Fresnel lenses based on dye-doped liquid crystals

We demonstrated transmissive- and reflective-type Fresnel lenses based on dye-doped liquid crystal using photoalignment technique. The former is a polarization-independent and electrically tunable. The maximum diffraction efficiency reaches 37%, which approaches the theoretical limit ∼ 41 %. Such a lens functions as a half-wave plate, and this feature could be well preserved under the applied voltage. The reflective-type Fresnel lens is based on dye-doped cholesteric liquid crystals (DDCLC). 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, and rewritable. Notably, both of the transmissive- and reflective-type Fresnel lenses are switchable between focusing and defocusing states, upon application of a voltage. In addition, these devices are simple to fabricate, and have fast switching responses between focusing and defocusing state.