Fig. 1: High-energy ultraviolet (a.k.a., UVC) irradiation (λ = 255 nm) transmission measurements on a 46 nm-thick polycrystalline TiO₂ film and a 500 μm-thick TiO₂ (100) single crystal as a function of UVC illumination density.

Our measurements on 46nm-thick TiO₂ polycrystalline film show that ~99% of UVC irradiation was absorbed within the first tens of nanometers. However, the optical transmission measurements of the 500 μm-thick TiO₂ (100) single crystal show that the rest of the UVC (~1%) can penetrate through the entire crystal for hundreds of micrometers. Our measurements for both 46nm-thick and 500 μm-thick samples display that with increasing UVC illumination power density, the optical transmission decreases, i.e., absorption increases. We believe that the enhanced absorption is associated with mid-gap states introduced by the photoinduced oxygen vacancies generated by UVC irradiation.