The Photorefractive Process

• The Photorefractive (PR) effect is the ability to change optical characteristics of a PRmaterial using light itself.
• In the case of two-wave mixing, the simplest geometry,  two coherent laser beams intersect on a PRmaterial creating a diffraction grating within the material.

This picture demonstrates the Two-Wave mixing geometry the distance between the interference fringes is dependent upon the angle between the two beams. 

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• The higher intensity of photons in the bright fringes leads to the generation of charge carriers (holes or electrons).

• As carriers are generated a concentration gradient causes the carriers to diffuse into the dark regions.
• Defects in the material trap the charge carriers in the dark regions.

• The trapped charge carriers create an electric field between the bright and dark regions.

• The Space-Charge fields cause a change in the refractive index.  The direction of the field determines the sign of the index change (red is a positive change).
• The result is a periodic varying of the index of refraction.

The index modulation allows the interacting of the two beams.  One interaction, for example, is the transfer of energy between  beams.  Through interference effects, a probe beam can increase
 the intensity of a weaker beam.  Applications of Photorefractive materials and electro-optic
effects include optical processing and signal amplification.