Fully leaky guided wave determination of the original alignment direction for the directors at the walls in a twisted nematic liquid crystal cell

Fully leaky guided optical modes are used to explore the director profile in a twisted nematic liquid crystal cell built using a pair of high-index glass plates. This allows access to the pseudocritical angles corresponding to the liquid-crystal indices. Optical data is recorded over a range of angles of incidence with the cell subject to ac voltages. At higher-applied fields, the director in the cell center aligns almost homeotropically. Then the two surface regions become elastically decoupled, the directors at the surfaces realigning along the original alignment axes. Beyond the pseudo-critical angle corresponding to the low index of the liquid-crystal two minima of the polarization conserving transmission signal, for transverse electric polarization, T_SS (over a particular range of incidence angles) are found as the cell rotation angle is changed. These two minima give the original alignment directions of the director, the easy axes, at the two substrates, information vital to the measurement of the torsional anchoring energies at the interfaces of the cells. © 2002 American Institute of Physics.