Optical characterization of liquid crystals by means of half-leaky guided modes

Abstract

Optical excitation of half-leaky guided modes has been used, for the first time to our knowledge, to characterize in detail the optical tensor profile in a liquid-crystal layer. The thin liquid-crystal layer is sandwiched between a high-index pyramid, with an index greater than the maximum index of the liquid crystal, and a glass substrate with an index lower than the minimum index of the liquid crystal. Analysis of this geometry shows that over a small angle range encompassed by a pseudocritical angle and a critical angle associated with the pyramid–liquid crystal and the pyramid–glass-substrate boundaries, respectively, there may exist sharp resonant features in the angle-dependent optical reflectivity. More particularly, if the director is tilted and twisted out of the plane of incidence there is strong TE-to-TM optical conversion over this small angular window, which is sensitive to details of the director profile. A 90° twisted, homogeneously aligned nematic liquid-crystal layer has been studied with this technique at two different wavelengths, 632.8 and 514.5 nm. Greater than 60% conversion from TM incident radiation to TE output has been recorded from sharp resonances in the half-leaky guided-wave angle window. If one fits these resonances with predictions from multilayer optics theory, one obtains extraordinary detail of the director profile in the cell. Since no metallic coatings are used, and all that is required are two glass plates of high and low index, respectively, the technique offers the potential of extremely useful applications in the examination of detailed director profiles in commercially fabricated cells.