Optical Bragg Diffraction of the Helical Smectic A Liquid Crystal

Abstract

Abstract Optical properties of the mixture of cholesteryl-n-nonanoate (CN) and n-nonyloxybenzoic acid (NOBA) in weight proportion 7:3 have been investigated. As it was early found by us (Ukranian Fiz. Zhurn., 35, 221 (1990)) this mixture possesses a new liquid-crystalline helical smectic A (SmA*) phase between the low-temperature smectic A (SmA) and high-temperature cholesteric (Ch) phases. It is shown experimentally that there are three temperature regions of the optical Bragg diffraction for the mixture. The first two regions correspond to the different states of the SmA* phase and the third one corresponds to the Ch phase. Inside each region the temperature dependence of the Bragg reflection wavelength γ is in exellent agreement with an empirical formula γ = γ0 + a/(T − T*)2, where T* = 82.4°C is the temperature of the SmA − SmA* transition and γ0 as well as a are constant within each region but differ for different regions.