Optical Properties of Certain Cholesteric Liquid-Crystal Films

Abstract

Optical scattering data have been used to investigate molecular order in certain cholesteric mixtures. When films of these materials are deposited on a reflecting substrate, their colors can be shifted by several thousand angstroms for any condition of illumination and observation by a mechanical disturbance. The dispersive color reflections in both the undisturbed and disturbed films can be attributed to Bragg‐type crystallite sites embedded in a material with refractive index n . When the films are mounted on a reflective substrate, there are effectively two angles of incidence. One angle corresponds to the actual angle of incidence and the other angle corresponds to the reflected beam. Accordingly, for some fixed combination of incidence and observation angles, two wavelength peaks should be observed for an isotropic distribution of scattering sites. Scattering data in both reflection and transmission indicate that the distribution is far from isotropic and that the effect of disturbance is to reorient the sites such as to produce a large increase in the intensity of light scattered from the direct beam and a corresponding decrease in the intensity of visible lightscattered from the reflected beam. There is evidence that the basic helical pitch parameter is not changed significantly by the mechanical disturbance.