4) Look, D. C.; Leedy, K. D. ZnO Plasmonics for Telecommunications. Appl. Phys. Lett. 2013, 102, 182107. (5) Jain, P. K.; Huang, X.; El-Sayed, I. H.; El-Sayed, M. A. Noble Metals on the Nanoscale: Optical and Photothermal Properties and Some Applications in Imaging, Sensing, Biology, and Medicine. Acc. Chem. Res. 2008, 41, 1578−1586. (6) Willets, K. A.; Van Duyne, R. P. Localized Surface Plasmon Resonance Spectroscopy and Sensing. Annu. Rev. Phys. Chem. 2007, 58, 267−297. (7) Knight, M. W.; Sobhani, H.; Nordlander, P.; Halas, N. J. Photodetection with Active Optical Antennas. Science 2011, 332, 702−704. (8) Fei Guo, C.; Sun, T.; Cao, F.; Liu, Q.; Ren, Z. Metallic Nanostructures for Light Trapping in Energy-Harvesting Devices. Light: Sci. Appl. 2014, 3, e161−e161. (9) Sun, M.; Xu, H. A Novel Application of Plasmonics: PlasmonDriven Surface-Catalyzed Reactions. Small 2012, 8, 2777−2786. (10) Zhou, L.; Swearer, D. F.; Zhang, C.; Robatjazi, H.; Zhao, H.; Henderson, L.; Dong, L.; Christopher, P.; Carter, E. A.; Nordlander, P.; Halas, N. J. Quantifying Hot Carrier and Thermal Contributions in Plasmonic Photocatalysis. Science 2018, 362, 69−72. (11) Ozbay, E. Plasmonics: Merging Photonics and Electronics at Nanoscale Dimensions. Science 2006, 311, 189−193. (12) Maier, S. A. Plasmonics: Fundamentals and Applications; Springer US: New York, NY, 2007. (13) Liberal, I.; Engheta, N. Near-Zero Refractive Index Photonics. Nat. Photonics 2017, 11, 149−158. (14) Silveirinha, M.; Engheta, N. Tunneling of Electromagnetic Energy Through Subwavelength Channels and Bends Using ε-NearZero Materials. Phys. Rev. Lett. 2006, 97, 157403. (15) Pendry, J. B.; Martín-Moreno, L.; Garcia-Vidal, F. J. Mimicking Surface Plasmons with Structured Surfaces. Science 2004, 305, 847− 848. (16) Stanley, R. Plasmonics in the Mid-Infrared. Nat. Photonics 2012, 6, 409−411. F


ACS Photonics

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(39) Mason, J. A.; Smith, S.; Wasserman, D. Strong Absorption and Selective Thermal Emission From a Midinfrared Metamaterial. Appl. Phys. Lett. 2011, 98, 241105. (40) Jung, D.; Bank, S.; Lee, M. L.; Wasserman, D. Next-Generation Mid-Infrared Sources. J. Opt. 2017, 19, 123001. (41) Schliesser, A.; Picqué, N.; Hänsch, T. W. Mid-Infrared Frequency Combs. Nat. Photonics 2012, 6, 440−449. (42) Runnerstrom, E. L.; Kelley, K. P.; Folland, T. G.; Nolen, J. R.; Engheta, N.; Caldwell, J. D.; Maria, J.-P. Polaritonic Hybrid-EpsilonNear-Zero Modes: Beating the Plasmonic Confinement vs Propagation-Length Trade-Off with Doped Cadmium Oxide Bilayers. Nano Lett. 2019, 19, 948−957. (43) Poddubny, A.; Iorsh, I.; Belov, P.; Kivshar, Y. Hyperbolic Metamaterials. Nat. Photonics 2013, 7, 948−957. (44) Caldwell, J. D.; Kretinin, A. V.; Chen, Y.; Giannini, V.; Fogler, M. M.; Francescato, Y.; Ellis, C. T.; Tischler, J. G.; Woods, C. R.; Giles, A. J.; Hong, M.; Watanabe, K.; Taniguchi, T.; Maier, S. A.; Novoselov, K. S. Sub-Diffractional Volume-Confined Polaritons in the Natural Hyperbolic Material Hexagonal Boron Nitride. Nat. Commun. 2014, 5, 31. (45) Aydin, K.; Ferry, V. E.; Briggs, R. M.; Atwater, H. A. Broadband Polarization-Independent Resonant Light Absorption Using Ultrathin Plasmonic Super Absorbers. Nat. Commun. 2011, 2, 517. (46) Yoon, J.; Zhou, M.; Badsha, M. A.; Kim, T. Y.; Jun, Y. C.; Hwangbo, C. K. Broadband Epsilon-Near-Zero Perfect Absorption in the Near-Infrared. Sci. Rep. 2015, 5, 12788. (47) Park, J.; Kang, J.-H.; Liu, X.; Brongersma, M. L. Electrically Tunable Epsilon-Near-Zero (ENZ) Metafilm Absorbers. Sci. Rep. 2015, 5, 205. (48) Wu, Z.; Haus, J. W.; Zhan, Q.; Nelson, R. L. Plasmonic Notch Filter Design Based on Long-Range Surface Plasmon Excitation Along Metal Grating. Plasmonics 2008, 3, 103−108. (49) Liang, Y.; Zhang, S.; Cao, X.; Lu, Y.; Xu, T. Free-Standing Plasmonic Metal-Dielectric-Metal Bandpass Filter with High Transmission Efficiency. Sci. Rep. 2017, 7, 4357. (50) Tittl, A.; Michel, A.-K. U.; Schäferling, M.; Yin, X.; Gholipour, B.; Cui, L.; Wuttig, M.; Taubner, T.; Neubrech, F.; Giessen, H. A Switchable Mid-Infrared Plasmonic Perfect Absorber with Multispectral Thermal Imaging Capability. Adv. Mater. 2015, 27, 4597− 4603. (51) Duempelmann, L.; Gallinet, B.; Novotny, L. Multispectral Imaging with Tunable Plasmonic Filters. ACS Photonics 2017, 4, 236−241. (52) Wei, D.; Harris, C.; Bomberger, C. C.; Zhang, J.; Zide, J.; Law, S. Single-Material Semiconductor Hyperbolic Metamaterials. Opt. Express 2016, 24, 8735−8745. (53) Sohr, P.; Wei, D.; Tomasulo, S.; Yakes, M. K.; Law, S. Simultaneous Large Mode Index and High Quality Factor in Infrared Hyperbolic Metamaterials. ACS Photonics 2018, 5, 4003−4008.

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