Unconventional photoluminescence effect of SrTiO3 thin film under UV-illumination

We find that PL intensity image of SrTiO3 thin film measured under UHV condition increases with UV-laser illumination over long time scale of image 2 h. The intensity increase takes place at lower sample temperature as well, 200, 100 K, and 20 K. When O2 and N2 gas are introduced into the sample chamber the PL intensity decreases with the UV-illumination time, opposite to the UHV-case. We consider a quantitative thermal energy flow model of the laser-power and heat absorption by the sample, but find that temperature change of the sample is not large enough to account for the time dependent image. We propose photo-catalysis effect on STO surface as possible scenario of the PL intensity change.