  Oxygen-containing yttrium hydride thin films exhibit a photochromic behavior:
transparent thin films switch from a transparent state to a photodarkened state
after being illuminated with UV or blue light. This feature has attracted much
attention in recent years due to its potential applications in smart
fenestration or in any device in which a response to intense light radiation is
needed. However, the process responsible for the reversible change of the
optical properties upon illumination is still not well understood. The
objective of the present work is to shed some light on the photochromic
mechanism by using an optical approach. On this basis, the optical properties
of oxygen-containing yttrium hydride thin films have been studied by optical
spectrophotometry and ellipsometry before (transparent state) and after UV
illumination (dark state). According to the observed results, the photochromic
optical change of the films can be explained quantitatively by the gradual
growth, under illumination, of metallic phases within the initial wide-band gap
semiconducting lattice.
