NONLINEAR OPTICAL RESPONSE OF CYANOBIPHENYL LIQUID CRYSTALS TO HIGH-POWER, NANOSECOND LASER RADIATION

Abstract

Results from investigations are summarized into: (1) transient refractive and absorptive (two-photon) nonlinearities at 0.532 μm by the Z-scan method, and (2) reflective nonlinearity 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 thermal/density refractive nonlinearities compete in changing the sign of the total transient refractive nonlinearity. For the different, given pulse durations, the influence of coupled thermal and density effects on nonlinear refraction depends, through buildup time, on the beam-waist diameter. Nonlinear absorption coefficients depend on the incident intensity. For planar nematic layers, cumulative effects 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 reflection conditions for circular polarized light at λ=1.064 μm 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 field. The results can find applications in laser-resonator mirrors, Q-switches and soft apertures for beam-profile formation, and also in showing the limits of use cholesteric optical elements in high-power laser beams.