Dynamic spatial structures of subharmonic beams and self-excitation of space charge waves in photorefractive sellinites

We provide a direct experimental observation of space charge waves in photorefractive crystals with point group 23 based on their penetration into an area with uniform light illumination. Those are the results of excitation of eigen modes of space charge filed excited by moving light fringes. We have found experimental conditions when the quality factor of the space charge waves attempts dramatical increase with respect to what current theory predicts. This leads to self- excitation of the waves and results in self-oscillation of light. The competition between space charge waves and fundamental grating results in appearance of K/2, K/4 components in the fundamental grating and to spontaneously occurring beams called subharmonics. The subharmonic beams are generated in a pattern of domains that evolve dramatically as they move through the crystal. The domains are result of non- homogeneity of the space charge waves and separated by narrow boundary regions, where the phase of subharmonics changes by pi. We experimentally prove the domains move with the group velocity of running space charge waves.