Engineering the Spectral and Spatial Dispersion of Thermal Emission Using Phonon Polaritons

Every object has a temperature greater than zero Kelvin can emit thermal emission according to the fundamental principles of statistical mechanics. Thermal emission, especially in the long-wave infrared region (LWIR), is of great importance in a variety of fields, including spectroscopy, imaging, communication, and energy management. Recently, the possibility of controlling thermal emission based on nanophotonic structures has been intensively investigated with a view towards a new generation of far-field thermal emitters. Such control is demonstrated in the aspects of engineering narrowband, polarized, and spatial coherent (directional) thermal emission. While narrow-band thermal emitters that provide IR LED-like performance have been demonstrated in photonic crystals, perfect absorber geometries, metallic bullseye structures, and periodic arrays of nanoantennas, spectral tunability and control of the spatial coherence within a given platform is challenging.<br/>We present phonon-polaritonic thermal emitter designs to control the spectral and spatial dispersion of thermal emission. Surface phonon polaritons (SPhPs) are quasiparticles comprising a photon and a coherently oscillating charge on a polar lattice. Localized SPhPs (LSPhPs) can be supported by periodic SiC nanopillars, offering exceptionally high Purcell enhancements that are in excess of plasmonic counterparts, which result from the narrow resonance line widths. We demonstrate that such large-scale narrowband thermal emitters, 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 [1]. Furthermore, increasing the complexity of the LSPhPs unit cell can serve to modify the resonant near-field and intra- and inter-unit-cell coupling as well as to dictate spectral tuning in the far-field. We also exploit more complicated unit-cell structures to realize new LSPhPs modes with additional degrees of design freedom [2]. Unlike LSPhPs, which offer no spatial coherence, spatially coherent thermal emission can be induced by a one-dimensional grating with propagating SPhPs. However, only a single spatially coherent mode is supported by purely periodic grating structures. We explore superstructure gratings to enable polaritons with different frequencies/wavevectors in a single grating, manifesting as additional spatial modes in the thermal emission profile [3]. By combining the corresponding virtues of both localized and propagating SPhPs through the introduction of strong coupling, we demonstrate a new hybrid mode with narrowband, spatially coherent thermal emission signature. Additionally, through coupling to a third zone-folded longitudinal optic phonon (ZFLO) mode, the realization of electrically driven emitters is possible [4]. Thus, our results show that phonon polaritons and strong coupling phenomenon enable thermal emitters, which meet the requirements for a host of LWIR applications in a simple, lightweight, and intense emitter.<br/>1. Lu, G., et al., <i>Narrowband Polaritonic Thermal Emitters Driven by Waste Heat.</i> ACS Omega, 2020. <b>5</b>(19): p. 10900-10908.<br/>2. Lu, G., et al., <i>Collective Phonon-Polaritonic Modes in Silicon Carbide Subarrays.</i> ACS Nano, 2021.<br/>3. Lu, G., et al., <i>Multi-frequency coherent emission from superstructure thermal emitters.</i> Applied Physics Letters, 2021. <b>118</b>(14).<br/>4. Lu, G., et al., <i>Engineering the Spectral and Spatial Dispersion of Thermal Emission via Polariton-Phonon Strong Coupling.</i> Nano Lett, 2021. <b>21</b>(4): p. 1831-1838.