UC San Diego

Thermal engineering plays an important role in our daily lives, such as thermal management for electronics, electric vehicles and buildings, solar thermophotovoltaic, refrigeration systems, thermal insulation, and so on. Photonic thermal engineering could give us some solutions to those application scenarios, for example, selective radiative coating on the roof could realize passive cooling for the house. This dissertation focuses on Nanophotonic thermal energy transport and conversion, particularly surface phonon polaritons (SPhPs) for heat conduction and molten meta-materials (MMM) for high-efficiency thermophotovoltaic (TPV) systems.The first part of my dissertation is about non-Fourier heat conduction by Surface Phonon Polaritons. In classical textbooks of thermal physics, there are three modes of thermal transport: conduction, radiation, and convection. Heat conduction in solids follows Fourier’s law, and is normally described by a diffusion process with a short mean free path associated with the main heat carriers, such as phonons and electrons. Photons for radiation, on the other hand, usually have very long propagation length unless they are scattered by disordered media, or absorbed by participating media. However, photons are typically not confined and their energy intensity is orders of magnitude lower than that of heat conduction in solids. These two separated ways can intersect in the form of surface phonon polariton (SPhP), which emerges due to the collective oscillation of atoms on the surface of a polar dielectric induced by electromagnetic (EM) waves. SPhPs are evanescent surface waves resulting from the coupling between optical phonons and photons, which have longer wavelengths and propagation lengths, making them ideal for extraordinary heat transfer. Our research demonstrated that SPhPs mediate thermal conductivity in SiO2 nanoribbon waveguides, showing non-Fourier behavior over distances of 50-100 μm at room and high temperatures. This was achieved by designing the waveguides to control the mode size of SPhPs, clearly differentiate the SPhP contribution from phonons, and ensure efficient coupling to thermal reservoirs. We observe an increased thermal conductivity in nanoribbons of SiO2, by as much as 34% over its well-known phonon thermal conductivity limit. This work establishes a foundation for manipulating heat conduction beyond traditional limits, with potential applications in advanced thermal management and energy conversion. The second part of this dissertation discussed the design and fabrication of Molten Meta-Materials selective emitter. We developed high-temperature plasmonic selective emitters using a novel concept of MMMs. These selective narrowband emitters can significantly boost thermophotovoltaic (TPV) efficiency by emitting photons around the TPV cell’s bandgap. Molten Al, with its superior plasmonic properties at high temperatures, enables a high Q-factor emission spectrum. Our emitter involves an array of nanoscale porous aluminum (Al) elements embedded in alumina, which acts as an oxygen barrier. This design solved the main problems for the previous high-temperature emitters in preventing reaction and releasing thermal stress. Aluminum and alumina are thermodynamically stable metamaterials at high temperatures. Besides, the voids in the porous aluminum will compensate for the volume expansion difference after the aluminum melts. In the future, we will integrate these MMM emitters into TPV systems, aiming for highly efficient thermal-to-electrical energy conversion exceeding 50%.