Radiative Contributions to Interfacial Thermal Transport

The continual drive for nanoscale electronic devices to shrink toward atomic length scales poses direct challenges to both our understanding of thermal processes and our implementation of heat dissipation techniques. At the nanoscale, not only does the power density forced through semiconductor devices increase, but the laws that govern thermodynamic processes begin to fail. In conventional theory, thermal transport in solids is mediated by vibrational resonances and electromagnetic waves, phonons and electrons, which store and propagate heat energy within the stretching of crystal bonds and electric charge. Focusing down beneath the characteristic length scale of the average carrier exchange, thermal gradients and even temperatures begin to take on different definitions. It is at these length scales where the intrinsic property of a material to dissipate heat energy, that is thermal resistance, becomes restricted by the boundaries of the system and understanding how to move heat across these boundaries is a major obstacle when engineering and designing microelectronics.

Typically, it is thought that the flow of phonons, or conductive transport, across interfaces dominates over any other thermal process, such as radiation or convection. However, recent innovations in calculating radiation across nanometer-scale vacuum gaps in several measurements have begun to challenge that claim. Some measurements show that the evanescent light available at distances below the Wien's Law characteristic wavelength can transmit fluxes that rival conduction.

The main driver in these intense fluxes is phonon resonances in the mid-infrared (MIR) spectral region, so-called optical phonons. Conductively speaking, because of their low group velocity, optical modes are often assumed to negligibly contribute to thermal conductivity. However, when exposed to the intense flux of near field radiation, these MIR oscillations can resonate with light and produce quasi-particles, a.k.a. polaritons, which are accelerated well beyond their group velocity. The investigation of these emergent quasi-particles is a highly active field of research in optics; most experimental studies neglect their contribution to thermal transport processes.

The goal of this thesis is to investigate how thermal energy can be transmitted by light across solid state interfaces via near-field transport of evanescent radiation. To this end, I will characterize and quantify the thermal transport processes such as thermal boundary conductance (TBC), radiative flux, mode-specific conductance, and polariton velocity at non-equilibrium interfaces in polaritonic systems. To begin, I will develop a theoretical framework for predictions of near-field radiative transport in solid state non-equilibrium systems. I will then design an experimental technique to probe thermally excited polaritonic modes. To demonstrate this effect, I will explore near-field radiative transport from a gold radiator into hexagonal boron nitride (h-BN). I will then engineer an ideal system to quantify the speed at which these modes travel while carrying heat in a cadmium oxide (CdO) structure, while demonstrating a mechanism for an ultrafast heat sink and thermal rectification.