Epsilon near zero modes offer extreme field enhancement that can be utilized for developing enhanced sensing schemes. However, demonstrations of enhanced spectroscopies have largely exploited surface polaritons, mostly due to the challenges of coupling a vibrational transition to volume-confined epsilon near zero modes. Here we fabricate high aspect ratio gratings (up to 24.8 µm height with greater than 5 μm pitch) of 4H-SiC, with resonant modes that couple to transverse magnetic and transverse electric incident fields. These correspond to metal-insulatormetal waveguide modes propagating downwards into the substrate. The cavity formed by the finite The electromagnetic field confinement offered by surface polaritons 1-2 and epsilon-near-zero (ENZ) 3-7 modes have long been discussed for applications in surface-enhanced sensing [8][9][10][11][12][13] , and vibrational coupling [14][15] . Whilst surface plasmon polaritons (SPPs) have been extensively explored, demonstrating enhanced spectroscopies using ENZ modes has remained challenging. This is largely because at optical frequencies ENZ modes are often realized by coupling light into a material where epsilon is close to zero 16-18 , or via waveguides that are not hollow and therefore incompatible with confining the analyte of interest within the region of highly confined electromagnetic fields [3][4] . In this letter we investigate high-aspect-ratio grating (HAG) structures designed to support surface phonon polaritons (SPhPs) 1 at the interface between the polar crystal grating surfaces and the surrounding environment 19 . We show that the modes supported by this structure behave like metal-insulator-metal (MIM) waveguide modes in a short cavity [20][21][22] .Furthermore, due to this architecture, these structures support an ENZ mode in the gap between the grating teeth. This enables the first colocation of strongly confined ENZ fields with an analyte of interest, including liquids, with large surface area. As proof of this, we demonstrate that the ENZ fields can coherently couple to vibrational transitions in a liquid. Thus, this constitutes a platform for studying ENZ and SPhP strong coupling at infrared (IR) frequencies, with potential applications in surface enhanced spectroscopies [8][9][10][11][12][13]23 as well as light-controlled chemistry [14][15] .This study exploits high aspect ratio gratings, which have a height (h) that is much larger than their period (Λ) (see Fig 1). In HAGs and nanopillars, surface polariton modes are supported between the teeth, propagating as MIM waveguide modes downwards into the grating [20][21][22][24][25] .The frequency of the modes can be controlled by changing the effective index of refraction (neff) of the polariton wave using the size of the air gap (g) between the teeth, or the height of the grating (h, see Fig. 1 a-c). Furthermore, polaritonic modes in these structures have been demonstrated to

Strong coupling between optical modes can be implemented into nanophotonic design to modify the energy− momentum dispersion relation. This approach offers potential avenues for tuning the thermal emission frequency, line width, polarization, and spatial coherence. Here, we employ three-mode strong coupling between propagating and localized surface phonon polaritons, with zone-folded longitudinal optic phonons within periodic arrays of 4H-SiC nanopillars. Energy exchange, mode evolution, and coupling strength between the three polariton branches are explored experimentally and theoretically. The influence of strong coupling upon the angle-dependent thermal emission was directly measured, providing excellent agreement with theory. We demonstrate a 5-fold improvement in the spatial coherence and 3-fold enhancement of the quality factor of the polaritonic modes, with these hybrid modes also exhibiting a mixed character that could enable opportunities to realize electrically driven emission. Our results show that polariton−phonon strong coupling enables thermal emitters, which meet the requirements for a host of IR applications in a simple, lightweight, narrow-band, and yet bright emitter.

Wavelength-selective thermal emitters (WS-EMs) are of high interest due to the lack of cost-effective, narrow-band light sources in the mid-to long-wave infrared. Cost-effective WS-EMs can be realized via Tamm plasmon polariton (TPP) structures supported by distributed Bragg reflectors (DBRs) on metal layers, however, optimizing TPP-WS-EMs is challenging because of the large number of parameters to optimize.To address this challenge, we use stochastic gradient descent (SGD) to optimize TPP-WS-EMs composed of an aperiodic DBR deposited on doped cadmium oxide (CdO) plasmonic films. While the SGD-optimized, aperiodic DBR offers extensive spectral control, the material choice, i.e., plasma-frequency-tunable doped CdO, enables the design capabilities not accessible with noble metals. Here, the individual layer thickness and carrier density of CdO are optimized by our SGD inverse design strategy. The resultant experimental designs demonstrate TPP-WS-EMs exhibiting isolated, high-Q (narrow bandwidth), and structures featuring multiple emission bands for applications such as free-space communications and gas sensing. Furthermore, we illustrate the deterministic design capability within the infrared, such as user-designated Q-factors (28 -10,127) at a desired frequency, multi-band emitters with user-defined Q, and the ability to directly match arbitrary chemical absorption spectra. Thus, the combination of our SGD inverse design and the broadly tunable plasma frequency of CdO enables lithography-free, CMOS-compatible, and wafer-scale solutions for WS-EMs with unprecedented spectral control.

There are a broad range of applications for narrowband long-wave infrared (LWIR) sources, especially within the 8− 12 μm atmospheric window. These include infrared beacons, free-space communications, spectroscopy, and potentially on-chip photonics. Unfortunately, commercial light-emitting diode (LED) sources are not available within the LWIR, leaving only gas-phase and quantum cascade lasers, which exhibit low wall-plug efficiencies and in many cases require large footprints, precluding their use for many applications. Recent advances in nanophotonics have demonstrated the potential for tailoring thermal emission into an LED-like response, featuring narrowband, polarized thermal emitters. In this work, we demonstrate that such nanophotonic IR emitting metamaterials (NIREMs), featuring near-unity absorption, can serve as LWIR sources with effectively no net power consumption, enabling their operation entirely by waste heat from conventional electronics. Using experimental emissivity spectra from a SiC NIREM device in concert with a thermodynamic compact model, we verify this feasibility for two test cases: a NIREM device driven by waste heat from a CPU heat sink and one operating using a low-power resistive heater for elevated temperature operation. To validate these calculations, we experimentally determine the temperature-dependent NIREM irradiance and the angular radiation pattern. We purport that these results provide a first proof-of-concept for waste heat-driven thermal emitters potentially employable in a variety of infrared application spaces.

For many industrial and manufacturing applications, detecting and identifying low concentrations of harmful gases and byproducts are performed using nondispersive infrared (NDIR) sensors. These simple devices utilize a broadband IR emitter, thermopile detector, and a spectrally narrow bandpass filter tuned to a vibrational resonance of the analyte of interest. However, such filters are expensive to fabricate and limit the NDIR to operation at only a single frequency, unless filter wheels are employed, which expand the size and complexity of the device considerably. Here, we create a nanophotonic infrared emitting metamaterial (NIREM) fabricated from thin films of doped CdO grown on patterned sapphire substrates (PSS) that exhibit narrowband thermal emission. By coupling a sufficiently narrow line width emitter with a simple broadband detector such as a thermopile, the functionality of the NDIR sensor can be replicated without the need for the narrow bandpass filter. Unlike many metamaterial-based emitters, our device emits both p- and s-polarized light with near-unity emissivity at angles ranging from 0° to 40° off the surface normal without complicated and expensive lithography steps. As a proof of concept, we implement this NIREM for CO2 gas detection within an FTIR spectrometer, demonstrating performance comparable with a conventional blackbody/filter combination. This demonstrates that the NIREM concept can provide a suitable plug-and-play replacement for NDIR devices as they can be implemented in a form-factor commensurate or significantly reduced in comparison to the current state of the art. In principle, by incorporating multiple NIREM dies tuned to emit at different frequencies, multiple vibrational modes could be sequentially detected, making the approach amenable to identification and quantification of complicated molecules within a single NDIR configuration.