Spatial subharmonics in photorefractive materials

Since the first observation of the spatial subharmonics in photorefractive BSO crystal by Mallick et al. 1988, the possible physical mechanism based on the Kukhtarev´s materials equations has been intensively investigated. However, most of the work in this field has been concentrated on obtaining the instability condition for the existence of the subharmonic resonances. In this paper we attempt to calculate K/2 spatial subharmonic resonances varying the detuning frequencies for several external electric fields and compare the theoretical results with experimental data. A simple theory based on a differential equation which is derived from the materials equations for the space charge field and the coupled mode analysis of the wave equation is developed to describe the subharmonic resonances. The mechanism which is responsible for the subharmonic resonance is assumed to be spontaneous generation caused by the inherent nonlinearity of the photorefractive effect