Behavior of the nonlinear refractive indices and birefringence in the neighborhood of first- and second-order phase transitions in lyotropic liquid crystals -: art. no. 051711

We investigated the behavior of the nonlinear refractive indices (n(2)) and birefringence (Deltan(2)) in the vicinity of the nematic-to-isotropic (N-I) and nematic uniaxial-to-nematic biaxial (N-N) phase transitions in a lyotropic liquid crystal. The single-beam Z-scan technique is used to measure n(2) in different relative configurations of the electric field of the laser beam and the symmetry axes of the phases. In the N-I transition, the nonlinear optical birefringence shows a discontinuity at the transition temperature (T-c), as observed in the linear birefringence. On the other hand, in the N-N transition, Deltan(2) was shown to be proportional to parallel toT-T(c)parallel to(-beta), with betasimilar to0.5, in both uniaxial and biaxial nematic domains. No discontinuity was observed on Deltan(2) in the N-N transition. The symmetric invariants of the order parameter were shown to be linear functions of the temperature in the uniaxial nematic domain, in good agreement with the mean-field prediction.