Nonlinear light scattering by laser- and dc-field-induced molecular reorientations in nematic-liquid-crystal films

We present here a detailed experimental study of nonlinear optical scattering in nematic liquid crystals using low-power cw lasers. The scattering is effected by the phase grating created by two intersecting same-frequency lasers. These lasers interfere spatially to create an index modulation via their reorientation effects on the molecules. When aided by a dc magnetic field, the reorientation and nonlinear responses of the medium are enhanced. We have quantitatively measured the dependence of the diffraction efficiency as a function of the optical intensity and the magnetic field ranging from well below to far above the critical orientation threshold values. The scattering is also analyzed in terms of its dependence on the temperature, sample thickness, and the scattering geometry. The experimental results are in good agreement with theoretical expectations. We also point out some high-intensity effects and applications of this phase grating, e.g., in wave-front conjugation and holographic imaging.