A new thin film photochromic material: Oxygen-containing yttrium hydride
  In this work we report on photochromism in transparent thin film samples of oxygen-containing yttrium hydride. Exposure to visible and ultraviolet (UV) light at moderate intensity triggers a decrease in the optical transmission of visible and infrared (IR) light. The photo-darkening is colour-neutral. We show that the optical transmission of samples of 500 nm thickness can be reduced by up to 50% after one hour of illumination with light of moderate intensity. The reaction is reversible and samples that are left in the dark return to the initial transparent state. The relaxation time in the dark depends on the temperature of the sample and the duration of the light exposure. The photochromic reaction takes place under ambient conditions in the as-deposited state of the thin-film samples. 
Trygve Mongstad, Charlotte Platzer-Bj\"orkman, Jan Petter Maehlen,   Lennard P. A. Mooij, Yevheniy Pivak, Bernard Dam, Erik ...
cond-mat.mtrl-sci