Photorefraction

The photorefractive effect is a nonlinear optical effect seen in certain crystals and other materials that respond to light by altering their refractive index. The effect can be used to store temporary, erasable holograms and is useful for holographic data storage. It can also be used to create a phase-conjugate mirror or an optical spatial soliton.

The photorefractive effect occurs in several stages:

The photorefractive effect can be used for dynamic holography, and, in particular, for cleaning of coherent beams. For example, in the case of a hologram, illuminating the grating with just the reference beam causes the reconstruction of the original signal beam. When two coherent laser beams (usually obtained by splitting a laser beam by the use of a beamsplitter into two, and then suitably redirecting by mirrors) cross inside a photorefractive crystal, the resultant refractive index grating diffracts the laser beams. As a result, one beam gains energy and becomes more intense at the expense of light intensity reduction of the other. This phenomenon is an example of two-wave mixing. It is interesting that in this configuration, Bragg diffraction condition is automatically satisfied.

The pattern stored inside the crystal persists until the pattern is erased; this can be done by flooding the crystal with uniform illumination which will excite the electrons back into the conduction band and allow them to be distributed more uniformly.

Photorefractive materials include barium titanate (BaTiO₃), lithium niobate (LiNbO₃), vanadium doped zinc telluride (ZnTe:V), organic photorefractive materials, certain photopolymers, and some multiple quantum well structures.

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