Photo-EPR study of light-sensitive impurity and defect centers in photorefractiveBaTiO3

Electron paramagnetic resonance (EPR) and photo-EPR measurements have been made on cobalt-doped ${\mathrm{BaTiO}}_3$ single crystals. Crystals illuminated at 25 K with light in the wavelength range 488–1010 nm showed an increase in concentration of the ${\mathrm{Co}}^{2+}$ centers relative to that observed at the same temperature in unilluminated crystals. Illumination with light of λ<690 nm produced a holelike intrinsic center that is stable below 50 K and that is assigned to ${\mathrm{O}}^{\mathrm{-}}$. Following exposure to continuous laser illumination, the photo-EPR response of ${\mathrm{BaTiO}}_3$:Co was measured at low temperatures with periodically chopped laser light. It was observed that the populations of the ${\mathrm{Co}}^{2+}$ and ${\mathrm{O}}^{\mathrm{-}}$ centers were modulated reversibly and in opposite ways. A qualitative discussion linking these observations to possible photoionization and recombination and trapping channels is presented. Also presented is a detailed analysis of the kinetic equations describing the time dependence of the photo-EPR response, which permits us to derive information on important optical interactions and photoinduced space-charge effects.