Photogenerated Carriers in SrTiO3 Probed by Mid-Infrared Absorption
  Infrared absorption spectra of SrTiO$_3$ have been measured under above-band-gap photoexcitations to study the properties of photogenerated carriers, which should play important roles in previously reported photoinduced phenomena in SrTiO$_3$. A broad absorption band appears over the entire mid-infrared region under photoexcitation. Detailed energy, temperature, and excitation power dependences of the photoinduced absorption are reported. This photo-induced absorption is attributed to the intragap excitations of the photogenerated carriers. The data show the existence of a high density of in-gap states for the photocarriers, which extends over a wide energy range starting from the conduction and valence band edges. 
H. Okamura, M. Matsubara, K. Tanaka, K. Fukui, H. Nakagawa, Y.   Ikemoto, T. Moriwaki, H. Kimura, T. Nanba