Super-Planckian Thermal Radiation from Nanophotonic Structures:|bTheory, Simulation and Experiment

Thermal radiation from macroscopic objects is limited by the well-known Planck's law. However, when the sizes of the objects or the gaps between the objects are in the micro- or nano-scale, Planck's law is no longer valid and the radiative power can exceed the blackbody limit by orders of magnitude. This super-Planckian thermal radiation phenomenon has attracted significant attention in the fields of the thermal management, energy conversion, infrared sensing and imaging, etc. Nevertheless, in comparison with the traditional thermal radiation under the framework of Planck's law, the understanding of super-Planckian thermal radiation is still relatively immature in the aspects of theoretical description, numerical modeling, and experimental characterization. In this dissertation, we discover new methodologies to design and manipulate the super- Planckian thermal radiation by using the nanophotonic techniques, such as metamaterial, plasmonics, optical cavity effects, etc. First, we present a broadband near-field thermal emitter based on hyperbolic metamaterials, which can significantly enhance near-field radiative heat ransfer with an infrared surface-polariton resonant materials and maintain the monochromatic characteristic of heat transfer. Second, we discover that the thermal graphene plasmons can be efficiently excited and have monochromatic and tunable spectra by graphene nanoribbons, which are resonant near-field thermal emitters. We further demonstrate that "thermal information communication" via graphene surface plasmons can be potentially realized by effectively harnessing thermal energy from various heat sources. To further understand the super-Planckian thermal radiation of the resonant emitters, we develop a general and self-consistent theory from fluctuational electrodynamics and Quasi-Normal Mode theory to describe the thermal radiation from microscale optical resonators made by lossy and dispersive materials. With our theory, we finally propose a general formalism to make the perfect resonant thermal emitters from the densely packed transmission line resonators, and experimentally demonstrate that the thermal emission from the transmission line resonator arrays can be maximized by tuning the waveguiding mode loss or bending the individual structure. In addition, we implement two numerical simulation methods (the Wiener Chaos Expansion method and the Fluctuating Surface Current method) to directly calculate the super- Planckian thermal radiation of arbitrary geometries. We also propose two highly efficient algorithms to expedite the simulations of periodic and symmetric structures and two-dimensional materials like graphene. These two numerical methods serve as our general computational tools and allow us to investigate the thermal radiation of complex nanophotonic structures in detail.