OPTICAL-PROPERTIES OF ANTIFERROELECTRIC LIQUID-CRYSTALS IN FREESTANDING FILMS

A theoretical explanation of the results of ellipsometric measurements , performed some time ago by Bahr and co-workers [Ch. Bahr, D. Fliegne r, C. J. Booth, and J. W. Goodby, Phys. Rev. E 51, R3823 (1995)] on fr ee-standing films of antiferroelectric liquid crystal (R)-1-methylhept yl-4-(4'-n-dodecyloxybiphenyl-4-yl carbonyloxy)-3-fluorobenzoate (in t he following 12F1M7), is given within a discrete phenomenological mode l of antiferroelectric liquid crystals. The structure of the smectic-C alpha(Sm-C alpha) phase is described as an incommensurate phase, an a verage dielectric tensor, in general biaxial, is determined and the el lipsometric parameter Delta is found for two orientations of the film structures, defined by external electric field. Results are in agreeme nt with observations: we show that temperature induced changes of the structure of the Sm-C alpha phase, as derived from the model, lead to the oscillatory dependence of the ellipsometric parameters Delta(-) an d Delta(+) on temperature.