Photo-induced polaron states in strontium titanate

Photo-induced absorption spectra are investigated under band-to-band excitation in SrTiO3 at low temperatures. A broad photo-induced absorption band is observed in the middle of the band gap. Persistent spectral hole burning and non-radiative recombination of photo-excited carriers are efficiently induced by stimulation into the photo-induced absorption band. It is suggested that the photo-induced absorption band is originated from small polaron states of electrons and holes.