Metal hydrides as a platform for reconfigurable photonic and plasmonic elements

Abstract

Metal hydrides often display dramatic changes in optical properties upon hydrogenation. These shifts make them prime candidates for many tunable optical devices from optical hydrogen sensors and switchable mirrors to physical encryption schemes. In order to design and fabricate optimized devices for any of these applications, we need to determine the optical and structural properties of these materials. In this dissertation, we design and implement an apparatus that dynamically measures the gravimetric, stress, calorimetric, and optical properties of metal hydrides as they are exposed to H2. We use this apparatus to measure the properties of 5 different pure metal hydrides (Pd, Mg, Ti, V, and Zr) and then use these properties to design tunable color filters and switchable perfect absorbers, among other devices. To widen our parameter space and to combine desirable characteristics of different metal systems, we use the same apparatus to investigate the properties of different metal alloy hydride systems including Pd-Au, Mg-Ni, Mg-Ti, and Mg-Al. We demonstrate many improved nanophotonic designs with these materials, including a thin-film physical encryption scheme with Pd-Au and a switchable solar absorber with Mg-Ti.

Many of these photonic devices can be further enhanced by tailoring the substrate of the device along with the metal hydride. In this dissertation, we also investigate combining the switchable optical properties of metal hydrides with near-zero-index substrates to further enhance the optical device changes. Near-zero-index materials are ones where the refractive index is below 1 and can lead to a variety of interesting optical effects, including high absorption in surrounding materials and enhanced non-linear effects. By combining an ITO substrate with a near-zero-index resonance at ~1250 nm with a thin Pd capped Mg film, we demonstrate a switchable absorption device with >76% absorption change at 1335 nm illumination. To further explore the possibility of large-scale fabrication of these devices, we survey the properties of commercially available near-zero-index materials and report the range of attainable optical properties, showing its feasibility.