Electrically switchable and optically rewritable reflective Fresnel zone plate in dye-doped cholesteric liquid crystals.

This work demonstrates a reflective Fresnel zone plate based on dye-doped cholesteric liquid crystals (DDCLC) using the photo-induced realignment technique. Illumination of a DDCLC film with a laser beam through a Fresnel-zone-plate mask yields a reflective lens with binary-amplitude structures - planar and focal conic textures, which reflect and scatter probed light, respectively. The formed lens persists without any external disturbance, and its focusing efficiency, analyzed using circularly polarized light, is ~ 23.7%, which almost equals the measured diffraction efficiency of the used Fresnel-zone-plate mask (~ 25.6%). The lens is thermally erasable, rewritable and switchable between focusing and defocusing states, upon application of a voltage.