Technique for characterizing azimuthal anchoring of twisted nematic liquid crystals using half-leaky guided modes

The half-leaky guided-mode technique is used to explore the director profile through a thin, twisted, homogeneously aligned nematic liquid-crystal cell (Merck-E7). Twisted alignment is realized by rubbed polyimide layers on both inner walls of the cell. By monitoring first a particular angle of incidence range of the p-to-s-conversion reflectivity for the twisted-nematic cell under high voltage, the direction of the liquid-crystal director on the top interface of the cell, the easy axis, is accurately determined. Then by removing the external voltage and modeling the complete director profile in the cell to fit the half-leaky guided-mode reflectivity data recorded, the real twist of the director from the top to the bottom of the cell is accurately determined. By comparing the surface director orientation at zero volts with the easy axis of the top interface, the azimuthal (in-plane twist) anchoring coefficient, WT, is quantified through knowledge of the twist elastic constant K22. At 26.5 °C we find that for E7, WT=(4.4±0.3)×10-5 J m-2.