Nonlinear Spectroscopy of Deep Levels in Ferroelectric SbSI Crystals

Local lattice defects in SbSI crystals and the optical transitions associated with defects, which drastically change under ferroelectric phase transformation, are investigated by means of the spectroscopy of photo-induced absorption (PIA). The thermal changes in the spectra are caused by the replacement of the predominating type of optical modulation of deep level occupation: by direct optical ionization of the center or by capture of non-equilibrium free carriers generated by two-photon absorption of laser radiation. The results are explained by considerable modifications in the band structure under phase transition. It is accompanied by the replacement of impurity absorption from one point of Brillouin zone to the other, thus reflecting a splitting of the valence band. These processes are confirmed by the band-structure calculations for SbSI.