Photorefractive energy exchange in BSO requiring both optical activity and an electric field

Optical activity is usually considered a negative factor that spoils the efficiency of energy transfer. Here it is shown that the gyration effect leads to energy exchange, which, in contrast to the usual nonlocal electrooptic mechanism, changes sign with reversal of the external electric field or the sense of circular polarization of the writing beams of the same frequency. An equation for the output intensity of the beam is derived. An experiment was performed using equal-intensity (≈1 mW/cm²) 514.5-nm beams polarized with the same circularity and incident on a 3-mm-thick BSO crystal. The direction of the coupling reversed with the sign of the electric field and with the sign of the circularity. An energy transfer of ±4-5% was obtained in general agreement with the above equation. The diffraction process was confirmed to be anisotropic with a 632.8-nm beam.