Spatial subharmonics in photorefractive Bi12SiO20 crystal with a square wave applied field

The instability leading to the appearance of subharmonics in the presence of a square wave applied field is investigated both theoretically and experimentally. The threshold conditions are found for a range of parameters and the temporal variation of the subharmonic amplitude is measured. It is shown that a relatively simple theory, based on a set of material equations, gives good agreement with some of the experimental results but other experiments indicate that the materials equations alone cannot describe all the observations.