Dissertation Defense: Guanyu Lu, Mechanical Engineering
Guanyu Lu, Mechanical Engineering
*under the direction of Dr. Josh Caldwell
“Phonon Polaritons for Engineering Thermal Emission and Heat Transfer”
03.01.23 | 1:00pm | Buttrick Hall 101
The field of nanophotonics is based on the confinement of electromagnetic energy to length scales an order of magnitude shorter than that of the free-space photon wavelength. In the infrared (IR) spectral region, which includes two atmospheric transmission windows and coincides with the molecular fingerprint region, such sub-diffractional confinement of long-wavelength photons is beneficial for applications like enhanced molecular sensing, light generation, free-space communication, and thermal management. However, it is impossible for conventional materials to achieve such confinement in the IR, because of the limitation of refractive index for those non-dispersive materials. Through the introduction of phonon polaritons, the quasi-particle comprising a photon and an oscillating ionic charge (phonon) in polar materials, sub-diffractional light-matter interactions can be achieved in the IR. In this dissertation, we seek to leverage phonon polaritons to engineer the spectral and spatial energy distribution of far-field thermal emission towards next-generation IR light sources, and to enhance conductive heat transfer in nanostructures towards counteracting the classic size effects by utilizing these optical modes to provide additional heat dissipation channel. First, we demonstrate waste-heat-driven, narrowband thermal emitters by leveraging low-loss localized surface phonon polaritons (LSPhPs), and we report such narrowband LSPhPs can be further engineered through complex unit cell designs of polar nanostructures. Next, we demonstrate multiple spatially coherent thermal emission modes from superstructure gratings by leveraging propagating surface phonon polaritons (SPhPs), and we further report that the spectral and spatial dispersion of far-field thermal emission can be controlled simultaneously via strong coupling between localized and propagating SPhPs with a phonon mode inherent to the polar lattice. In addition to the far-field response, the near-field phenomenon is also of importance for phonon polaritons, especially for engineering nanoscale energy transport. We thus report the study of phonon-polaritonic standing waves and manipulation of higher-order in-plane hyperbolic phonon polaritons in the near-field. Finally, by combing the nanoscale real-space mapping of SPhPs supported within SiC nanowires (NWs) and direct thermal transport measurements of the same NW, SPhP-mediated conductive heat transfer has been unambiguously demonstrated.