Control electromagnetic heat transfer using photonic structures [electronic resource]
Keywords: Applied Physics.

Electromagnetic heat transfer is a fundamental and ubiquitous energy process. Controlling electromagnetic heat transfer is essential for improving the energy efficiency and can point to novel applications. Photonic structures provide possibilities to achieve electromagnetic heat transfer properties that are unattainable using naturally occurring materials. This thesis explores the opportunities for controlling electromagnetic heat transfer using nonreciprocal photonic structures, tailoring thermal radiation for the applications of radiative cooling, active control of near field heat transfer, and understanding thermal radiation from a single thermal emitter. I will also discuss angle-selective perfection with 2D materials, in particular in the thermal infrared. In Chapter 1, I will introduce electromagnetic heat transfer, discuss different numerical methods for calculating electromagnetic heat transfer in nano-structures, and introduce the general consideration in radiative cooling. In the first part, including Chapter 2 and Chapter 3, I will discuss new opportunities in controlling electromagnetic heat transfer by using non-reciprocal photonic structures. In Chapter 2, we discuss the possibility to maximally violate detailed balance of thermal radiation. Violation of detailed balance points to a pathway for fundamental improvement for energy conversion processes such as solar cells. We will introduce the general conditions to achieve maximal violation of detailed balance in thermal radiation, and numerically demonstrate a magneto-optical photonic crystal structure that exhibits near-complete violation of detailed balance. In Chapter 3, we discuss the theoretical discovery of thermal supercurrent, i.e. persistent directional heat current at thermal equilibrium, in non-reciprocal many-body near field electromagnetic heat transfer. Transport processes including charge, mass etc. usually signify that the system is away from equilibrium. Similarly, for radiative heat transport, typically a temperature gradient is needed in order to have a non-zero net radiative heat transfer. On the other hand, the discoveries of superconductivity and superfluidity represent some of the most important discoveries of physics, where there is supercurrent at zero bias. Thus, it is important to examine whether there can exist thermal supercurrent in heat transfer. The demonstration of thermal supercurrent in near field heat transfer also points to new method for controlling heat flow in the nanoscale. In the second part, including Chapter 4, Chapter 5 and Chapter 6, I will discuss controlling thermal radiation for the application of radiative cooling. Earth's atmosphere has a transparency window for electromagnetic waves between 8 and 13 microns, which coincides with the peak wavelength of the blackbody radiation from an object at typical terrestrial temperatures. By using the cold outer space as a cold heat sink, a terrestrial object can send out its heat through this transparency window to achieve passive radiative cooling. In Chapter 4, we discuss color-preserving daytime radiative cooling, in which one aims to lower the temperature of a structure as much as possible, while the amount of sunlight absorption needs to be maintained for functional or aesthetic considerations. In Chapter 5, we theoretically study radiative cooling for solar cells. A solar cell, under the sun, heats up. The heating is undesirable, and leads to efficient degradation and also reduces stability. A solar cell by necessity naturally has radiative access to the sky. Thus, it would be very attractive to lower the temperature of a solar cell by sending its heat to the cold outer space, while maintaining its amount of sunlight absorption in the same time. In Chapter 6, we experimentally demonstrate radiative cooling of solar absorber using a visibly transparent thermal blackbody based on a photonic crystal, and show a temperature reduction as large as 13 ˚C using radiative cooling. We also show that radiative cooling can synergize with other cooling mechanisms. In the third part, we discuss active control of near field electromagnetic heat transfer, including ultrahigh contrast and large bandwidth thermal rectification in near field heat transfer between nanoparticles in Chapter 7, and negative differential thermal conductance in near field heat transfer in Chapter 8. In Chapter 9, we introduce a simple temporal coupled mode theory to discuss thermal emission from a single thermal emitter, with arbitrary geometry and composition complexity. In Chapter 10, we introduce a general approach to achieve angle-selective perfect absorption with two-dimensional materials, and experimentally demonstrate record-high absorption of mid-infrared light in single-layer graphene. Such an angle-selective perfect absorption can be important for energy, photo-detection and sensing applications. In Chapter 11, we summarize the studies of the thesis, and provide suggestions for future work.