Anomalous transmission (Borrmann effect) in absorbing cholesteric liquid crystals

Abstract

The celebrated Borrmann effect is an anomalous increase in transmitted X-ray intensity when a crystal is set for Bragg reflexion. In this paper, it is shown that a similar effect occurs in absorbing cholesteric liquid crystals in the vicinity of the reflexion band. However, in contrast to the x-ray case, the polarization of the wave field and the linear dichroism of the molecules play an essential part. Numerical computations are presented, based on the general theory of reflexion and transmission by oriented cholesteric films. They illustrate the role of dichroism and sample thickness in determining the magnitude of the effect. The existence of the effect is established by experimental studies on cholesteryl nonanoate mixed with small quantities of p-azoxyanisole or n-p-methoxybenzylidene-p-phenylazoaniline. When the reflexion band is adjusted to overlap with the strongly dichroic band of the solute molecules, the transmission spectrum exhibits the features predicted by theory.