Ultraviolet Triggered Switchable Mirrors

Trivalent rare earth hydrides stabilized in thin film form demonstrate spectacular optical and electronic properties. A thin film of YH x or LaH x can be transformed rapidly from metal to insulator, from shiny mirror to transparent window, simply by changing the surrounding hydrogen gas pressure or an electrolytic cell potential at room temperature (RT). At low temperatures, in‐situ doping is not possible in this way as hydrogen cannot diffuse. However, our finding of persistent photoconductivity under ultraviolet illumination permits tuning through the T = 0 metal–insulator transition and reveals the important role played by strong electron correlations. We discuss the optical, electronic, magnetic, and structural properties of switchable mirrors from both the technological and scientific perspectives.