Molecular motion analysis in nematic liquid-crystal cells using linear and nonlinear optical techniques

Abstract

The molecular motion of nematic liquid-crystal (NLC) molecules in a NLC cell was analyzed upon the switching on, the switching off, and the sign reversal of an external electric field. Both time-resolved nonlinear optical second-harmonic generation, which is sensitive to induced molecular dipole moments, and time-resolved linear optical transmission measurement, which is sensitive to the birefringence of the NLC molecules, were used. The combination of these techniques allowed a detailed analysis of both the molecular reorientation in the external field (through transmission experiments) and the dynamics of the electrically induced molecular dipole moments (through second-harmonic generation), which are again responsible for the molecular reorientation. Different time constants for the linear and the nonlinear optical experiments that correspond to different physical processes were observed. As a new second-harmonic-generation technique, the method of time-resolved phase measurement is introduced to analyze the sign reversal of the susceptibility of the NLC molecules induced by the polarity reversal of an external electric field.