Nanotextured surfaces: platform for tailored light-matter interaction and optical sensing

Texturation of material interfaces provides a purely topological mechanism for altering or controlling the way a given object interacts with a range of physical phenomena, defined by the relative lengthscales of surface features to those of the relevant effects. For electromagnetic radiation, scattering and interference represent the two main points of leverage that can be employed to alter its propagation behavior at an interface. Visible or near-infrared light, of primary interest here, tends to possess wavelengths on the nanometer range, hence, necessitating nanotexturation to exert control over its interaction with mater. This work is primarily geared towards development of methods and techniques for the creation of effective and affordable nanotextured surfaces exhibiting a range of optical functionalities. It is divided into three parts, based on the class of materials onto which nanotexturing is applied, and deals with solid-state semiconductors/dielectrics like silicon, polymers of natural origin with the primary focus on silk, and plasmonic noble metals such as gold. The problem of matching the impedances of radiation propagating in free space and within high refractive index materials is of primary relevance to technological fields pertaining to photovoltaics, photodetectors as well as solid state light sources. Increasing the efficiency of such devices involves creating effective methods for suppressing reflectance-related loss. Here, after overviewing the major ways for the realization of anti-reflectance, we focus on black silicon, a specific type of stochastic nanotextured surface, created on different types of substrate materials by way of a self-masking reactive ion etching process. The relationship between topology and optical behavior of black silicon is investigated via experimental and numerical experiments, and methods of extending its anti-reflectance towards the infrared spectral region by changing the scale of structural features or by creating a hierarchical texture, are proposed. Lastly, ways of inducing absorbance in the black silicon transparency region by way of metal nanoparticle decoration and infrared absorber layer creation are sought out, and new prototype photothermovoltaic as well as hybrid photovoltaic and thermoelectric full solar spectrum energy harvesting devices are proposed. Nanotexturation was first pioneered in the natural world, however, so it is unsurprising that for most of the contemporary artificial functional interfaces an equivalent in both form and function can be found among various organisms. Following a short summary of notable examples of nanotexturing in the biosphere, an in-depth investigation of the behavior of nanotextures overlaying the wings of cicadas, similar in appearance and behavior to the aforementioned black silicon, is presented. Furthermore, biopolymers represent an important class of materials in their own right, since they tend to be inherently compatible with human physiology, hence, suitable to a wide range of important biomedical uses. Here the focus is primarily on silk – a protein-based material with a range of exceptional properties. First, optical and electromagnetic absorbance of natural silk fibers is probed throughout an unprecedentedly wide spectral region, ranging from the ultraviolet down to terahertz frequencies. Also,