Optical: systems and elements – Holographic system or element – Using a hologram as an optical element
Patent
1994-10-21
1996-06-18
Gross, Anita Pellman
Optical: systems and elements
Holographic system or element
Using a hologram as an optical element
359100, G02F 11343, G02F 113
Patent
active
055283942
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention relates to a ferro-electric liquid crystal device.
EP-A-0032362 discloses a surface stabilized FLC device which possesses bistability and fast switching. However, in most real devices as cooled from the isotropic phase to the S.sub.c.sup.. phase, instead of a structure consisting of smectic layers normal to the substrate interface as disclosed in EP-A-0032362, a tilted layer structure called a chevron is obtained. Consequently, this gives rise to a relatively small effective bistable switching angle which tends to reduce the overall optical performance of the device. The existence of two different forms of the chevron structure also provides an alignment texture consisting of visible defects called `zig-zags`. These defects usually degrade the appearance of the device.
In order to overcome the above problems of zig-zag defects and a reduced switching angle, a liquid crystal device has been proposed in GB2191870-B in which the surface pre-tilt angle matches the half cone angle of the ferro-electric liquid crystal material. However this technique presently requires the use of a vacuum deposited surface alignment layer in order to generate the required pre-tilt angle of 22.5.degree., which is costly for use in high volume production.
GB2183054 B, discloses the application of a high voltage, low frequency electrical signal to a ferro-electric liquid crystal device to irreversibly reduce the chevron angle so that at the end of the electrical treatment, a layer structure known as the Quasi Bookshelf Geometry (QBG), very similar to the idealised bookshelf, is obtained. Such samples not only have large electrical threshold but also contain defects which scatter light and reduce optical performance.
Based on the relative orientation of the surface pre-tilt, .beta. and the chevron inclination, two forms of tilted layer structure can be realized; Chevron 1 (C1) and Chevron 2 (C2) as disclosed in an article by J. Kanbe et el. Ferro-electrics 14,(1991)P3. The geometrical conditions for these forms are given in FIG. 1 where .delta. and .theta..sub.c are the chevron angle and half of the S.sub.c.sup.. cone angle respectively. The value of .theta..sub.c is usually arranged to be .apprxeq.22.5.degree. and from X-ray diffraction measurements, .delta. is typically 90% of .theta..sub.c i.e. .apprxeq.20.degree..
In devices with low values of the surface pre-tilt angle (.beta.), both C1 and C2 chevrons are stable i.e. both .theta..sub.c +.delta.>.beta. and .theta..sub.c -.delta..ltoreq..beta. can be satisfied respectively. Under these conditions both chevrons are found; zig-zag defects occur at the boundary between the two chevron states and there is very little twist of the director profile in the plane of the substrate through the thickness of the cell. The bistable switching angle (2.theta..sub.s) which is defined as the angle between the projection of the optic axis of tile two memory states onto the substrate plane is given approximately by .theta..sub.s is typically between 6.degree. and 10.degree..
As the surface pre-tilt is increased, zig-zag defects cease to occur because only the C1 chevron is stable i.e. .theta..sub.c +.delta.>.beta. can be satisfied but .theta..sub.c -.delta.>.beta. cannot be satisfied. In addition, the director profile within the bistable domains becomes twisted. When such samples are switched by electrical pulses, two types of bistable states can be obtained: bistable twisted states and bistable planar states. The former correspond to the switching of the director only at the chevron interface. The latter, which correspond to the director switching at the substrate interface, require very large electrical switching pulses (i.e. high voltages and long pulse widths).
In order to achieve large bistable switching angle in low pre-tilt samples, there is disclosed in tile reports (W. J. A. M. Hartman Ferro-electric, 122, 1991 and H. Reiger, et. al. SID 91 Digest (1991), P.137), the application of a low frequency (1-200 Hz) electric field treatment whic
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White Philip G.
Yeoh Colin T. H.
GEC - Marconi Limited
Gross Anita Pellman
Ton Toan
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