The Advantages of Dye-Doped Liquid Crystals in Constructing a Fresnel Zone Plate Lens

A Fresnel zone plate lens can be constructed using a dye-doped liquid crystal filled cell and photo-induced alignment techniques. Light passing through a Fresnel zone patterned photo-mask causes the dye-doped liquid crystal to align in such a way to produce odd and even zones on the lens. Diffraction of incident light through the odd and even zones of the lens allows the light to constructively interfere and come to a focus. The properties of the odd and even zones can be electronically controlled resulting in a tunable lens. The lens exhibits the properties of being scatter free, having a very low operating voltage (~5Vrms), producing separable zero and first focus orders, having adjustable diffraction efficiency, and having controllable polarization. The lens can be easily fabricated in comparison to other Fresnel zone plate lens designs. Introduction: Most lenses utilize the concept of refraction in order to focus light. Refraction occurs when light passes through an interface where the index of refraction changes. The larger the change in the index of refraction the more the light bends or changes its path. The index of refraction of a material depends on the wavelength of light it is transmitting. Some wavelengths outside of the visible spectrum cannot be focused using lenses that rely on refraction since materials with the necessary change in index of refraction do not exist. Therefore, lenses can also be constructed using the concept of diffraction in order to focus light. Diffraction occurs when light encounters an obstruction and appears to “bend” around the object. In fact, according to the Huygens-Fresnel principle, each unobstructed point of the wavefront appears as a secondary source of spherical waves when passing through or around an obstruction. In the case of Fresnel diffraction, either the incident or the received light deviates from being plane waves and has significant wavefront curvature. In this region, Fresnel or half-period zones exist where