Thermal emission control with periodic microstructures.

Abstract

In this thesis, the control of thermal emission with periodic microstructures is investigated. An important class of these structures, known as photonic crystals, are considered and Kirchhoff's law for photonic crystal films is discussed. Using fluctuational electrodynamics and a Green's function formalism, it is proved that Kirchhoff's law is obeyed for any photonic crystal films. This formalism allows the calculation of optical coherence for periodic structures. Moreover, a generalized form of Kirchhoff's law is derived for non-uniform temperatures. Using this, the control of thermal emission by selective heating of periodic structures is explored. It is found that local periodic heating allows control over which peaks appear in the thermal emission spectrum. The modification of thermal emission using a self-assembled metallic photonic crystal called inverse opal is also discussed. Despite its simplicity in fabrication, strong absorption in inverse opals prevents any influence of the periodicity. The origin of this effect is considered and it is shown how to tailor both the absorption and the surface coupling in experimentally realizable metallic inverse opals. The results show that the optical properties of tailored tungsten inverse opals can be similar to the tungsten woodpile, where modified thermal emission is already seen. In addition to these structures, structured metal surfaces, which are even easier to fabricate, are discussed. In particular, thermal emission from the surfaces of metal films that are patterned with a series of circular concentric grooves (a bull's eye pattern) is examined. Due to thermal excitation of surface polaritons, theory predicts that a single beam of light can be emitted from these films in the normal direction that is amazingly narrow, both in terms of its spectrum and its angular divergence. Experiments of tungsten bull's eyes verify this effect in the infrared. This shows that metallic films can generate laser-like beams of infrared light by a simple thermal process.