Critical slowing down of space-charge field relaxation in photorefractive sillenites

We propose a theoretical model for the recently reported anomalously long d ecay of a space-charge field that was resonantly excited in a photorefracti ve sillenite crystal [Phys. Rev. Lett. 79, 67 (1997)]. The model is based o n the conventional concept of charge transfer and does not require excessiv ely large values of the quality factor for space-charge waves. The effect i n question is qualified as a critical slowing of the relaxation process as the threshold of optical oscillation is approached. (C) 1999 Optical Societ y of America.