UC Davis

The study of optical materials for extreme environments is a rapidly growing field, with a number of possible applications from space science to energy efficiency to defense. In order to meet the growing demand for optical devices which can operate at very high temperatures and in different environments, we must first characterize the optical properties of different materials which might be used in the design of such devices at these elevated temperatures. Given their high-temperature stability, this makes refractory materials the ideal candidates for such a study. Refractory materials have previously been analyzed in great detail for their high-temperature structural and chemical stability. However, their usage as optical materials has been comparably limited.

In this thesis, I begin to analyze the optical properties of multiple classes of refractory materials: refractory metals and their oxides, refractory nitrides, and refractory carbides. In addition to analyzing their optical and chemical responses to high-temperature treatment, I present in situ high-temperature data up to 1,500°C, which are used to analyze their stability–or lack thereof–at very high temperatures. This thesis will also study multiple possible applications of each structure, including structural color, superabsorption, and thermophotovoltaics.

Structural color is used in a variety of applications, from conformal coatings for spacecraft, to sensing, to anticounterfeit technology. However, in many cases, the structural color systems proposed are either too complicated to fabricate making them difficult to scale for large-scale production, or would not be able to survive in the extreme environments required by some of these applications. We propose a simple multi- layer structure composed of a refractory metal and its oxide on silicon, in which the oxide is grown via high-temperature treatment in an oxygen-rich environment. During this high-temperature treatment (up to 600C) we measure the samples’ optical properties in situ using ellipsometry, which can give us very precise control over the thickness of the generated oxide layer and thus the generated color of the structure. In addition to our experimental results for three refractory metals (Ru, Ta, and W), we present simulated results demonstrating the full range of possible colors achievable with these metals and their oxides.

Many technological applications in photonics require devices to function reliably under extreme conditions, including high temperatures. To this end, materials and structures with thermally stable optical properties are indispensable. State-of-the-art thermal photonic devices based on nanostructures suffer from severe surface diffusion-induced degradation, and the operational temperatures are often restricted. Here we report on a thermo-optically stable superabsorber composed of bilayer refractory dielectric materials. The device features an average absorptivity 90% over >500 nm bandwidth in the near-infrared (NIR) regime, with minimal temperature dependence up to 1,500°C. Our results demonstrate an alternative pathway to achieve high-temperature thermo-optically stable photonic devices.

As the impacts of extended fossil fuel use are felt by our environment, economy, and public health, it becomes increasingly crucial to minimize our dependence on fossil fuels in favor of reusable, clean energy generation techniques. This requires the optimization of efficiency for each technique, thus requiring the minimization of waste heat. One method through which we can recover this waste heat is using thermophotovoltaics (TPV): converting the waste heat into usable electricity utilizing a thermal emitter and a photovoltaic cell. We propose a simple bi-layer SiC/AlN structure as a broadband thermal emitter for thermophotovoltaic applications, demonstrating ex situ and in situ its optical stability in temperatures up to 1,200°C in air and 1,500°C in inert environments. Our results demonstrate a scalable, broadband thermal emitter which can be applied for waste heat recovery across a myriad of high-temperature energy generation processes.