Optically induced space-charge fields, dc voltage, and extraordinarily large nonlinearity in dye-doped nematic liquid crystals

We have observed extraordinarily large optical nonlinearity in Methyl Red–doped nematic liquid-crystal film. Grating diffraction can be generated with an optical intensity as low as $40 \mathit{\mu }\mathrm{W}/{\mathrm{cm}}^2$, and a refractive-index change coefficient of more than $6 {\mathrm{cm}}^2/\mathrm{W}$ is obtained. The effect is attributed to formation of an optically induced dc space-charge field and to the resulting reorientation of the highly birefringent nematic director axis.