PRETRANSITIONAL ELECTROOPTIC RESPONSE OF 6CB LIQUID-CRYSTAL IN THE ISOTROPIC-PHASE

In this work, the electro-optic response of a 6CB liquid crystal layer is studied using a sensitive differential technique. The layer is hel d at a temperature just above the nematic to isotropic phase transitio n. Transverse magnetic (p) polarized light incident on the cell is cou pled to guided modes in the liquid crystal layer using prism coupling. The modes manifest themselves as sharp dips in the reflectivity as th e angle of incidence is scanned. When a low frequency sinusoidal volta ge is applied to the cell, the resonant mode shapes and excitation ang les are altered at a frequency which is twice that of the applied fiel d, resulting in a modulation of the reflectivity for a given angle of incidence. By synchronous observation of the modulated signal, a diffe rential signal is recorded. Comparing the data with modelling generate d from multilayer optics theory, two effects are then quantified. The first of these is an induced birefringence, varying quadratically with applied voltage, which is well understood and can be expressed in ter ms of Landau-de Gennes theory. The second is a field induced perturbat ion in the imaginary part of the optical permittivity, deltaepsilon(i) , which implies a modification of the light scattering properties of t he liquid crystal. The measurement of the latter effect is, as far as we know, a novel one, being only made possible by the remarkable sensi tivity of the synchronous differential technique.