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# Engineering Radiative Heat Transfer with Thermal Polaritonics

• Author / Creator
Starko-Bowes, Ryan C.K.
• This thesis explores the concept of polaritons, collective excitations of light and matter, for applications in thermal photonics. Emerging technologies such thermophotovoltaics and passive radiative cooling would greatly benefit from advancements in thermal photonics where complete control over spectral shape, polarization state, directionality and coherence of thermally generated radiation is the ultimate goal. Conventional thought does not associate these properties of light with photons that are thermally generated. However, recent advances in nanophotonics has allowed us to challenge these longstanding assumptions of thermal light. First, we propose the use of plasmon polaritons in metallic nanowire metamaterials for the purpose of controlling visible frequency light. These metamaterial structures possess strong anisotropic and tunable absorption behavior at visible frequencies. The obvious roadblock that is encountered when moving to high temperature environments is that the metal constituents in plasmonic metamaterials fail. To rectify the problem of high temperature operation, we subsequently move to polar diatomic ceramic materials, which are known to support phonon polaritons at mid infrared frequencies. Bound surface phonon polaritons in silicon carbide are coupled to free space with the aid of a 2D bi-periodic grating. With this structure patterned in to a single crystal wafer, we are able to achieve two independently controllable emission bands that exist within the Reststrahlen region of silicon carbide. Localized surface phonon polariton resonances within $\mu$-particles of SiO$_2$ and SiC are also explored as a means to control mid infrared thermal radiation. The strong overlap between the resonant emission bands of the polaritonic particles and the atmospheric transmission window establishes them as excellent candidates for passive radiative cooling applications. Their emissivity spectrum is determined through direct thermal emission measurements and used to calculate a potential radiative cooling result of $\approx6^\circ C$ below ambient temperature. Lastly, we investigate the potential of boron nitride nanotubes (BNNTs) for high temperature mid infrared thermal emission. BNNTs are unique emerging materials that possess optical phonons in two distinct mid infrared bands due to the particles extreme optical anisotropy. We characterize the polaritonic modes of BNNTs through thermal emission and absorption measurements and show that there is strong agreement with Mie theory simulations. This is the first study exploring the thermal radiative behavior of BNNTs and the results tout them as a strong candidate for high temperature thermal photonics applications. The findings of this thesis help pave the way for future technologies to transform the world we live in through the advancement of thermal photonics.

• Subjects / Keywords
Fall 2019
• Type of Item
Thesis
• Degree
Doctor of Philosophy
• DOI
https://doi.org/10.7939/r3-76zs-bc56