Time-resolved Total Internal Reflection Raman Scattering Study on Electric-Field-Induced Reorientation Dynamics of Nematic Liquid Crystal of 4-Hexyl-4′-Cyanobiphenyl

Abstract

Total internal reflection (TIR) Raman scattering spectroscopy was performed using a cell combined with a high index hemicylindrical substrate and a glass plate with obliquely evaporated SiO alignment layers to study an electric-field-induced reorientation process of 4-n-hexyl-4′-cyanobiphenyl (6CB) in a nematic phase. Two types of cells having different director configurations were studied; one has the orientation of the director parallel to the axis of the hemicylinder (s-configuration) and the other the director perpendicular to the axis (p-configuration). Squared relative Raman scattering tensor components of the LC layer were determined by analyzing the polarized TIR Raman spectra observed for the cells. Time-resolved TIR Raman measurements were performed on the p-configuration cell under application of squared electric fields (duration = 10 ms, repetition rate = 10 and 5 Hz, amplitude = 3–8 V) by using a pulsed Nd-YAG laser (Λex = 532 nm, pulse width = about 15 ns). The results elucidated characteristics of the electric field-induced reorientation dynamics of a layer of the LC molecules (with a thickness of about 50 nm) adsorbed on the evaporated SiO film: (i) Upon application of the electric field with the amplitudes of 3–5 V the director of the surface layer keeps its original orientation for a certain period, although the director of the bulk phase immediately responds to the fields; the period becomes longer as the amplitude decreases, being ca. 4 ms for the amplitude of 5 V and about 6 ms for 3 V: (ii) After switching off the electric fields the surface layer exhibits a rapid recovery process with the time constant for 3–6 ms, which is appreciably shorter than that of the bulk phase of almost 350 ms (measured under application of the squared electric field of 9 V). An elastic interaction force within the former