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Nonlinear birefringence of nematic liquid crystals

Liquid crystals consist of anisotropic organic molecules which are orientationally ordered in the nematic phase. The birefringence of these materials gives a measire of the degree of orientational order. In addition to the linear birefringence, liquid crystals possess large third order nonlinear susceptibilities. Due to the bulk anisotropy of these materials, the nonlinear refractive index 2 depends on the direction of polarization of light with respect to the nematic director. We have used the Z-scan technique' to determine the third-order nonlinear refractive indices n2 and nonlinear absorption coefficients j3 of the pure liquid crystal 5CB (4-cyano-4'--npentylbiphenyl) in both the nematic and the isotropic phases. We have measured the nonlinear refractive indices n211 and n21 for light polarized parallel and perpendicular to the director, using both a CW Ar laser and a nanosecond frequecy-.doubled Nd:YAG laser. The nonlinear index n2 of 5CB in its nematic phase, measured with 7ns Nd:YAG laser pulses, is more than two orders of magnitude larger than that of CS2, and originates in a self-defocusing process. We have also observed strong nonlinear birefringence, the divergence of the nonlinear refractive indices as the nematic-isotropic transition is approached from below, and strong polarization dependent nonlinear absorption.