MECHANISM OF PHOTOREFRACTIVE ENHANCEMENT OF PHOTOCHROMIC GRATINGS IN BSO - EXPERIMENTAL RESULTS AND PHENOMENOLOGICAL MODELING

We report an increase of more than a factor of 100 in the diffraction efficiency from photochromic gratings written in BSO, by uniformly ill uminating these gratings with 488 nm light to induce secondary photore fractive gratings. Upon application of an electric field, the phase sh ift between the two gratings can be varied and under appropriate condi tions the diffraction efficiency can increase. The dependence of the e nhanced diffraction efficiency as a function of the applied electric f ield and readout beam polarisation was experimentally investigated for two different crystal orientations. Phenomenological modelling is pre sented requiring modifications to the phase angle between the photochr omic and induced photorefractive gratings in order to reproduce the ex perimentally observed asymmetry in the increase in the diffraction eff iciency. This suggests a periodic modulation of the photorefractive do nor/acceptor density, due to the presence of the photochromic grating.