Charge Carrier Photoexcitation and Two-Wave Mixing in Dichroic Materials

We give experimental and theoretical evidence that two-wave mixing light amplification in electro-optic and photoconductive materials is dramatically influenced by an anisotropic photoexcitation cross section of mobile charge carriers. An enhancement of the two-wave mixing gain is predicted and an increase of 90% is observed in a dichroic Ni doped ${\mathrm{KNbO}}_3$ crystal in a geometry with large angles between the two wave propagation directions. The enhancement is possible because in anisotropic materials the photogeneration can lead to higher modulation depths as compared to the depth of light intensity modulation.