Technique for Measuring Electronic-Based Electro-Optic Coefficients of Ferroelectric Liquid Crystals
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KENNETH E. ARNETT*, DAVID M. WALBA, AND JOEL A. DREWES Department of Chemistry, University Colorado-Boulder, Boulder, Colorado 80309 * also at Displaytech, Inc., 2200 Central Ave., Boulder, Colo. 80303
ABSTRACT Among soft organic nonlinear optical materials is a class of recently developed: X(2)_ enhanced ferroelectric liquid crystals (FLCs). The FLC phase nonlinear susceptibility is enhanced by synthesizing onto the molecules constituting the FLC phase a moiety with an increased hyperpolarizability. The hyperpolarizability of the FLC molecules couples into the permanent, thermodynamically stable, polar order of the FLC phase resulting in a material with an enhanced nonlinear susceptibility. Like other soft organics, the linear and nonlinear optical materials characteristics can be altered by chemical synthesis and mixing. We report on our technique to evaluate the nonlinear optical properties of X(2)-enhanced FLCs by measuring their high-frequency electro-optic r-coefficients. The technique is broad-band, readily allowing electro-optic coefficient measurement between 100 KHz and 200 MHz. Although the experimental geometry is not conducive for practical device application, it offers a compromise between ease of fabrication and magnitude of nonlinear response. This technique can also be used to evaluate other organic materials such as poled polymers.
FERROELECTRIC LIQUID CRYSTAL PHASE The ferroelectric liquid crystal (FLC) phase results from symmetry-enforced orientations of Z-shaped, optically anisotropic, chiral molecules in a polar phase. The Z-shaped molecules contain a "core" - one or more cyclic compounds - surrounded on each end by a "tail". At present, cyclic cores known to produce the smectic C* phase are benzene, cyclohexane, pyridine, and pyrimidine. The tails could be n-alkyl chains containing one or more chiral centers. Locally, the molecules self-organize into smectic liquid crystal layers with the director n tilted with respect to the smectic layer normal z, figure 11. Within a smectic layer, the average orientation of the long axis of the molecules defines the director n, which is approximately parallel to the extraordinary axis of the birefringent FLC medium (An = 0.15). The director n makes an angle TP (tilt angle), typically 20 to 45 degrees with the layer normal z. Since the molecules are chiral the liquid crystalline phase has a polar order manifested through a spontaneous polarization P, defined by: P = (z x n)P. The spontaneous polarization magnitude is a function of the chiral center and the net orientation of the dipole moment with respect to the polar axis of the FLC phase. A typical value of P for FLC mixtures optimized for surface stabilized switching is 30 nC/cm 2.
135 Mat. Res. Soc. Symp. Proc. Vol. 392 ©1995 Materials Research Society
Figure 1
X(2) -ENHANCED FERROELECTRIC LIQUID CRYSTALS
Hyperpolarizable cyclic cores The molecular second order hyperpolarizability P3 of the molecules in the FLC phase originates from electron donor (D) and acceptor (A) groups synthesized onto the
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