Fast electrooptical modes in ferroelectric liquid crystal and their applications.

摘要

The ferroelectric liquid crystal (FLC) which has the advantage of a fast response under an electrical field has become one of the most promising candidates of the next generation liquid crystal display (LCD) as well as photonic devices. In the display area, sub-millisecond switching is needed to enable field sequential color (FSC) display technology, which improves resolution and efficiency by three times when using sequentially coming RGB colors instead of color filters. The FLC allows electrical control of the response time in the range from several microseconds to several seconds. FLC devices also have a great potential in the sensing and detection area since a microsecond response is in high demand for real time sensors. Gratings, optical switches and phase modulators can also be fabricated with FLC, and can be applied in the optical communication area.;Among various electrooptical modes of FLC, the surface stabilized ferroelectric liquid crystal (SSFLC) shows intrinsic bi-stability and response time of order microsecond. But a continuously tunable threshold free phase shift of light, which is important for photonic and display application is rarely possible with SSFLC. On the other hand, deformed helix ferroelectric (DHF) mode can provide a continuously tunable and hysteresis-free phase modulation. The characteristics of DHF cells in a transmissive and reflective regime are investigated. Several application cases of DHF mode are discussed.;Recently, we have proposed a novel electrooptical mode, the so called electrically suppressed helix (ESH) mode, which offers good alignment quality, high contrast ratio and low driving voltage. The self-diffractive scattering of ESH cells is extremely low. Owing to the good alignment quality, we can apply the patterned alignment method to generate the periodic distribution of the refractive index and thus the switchable grating. The FSC display based on ESH mode exhibits a high contrast ratio and fast response, as well as a wide viewing angle.;The aligning method is a critical issue for FLC devices. The photoaligning technique is in high demand compared with the traditional rubbing method, since photoaligning is a non-contact process which avoids static charges, particles, and contacting damages. Furthermore, the photoaligning technique can realize controllable anchoring energy by exposure dose. So far, among the photosensitive materials, azo-dye material SD1 provides best alignment quality for both ESH and DHF cells. A method to obtain optimal alignment quality by balancing the elastic energy of helix and the anchoring energy of the alignment layer is proposed. The photo-stability of azo-dye SD1 is improved using a composite layer with SD1 and photosensitive polymer.