Thermal grating with threshold in polymer dispersed liquid crystals

Measurements of the degenerate wave mixing phenomenon that occurs between a pump and signal beam after the onset of transparency in a polymer dispersed liquid crystal (PDLC) are reported. The experimental procedure and results are described. These materials contain nematic microdroplets in a polymeric matrix; therefore, they produce a strong light scattering that depends on the difference between the refractive indices of the nematic droplets and the polymer. In a dye-doped sample, the light absorption produces thermal indexing. A strong change in the transmission occurs when the liquid-crystal droplets become isotropic, thus approaching the refractive index of the polymeric matrix. In this way, the effect of self-transparency can be observed. The results are believed to be the first demonstration of nonlinear light diffraction in a PDLC sample.