Investigation of the helix unwinding process in thick freely suspended smectic films

Optical and electro-optical measurements have been performed on free-standing chiral smectic films sufficiently thick (similar to 10 000 layers) to preserve the natural smectic helix. The Goldstone mode appears at about 200 Hz, showing that these films are a much better approximation of the 'ideal' smectic bulk state than a thick planar sample between glass plates (where the Goldstone mode is found at about 3 kHz). In these films the unwinding of the helix is studied, as a function of applied electric field, by monitoring Bragg reflections and their Fourier components. When the helix deforms, a reflection appears which at first sight might be taken for a subharmonic, but must be interpreted as the main 'full pitch mode' reflection relative to the 'half pitch' reflection from the undeformed helix. Our measurements further confirm that in anticlinic materials no helix unwinding takes place prior to the antiferroelectric-ferroelectric transition.