Complex permittivities of a nematic liquid crystal in a hybrid-aligned cell

The half-leaky guided-mode (HLGM) technique has been used to explore a hybrid-aligned nematic (HAN) liquid-crystal cell. With a suitably oriented cell the optical response of the system, as a function of incident angle, has some unusual features. It is found that beyond a particular angle of incidence, with light incident from the homeotropically aligned side, the apparent mode spacing increases with increasing angle, which is entirely opposite to that found for an isotropic slab of material. This clearly indicates that the effective cell thickness varies with incident angles, an effect caused by the director profile within the cell, leading to bent trajectories for the beams propagating inside the cell. For this situation the depths of the reflectivity minima associated with excitation of the waveguide modes are particularly sensitive to the imaginary parts of the permittivities of the liquid crystal. This has enabled the two imaginary parts of the parallel and perpendicular permittivities of the liquid crystals to be determined separately. The technique is a very useful and accurate method for determining the whole optical tensor of the liquid crystal.