Double peaks of fundamental diffraction efficiency versus biasing of electric fields through the degenerate four-wave mixing process in nematic liquid-crystal films.

The observation of double peaks of diffraction efficiency through the degenerate four-wave mixing process as a function of the biasing electric-field strength in a nematic liquid-crystal film is described. This behavior is due to the high optical nonlinearity of liquid-crystal media and the dramatic modulating effect of an ac voltage on the reorientational Freedericksz transition. The dependence of the peak occurrence on grating periods, electric fields, and optical intensities is presented. Theoretical calculations show the same characteristic behavior as do the experimental results.