Quasi-Linearly Polarization-Independent Liquid Crystal Lens by Means of Flow-Induced Alignments in the Cells

Given their specific electro-optical characteristics, liquid crystals (LCs) are usually used to study and fabricate optical devices for applications. The early study of LC lenses began in the 1970s and immediately became an interesting topic because of their unique optical capabilities. Especially, several innovative types of LC lenses had been proposed including a lens with circular hole-patterned electrode [1], spherical-shaped LC lens [2], and LC lens with inhomogeneous polymer network [3] etc. However, most LC lenses are polarization-dependent with respect to the incident light due to LC birefringence. In order to conveniently achieve polarization-independent LC lenses, the cells composed of two LC layers with orthogonal alignments are usually used to achieve the purpose [4]. In the paper, we used a way of flow-induced LC alignments with a radial symmetrical distribution [5], with which the LC lenses with hole-patterned electrodes can be realized with the polarization-independent characteristics. The fabrication processes of LC lenses are briefly described as follows. Two small LC droplet (E7, Merck) from a micro-dropper fell on individual two glass substrates, which one was coated with ITO-polyimide films and the other was only coated with a polyimide film. Due to a symmetrical circular shape of LC droplet on the substrates, LC orientations aligned in a form of radial symmetry was possibly achieved. We combined these two substrates and accurately aligned via the observation with a microscope. Finally, we more combined a glass substrate with a hole-patterned aluminum (Al) electrode next to the former substrate without an ITO film in order to complete a LC lens with an Al electrode in the outside surface of the LC cell. For a completed LC lens, a crossed dark optical texture was observed with a pair of crossed polarizers as shown in Fig. 1(a), which prove that LCs were aligned in a form of radial symmetry in the cell. When an applied voltage of 100 Vrms was used in the LC lens, the LC reorientations also maintained a form of radial symmetry and simultaneously possessed an ideal quadric gradient distribution of refractive indices to emerge a concentric interference pattern as shown in Fig. 1(b). A quasi-linearly polarization-independent characteristic in the LC lens was also obvious. In Fig. 2, it shows that the features of focal points were almost the same with respect to the cell rotations when a normal incident light beam passing through the LC lens. In conclusion, we use a simple way of flow-induced LC alignments to realize a quasi-linearly polarization-independent LC lens. In the future, we will evaluate its performance used in an imaging system.