Photoinduced charge transfer in near-stoichiometric LiNbO₃

We have observed that near-stoichiometric LiNbO₃ (nominally pure and Tb-doped) exhibits a strong and stable photoinduced absorption band extending from λ ≈650 nm to the absorption edge when illuminated with ultraviolet light at 313 nm. It was found that crystals have both shallow electron traps close to the conduction band and deep traps located about 1.9 eV (λ ≈650 nm) below it and that the deep-trapped charges give rise to this induced absorption band. The measured shallow-trap electron lifetime was about 4 s at room temperature both in nominally pure and Tb-doped crystals. The deep-trapped charges were thermally stable at room temperature and decayed in a near-exponential manner at high temperatures. The deep-trap decay time estimated at room temperature increased with increasing Tb concentration, being over 60 years in Tb 200 ppm doped crystal. These provide a favorable energy structure for effective two-color holographic recording at near-infrared ranges.