  Metal hydrides often display dramatic changes in optical properties upon
hydrogenation. These shifts make them prime candidates for many tunable optical
devices, such as optical hydrogen sensors and switchable mirrors. While some of
these metals, such as palladium, have been well studied, many other promising
materials have only been characterized over a limited optical range and lack
direct in situ measurements of hydrogen loading, limiting their potential
applications. Further, there have been no systematic studies that allow for a
clear comparison between these metals. In this work, we present such a
systematic study of the dynamically tunable optical properties of Pd, Mg, Zr,
Ti, and V throughout hydrogenation with a wavelength range of 250 - 1690 nm.
These measurements were performed in an environmental chamber, which combines
mass measurements via a quartz crystal microbalance with ellipsometric
measurements in up to 7 bar of hydrogen gas, allowing us to determine the
optical properties during hydrogen loading. In addition, we demonstrate a
further tunability of the optical properties of titanium and its hydride by
altering annealing conditions, and we investigate the optical and gravimetric
hysteresis that occurs during hydrogenation cycling of palladium. Finally, we
demonstrate several nanoscale optical and plasmonic structures based on these
dynamic properties. We show structures that, upon hydrogenation, demonstrate
five orders of magnitude change in reflectivity, resonance shifts of >200 nm,
and relative transmission switching of >3000%, suggesting a wide range of
applications.
