3RD-ORDER OPTICAL NONLINEARITIES OF NEMATIC LIQUID-CRYSTALS

Liquid crystals can exhibit large third order optical non-linearities. Using the Z-scan technique, we have measured the intensity dependence of the non-linear absorption and non-linear refractive indices on tim e-scales ranging from milliseconds to picoseconds for nematic liquid c rystals. This method allows the determination of the non-linear absorp tion coefficients as well as the signs and magnitudes of the non-linea r refractive indices for different polarizations. A two pulse techniqu e further enables estimation of the response time of the dominant mech anism. Using CW argon and Q-switched and mode-locked pumped dye lasers , we have carried out Z-scan measurements on aligned liquid crystal sa mples as a function of temperature, as well as wavelength. In the geom etries studied, director reorientation is not expected to take place. On the nanosecond time-scale, all materials studied were self-focusing for polarization perpendicular to the director, and self-defocusing f or parallel polarization. On the picosecond timescale, the samples wer e self-focusing for all polarizations, but strong non-linear birefring ence was typically observed. An attempt is made to relate the bulk res ponse to the molecular structures.