Flat liquid crystal diffractive lenses with variable focus and magnification

Non-mechanical variable lenses are important for creating compact imaging devices. Various methods employing dielectrically actuated lenses, membrane lenses, and/or liquid crystal lenses were previously proposed^1-4. Here we present tunable-focus flat liquid crystal diffractive lenses (LCDL) employing binary Fresnel zone electrodes fabricated on Indium-Tin-Oxide using conventional micro-photolithography. The phase levels can be adjusted by varying the effective refractive index of a nematic liquid crystal sandwiched between the electrodes and a reference substrate. Using a proper voltage distribution across various electrodes the focal length can be changed. Electrodes are shunted such that the correct phase retardation step sequence is achieved. If the number of 2π zone boundaries is increased by a factor of m the focal length is changed from f to f/m based on the digitized Fresnel zone equation: f = r_m ²/2mλ, where r_m is m^th zone radius, and λ is the wavelength. The lenses operate at very low voltage levels (±2.5V ac input), exhibit fast switching times (20-150 ms), can have large apertures (>10 mm), and small form factor, and are robust and insensitive to vibrations, gravity, and capillary effects that limit membrane and dielectrically actuated lenses. Several tests were performed on the LCDL including diffraction efficiency measurement, switching dynamics, and hybrid imaging with a refractive lens. Negative focal lengths are achieved by adjusting the voltages across electrodes. Using these lenses in combination, magnification can be changed and zoom lenses can be formed. The promising results make LCDL a good candidate for non-mechanical auto-focus and zoom lenses.