Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
1995-09-13
2002-09-24
Parker, Kenneth (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S172000
Reexamination Certificate
active
06456349
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal-element such as a liquid crystal display element or an liquid crystal optical shutter and, more particularly, to a ferroelectric liquid crystal element and a liquid crystal apparatus using the same. More specifically, the present invention relates to a liquid crystal element and a liquid crystal apparatus using the same, in which display characteristics are improved by improving an orientation state of liquid crystal molecules.
2. Related Background Art
A display element for controlling a transmission beam by a combination with a polarization element by Utilizing refractive index anisotropy of ferroelectric liquid crystal molecules is proposed by Clark and Lagerwall (Japanese Laid-Open Patent Application No. 56-107216 and corresponding U.S. Pat. No. 4,367,924). This ferroelectric liquid crystal has a chiral smectic C phase (SmC*) or H phase (SmH*) having a non-helical structure. In this state, the ferroelectric liquid crystal has the first or second optically stable state in response to an electric field applied thereto. When no electric field is applied to this ferroelectric liquid crystal, it maintains the current state. In this manner, the ferroelectric liquid crystal has a bistable property and has a high response speed to a change in electric field. It is expected as a high-speed storage type display element in a variety of applications. In particular, this ferroelectric liquid crystal is expected for an application as a large, high-precision display due to the functions of the liquid crystal.
In order to enhance a predetermined drive characteristic of an optical modulation element using this bistable liquid crystal, the liquid crystal sealed between a pair of parallel substrates is required to be kept in a molecular orientation state to cause effective phase transition between the above two stable states.
A transmittance of a liquid crystal element utilizing a birefringence of the liquid crystal under the presence of crossed Nicols is given as follows:
I
/
I
0
=
sin
2
4
θ
sin
2
δ
nd
λ
π
where I
0
is the incident light intensity, I is the transmitted light intensity, &thgr; is the tilt angle, &Dgr;n is the refractive index anisotropy, d is the thickness of a liquid crystal layer, and &lgr; is the wavelength of incident light.
The tilt angle &thgr; in the non-helical structure appears as an angle of an average molecular axis of liquid crystal molecules having helical arrangements in the first and second orientation states. According to the above equation, a maximum transmittance can be obtained when the tilt angle &thgr; is given as 22.5°. The tilt angle &thgr; in the non-helical structure for realizing the bistable property must be as close as to 22.5°.
Of orientation methods of ferroelectric liquid crystals, a method capable of aligning a molecular layer consisting of a plurality of molecules constituting a smectic liquid crystal along a uniaxial direction along a normal to the molecular layer, and capable of achieving such orientation by simple rubbing is preferable.
An example of the method of aligning a ferroelectric liquid crystal and, particularly, a chiral smectic liquid crystal having a non-helical structure is described in U.S. Pat. No. 4,561,726.
The following problem is posed when the conventional orientation method and, particularly, an orientation method using a rubbed polyimide film is applied to a ferroelectric liquid crystal having a non-helical structure exhibiting the bistable property and announced by Clark and Lagerwall.
According to experiments of the present inventors, it is found that an apparent tilt angle &thgr; (i.e., ½ an angle formed by two molecular axes of the two stable states) of a ferroelectric liquid crystal having a non-helical structure aligned by a conventional rubbed polyimide film is smaller than a tilt angle (i.e., an angle &thgr; which is ½ a vertex angle of a triangular cone, shown in
FIG. 4A
to be described later) of a ferroelectric liquid crystal having a helical structure. In particular, the tilt angle &thgr; of the ferroelectric liquid crystal having the non-helical structure oriented by the conventional rubbed polyimide film generally falls within the range of about 3° to 8°, and its transmittance falls within a maximum range of about 3% to 5%.
As can be apparent from the above description, according to Clark and Lagerwall, although the tilt angle of a ferroelectric liquid crystal having a non-helical structure for realizing the bistable property must be equal to the tilt angle of a ferroelectric liquid crystal having a helical structure, the tilt angle &thgr; of the non-helical structure is smaller than the tilt angle &thgr; of the helical structure. In addition, it is also found that a cause for setting the tilt angle &thgr; of the non-helical structure to be smaller than that (&thgr;) of the helical structure is based on helical orientation of liquid crystal molecules in the non-helical structure. More specifically, in a ferroelectric liquid crystal having a non-helical structure, liquid crystal molecules are continuously twisted from an axis of the liquid crystal molecules adjacent to the upper substrate to the axis of the liquid crystal molecules adjacent to the lower substrate with respect to the normal to the substrates. For this reason, the tilt angle &thgr; of the non-helical structure is smaller than the tilt angle &thgr; of the helical structure.
Since a polyimide orientation film serving as an insulating layer is present between each electrode and the liquid crystal layer, when a voltage having one polarity is applied to the chiral smectic liquid crystal which is oriented by the conventional rubbed polyimide orientation film so as to switch the orientation state from the first optically stable state (e.g., a white display state) to the second optically stable state (e.g., a black display state), a reverse electric field V
rev
having the other polarity is applied to the ferroelectric liquid crystal after the voltage having one polarity is withdrawn. The electric field V
rev
causes an after image at the time of display.
This reverse electric field generation phenomenon is described in Akio Yoshida, “SSFLC Switching Characteristics”, Lecture Papers on Liquid Crystal Forum, October, 1987, pp. 142-143.
One of the present inventors found the following phenomenon associated with an orientation state of a ferroelectric liquid crystal.
An LP 64 (tradename) having a relatively small pretilt angle available from TORAY INDUSTRIES, INC. or the like is applied to substrates to form orientation films thereon, and the substrates respectively having the orientation films are rubbed and adhered to each other with a gap of about 1.5 &mgr;m such that the rubbing direction of one substrate is the same as that of the other substrate. A ferroelectric liquid crystal CS1014 (tradename) available from Chisso Kabushiki Kaisha is injected into the space defined between the two substrates of the obtained cell and is sealed. When the temperature of the ferroelectric liquid crystal is decreased, phase transitions shown in
FIGS. 2A
to
2
E are obtained. More specifically, in a state shown in
FIG. 2A
immediately after the phase transition from a high-temperature phase to an Sc* phase, the ferroelectric liquid crystal takes orientation states (C
1
orientation states)
21
and
22
having a small contrast value. When the temperature is further decreased and reaches a given temperature region, zig-zag defects
23
are generated, and orientation states (C
2
orientation states)
24
and
25
having large contrast values with respect to these defects appear, as shown in FIG.
2
B. When the temperature is further decreased, the C
2
orientation state propagates (FIGS.
2
C and
2
D), and the entire liquid crystal is set in the C
2
orientation state (FIG.
2
E).
These C
1
and C
2
orientation states ca
Asaoka Masanobu
Hanyu Yukio
Inaba Yutaka
Shinjo Kenji
Taniguchi Osamu
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Parker Kenneth
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