ACCURATE METHOD FOR DETERMINING TILT BIAS ANGLES IN THIN-FILMS OF NEMATIC LIQUID-CRYSTALS

We have developed a new method for measuring tilt bias angles in spati ally uniform and nonuniform thin films of nematic liquid crystals. The method employs modulation ellipsometry, based on the use of an expone ntially decaying light wave to probe the boundary layer. Oscillations of the director of the liquid crystal, which are induced by the flexoe lectric torque, are excited with an external periodic field. A periodi c variation of the ellipticity of the light wave reflected from the in terface is detected at both the first and second harmonics of the exci ting electric field. When these two Fourier components of the electroo ptic response are known, it is possible to calculate both the tilt bia s angle theta(0) of the director and the dynamic deviation se of the t ilt bias angle. The angles theta(0) and delta theta measured by this m ethod on the surface of an electrode (ITO) and on the surface of a fer roelectric film (a copolymer of vinylidene fluoride and trifluoroethyl ene), oriented in a corona discharge, were equal to theta(0) = 5.1 deg rees, delta theta = 0.5 degrees and theta(0) = 89 degrees, delta theta = 0.06 degrees, respectively. (C) 1998 American Institute of Physics. [S0021-3640(98)00606-9].