Optimizing magnesium thin films for optical switching applications: rules and recipes

Abstract

While magnesium holds great potential as hydrogen storage material, interest has recently shifted to its use in optical switching applications. The hydrogen-induced phase transition from metallic magnesium to dielectric magnesium hydride is a promising candidate for switchable and active plasmonic systems. Most studies in the past have been performed on magnesium thin films and were directed to the investigation and optimization of hydrogen storage rather than to the optical properties. While these studies found a strong influence of the material morphology and crystallinity on the bulk and thin film properties, no in-depth study has revealed rules and recipes to tune and control the nanoscale morphology. Here, we demonstrate that the nanocrystallinity, that is, the crystallite size and morphology on the nanoscale, as well as the surface roughness of magnesium thin films in an optically switchable geometry, can be tuned and adjusted by a comprehensive set of evaporation parameters. The required film geometries, optical properties, and the applications at hand determine the deposition parameters and need to be chosen accordingly. Further, we find that the surface roughness changes drastically upon hydrogenation. Our results have an immediate impact on the understanding as well as the fabrication of optically active devices where magnesium is being used.