Optically guided mode study of nematic liquid crystal alignment on a zero-order grating

The characterization of a liquid crystal cell, which comprises one zero-ord er (that is, at the wavelength of study it is nondiffractive) diffraction g rating and one rubbed polyimide-coated substrate, has been performed using an optically guided mode technique. The cell is filled with nematic liquid crystal E7 (manufactured and sold by Merck, Poole, U.K.). The excitation of fully leaky guided modes within the liquid crystal layer has allowed the o ptical director profile to be quantified under the application of weak in-p lane electric fields. The fitting of angle-dependent optical data to multil ayer optical theory yields the accurate twist profile of the liquid crystal for different field strengths. Comparisons with profiles predicted from el astic continuum theory, assuming a Rapini-Papoular-type anchoring at the su rfaces, allow both the azimuthal anchoring strength at each surface and the twist elastic constant of the bulk to be accurately determined. Repeating these measurements as a function of temperature allows the surface and bulk order parameters of the grating-aligned liquid crystal to be deduced.