Wide-band reflective polarizers from cholesteric polymer networks with a pitch gradient

Abstract

CHOLESTERIC liquid crystals, in which the orientation of the molecules varies in a helical fashion, are used for optical filtering of circularly polarized light, for example in liquidcrystal displays. They reflect circularly polarized incident light of the same handed ness as the cholesteric helix, and in a wavelength band that depends on the helical pitch (repeat distance). This pitch can be selected by careful design of the liquidcrystal molecules or by mixing cholesteric materials with nematic (linearly oriented) liquid crystals, which tend to increase the pitch. Stable optical filters are produced by crosslinking the cholesteric molecules by photopolymerization1; these filters typically have a reflection wavelength bandwidth of ∼50 nm. Here we show that, by introducing a gradient in the pitch of the cholesteric helix, we can obtain reflection of one of the two circularly polarized components over the entire visible spectrum. Polarizers with such broadband reflectivity would greatly improve the light yield and energy efficiency of liquidcrystal display devices.