Dynamic spatial structure of spontaneous beams in photorefractive bismuth sillicon oxide

We report the domain structure of spontaneously occuring beams (subharmonics) in photorefractive bismuth silicon oxide with an applied electric field from 1 to 6 kV/cm and a running grating. The subharmonic beams are generated in a pattern of domains that evolve dynamically as they move through the crystal. We find that the domains move as a whole with a speed approximately equal to that of the primary grating, but in the opposite direction. The domains are separated by narrow boundary regions, where the phase of the subharmonic waves changes by π. The domain motion is consistent with the group velocity for running space-charge waves.