Enhanced Four-Wave Mixing by Use of Frequency-Shifted Optical Waves in Photorefractive BaTiO3

We present theory and experiments to show that the efficiency of four-wave mixing in photorefractive materials can be enhanced by use of optical waves of slightly different frequencies. The optimum frequency shift scales inversely with the response time of the material. We show that the ∼1-Hz frequency shift previously observed in the output of a self-pumped phase conjugator that used photorefractive BaTi${\mathrm{O}}_3$ is the result of this inherent property of four-wave mixing.