Shine a helium‐neon laser at a crystal of barium titanate, and at first the beam merely passes through the crystal. After a few seconds, however, the incident beam begins to scatter into a broad fan of light. After a few more seconds, a weak beam of light emerges from the crystal, traveling back exactly along the direction of the incident beam. As the laser continues to illuminate the crystal, the fan of scattered light disappears and the backscattered beam grows in strength, until it has stolen over half the power of the incident beam.
Photorefractive Nonlinear Optics
Jack Feinberg; Photorefractive Nonlinear Optics. Physics Today 1 October 1988; 41 (10): 46–52. https://doi.org/10.1063/1.881157
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