Ferroelectric liquid crystal with positive dielectric anisotropy

Details Ferroelectric liquid crystal with positive dielectric anisotropy Ferroelectric liquid crystal with positive dielectric anisotropy

1991-12-13

1994-10-04

Sikes, William L.

Optical: systems and elements

Holographic system or element

Using a hologram as an optical element

359 84, 359100, 345 89, 345 97, G02F 113, G02F 11343, G02F 1137

Patent

active

053531361

DESCRIPTION:

BRIEF SUMMARY
Switching and display elements in which ferroelectric liquid crystals serve as switching and display medium (FLC displays) are described, for example, in U.S. Pat. No. 4,367,924. They comprise a layer composed of a ferroelectric liquid-crystalline medium (FLC) which are enclosed on both sides by electrically insulating layers, electrodes and boundary sheets, usually glass sheets. In addition, they contain a polarizer when operated in the guest-host mode and two polarizers when operated in the birefringence mode. The electrically insulating layers are intended to prevent electric short-circuits between the electrodes and the diffusion of ions out of the glass of the boundary sheets into the FLC. Furthermore, at least one and preferably both insulating layers serve as an orientation layer which brings the FLC into a configuration at which the longitudinal axes of the molecules of the FLC are parallel to one another and in which the smectic planes are arranged perpendicular to or at an angle relative to the. orientation layer. In this arrangement, the FLC molecules can adopt two possible and equivalent orientations, into which they can be brought by applying pulses of an electric field. In each case, they remain in the last-generated orientation, even if the field is switched off or the display is short-circuited. This means that FLC displays can be switched between two stable states. The switching times are in the region of .mu.s and decrease with increasing spontaneous polarization of the FLC used.
Compared with the previously used liquid crystal displays, which are all non-ferroelectric, FLC displays have in particular the advantage that the obtainable multi-plexing ratio, i.e., the maximum number of lines which can be addressed in a time sequence ("multiplexing") is considerably higher than in the known non-ferroelectric displays.
A serious disadvantage of FLC displays is their hitherto too low optical contrast of multiplex-driven displays. In this form of addressing, the maximum possible switching angle is not fully utilized, but only a so-called effective switching angle .theta..sub.eff. This is explained by undesirable twist states or by the presence of tilted smectic layers [see N. Hiji, Y. Ouchi, H. Takezoe, A. Fukuda, Jap. J. Appl. Phys. 27, 8 (1988)]. In order to improve the contrast, special addressing modes have been developed, which in the case of dielectrically negative FLC material (i.e., LC material having a .DELTA..epsilon.<0) lead to a widening of the effective switching angle .theta..sub.eff [see T. Umeda, T. Nagata, A. Mukoh, Y. Hori, Jap. J. Appl. Phys. 27, 2187 (1988) and Y. Sato, T. Tanaka, M. Nagata, H. Takeshita, S. Morozumi, Proc. 6th Intl. Display Res. Conf. 1986, p. 348].
Although this method produces certain improvements in the contrast, they are by no means sufficient. In addition, undesirable side effects are observed especially when the attempt is made to maximize the effect of contrast improvement by suitable addressing. These undesirable effects are irreversible transformations in the geometry of the FLC cell, so-called textural transformations [see H. -R. Dubal, C. Escher, D. Ohlendorf, Proc. 6th Intl. Symp. on Electrets, Oxford, England 1988, p. 334]. These transformations are unfavorable because they considerably lengthen the switching times of the displays.
Surprisingly, it has now been found that, even in the case of dielectrically positive FLC material (FLC material having a .DELTA..epsilon.>0), the effective switching angle can be enlarged and thus the contrast increased by suitable multiplex addressing. A particular advantage of this method is that a textural transformation does not occur or at least only very slightly even when the effect is maximized.
A liquid-crystalline material which forms an S.sub.C.sup.*, S.sub.F.sup.*, S.sub.G.sup.*, S.sup.* or S.sub.J.sup.* phase proves to be particularly suitable. Another particularly suitable material is an FLC material having N.sup.* phase in a temperature range above the ferroelectric phase, since in this case the

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Structures and Applications of SSFLC devices, Japan Display 1986, Clark et al, Oct. 1986.

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