Nanophotonic Engineering of Thermal Emitters

Thermal emission is our most ubiquitous light source, as all objects with non-zero temperature emit this type of radiation. Consequently, our ability to shape the spectral and directional properties of thermally emitted and absorbed light by structures is both intriguing at a fundamental level and has practical implications for infrared light sources, radiative cooling, and energy harvesting systems. To impart desired properties to emitted radiation, nanophotonic designs where subwavelength features are patterned into structures have proved effective in preliminary demonstrations of engineered nanoscale control of thermal emission. In this thesis, we leverage nanophotonic designs to demonstrate new phenomena in the context of thermal emission. We first use a guided-mode structure made of α-Si to resonantly couple to magneto-optically active InAs. The magneto-optic response is a common effect used in nonreciprocal optical elements, which we use here to directly observe a violation of the Kirchhoff thermal radiation law, a strict equality in the spectral, directional absorptivity and emissivity. This demonstration is significant in two ways: first, it opens new avenues to design thermal emitters with distinct spectral, directional emissivity and absorptivity properties, and second, it confirms theoretical predictions which have long lacked experimental confirmation. We then extend this experimental Kirchhoff violation to a broadband, directive thermal emitter. The nanophotonic design to achieve this is a deeply subwavelength structure of gradient epsilon-near-zero InAs layers that couple to a Berreman mode. The angular selectivity is determined by the stack thickness, while the broadband spectral range of the effect is imparted by the closely spectrally separated epsilon-near-zero wavelengths. Finally, we theoretically and experimentally lay the groundwork for a thermal lens, where emitted radiation is directed to a focus a given distance above the surface of the structure. Using a combination of coupled dipole approximation, global optimization, and experimental measurements, we realize the necessary collective and local resonance conditions for this effect.