EXPERIMENTAL INVESTIGATION OF THE PHOTOCHROMIC EFFECT AND TWO-COLOR HOLOGRAPHIC RECORDING IN NEAR-STOICHIOMETRIC LiNbO3:Fe:Mn CRYSTALS

Near-stoichiometric LiNbO3:Fe:Mn crystals are grown, then annealed in the oxygen atmosphere at 900 degrees C for 6, 10 and 18 h. The photochromic effect of samples are investigated by measuring UV/VIS/NIR spectra before and after illumination of sensitizing light of 405 nm, and the dynamic characteristics were evaluated by use of the pump-probe method. Both the saturated absorption change and the photochromic speed increase with increases in the sensitizing intensity and the Fe concentration. In the two-color recording experiment, the holographic gratings were recorded by a red light of 633 nm, simultaneously sensitized by a violet light of 405 nm. The results show that the saturated diffraction efficiency increases with an increase in the ratio of recording light to sensitizing beam intensities, however, the sensitivity decreases. Higher concentration of Fe enables higher diffraction efficiency and higher sensitivity. In our investigation, the photochromic effect, and the two-color recording properties have little dependence on the oxidation time in the oxygen atmosphere when the oxidation time is longer than 6 h. An optimization may be obtained by further modifying these conditions appropriately.