Method of preparing a tunable-focus liquid crystal (LC) lens

A liquid crystal (LC) lens is prepared by controlling the alignment of a LC using a homogeneous polyimide (PI) layer and a homeotropic PI layer. The rubbed homogeneous PI layer has a concave surface and the homeotropic PI layer is flat. The LC sandwiched between the two PI layers obtains a hybrid alignment which has the largest gradient of refractive index (GRIN) distribution. The LC layer exhibits a lens character because of its convex shape. Since the effective refractive index of the LC is larger than that of the homogeneous PI, the LC lens can focus a light with the shortest focal length in the voltage- off state. By applying an external voltage, the LC molecules can be reoriented along the electric field. As a result, the focal length of the LC lens is reduced. The focal length of the LC lens can be tuned from similar to 30 to similar to 120 mu m when the voltage is changed from 0 to 7 V-rms. This LC lens has the advantages of no threshold, low operating voltage, and simple fabrication.