Nanoscale Thermal Transport with Photons and Phonons

Recent progress in nanosciences challenges the conventional understanding of Fourier's law for heat conduction and Planck's law for thermal radiation, calling for theoretical and experimental advancement to improve our understanding at these length scales. Advances in both theoretical and experimental progress at these length scale have been made in the past two decades, but there are still many challenges and possibilities in further understanding how heat conducts or radiates at these length scales. The first half of this thesis focuses on topics in nanoscale thermal radiation. First, we will discuss an effort to modify thermal emission using a hyperbolic metamaterial (HMM). Recent efforts in utilizing different metamaterial designs to modify thermal emission has led to greater control over the spectral and directional properties of thermal radiation, and the HMM is one such metamaterial. HMM is typically made up of sub-wavelength alternating layers of metal and dielectric that result in an anisotropic permittivity. Here we demonstrate that an annular, transparent HMM lens enables selective controlling of the plasmonic resonance such that a nanowire emitter, surrounded by an HMM, appears dark to incoming radiation from an adjacent nanowire emitter unless the second emitter is surrounded by an identical lens. While many metamaterial schemes exist to modify thermal emission, these schemes are ultimately limited by the maximum possible emission of a blackbody. In an effort to further increase radiative thermal emission, we made another effort to explore the possibility of removing the enhanced but trapped thermal radiation energy density at sub-wavelength distances. Here, we propose and numerically demonstrate an active scheme that exploits the monochromatic nature of near-field thermal radiation to drive a transition in a laser gain medium, which, when coupled with external optical pumping, allows the resonant surface mode to be emitted into the far-field. We compare this proposed active radiative cooling (ARC) approach to the better-understood laser cooling of solids (LCS) technique, which achieves cooling by extracting phonons instead of thermal radiation. We show that LCS and ARC can be described with the same mathematical formalism and find that ARC can achieve higher efficiency and extracted power over a wide range of conditions. In the second half of thesis, we switch our attention to nanoscale heat conduction where phonons are the dominant heat carriers. Phonons require a medium to travel, unlike thermal radiation, and thus experience much stronger interaction with the medium. Typical assumptions of many scattering events of phonons at the larger length scales break down at the nanoscale when phonon transport can no longer be accurately described by diffusion theory. Here, we present a numerical modeling effort using the Boltzmann Transport Equation to accurately model nanoscale phonon transport of a recent experiment. We show a calculated trend of pump beam size dependence on thermal conductivity similar to results from the time-domain thermal reflectance (TDTR) experiment. We also identify the radial suppression function that describes the suppression in heat flux, compared to Fourier's law, that occurs due to quasiballistic transport and demonstrate good agreement with experimental data. While time-domain thermal reflectance (TDTR) experiment is widely used to characterize thermal transport, it is not ideal for in-plane thermal measurements compared to the transient grating (TG) techniques which utilize interference of two beams to create a in-plane grating pattern for thermal measurements. In the last part of my thesis, we highlight details of an experimental effort to develop the ultra-fast transient grating (TG) technique capable of measuring fast thermal decays. We will then highlight the results of thermal and acoustic measurements of molybdenum disulphide that can be obtained from this technique. Our results are in good agreement with other measurements and calculations. With nanosciences paving way for the future of technology, understanding thermal management at the nanoscale is crucial for device performance and reducing energy waste. We believe that these results in thermal radiation and conduction will benefit thermal management at the nanoscale.