The soft-mode ferroelectric effect

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Abstract
In this paper a presentation is given of some of the basic physics of the soft-mode ferroelectric effect, along with its experimental background. Further, possible applications in electrooptic devices are elucidated. A fast (sub-microsecond) electrooptic switching can be achieved in the A* phase, and in similar orthogonal smectic phases made up of chiral molecules. Instead of using the phase variable Φ, it uses the tilt angle θ, which in principle is a “hard” variable, but is expected to soften on approaching the transition to a lower-lying adjacent tilted smectic phase. However, the switching is efficient in the whole range of the orthogonal phase (in fact, less efficient near the tilting transition) and is observed whether an adjacent tilted phase is present or not. As compared to the surface-stabilized (SSFLC) electrooptic mode, this soft mode (SMFLC) is based on the electro clinic, effect of essentially ferroelectric nature very closely related to the presently more investigated ferroelectric, effect in tilted phases. It uses the same bookshelf geometry and has the same symmetry in its ON-OFF switching characteristics. It possesses no bistability but can generate a continuous grey-scale or even a continuous colour-scan controlled by an applied voltage. Depending on polarizer and retarder arrangement, it may be used for modulating a light signal with a transmitted intensity linear in the applied electric field, or it may be used as a tunable colour filter. One of the obvious potential applications is in active matrices to replace the present nematics; another one is in spatial light modulators.

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