In Situ Optical Analysis of the High-Temperature Behavior of Refractory Materials

The degree of control we have over the electromagnetic spectrum <i>via</i> tailoring of optical material properties and geometries governs our ability to advance cutting-edge photonic devices ranging from structural color pixels to thermophotovoltaic emitters to high-temperature sensors. A crucial step of improving this control is understanding how the dielectric functions of different materials change at high temperatures. Though the field has recently been of great interest to the photonics community for a variety of applications, most research in materials under extreme conditions have focused on structural properties. To address the growing need for high-temperature optical material properties, we present an <i>in situ</i> optical analysis of three refractory metals (Ru, Ta, and W) at elevated temperatures in an oxidizing environment. These samples preferentially oxidize when held at high temperatures (&gt;500°C) in an oxidizing environment: by employing <i>in</i> <i>situ</i> ellipsometry measurements during a controlled heating treatment, we gain very precise control over the thickness of the oxide layers. We consider the application of these thin films to a structural color system, given their vibrant coloration: with our initial heating treatments our three structures generated vibrant pink (RuO₂/Ru/Si), yellow (Ta₂O₅/Ta/Si), and teal (WO₃/W/Si) hues. These structural color devices have the added benefit of being stable up to 1,200°C in an inert environment, showing the capability of these materials for use in high-temperature optical systems. We extend our technique to carbides, where a quantitative analysis of high temperature effects on permittivity will also be presented. These two sets of data represent a crucial advance in the development of robust materials for applications in extreme environments.