Amazing nonlinear optical response in thin unoriented nematic liquid crystal cells

This work reports the highest optical nonlinearity (to the best of the authors knowledge) ever observed. The samples showing this behaviour are unoriented methyl-red doped 5CB cells with thickness of 1-2 μ. Dynamic gratings are obtained with two cw coherent laser beams (λ = 532 nm) linearly polarised in the same direction, which impinge on the samples at normal incidence (i.e. the bisector of the crossing angle is parallel to the cell normal). The diffraction efficiency η = I_+I/I_O has been evaluated by means of a low power He-Ne probe beam. Diffraction efficiencies of the order of 20-30% are easily reachable with a total incident intensity of a few μm/cm². By fitting the experimental data using the expression valid for thin gratings η = |J₁(ξ)|², values in the range (1250 - 1325) cm²/W can be obtained for the nonlinear coefficient n₂. It is worth noting that the small thickness is a stringent condition for observing such an amazing nonlinear response.