Application
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), organic photorefractive materials, certain photopolymers, and some multiple quantum well structures.
There were even claims that an amplifier based on photorefractive crystals can have less than the minimum quantum noise that is typical for optical amplifiers of any kind.
Read more about this topic: Photorefractive Effect
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