c©World Scientic Publishing Company NONLINEAR OPTICAL RESPONSE OF CYANOBIPHENYL LIQUID CRYSTALS TO HIGH-POWER, NANOSECOND LASER RADIATION

Results from investigations are summarized into: (1) transient refractive and absorptive (two-photon) nonlinearities at 0.532 m by the Z-scan method, and (2) reflective non-linearity in the near-IR, of linearly nonabsorbing cyanobiphenyl liquid crystals under nanosecond laser irradiation. (1) For isotropic liquid crystals at the several-nanosecond time scale and several tens-micrometers beam-waist-diameter, transient molecular-reorientation and ther-mal/density refractive nonlinearities compete in changing the sign of the total transient refractive nonlinearity. For the dierent, given pulse durations, the influence of cou-pled thermal and density eects on nonlinear refraction depends, through buildup time, on the beam-waist diameter. Nonlinear absorption coecients depend on the incident intensity. For planar nematic layers, cumulative eects in heating (and in refractive nonlinearity) were observed even at low, 2{10 Hz pulse repetition rate. These results are useful for optical power limiting applications, and for intensity and beam-quality sensors of pulsed, high-power lasers. (2) Reflective nonlinearity of chiral-nematic (cholesteric) mirrors near selective reflec-tion conditions for circular polarized light at = 1:064 m was studied both under free space irradiation and inside a laser resonator. Specially chosen experimental irradiation conditions make it possible to attribute the observed changing of reflectivity to athermal helix unwinding by the optical eld. The results can nd applications in laser-resonator mirrors, Q-switches and soft apertures for beam-prole formation, and also in showing the limits of use cholesteric optical elements in high-power laser beams. 1