The LCDs described so far are all based on nematic liquid crystals. A second major class of LCDs under development is called the ferroelectric LCD because the molecules have a permanent polarization. Ferroelectric LCDs are smectic liquid crystals that have a natural layered order. Most ferroelectric LCDs are in fact of the smectic C phase (SmC(superscript *)) and possess a chiral behavior. Typically, the ferroelectric LCDs are built with very small cell gaps (1-2 microns) to stabilize the alignment of the molecules. The alignment layers cause a perpendicular alignment of the smectic phases in a "bookshelf" geometry. In the thin cells, there is a net polarization that is perpendicular to the glass plates; this polarization can couple to an externally applied field to rotate the molecules either "up" or "down." Because the liquid crystal molecules have an optical anisotropy, the state of the molecules can be "read" with external polarizers. Ferroelectric LCDs have several interesting properties that make them desirable for display applications: (a) the molecular rotation is fast--50-100 micro sec, (b) the effect is bistable, and (c) the viewing angle is wide because of the small cell gap. LCDs with up to 1000 lines have been produced in both color and monochrome (Canon). One drawback for some applications is the lack of gray scale except by time multiplexing; this severely limits applications for TV. Another drawback is the sensitivity of the alignment to shock and vibration. Because they allow high-speed switching, ferroelectric LCDs are also of interest to the electro-optic community as optical switches. Another interesting application is for print heads to replace scanning laser beams; in this case, the LCD acts as a shutter and is used with a light bar as a fluorescent lamp.

Published: June 1992; WTEC Hyper-Librarian