Thermal Emission from Structured Media

Abstract

This dissertation covers a study of the use of macroscopic structure as a means of controlling thermal emission in the THz and mid-IR frequency regions. Chapter 1 presents a brief introduction to the THz frequency region and to the concept of the photonic crystal, the primary type of geometry used. Chapter 2 compares the two most common methods used to calculate the thermal emission of a structure whose components are all at the same temperature. These methods are compared in terms of the results they give and in terms of how computationally involved the methods are. The first method explored involves using Kirchhoff's law of thermal emission which equates the absorptivity and emissivity of a structure. The second method is to calculate the emission directly from the Green's function using the microscopic thermal currents given by the Fluctuation-Dissipation theorem. A derivation of the second method is given, and the equality between the two methods is proven in 1D. It is shown that the Kirchhoff's law method is much more computationally efficient, and it is therefore used for the parametric studies of the structures which make up the remainder of this document. Chapter 3 covers work done in the THz regime. In the THz frequency regime, where a historic lack of sources has in part impeded full exploration and utilization, a photonic crystal design is proposed to control the thermal emission. It is shown that using a 1D bi-layered photonic crystal, composed of alternating section of silicon wafers and vacuum sections, it is possible to tailor many narrowband emission features over a broadband frequency range. In simulation both spectral and directional thermal emission control is demonstrated, and a parametric study is performed to explore how changes in the geometry of the photonic crystal change its thermal emission signature. A description is then given of how the photonic crystal is constructed and how its thermal emission is measured using Fourier transform spectroscopy. The photonic crystal is placed in a Michelson interferometer, and the normal direction thermal emission across as broad range of the THz spectrum is measured. The positions of these measured emission peaks are shown to be consistent with prediction when the physical sizes of the constituent components are included in the simulation. The focus of this document then shifts to the mid-IR in Chapter 4. The mid-IR study is done entirely in simulation. The thermal emission from a plane of plasmonic coated spheres is simulated and the effects of the lattice geometry and spacing on the spectrum as well as on the direction of emission are studied. The lattice constants modeled range from much smaller than a wavelength, designated the effective medium regime, to on the order of the wavelength of light, designated the photonic crystal regime. The thermal emission exhibits significantly different spatial and spectral dependencies on the lattice spacing and geometry in these two regimes. Finally more complex situations are studied. A second layer of spheres is added, and the effects on the emission are explored. A single layer of spheres embedded in a generic plastic bonding matrix is also simulated, and the modifications to the thermal emission caused by this bonding matrix are briefly discussed.