Electrically tuneable cholesteric mirror

Chiral nematic liquid crystals which possess a large negative dielectric an isotropy (Delta epsilon < 0), exhibit a larger dielectric constant parallel to the helical axis than perpendicular to it. In a liquid crystal eel with homeotropic boundary conditions, the direction of the helical axis is para llel to the cell surfaces and the sample adopts a pseudo focal conic textur e. On the application of a suitable AC field, the director is rotated, such that the helix direction is now perpendicular to the surface and the sampl e adopts a Grandjean texture. The wavelength of reflected light in this "fi eld On" state is a function of temperature and the magnitude of the applied field. In this paper we show that highly twisted chiral nematic liquid cry stals, which possess a large negative dielectric anisotropy are potentially useful as optical modulators and in electrically tuneable mirrors.