Wavelength dependence of visible and near-infrared photorefraction and phase conjugation in Sn_2P_2S_6

The photorefractive properties of Sn2P2S6 crystals in the wavelength range from 633to1064 nm are investigated. Conventional yellow and modified brown crystals with a variation of nonstoichiometric defects are examined. Brown crystals respond much faster and exhibit substantially higher photorefractive gain than yellow crystals, up to 18 cm−1 at 780 nm and 8.5 cm−1 at 1064 nm. Ring-cavity self-pumped phase conjugation is demonstrated using both types of crystal. The phase-conjugate response of brown Sn2P2S6 is 2 orders of magnitude faster than in yellow Sn2P2S6 or Rh-doped BaTiO3, with a grating recording time below 50 ms for 30 mW power at 860 nm. Various thresholding effects are analyzed to determine the optimal wavelength range for the two types of crystal. We find an optimum in the range of 650–950 nm for yellow and in the range of 850–1100 nm for brown Sn2P2S6.