Nonlinear Optics of Nematic Liquid Crystals.

The fields of nonlinear optics and liquid crystal physics have much to offer each other. In the present work we show how a nonlinear optical technique may be used to study aspects of the microscopic structure of nematic liquid crystals, and also that these materials may have an extraordinarily large nonlinear response, which is manifested in a number of novel effects. A two-photon dichroism measurement is used for the first time to deduce the second non-trivial moment <P(,4)> of the molecular orientational distribution function of the liquid crystal 4-cyano-4'-pentylbiphenyl (5CB) in the nematic phase. An anisotropic local-field model is developed for the data analysis. The temperature variation of <P(,4)> that is found is in fair agreement with that deduced from Raman depolarization results. The negative <P(,4)> near the nematic-isotropic transition temperature, a finding difficult to reconcile with conventional theories of liquid crystal structure, is believed to be due to the nature of the strong local ordering in the medium. Nematic liquid crystals, by virtue of their orientational ordering and fluid nature, are very sensitive to the effects of an applied optical field. The collective molecular reorientation results in a large laser-induced change of refractive index. Light intensities of the order of 100 W/cm('2) can cause index changes as great as (DELTA)n (TURN) 0.1. In a symmetric geometry, the reorientation shows threshold behavior, the optical analog of the Freedericksz transition. The huge nonlinearity means that even with thin layers of material, one can easily observe such exotic phenomena as strong spatial self-phase modulation, high -order diffraction from a laser-induced phase grating, and high-order bistability and self-oscillation in a nonlinear Fabry-Perot interferometer containing a nematic layer. These effects are investigated quantitatively and explained theoretically using a free energy formalism to calculate the distortion, and non-perturbative methods to calculate the propagation of the optical field.