A Method for Studying Optical Eigen Modes in Cholesteric Liquid Crystals by Using Stress Plate Modulators in Tandem

Abstract

An apparatus has been constructed for the direct study of the optical eigen modes (OEM's). The apparatus consists of an He–Ne laser, a linear polarizer (LP), a stress plate modulator (SPM), another SPM, another LP and a photomultiplier tube (PMT). A sample is placed between the SPM's, and the first combination of LP and SPM acts as a dynamic elliptical polarizer and the second one constitutes the analyzer which is always perpendicular to the polarizer. The output signal from the PMT is recorded in a transient memory and is accumulated and processed by a microcomputer. Taking the OEM propagating along the cholesteric helical axis as a test example, we calculated the temporal change in the PMT output signal; it indicates the existence of an ellipticity which gives zero transmittance. The calculated dependence of the ellipticity on wavelength agrees qualitatively with that of the true OEM, except for the region near or in the characteristic reflection. By performing actual measurement with a monodomain cholesteric film at a certain wavelength, we confirmed that the apparatus works well.