Sexy liquid crystal (LC): Apllication in LCD

Liquid crystals find wide use in liquid crystal displays, which rely on the optical properties of certain crystalline substances in the presence or absence of an electric field. In a typical device, a liquid crystal layer (typically 10 micro meters thick) sits between two polarizers that are crossed. The liquid crystal alignment is chosen so that it relaxed phase is a twisted one. This twisted phase reorients light that has passed through the first polarizer, allowing it to be transmitted through the second polarizer (and reflected back to the observer if a reflector is provided). The device thus appears transparent. When an alectric field is applied to the liquid crystal layer, the long molecular axes tend to align parallel to the electric field thus gradually untwisting in the center of the liquid crystal layer. In this state, the mesogens do not reorient light, so the light polarized at the first polarizer is absorbed at the second polarizer, and the device loses transparency with increasing voltage. In this way, the electric field can be used to make a pixel switch between transparent or opaque on command. Color LCD systems use the same technique, with color filters used to generate red, green, and blue pixels. Similar principles can be used to make other liquid crystal based optical devices.