ABSTRACT
Since its žrst discovery by Ashkin et al. [1] in the mid-1960s, a tremendous amount of research has been carried out to study the photorefractive (PR) effect and apply it to real-time image processing [2], beam ampližcation [3], self-pumped phase conjugation [4], four-wave mixing [5], and optical computing [6], to name a few applications. When two coherent plane waves of light intersect in a PR material (see Figure 6.1), they form an intensity interference pattern comprising bright and dark regions. Assuming that the PR material is predominantly n-doped, the electrons migrate from bright to dark regions, thus creating an approximately sinusoidal charge distribution. This diffusion-controlled PR effect in turn creates an electrostatic želd that is ideally 90° phase shifted from the intensity pattern and modulates the refractive index of the crystal via the electro-optic effect. The incident plane waves are, in turn, scattered by the grating in a way that one wave may have constructive recombination, while the other may encounter a destructive recombination. This effect leads to energy coupling between the beams through what is commonly referred to as the two-beam-coupling effect [3].
