Realizing Directional, Narrowband Thermal Emission through Control of Spectral Dispersion and Strong Coupling
Nolen, Joshua Ryan
The mid-to long-wave infrared (λ~2.5– 12 µm) is home to a large number of fundamental molecular vibrations as well as two atmospheric transmission windows (λ ~ 3–5 and 8–12 µm). Accordingly, research focused on developing compact mid- to long-wave infrared optical sources of sufficiently narrow spectral bandwidths and minimal power demands is of great interest for potential spectroscopic and sensing applications. Further, with added spatial and polarization control, applications such as free-space communications can also be enabled. One approach that has garnered significant recent attention has been the realization of frequency-selective thermal emitters. Here, by judiciously selecting and/or structuring materials, the thermal photonic density of states can be tailored such that frequency, direction, and polarization-dependent emissivity can be achieved. In this dissertation, such emissivity control is realized through the thermal excitation of polaritonic modes, with polaritons being quasiparticles that arise due to strong coupling between light and coherent charge oscillations. Recent work has demonstrated that such polaritonic thermal sources can be designed to produce narrowband, polarization, and angular-selective thermal emission, all while maintaining low power demands. In some cases, these devices can even be driven by waste heat alone. Here, multiple demonstrations of highly tunable, narrowband thermal emission are realized in potentially scalable platforms. Further, strong coupling between polaritons in multilayer films and resonators is explored as a route towards achieving control over the emitted radiation patterns. Combined, methods of achieving both spatial and spectral control over thermal emission are demonstrated, which are critical for realizing next generation compact, low-cost infrared sources.