Optimized Aperiodic Multilayer Structures for Absorbers and Thermal Emitters

Abstract

In this dissertation, we begin with a brief introduction to nanophotonics. In particular, we will focus on the theory of computational electricity and magnetism, specifically, the method used for this research, the transfer-matrix method. We will also provide discussions of the principles which motivate the analysis of the multilayer structures: Planck's blackbody distribution, Kirchoff's law, and thermodynamics. Finally, we will provide a brief discussion of computer-based optimization algorithms, specifically NLOPT (a library which provides computational packages for nonlinear optimization) and genetic algorithms. This background information is necessary to understand discussions which occur in later chapters. Chapter 2 will discuss multilayer structures which are optimized by a genetic algorithm to provide both narrowband and narrow-angle thermal emission for selected wavelengths in the infrared wavelength range. This chapter will compare the performance of aperiodic multilayer structures with more widely researched periodic multilayer structures. It will also provide a detailed analysis of how this emission profile is achieved via an electromagnetic field analysis. Finally, Chapter 2 will present a concrete application for such optimized structures as a carbon monoxide detector via absorption spectroscopy. Chapter 3 will focus on the development of aperiodic multilayer structures for use as narrow-angle absorbers. Both the layer thicknesses and materials used are optimized by a genetic optimization algorithm coupled to a transfer matrix code. We find that utilizing silicon and silica above a thick tungsten substrate provides a structure capable of near unity absorptance at a single wavelength. Finally we show that structures with almost perfect absorptance at multiple wavelengths for normally incident light can be achieved. Chapter 4 presents optimized aperiodic structures for use as broadband, broad-angle thermal emitters. These structures are capable of increasing the thermal emittance by nearly a factor of two when compared to bulk tungsten. We utilize a hybrid optimization algorithm coupled to a transfer matrix code to maximize the power emitted in the visible wavelength range in the normal direction. Chapter 4 also discusses the mechanisms present which allow these structures to possess properties, which could lead to a decrease in incandescent lightbulb power consumption by nearly 50%.