Abstract

Liquid crystals can exhibit large third order optical nonlinearities.^[1] The nonlinear response is a strong function of laser pulsewidth. Using the Z-scan technique,^[2] we have measured the intensity dependent absorption and refractive indices on timescales ranging from milliseconds to picoseconds for eight nematic liquid crystals, as a function of temperature. This method allows the determination of the nonlinear absorption coefficients as well as the signs and magnitudes of the nonlinear refractive indices for different polarizations, and hence the third order susceptibility χ³(-ω; -ω, ω, ω). A multiple pulse technique further enables estimation of the response time of the dominant mechanism. Using CW Argon and Q-switched and mode-locked Nd:YAG lasers, we have carried out Z-scan measurements on aligned liquid crystal samples as a function of temperature in the nematic phase. The sample materials, low molecular weight nematic liquid crystals, were contained between glass plates separated by mylar spacers; the plates were coated with poly imide and buffen to obtain homogeneous alignment. The sample cells, varying in thickness from 25-125μm, were housed in a thermostatted stage with optical access ports.

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