Four wave cross scattering

The real samples used in holographic experiments have limited dimensions and usually are enough transparent so that there always exists reflection by rear side of the crystal.

Such a reflection can play a role of a second pump inducing its own scattering. The both pumps record their own set of noise holographic gratings. But some of them can coincide and thus can have the same holographic vectors. If the following condition of phase synchronism K 1p - K 1s = K 2p – K 2s is realized each corresponding scattering component will use for its amplification not only its own holographic grating but all the gratings with holographic vectors determined by the condition. Such a four-waved interaction brings additional amplification for initial scattering components meeting the conditions of the phase synchronism and accordingly having indicatrix in the form of cone surface with angle 2α. As a result a bright scattering appears at surfaces of two cones in both straight and backward direction.

On the screen four wave cross scattering appears in the form of bright ring against a background of usual photorefractive light scattering