Concealed Holograms based on Cholesteric Liquid Crystals

A hologram element that is transparent to visible light is demonstrated using a cholesteric liquid crystal. The operation of the hologram is based on the Berry phase effect in which the phase of reflected light varies depending on the orientation direction of the liquid crystal director on the substrate surface. Transparency in the visible is achieved by employing a cholesteric material with the reflection band in the infrared. Because of the sinusoidal modulation of the dielectric tensor, higher order reflections are suppressed for normal incidence and occurs over a very narrow region even for oblique incidence. This leads to a novel class of holographic optical elements where the element appears completely transparent when information is encoded in its phase.