Nanophotonics for tailoring the flow of thermal electromagnetic radiation

Abstract

In this thesis, we explore the interaction of thermal radiation with nano-scale structures. First, we introduce the concept of radiative energy transfer between two objects of different temperatures in the near field, and theoretically argue that the radiation tunneling of evanescent surface modes can enable energy transfer that is orders of magnitude stronger than the energy transfer in the far field. Specifically, we develop a new computational approach-based on a finite-difference time-domain (FDTD) method that incorporates the Langevin approach to Brownian motion-which enables calculations of heat transfer for arbitrary geometries and materials. Second, we study the near-field heat transfer between two sheets of graphene and show that thermally excited plasmon-polariton modes can strongly mediate, enhance, and tune the energy exchange in this system. We predict maximum transfer at low doping and for plasmons in two graphene sheets in resonance, with orders-of-magnitude enhancement over the Stefan-Boltzmann law. Third, we develop the concept of a near-field thermophotovoltaic (NFTPV) system, and analyze several different implementations that use plasmonic materials as thermal emitters. In particular, we quantify the properties of an optimal near-field photovoltaic cell, argue that large plasmonic losses can-contrary to intuition-be helpful in enhancing the overall heat transfer, and propose and develop the concept of graphene as a tunable thermal emitter for a NFTPV system. Fourth, we tailor the far-field thermal emission from objects at high temperatures and experimentally demonstrate a method where the emission spectrum is controlled on the cold-side by implementing a nano-layer structure that surrounds the hot emitter and recycles unwanted emission. We find that this approach can enable lighting sources with luminous efficiencies close to the fundamental limit for lighting applications. Finally, we study opto-thermal effects in asymmetric nanoparticles. Specifically, we show that a type of metal-dielectric (Janus) particle in uniform light field exhibits a new class of stable rotational dynamics. We demonstrate (in a simulation) opto-thermal guiding of a composite asymmetric particle by switching the light beam frequency, without regard to the direction or the shape of the light beam.