GENERALIZED TRANSMISSION ELLIPSOMETRY FOR TWISTED BIAXIAL DIELECTRIC MEDIA - APPLICATION TO CHIRAL LIQUID-CRYSTALS

We report on the application, for the first time to our knowledge, of spectroscopic generalized ellipsometry to liquid crystal materials. We have measured the three normalized elements of the Jones transmission matrix t at various sample temperatures within the spectral range fro m 340 to 1700 nm (0.73 to 3.65 eV) on thin cells filled with twisted n ematic mixtures 4-cyano-4'-pentylbiphenyl (5CB) and 4-cyano-4'-(2 meth yl)-butylbiphenyl (CB15) (Ref. 4). The Berreman 4 x 4 matrix for elect romagnetic plane waves in a biaxial medium homogeneously twisted along the sample normal is derived and presented. Analytic expressions in t he case of light propagation along the helical axis permit the calcula tion of the transmission and reflection coefficients simultaneously wi thout numerical approximations. This solution is valid for any biaxial configuration of chiral liquid crystals, including the case of absorp tion. We have fully analyzed the measured Jones transmission matrix el ements and obtained the geometrical sample properties and, as a functi on of the photon energy and the temperature, the refractive indices n( o), and n(e) of the chiral liquid crystals. We found that within the e xperimental error the main refractive indices of the mixtures 5CB and CB15 are those of pure 5CB at the same reduced temperatures. The hande dness of the optical activity of the samples can be obtained immediate ly from the phase information of the Jones transmission matrix coeffic ients. (C) 1996 Optical Society of America.