Modulational instability of optical beams in photorefractive media in   the presence of two wave and four wave mixing effects
  Modulational instability in a photorefractive medium is studied in the presence of two wave mixing. We then propose and derive a model for forward four wave mixing in the photorefractive medium and investigate the modulational instability induced by four wave mixing effects. By using the standard linear stability analysis the instability gain is obtained. In both the cases, the geometry is such that the effect of self-phase-modulation self-focusing is suppressed and only the holographic focusing nonlinearity is acting. 
C. P. Jisha, V. C. Kuriakose, K. Porsezian and B. Kalithasan