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.

Nonlinear optical properties of liquid crystals probed by Z-scan technique

“…The only shortcoming, a value of n 2 independent on T on approaching the transition temperature T c to the isotropic phase, could be explained by a slowing down of the orientational relaxation for T → T c [17]. At these short time scales, many puzzles remain, like the unexpected large value of β at nanosecond time scale, 10-to 10 2 times larger than that in the picosecond time scale [16]. To account for this result, it was suggested the possibility of formation of µm-sized bubbles and microscopic carbonization in the sample during the laser exposure.…”

“…Other important features that influence the magnitude and sign of the nonlinear refraction index are the beam waist and the thickness of the sample (ref. [16] and references therein). In the case of the isotropic phase of the 5CB it was observed that increasing the beam waist from 7 µm to 35 − 50 µm, n 2 changes from negative to positive.…”

Optical properties of a mesogen by image analysis

“…Sharp texture transformations of SmA-SmC * , SmC * -Cr2, and Cr2-Cr1 give abrupt changes in the values of AC. The optical transmittance of the sample is inversely proportional to AC [26,27].…”

Dye-doped cholesteric lasers: Distributed feedback and photonic bandgap lasing models

“…These experiments have shown the stability of CLCs with mixtures of cholesterol esters in the field of high-powered nanosecond single pulsed lasers. Other authors have observed the untwisting of a cholesteric helix in the field of light waves [73][74][75][76][77][78][79], which is manifested by a reduction of the selective reflection under irradiation by laser pulses with a duration of §500 ns, at a repetition rate of 4.5 kHz, and at a power density of more than 1 MW/cm 2 [73].…”