Quantification of the surface- and bulk-order parameters of a homogeneously aligned nematic liquid crystal using fully leaky guided modes

An optical waveguide technique, involving the excitation of fully leaky gui ded modes, has been used to quantify the director distortions induced by th e application of weak in-plane electric fields to a homogeneously aligned n ematic liquid crystal, E7. The twist profile of the nematic is determined b y fitting angle-dependent reflectivity and transmission data to multilayer optical theory. By considering the Frank-Oseen free energy in the bulk, wit h Rapini-Papoular-type anchoring at the surfaces, we have determined accura te values for both the azimuthal anchoring strength W-a and the twist elast ic constant k(22) as a function of temperature. Values for the temperature- dependent surface and bulk average-order parameters have also been deduced by allowing for a spatially varying nematic-order parameter. (C) 1999 Ameri can Institute of Physics. [S0021-8979(99)03424-6].