Measurement of the refractive indices of a ferroelectric liquid crystal

Optical excitation of half-leaky guided modes has been used to determine the refractive indices of a ferroelectric liquid crystal (Merck-SCE13) in a homeotropically aligned state. The uniform homeotropic alignment is realized, with no surface treatment, by the application of an in-plane dc electric field. This applied field, of the order of 5×105 V m−1 is sufficient to fully unwind the S*c helix giving a uniformly tilted homeotropic monodomain for optical characterization. Analytic and numerical modeling results indicate that, for a slab with its optic axis tilted in a plane orthogonal to the plane of incidence, two distinct critical angles appear in the half-leaky guided mode response. These independently relate simply to the ordinary and extraordinary refractive indices of the S*c material. By fitting theoretical angle dependent reflectivities to those recorded experimentally the two refractive indices have been obtained for a range of temperatures in both the S*c and SA phases.