Negative orientational nonlinearity

Khoo, I. C., H. Li, and Yu Liang. 1993. Optically induced extraordinarily large negative orientational nonlinearity in dye-doped liquid crystal. lEEEJ. Quantum Electron. QE-29 1444. 

The minus sign in (17) does not mean that the nonlinear phase shift is negative—indeed, the orientational nonlinearity always results in self-focusing of the light, i.e., > 0. 

Second-order optical nonlinearities result from introduction of a cubic term in the potential function for the electron, and third-order optical nonlinearities result from introduction of a quartic term (Figure 18). Two important points relate to the symmetry of this perturbation. First, while negative and positive p both give rise to the same potential and therefore the same physical effects (the only difference being the orientation of the coordinate system), a negative y will lead to a different electron potential than will a positive y. Second, the quartic perturbation has mirror symmetry with respect to a distortion coordinate as a result, both centrosymmetric and noncentrosymmetric materials will exhibit third-order optical nonlinearities. If we reconsider equation 23 for the expansion of polarization of a molecule as a function of electric field and assume that the even-order terms are zero (i.e., that the molecule is centrosymmetric), we see that polarization at a given point in space is ... 

The negative Ni is the result of the coupling of molecular tumbling under flow and the local molecular-orientation distribution. At low shear rates, the director tumbles with the flow and Ni will be positive. At intermediate shear rates, nonlinear viscoelastic effects are important. The director tumbling competes with the steady director alignment along... 

Sinclair et al. [505] studied both P(Ac) and P(DiAc)-4BCMU. Their THG measurements on oriented, trans-P(Ac) parallel to the fiber direction at an energy of 1.17 eV yielded a value of X xxxx of ca. 4 X 10 esu, more than 15 times larger than the corresponding measurement on the cis-isomer. Their study on the P(DiAc) yielded a nonlinear refractive index (n which was negative, with a value of 10" (MW/cm ) with a decay time for this nonlinearity of ca. 2.5 ps. Chollet et al. [506] studied the blue and red forms of several P(DiAc) films via THG, and observed two and three photon resonances, with X xxxx of ca. 5 X 10 " esu at ca. 2 pm, Kanetake et al. [507] measured THG on highly oriented, urethane-based P(DiAc) films, prepared by vacuum deposition, at a wavelength of 1.9 pm. They found that for a pump direction parallel to the polymer chain, a three-photon resonance to the exciton, was observed, with a x of 3.8 X 10 esu (vs. 0.2 X 10 ° esu for the perpendicular direction). Their frequency-dispersive measurements... 

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