Theory of nonlinear multiple-grating interaction in diffusion-dominated photorefractive media

Using a new model based on the so-called perturbational approach, we present a theoretical analysis of nonlinear multiple-grating interaction in diffusion-dominated photorefractive materials. This process appears when more than one grating at a time is induced in a photorefractive crystal. The interaction is caused by the nonlinear terms in the band-transport equations, and it is highly dependent on the intensity ratios of the writing beams. We show that, in a three-wave mixing scheme in which one reference beam and two nearly coincident object beams are incident upon a photorefractive crystal, one holographic grating is strongly influenced by the presence of the other. It appears that higher harmonic gratings, together with so-called sideband gratings, have a significant influence on the primary gratings. We show, for the first time to our knowledge, that in the transient case the temporal grating buildup exhibits a much more complex behavior compared with the linear case, in which only one grating is present. We also show that in the steady state one grating can experience everything ranging from total extinction to an enhancement of more than 50% when a second grating is induced. Our theoretical results are in good agreement with previously presented experimental results.