New insights into the photochromic mechanism in oxygen-containing   yttrium hydride thin films: an optical perspective
  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. 
J. Montero, F. A. Martinsen, S. Zh. Karazhanov, B. C. Hauback, and E.   S. Marstein
cond-mat.mtrl-sci