U.S. patent number RE44,889 [Application Number 12/039,697] was granted by the patent office on 2014-05-13 for bistable liquid crystal devices.
This patent grant is currently assigned to F. Poszat Hu, LLC. The grantee listed for this patent is John Clifford Jones, Richard Jonathan Miller. Invention is credited to John Clifford Jones, Richard Jonathan Miller.
United States Patent |
RE44,889 |
Miller , et al. |
May 13, 2014 |
Bistable liquid crystal devices
Abstract
In a liquid crystal device a substrate (1) favors at least two
stable or metastable differently directed liquid crystal
alignments, and switching means for causing the liquid crystal
material to switch between the alignments includes means arranged
for optically irradiating said device. The latter may provide
linearly polarized light (3) for inducing a torque on the liquid
crystal to determine the alignment direction, and may optionally
cooperate with a second energy supplying means such as an electric
field (V) for assisting and switching. Alternatively, the alignment
of the liquid crystal may be switched by a second energy supplying
means such as an electric field, the light serving to produce heat
to aid the switching. Either or both energy sources may be applied
locally for switching of selected areas or pixels. Energy levels at
the bistable substrate may be adjusted by the use of an oligomeric
additive (slippery surface). As shown, the alignment at the surface
of an opposed substrate (2) optionally follows the alignment at
substrate (1).
Inventors: |
Miller; Richard Jonathan
(Malvern, GB), Jones; John Clifford (Malvern,
GB) |
Applicant: |
Name |
City |
State |
Country |
Type |
Miller; Richard Jonathan
Jones; John Clifford |
Malvern
Malvern |
N/A
N/A |
GB
GB |
|
|
Assignee: |
F. Poszat Hu, LLC (Wilmington,
DE)
|
Family
ID: |
9895666 |
Appl.
No.: |
12/039,697 |
Filed: |
February 28, 2008 |
PCT
Filed: |
July 11, 2001 |
PCT No.: |
PCT/GB01/03112 |
371(c)(1),(2),(4) Date: |
February 11, 2003 |
PCT
Pub. No.: |
WO02/06888 |
PCT
Pub. Date: |
January 24, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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Reissue of: |
10332902 |
Jul 11, 2001 |
7006165 |
Feb 28, 2006 |
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Foreign Application Priority Data
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Jul 15, 2000 [GB] |
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0017312 |
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Current U.S.
Class: |
349/24; 349/22;
349/123; 349/191; 349/129 |
Current CPC
Class: |
G02F
1/133362 (20130101); G02F 1/1391 (20130101) |
Current International
Class: |
G02F
1/133 (20060101); G02F 1/139 (20060101); G02F
1/1337 (20060101) |
Field of
Search: |
;349/22,24,123,124,128,129,130,160,163,165,177,182,187,191
;252/299.01,299.1 ;430/20,321 ;345/50,87 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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2318422 |
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Apr 1998 |
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GB |
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95/22077 |
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Aug 1995 |
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WO |
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96/31876 |
|
Oct 1996 |
|
WO |
|
97/14990 |
|
Apr 1997 |
|
WO |
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WO 97/14990 |
|
Apr 1997 |
|
WO |
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99/00993 |
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Jan 1999 |
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WO |
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99/18474 |
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Apr 1999 |
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WO |
|
99/19767 |
|
Apr 1999 |
|
WO |
|
99/34251 |
|
Jul 1999 |
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WO |
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02/06888 |
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Jan 2002 |
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WO |
|
0206888 |
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Jan 2002 |
|
WO |
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Other References
Kim, G.H. et al., "Optical Switching of Nematic Liquid Crystal by
Means of Photoresponsive Polyimides as an Alignment Layer", Applied
Physics Letters, American Institute of Physics, New York, vol. 75,
No. 21, Nov. 22, 1999, pp. 3458-3460, (HP000875709). cited by
applicant .
Wang, Y. et al., Alignment of a Nematic Liquid Crystal Induced by
Anisotropic Photo-Oxidation of Photosensitive Polyimide Films:,
Journal of Applied Physics, Oct. 15, 1998, vol. 84, No. 8, pp.
4573-4578, (XP02180280). cited by applicant .
Juodkazis, S. et al., "Size Dependence of Rotation Frequency on
Individual Laser Trapped Liquid Crystal Droplets", Japanese Journal
of Applied Physics, Tokyo, Japan, vol. 38, No. 5A, May 1, 1999, pp.
L518-L520, (XP000890579). cited by applicant .
Kurihara, S. et al., "Optical Shutter Driven Photochemically
Anisotropic Polymer Network Containing Liquid Crystalline and
Azobenzene Molecules", Applied Physics Letters, American Institute
of Physics, New York, vol. 73, No. 2, Jul. 13, 1998, pp. 160-162
(XP000771190). cited by applicant .
Feringa, B.L. et al., "Chiroptical Switching Between Liquid
Crystalline Phases", Journal of the American Chemical Society,
Washington, D.C., vol. 117, No. 39, 1995, pp. 9929-9930,
(XP000619719). cited by applicant .
Docchio, F. et al., "Light-Induced Transmission Changes in Liquid
Crystal Displays for Adaptive Pattern Projection", IEEE
Transactions on Instrumentation and Measurement, New York, vol. 41,
No. 5, Oct. 1, 1992, pp. 629-632, (XP000323835). cited by applicant
.
European Patent Office, International Search Report for
PCT/GB01/03112, dated Oct. 16, 2001, 2 pages. cited by applicant
.
Stolowitz Ford Cowger LLP, "Listing of Related Cases", Feb. 26,
2012, 1 page. cited by applicant.
|
Primary Examiner: Glick; Edward
Assistant Examiner: Duong; Tai
Attorney, Agent or Firm: Stolowitz Ford Cowger LLP
Claims
What is claimed is:
1. A liquid crystal device comprising liquid crystal material in
contact with the surface of a substrate, said surface
.[.favouring.]. .Iadd.favoring .Iaddend.at least first and second
stable or metastable liquid crystal alignments thereat with
respective first and second different directions, and switching
means for causing the liquid crystal material to switch between
said alignments, wherein said switching means includes first energy
supplying means arranged for applying optical radiation to said
liquid crystal material, characterized in that said liquid crystal
material and said optical radiation are selected for significant
absorption of the radiation by the liquid crystal material.
2. A device according to claim 1 wherein said first energy
supplying means is arranged to provide linearly polarised light of
a polarisation direction and spectral composition selected or
selectable for effectively rendering one of the first and second
alignments less energetically .[.favourable.]. .Iadd.favorable
.Iaddend.than the other of said .[.the fire.]. .Iadd.first
.Iaddend.and second alignments.
3. A device according to claim 1 wherein said first energy
supplying means is arranged to provide linearly polarised light of
a polarisation direction and spectral composition selected or
selectable for effectively inducing a torque on .Iadd.molecules of
.Iaddend.the liquid crystal .[.molecules.]. .Iadd.material
.Iaddend.to alter the liquid crystal alignment between the first
and second alignments.
4. A device according to claim 2 wherein the liquid crystal
material contains no dichroic additive, and said spectral
composition includes an absorption band of the liquid crystal
material.
5. A device according to claim 1 wherein the liquid crystal
material comprises a dichroic additive in a liquid crystal
host.
6. A device according to claim 5 wherein .[.the said.]. .Iadd.a
.Iaddend.spectral composition .Iadd.associated with the optical
radiation .Iaddend.includes an absorption band of the dichroic
additive.
7. A device according to claim 5 wherein .[.the said.]. .Iadd.a
.Iaddend.spectral composition .Iadd.associated with the optical
radiation .Iaddend.includes an absorption band of the liquid
crystal host.
8. A device according to claim 2 wherein said switching means
further comprises second energy supplying means to assist in the
switching of the liquid crystal material between said
alignments.
9. A device according to claim 8 wherein the second energy
supplying means is arranged to apply energy to the liquid crystal
material to destabilise the existing liquid crystal alignment.
10. A device according to claim 8 wherein the energy from said
second energy supplying means induces a homeotropic alignment at
the surface of the substrate.
11. A device according to claim 8 wherein the energy from said
second energy supplying means induces a planar alignment at the
surface of the substrate.
12. A device according to claim 8 wherein the said second energy is
provided by an electric field.
13. A device according to claim 8 wherein said second energy
supplying means is arranged to promote said switching of the liquid
crystal material between said alignments, but is insufficient of
itself to cause said switching.
14. A device according to claim 1 and further comprising second
energy supplying means to assist in the switching of the liquid
crystal material between said alignments.
15. A device according to claim 14 wherein said second energy
supplying means is arranged to determine which of said alignments
is adopted.
16. A device according to claim 15 wherein said first energy
supplying means cooperates with the device to produce heat by light
absorption, to cause said switching in cooperation with said second
energy supplying means.
17. A device according to claim 15 wherein the energy from said
second energy supplying means .[.favours.]. .Iadd.favors .Iaddend.a
homeotropic alignment of the .[.LC.]. .Iadd.liquid crystal
.Iaddend.material at the substrate.
18. A device according to claim 15 wherein the energy from said
second energy supplying means induces a planar alignment of the
.[.LC.]. .Iadd.liquid crystal .Iaddend.material at the
substrate.
19. A device according to claim 14 wherein the said second energy
is provided by an electric field.
20. A device according to claim 1 wherein the liquid crystal
material comprises an oligomer for reducing the energy between the
first and second alignments.
21. A device according to claim 1 wherein the first energy
supplying means is arranged for local irradiation of the
device.
22. A device according to claim 1 wherein the first alignment is
planar.
23. A device according to claim 1 wherein the second alignment is
planar.
24. A device according to claim 1 wherein the second alignment is
homeotropic.
25. A device according to claim 1 wherein the .[.alignment is
comprised of.]. .Iadd.surface comprises .Iaddend.a grating
structure.
26. A display comprising a device according to claim 1.
27. An optical system or display comprising a plurality of optical
devices, at least one said device being a device according to claim
1.
28. An optical system comprising a plurality of said devices each
according to claim 1.
29. A system according to claim 27 wherein said plurality .Iadd.of
optical devices .Iaddend.is tiled in a common plane.
30. A method .[.of controlling the alignment of liquid crystal
material in contact with a substrate surface which favours at least
first and second stable or metastable liquid crystal alignments
thereat with respective first and second different directions,
including the step of.]..Iadd., comprising of: .Iaddend. optically
irradiating .[.said.]. .Iadd.a liquid crystal .Iaddend.material
with radiation, .Iadd.wherein .Iaddend.a significant portion of
said radiation .Iadd.is .Iaddend.absorbed by said liquid crystal
material.Iadd.; and controlling, based at least in part on said
optically irradiating, an alignment of said liquid crystal material
in contact with a substrate surface which favors both a first
stable or metastable alignment associated with a first direction
and a second stable or metastable alignment associated with a
second direction, wherein the second direction is different than
the first direction.Iaddend..
31. The method according to claim 30 wherein the step of optically
irradiating provides radiation selected for significant absorption
by the liquid crystal material.
32. The method according to claim 30 wherein said step of optically
irradiating includes the provision of linearly polarised light.
33. The method according to claim 32 wherein said linearly
polarised light is .[.such as to effectively.]. .Iadd.configured to
.Iaddend.exert a torque on .Iadd.molecules of .Iaddend.the liquid
crystal .[.molecules.]. .Iadd.material .Iaddend.or to effectively
rotate the .[.liquid crystal.]. molecules.
34. The method according to claim .[.33.]. .Iadd.30
.Iaddend.wherein said step of optically irradiating includes the
provision of unpolarised light.
35. The method according to claim 34 wherein the unpolarised light
produces heating in the liquid crystal material.
36. The method according to claim 30 including .[.the additional
step of.]. providing a further energy input .[.to the device.]. for
controlling the alignment.
37. The method according to claim 36 wherein the further energy
input is an electric field.
38. The method according to claim 36 wherein the optical
irradiation continues after the further energy input has
ceased.
39. The method according to claim 36 wherein the further energy
input continues after the optical irradiation has ceased.
40. The method according to claim 36 wherein the further energy
input is applied locally.
41. The method according to claim 30 wherein the optical
irradiation is applied locally.
42. The method according to claim 30 wherein one .[.said
alignment.]. .Iadd.of the liquid crystal alignments .Iaddend.is
planar.
43. The method according to claim 42 wherein another .[.said
alignment.]. .Iadd.of the liquid crystal alignments .Iaddend.is
planar.
44. The method according to claim 42 wherein another .[.said
alignment.]. .Iadd.of the liquid crystal alignments .Iaddend.is
homeotropic.
45. The method according to claim 30 including the step of
providing an oligomer in .[.the.]. .Iadd.a .Iaddend.liquid crystal
phase of the material.
46. The method according to claim 30 wherein said substrate surface
is provided as a grating structure.
.Iadd.47. A device, comprising: a substrate configured to promote a
phase alignment in a first direction and a second direction; and a
liquid crystal material located adjacent the substrate, wherein
different portions of the liquid crystal material are configured to
be coincidentally aligned with the first and second directions of
alignment when localized areas of the device are optically
addressed based, at least in part, on a significant absorption of
optical radiation by the liquid crystal material. .Iaddend.
.Iadd.48. The device of claim 47, wherein the first direction of
alignment is a favored alignment of the substrate, and wherein
metastable portions of the liquid crystal material that correspond
to the localized areas of the device are configured to be aligned
with the favored alignment when the device is optically addressed.
.Iaddend.
.Iadd.49. The device of claim 48, wherein the favored alignment is
more stable than a state of the metastable portions. .Iaddend.
.Iadd.50. The device of claim 47, wherein the localized areas of
the device are both optically addressed and electrically addressed.
.Iaddend.
.Iadd.51. The device of claim 47, wherein an electric field is
configured to be applied universally to the device while the
localized areas of the device are optically addressed.
.Iaddend.
.Iadd.52. The device of claim 47, wherein energy states associated
with the first and second directions of alignment are equal.
.Iaddend.
.Iadd.53. The device of claim 52, further comprising another
substrate, wherein the liquid crystal material is located between
the two substrates, and wherein a direction of alignment of the
liquid crystal material at the other substrate is midway between
the first and second directions of alignment. .Iaddend.
.Iadd.54. The device of claim 47, wherein the different portions of
the liquid crystal material are configured to switch between the
first and second directions of alignment whereas other portions of
the liquid crystal material do not switch. .Iaddend.
.Iadd.55. The device of claim 54, wherein the other portions of the
liquid crystal material are not optically addressed. .Iaddend.
.Iadd.56. The device of claim 54, wherein at least some of the
different portions are aligned in the first direction and at least
some of the other portions are aligned in the second direction.
.Iaddend.
.Iadd.57. A method, comprising: optically addressing a liquid
crystal material located adjacent a substrate, wherein the
substrate is configured to promote a phase alignment in both a
first direction of alignment and a second direction of alignment;
and switching different portions of the liquid crystal material
coincidentally between the first direction of alignment and the
second direction of alignment based, at least in part, on a
significant absorption of optical radiation by the liquid crystal
material. .Iaddend.
.Iadd.58. The method of claim 57, wherein the different portions of
the liquid crystal material are configured to switch according to a
localized absorption of the optical radiation in the liquid crystal
material. .Iaddend.
.Iadd.59. The method of claim 57, wherein the different portions of
the liquid crystal material are selectively addressed by the
optical radiation. .Iaddend.
.Iadd.60. The method of claim 59, wherein certain portions of the
liquid crystal material are not optically addressed. .Iaddend.
.Iadd.61. The method of claim 60, wherein a first portion of the
liquid crystal material is aligned differently than a second
portion of the liquid crystal material after the liquid crystal
material is optically addressed. .Iaddend.
.Iadd.62. The method of claim 57, further comprising supplying a
secondary source of energy to the liquid crystal material to assist
said switching. .Iaddend.
.Iadd.63. The method of claim 62, wherein the secondary source of
energy is provided by an electric field. .Iaddend.
Description
The present application is a U.S. National Phase (371 application)
of PCT/GB01/03112, filed 11 Jul. 2001.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to liquid crystal devices, and has
particular but not exclusive relevance to spatial light
modulators.
2. Discussion of Prior Art
It is known to incorporate anisotropic dichroic materials, or dyes,
in a liquid crystal host material, and/or to provide liquid crystal
material in which at least one liquid crystal component has a
significant dichroism. In one type of device, known as a
"guest-host" device, guest dye molecules align co-operatively with
the molecules of a host liquid crystal material. Alteration of the
liquid crystal alignment, for example by application of an electric
field, causes the dye molecules to re-orientate, thereby altering
the optical properties of the device, and in particular the
absorption or colour.
However, it is also possible to manipulate the molecules, for
example by exposing the dye molecules to linearly polarised light
within the absorption band of the dye, so as to affect the liquid
crystal alignment. The polarised light tends to produce an
effective re-orientation of the dye molecules, and this in turn
produces a small torque on the liquid crystal host material,
pushing its director away from the polarisation direction of the
incident light. Where the liquid crystal molecules absorb the
polarised light directly, a similar effect can occur, but without
the intermediation of the dye.
It is also known that the alignment of a liquid crystal phase
adjacent the surface of a substrate is dependent inter alia on the
properties of the substrate surface. Although a substrate surface
is often treated, for example by rubbing a polymer film or by
vapour deposition, so as to induce a single energetically preferred
type of alignment at the surface, it is known that it is possible
to treat a substrate surface so that there is more than one
energetically favourable alignment thereat. The favoured alignments
will be separated by intervening alignments which are less favoured
energetically, i.e. there is an energy barrier between the favoured
alignments. The favoured alignments may or may not be energetically
equal, or equally favourable relative to the intervening
alignments, and there is a degree of control of the height of the
energy barrier as will be exemplified later. In certain cases,
particularly where a first alignment is more energetically stable
than a second alignment, the energy barrier may be sufficiently low
relative to the second alignment that relaxation to the first
alignment may occur in response to ambient or supplied thermal
energy, for example, so that the second alignment is considered to
be metastable.
In most cases at least, the surface will receive treatments
corresponding to each stable alignment. The treatments may use the
same type of treatment for each alignment, for example two
differently aligned gratings or oblique vapour deposition from two
different angles, or different types of treatment may be used for
different ones of the alignments, for example a surface profile
such as a grating for a planar alignment and a coating of material
to induce homeotropic
Thus, one way of providing at least two favoured alignments on a
substrate surface is to provide a suitable surface relief pattern,
such as one defined by two sets of parallel lines (gratings)
extending in different directions over a common area. Depending on
depth, the resulting pattern may vary front an array of isolated
pillars on a flat surface to a smoothly varying "eggbox" pattern. A
typical spacing for each set of parallel lines is around one
micron, and for two favoured alignments the two sets of parallel
lines could intersect at 90.degree. or a lower angle. The two
azimuthal directions of the preferred liquid crystal phase
alignments thereat would be expected to differ by a corresponding
angle. This type of arrangement is known as "azimuthally bistable"
and further details thereof will be found in our UK Patent No.
0744041.
In an alternative arrangement, the substrate comprises a surface
relief pattern such as a one dimensional sinusoidal grating, which
is designed to give stability to a planar alignment with a
predetermined direction. Typically the surface corrugations of the
grating are 1 micron or less in height and are formed using
photolithography or embossing. Provided over the grating is a layer
for inducing homeotropic alignment of the adjacent liquid crystal
material.
The homeotropic alignment layer and the surface relief pattern
compete in defining the bulk alignment of the adjacent liquid
crystal. The alignment tends to be homeotropic or planar depending
on whether the depth of the grating is respectively far less or far
greater than its pitch, but by tailoring the depth to pitch ratio
to lie between these two extremes a bistable region is entered
where either alignment has a degree of stability (corresponding to
respective energy minima as the alignment direction is altered, as
indicated schematically in FIG. 1 as a plot of energy against
liquid crystal tilt angle (shown by way of example only with
homeotropic alignment energy minima lying either side of a planar
alignment energy minimum). The two alignment directions may have
the same azimuthal direction (i.e. lie in the same azimuthal
plane), or they may not. Generally, the energy minima for the two
types of alignment may or may not be equal, and in the latter case,
either alignment may have the lower energy minimum corresponding to
the more preferred alignment--this will, in part, be determined by
the depth to pitch ratio of the surface relief pattern.
While the exact profile of the surface relief pattern appears to be
relatively unimportant in achieving bistability in tilt angle, it
may contribute to the energy barrier between the two preferred
alignments and/or to the associated energy minimum for the planar
alignment. It should be noted that FIG. 1 is an example of the more
general case where two stable orientations, not necessarily planar
and homeotropic, have different values of "tilt" or "zenithal
angle", and that the invention is not limited to the situation
shown in FIG. 1, but extends to this more general case.
In this arrangement, the "planar" alignment may sometimes involve a
relatively high tilt angle, and is sometime referred to as the
defect state, because it is also characterised by a pair of line
defects in the liquid crystal or nematic director. Arrangements
having stable planar and homeotropic states in which the liquid
crystal director lies in the same azimuthal plane are known as
"zenithally bistable arrangements", and further details thereof
will be found for example in our UK Patent No. 2318422, and our
published patent application PCT/GB98/03787 (WO 9934251).
It should be understood that other bistable surfaces will possess
at least two preferred alignment directions which differ in both
zenithal and azimuthal angle. For example, silicon oxide could be
deposited obliquely from two different directions which also differ
in both zenithal and azimuthal angle.
Although surface profiles in the form of gratings, formed for
example by photolithographic techniques, have been specifically
mentioned above, any other method of providing a surface profile
could be used, such as by oblique evaporation.
Furthermore, it should be noted that although the two bistable
arrangements specifically described above both involve a surface
profile, the latter is not a necessary requirement for providing a
plurality of stable alignments at a substrate. Any arrangement
where more than one alignment is energetically favourable, whether
imposed by use the same technique adapted for each preferred
direction as in the example of crossed gratings above, or by the
use of different techniques for each preferred direction as in the
case of the grating and homeotropic surface treatment exemplified
above, may be used.
In addition, it is not an absolute requirement that each part of
the substrate surface is adapted to favour both alignments. For
example, each two commingled sets of suitably dimensioned and/or
shaped areas of a substrate may be treated to produce the different
alignments, whereupon it may be arranged that either of the
alignments, once adopted and favoured by one set of areas, will
prevail over the alignment favoured by the other set of areas. Thus
two interleaved sets of stripes may be treated differently to
produce a favoured planar alignment for one set and either a
favoured differently directed second planar alignment or a
homeotropic alignment for the other set, using any of the
techniques mentioned above, such as surface treatment, coating,
and/or profiling.
By careful choice of the liquid crystal material, including the
provision of appropriate additive(s) to the liquid crystal
material, and as indicated above by suitably selecting and/or
treating the substrate surface in known ways, it is possible to
control the energy barrier(s) between the two preferred alignment
states. Also, in an assembled device comprising a layer of liquid
crystal material between the one bistable (or polystable) substrate
and a further substrate, the alignment imposed at the surface of
the other substrate may modify the energies of the stable states at
the one substrate.
SUMMARY OF THE INVENTION
A relatively recent development in modifying the interaction
between a liquid crystal phase and a surface goes under the name of
"slippery surfaces", as described for example in International
Patent Application PCT/GB98/03011 (WO 9918474) (Hewlett-Packard).
An additive in the liquid crystal material, commonly an oligomer,
provides large molecules which tend to concentrate near the
substrate surface. The resulting reverse directed concentration
gradient of the liquid crystal phase away from the substrate(s) is
believed to reduce the interaction between the substrate and/or any
alignment layer thereon and the liquid crystal material, and
accordingly reduces the energy required to alter the alignment of
the adjacent liquid crystal material from that induced by the
substrate and/or any alignment layer thereon. In fact, at certain
concentrations, such an additive will permit a planar aligned
surface to have complete degeneracy with regard to its alignment
direction. The presence of such additives can facilitate
performance of the present invention by reducing the energy barrier
between two or more different favoured alignments induced by a
substrate surface, e.g. as described above.
The invention provides a liquid crystal device comprising liquid
crystal material in contact with the surface of a substrate, said
surface favouring at least first and second stable or metastable
liquid crystal alignments thereat with respective first and second
different directions, and switching means for causing the liquid
crystal material to switch between said alignments, wherein said
switching means includes irradiating means for illuminating said
device. The invention extends to a display comprising such a
device, and to a display or optical system comprising a plurality
of such devices, for example tiled in a common plane.
The invention also provides a method of controlling the alignment
of liquid crystal material in contact with a substrate surface
which favours at least first and second stable or metastable liquid
crystal alignments thereat with respective first and second
different directions, including the step of optically irradiating
said device.
The light from the irradiating means provides the or a first energy
input to the device, and may act directly or indirectly on the
liquid crystal material. Thus in some embodiments of the invention
an unpolarised light beam alone might provide sufficient thermal
energy to convert from a metastable alignment as described above to
a more stable alignment; or the thermal energy may be sufficient to
destroy the existing alignment, or the liquid crystal phase, so
that a second alignment may be preferentially adopted under a
directional influence of some other energy input (such as an
electric or magnetic field) as described in greater detail
below.
In other embodiments of the invention, this light may be linearly
polarised to impose an effective torque on the liquid crystal
molecules either directly or indirectly, or to render one of the
first and second alignments more energetically favourable relative
to the other of the first and second alignments (this may be
regarded as effective rotation), as is known in the art.
For example, a material which comprises a double bond link such as
an azo compound, a stilbene or a Schiff base will normally have a
low energy trans isomer and a higher energy cis isomer. The double
bond will absorb light at wavelengths in or close to the visible
range, but preferentially for light polarised in one direction
relative to the double bond, and the material is positively
dichroic. In the excited state the molecule can undergo a series of
changes resulting in conversion to the cis isomer. Consequential
relaxation to the energetically favourable trans isomer can lead to
a molecular alignment similar to the initial alignment, or to an
alignment which is effectively rotated relative to the initial
alignment. Under isotropic conditions, there is nothing to
distinguish the initial and rotated alignments, but with polarised
illumination one of the alignments of the trans isomer
preferentially absorbs light, eventually leading to the majority of
molecules ending up with a trans orientation that minimises the
absorption of the incident polarised light.
A typical sequence of events is illustrated schematically in FIG.
2, in which (a) shows an azo molecule in its original alignment in
the energetically favoured trans state, and (b) shows the higher
energy cis state arising from absorption of polarised light hyl.
Via any of a number of mechanisms including thermal and radiative
mechanisms (hv.sub.2) the molecule (b) may revert to the original
state (a) or may proceed to a trans state in which the direction of
the long molecular axis has effectively rotated (although as shown
this is not a true rotation in the plane of the paper, the reader
will appreciate that rotation of the molecule about the long axis
is energetically relatively easy, and whether or not this occurs is
in any case irrelevant in relation to the liquid crystal alignment
to be induced).
For example, where the spectral composition of the irradiating
light includes an absorption band of the liquid crystal material,
it may act directly on the liquid crystal material. The latter
therefore need contain no dichroic additive for enabling the change
of liquid crystal alignment Nevertheless, a dichroic additive
having an absorption spectrum lying outside the spectrum of the
irradiating light may be provided for other purposes, for example
for exhibiting a desired optical change as in guest-host variable
absorption/colour devices.
Alternatively, the liquid crystal material may be in the form of a
dichroic additive in a liquid crystal host, wherein the said
spectral composition of the irradiating light includes an
absorption band of the dichroic additive. In this case the dichroic
additive responds directly to the irradiation, and in turn exerts
an effective torque on the liquid crystal molecules.
Although reference has been made to positively dichroic material,
which tends to cause the liquid crystal material to align
transversely to the optical polarisation direction, it is within
the ambit of the invention to use negatively dichroic materials,
which tend to rotate the liquid crystal material so that it lies
parallel to the polarisation direction.
The direct or indirect torque induced by the polarised light may of
itself be sufficient to overcome the energy barrier between the
favoured alignment. Where this is not the case, it is possible
either to modify the interaction between the liquid crystal
material and the substrate to reduce the energy barrier so that a
light energy input is sufficient, or to assist the realignment by
inputting a second form of energy.
With respect to the first option, the preceding description has
already briefly discussed two ways of reducing the energy barrier
between alignments, viz. by modifying the surface geometry, and by
incorporating an additive to provide a "slippery surface", and
either or both of these may be utilised in the performance of the
invention.
With respect to the second option, realignment may be assisted by
effectively putting energy into the liquid crystal material over
and above that supplied by the light from the irradiating means. In
the case of a liquid crystal host-guest additive material, the
co-operative alignment means that energy may be supplied either to
the host or to the guest material regardless of which of these is
being energised by the polarised light.
The second energy may be sufficient per se to alter the alignment
of the liquid crystal material, although the final alignment may
still depend on the optical input from the irradiating means (see
case A two paragraphs below), or realignment may require the
optical input from the irradiating means working in combination
with the second energy input (see case B two paragraphs below).
Many ways are known for energising a liquid crystal material to
alter its alignment, including the application of an electric or
magnetic field, passage of an electric current (ion transport), and
acoustic irradiation.
Of these perhaps the application of an electric field is probably
the most easily accomplished, and is often the most effective. For
example, where the liquid crystal alignment is to be altered
between first and second homogeneous (parallel) states, and has
positive dielectric anisotropy, application of an electric field
across the thickness of a layer in the first state to induce a
homeotropic (perpendicular) alignment will then enable the liquid
crystal material preferentially to assume the second state on
cessation of the field under the influence of light of an
appropriately directed linear polarisation (case A). Indeed, even
if the magnitude of the field (or the magnitude of the effect of
any other energy supplying means) is insufficient of itself to
produce an alternative alignment, the energy of the original
preferred alignment may be raised sufficiently to assist in
realignment of the liquid crystal material to the second alignment
when polarised light is applied (case B).
When the layer is sandwiched between two substrates, each substrate
may carry a continuous electrode for applying an electric field
across the thickness of the layer. Either or both continuous
electrodes may be replaced by an interdigitated electrode for the
same purpose, particularly if both parts of the interdigitated
electrode are energised in the same way.
An interdigitated electrode may also be used to apply a field in
the plane of the layer. For example, an interdigitated electrode on
the substrate having two favoured alignment directions may be
employed to alter a planar alignment of a positive dielectric
anisotropy liquid crystal material so that it lies at or close to a
maximum energy state between the favoured alignments. Removal of
the field in the presence of linearly polarised light of an
appropriate polarisation direction and intensity will induce the
liquid crystal to relax to the favoured alignment as determined by
the polarisation direction.
In many cases, the polarisation direction of the irradiating light
may alone determine the resulting liquid crystal alignment,
particularly where no other energy input is provided. This also
applies where a second energy input is provided but has no
directional property capable of affecting the resulting
alignment.
However, in certain other cases, the second energy input is
directional and can affect the resulting alignment, for example an
electric field. Where this is not desired, the second energy input
should normally cease before the irradiating light is turned
off.
Alternatively, maintaining the second energy input after cessation
of the irradiating light may lead to determination of the resulting
alignment by the second energy input, if this is desired.
Furthermore, where the irradiating light is unpolarised, it cannot
per se determine the resulting alignment. In such a case, it will
be the second energy input which will determine the resulting
alignment, with the irradiating light being relegated to performing
the lesser but necessary function of assisting in the realignment,
for example by heating the liquid crystal material either directly
or indirectly, as by optical absorption by the liquid crystal
material, or a component thereof, or by another component of a
liquid crystal cell.
In many cases the same type of method, including providing an input
from the irradiating means, will be used for switching between the
preferred alignments in both directions. However, in other cases
different methods will be used for the different switching
directions, provided that at least one switching direction involves
irradiation from the irradiating means.
This may be particularly so where the energy minima for the
preferred alignments are not equal, and including the case where
one alignment is metastable, so that the energy barrier depends on
switching direction. For example switching in one direction, from
the less stable or metastable alignment, may require only
unpolarised or polarised light from the irradiating means.
Switching in the reverse direction may require a second energy
input, such as an electric field, used alone or in conjunction with
appropriate light from the irradiating means. Alternatively,
switching in the one direction could be effected by a second energy
input, with reversal requiring a combination of the second energy
input and suitable light from the irradiating means.
The light from the irradiating means may be applied locally so as
to produce different alignments across the area of the device.
Where a second energy input is also required, such as an electric
field, this may also be applied locally so that only a restricted
area of the device may be written at any time. In this case, one of
the irradiation and second energy input may be applied universally
and the other may be applied locally, so that the local input
determines which areas are switched. Alternatively both inputs may
be applied locally, possibly with different distributions so that
only areas where both inputs occur are switched, i.e. an AND
logical function. Other logical arrangements may occur to the
skilled person.
The ability to alter selected areas of the device means that it can
be used for a number of purposes, including displays and optical
data processing. For example, our copending International Patent
Application No. PCT/GB98/01866 (WO 9900993) describes an
autostereoscopic display in which an assembly of displays is tiled,
and International Patent Application No. PCT/GB98/03097 (WO
9919767) shows an assembly of individual displays for holographic
purposes.
A device is described by Kim et al in "Optical Switching of Nematic
Liquid Crystal by Means of Photosensitive Polyimides as an
Alignment Layer" Applied Physics Letters, 29 Nov. 1999, pp 3458-60.
In this device the threshold voltage for switching between two
liquid crystal alignments is influenced by irradiation of the
polyimide layer, and it is suggested that the latter undergoes
"photophysical changes". It is possible to switch the liquid
crystal by varying the radiation in the presence of a constant
voltage. However, the alignment layer itself is not described as
favouring more than one alignment direction. Moreover as described
this device requires irradiation of the alignment layer to change
its properties, whereas in embodiments of the present invention the
spectral composition of the light is selected with respect to the
liquid crystal material so as to exert a torque thereon or to alter
its temperature for switching purposes.
Similarly, Wang et al in "Alignment of a Nematic Liquid Crystal
Induced by Anisotropic Photo-Oxidation of Photo-sensitive Polyimide
Films" in Applied Physics Letters, 15 Oct. 1998, pages 4573-8,
describe a process in which the alignment properties of a polyimide
film can be altered by irradiation with linearly polarised laser
light. Again, however, there is no suggestion that the layer is
ever capable of supporting either of two alignments at the same
time.
BRIEF DESCRIPTION OF THE DRAWINGS
Other features and advantages of the invention will become apparent
upon consideration of the appended claims, to which the reader is
referred, and upon a reading of the following description of
exemplary embodiments of devices according to the invention, made
with reference to the accompanying drawings in which:
FIG. 1 schematically illustrates the variation of energy with
zenithal angle of liquid crystal material on a zenithally bistable
surface;
FIG. 2 schematically illustrates the effect of light on an azo
compound;
FIGS. 3(a) to 3(d) schematically illustrate operation of Embodiment
2 below, which is an example using an azimuthally bistable
surface;
FIGS. 4 and 5 are voltage-time graphs illustrating the response of
the cell of Embodiment 2;
FIGS. 6 and 7 are voltage-time graphs further illustrating the
response of the cell of Embodiment 2, and the need for the electric
field to be applied in this embodiment;
FIG. 8 schematically illustrates a zenithally "bistable" surface
with a modified energy barrier relative to that of FIG. 1 so that
it is effectively monostable;
FIGS. 9(a) to 9(c) illustrate the operation of a liquid crystal
cell incorporating the surface of FIG. 8;
FIG. 10 is generally similar to FIG. 1, and is used to illustrate
conjoint electrical and optical addressing; and
FIG. 11 illustrates an embodiment of the current invention in which
a plurality of cells are employed in a common plane.
DETAILED DISCUSSION OF EMBODIMENTS
Embodiment 1
A layer of liquid crystal material containing a dichroic additive
is sandwiched between first and second substrates. The first
substrate has a surface profile for azimuthal bistable surface
alignment (two stable, or favoured, planar alignments having
different azimuthal directions), and the second surface comprises a
non-rubbed polymer surface of the type which, if rubbed, would
promote planar alignment. The liquid crystal material contains a
slippery surface additive of a sufficient concentration to allow
degeneracy of (planar) alignment at the second substrate surface
but insufficient to destroy the bistability of alignment at the
first substrate surface. This construction means that either of the
two bistable alignments at the first substrate surface generates a
respective uniform planar texture across the liquid crystal layer
with a different azimuthal direction.
Application of linearly polarised light of a sufficient intensity
and an appropriate polarisation direction to the liquid crystal
material suffices to convert one uniform planar texture to the
other or vice versa. The intensity of the polarised light will be
determined, inter alia, by the energy barrier between the bistable
alignments as determined by the surface geometry of the first
substrate, and the concentration of slippery surface additive,
amongst other factors.
Embodiment 2
This is similar to embodiment 1, but (a) the energy barrier between
the bistable alignments is of a magnitude such that the polarised
light alone is insufficient to cause conversion between the two
planar textures (or if a source of polarised light of sufficient
light intensity is available to cause conversion it would be
detrimental to the cell), and (b) each substrate is provided with a
continuous electrode for applying a field across the liquid crystal
layer thickness. When a field is applied of sufficient intensity to
induce a homeotropic alignment in the layer and is then removed in
the presence of linearly polarised light, the alignment of the
liquid crystal material at the first substrate relaxes to the
preferred alignment determined by the polarisation direction.
FIGS. 3(a) to 3(d) schematically illustrate operation of this
embodiment, for a cell comprising a liquid crystal layer 3 between
an azimuthally bistable first electrode providing substrate 1 and a
(non-aligning) second electrode providing substrate 2. FIGS. 3(a)
to 3 (c) cover a period when linearly polarised illumination L is
being applied (effectively to induce a torque on the liquid crystal
molecules). Within that period, FIG. 3(b), an electric field is
applied across the cell by applying a potential difference V
between the electrodes of the substrates, to produce a homeotropic
orientation. On removal of the field the alignment at the substrate
1 relaxes to that determined by the polarisation direction of the
illumination L, FIG. 3(c). The liquid crystal orientation at the
bistable substrate is transferred through the bulk liquid crystal
layer to the other substrate, as permitted by the slippery surface
effect thereat, FIG. 3(d).
This embodiment has the advantage that the device can only be
updated when an electric field is applied, and is otherwise stable.
It is possible to arrange that the electric field per se is
sufficient to produce the homeotropic orientation for subsequent
relaxation to a state determined by application of polarised light.
By selectively illuminating different areas with different optical
polarisations, it is possible to write an image.
However, it is also possible to arrange that only those portions of
the liquid crystal which are addressed both optically and
electrically are brought to the homeotropic state, in which case
selective spatial optical addressing of the cell enables only
selected areas of the cell to be written and latched, giving rise
to the possibility of writing a complex image with a single
electrode pair, e.g. for high density images and image or data
storage.
In an experiment, an azimuthally bistable grating surface was
prepared by coating a clean glass substrate with etched indium tin
oxide. A photoresist Microposit S1805 as supplied by Shipley Europe
Limited was spin coated onto this coating giving a layer nominally
0.5 microns thick. A 1 micron pitch binary grating mask was then
placed in contact with the photoresist and exposed using a
broadband UV light source (365 nm, 404 nm and 435 nm at about 150
mJ/cm.sup.2). The grating was then rotated by 90.degree. and the
exposure repeated. Development of the exposed photoresist according
to specification was followed by a soft bake in UV light for 15
minutes (254 nm light at about 9 mW/cm.sup.2) and a hard bake at
180.degree. C. for 2 hours. A cell was constructed using the above
substrate and a similarly processed substrate in which the
photoresist, however, was not exposed to create a grating. The gap
between the substrates was nominally 5 microns and was filled using
a standard mixture E63 from Merck, which has a nematic phase at
room temperature. To this mixture was added 2 weight percent of a
dichroic dye D2, also from Merck, and 2 weight percent of an
oligomer 3M Flourad FC430 as supplied from 3M Belgium N.V. The dye
has peak absorption at 487 nm, dropping to zero at close to 600
nm.
A 20 mW argon ion laser (448 nm wavelength) displaying a 1 mm.sup.2
Gaussian beam profile at the cell was used to address the cell via
a rotatable half-wave plate to control its polarisation direction.
The optical response of the cell was monitored using a linearly
polarised beam from a 5 mW HeNe laser (663 nm wavelength) plus
linear polariser focussed to a spot size at the cell smaller than
that of the argon ion laser. The transmitted intensity of the 633
nm light was detected by a photodiode fitted with a linear analyser
and a notch filter to exclude virtually all visible wavelengths
other than the HeNe light. The device was addressed with bipolar
electrical pulses to switch the cell into a homeotropic state.
Bistable liquid crystal cell orientations at 90.degree. to each
other were set vertical and horizontal while the polariser and
analyser were set at plus and minus 45.degree. respectively. Since
the stable states have optically equivalent transmissions between
crossed polarisers a quarter wave plate was incorporated after the
cell to distinguish the two states.
Switching of the device is illustrated in FIGS. 4 and 5, showing
transmission T as a function of time t over a period of around 50
seconds. During application of 488 nm illumination of either
vertical (FIG. 4) or horizontal (FIG. 5) linear polarisation, an 80
mV 10 msec bipolar electric pulse VP was also applied, which
triggered data collection by the oscilloscope. Subsequent removal
of the illumination around 5 seconds later at a point T1 allowed
the liquid crystal to relax (over possibly hundreds of
milliseconds) to a final orientation, and as can be seen by a
comparison of the final parts of FIGS. 4 and 5, this can be
selected by selection of the polarisation direction of the 488 nm
illumination.
A further insight into the behaviour of the cell may be gained from
a consideration of FIGS. 6 and 7, showing transmission T as a
function of time t over a period of around 200 seconds. In FIG. 6
the cell is taken through the following series of steps: 11.
Horizontal polarised light ON. 12. Voltage pulse applied. 13.
Horizontal polarised light OFF. 14. Horizontal polarised light ON.
15. Voltage pulse applied. 16. Horizontal polarised light OFF. 17.
Vertical polarised light ON. 18. Voltage pulse applied. 19.
Vertical polarised light OFF. 20. Vertical polarised light ON. 21.
Voltage pulse applied. 22. Vertical polarised light OFF.
This shows that there is no net effect of repeating the addressing
cycle, viz. steps 4 to 6 and 10 to 12. However, when an addressing
cycle specifying a new orientation is carried out, as in steps 17
to 19, the liquid crystal alignment changes to a second stable
state (step 18).
FIG. 7 illustrates the need for the voltage pulse and shows the
following sequence of events: 31. Horizontal polarised light ON.
32. Voltage pulse applied. 33. Horizontal polarised light OFF. 34.
Vertical polarised light ON. 35. Vertical polarised light OFF. 36.
Vertical polarised light ON. 37. Voltage pulse applied. 38.
Vertical polarised light OFF.
Steps 31 to 33 with horizontally polarised illumination are not
intended to affect the state of the cell but to ensure that it is
in the state induced by horizontally polarised illumination. Steps
34 and 35 with vertically polarised illumination in the absence of
an applied voltage likewise do not alter the state of the cell.
However, the insertion of a voltage pulse 37 during illumination
with vertically polarised light, steps 36 to 38, triggers a change
of state and on removal of the illumination at step 38 the liquid
crystal alignment is altered.
Embodiment 3
A layer of liquid crystal material containing a dichroic additive
is sandwiched between first and second substrates, the first
substrate having a surface profile providing an azimuthal bistable
surface alignment, and the second surface comprising a polymer
surface which has been rubbed to promote planar alignment thereat
parallel to one of the preferred alignments at the first substrate.
The planar alignment at the second substrate persists, so that in
one state of the device there is a uniform planar texture across
the liquid crystal layer, and in the other state the liquid crystal
is twisted. The liquid crystal material also contains a chiral
additive to render the energies of the uniform and twisted states
generally (and preferably substantially) equal, so as to facilitate
switching therebetween on application of light of the appropriate
linear polarisation direction and intensity.
Optionally, the liquid crystal material may include a slippery
surface additive as in embodiments 1 and 2, but not at such a high
concentration that the planar alignment at the second surface is
rendered ineffective, and/or the device may include electrodes for
applying a field across the layer thickness as in embodiment 2.
Embodiment 4
A layer of liquid crystal material containing a dichroic additive
is sandwiched between first and second substrates, the first
substrate having a surface profile providing an azimuthal bistable
surface alignment, and the second surface comprising a polymer
surface which has been rubbed to promote planar alignment thereat
between the preferred alignments at the first substrate. The
construction is such that the alignment twists in one direction or
the other on passing across the layer thickness, depending on the
alignment adopted at the first substrate.
Preferably the energies of the two states of the device are
substantially equal, although this depends on the energy relation
between the favoured alignments at the first substrate. It is
preferred that the alignment at the second substrate is
substantially parallel to the direction corresponding to the energy
maximum between the favoured alignments at the first substrate.
Where the energies of the two favoured alignments at the first
substrate are equal, this direction may be midway between the
favoured alignments. Where the energies of the two favoured
alignments at the first substrate are unequal, this direction may
or may not be midway between the favoured alignments. However, the
alignment at the second substrate can deviate from the ideal
position provided that the intensity of the polarised light (and
any other energy input) is sufficient for realignment.
Optionally, the liquid crystal material may include a slippery
surface additive as in embodiments 1 and 2, but not at such a high
concentration that the planar alignment at the second surface is
rendered ineffective, and/or the device may include electrodes for
applying a field across the layer thickness as in embodiment 2.
Embodiment 5
A layer of liquid crystal material containing a dichroic additive
is sandwiched between first and second substrates, the first
substrate having a surface profile providing an azimuthal bistable
surface alignment, and the second surface comprising a surface
which has been treated to promote homeotropic alignment thereat.
There is twist in alignment direction on passing from one substrate
to the other, regardless of the alignment direction at the first
substrate, so that the energies of the two states of the device are
generally or substantially equal (depending on the energy relation
between the alignments at the first substrate).
Optionally, the liquid crystal material may include a slippery
surface additive as in embodiments 1 and 2, but not to the extent
that the alignment at the second surface is rendered ineffective,
and/or the device may include electrodes for applying a field
across the layer thickness as in embodiment 2.
In each of embodiments 1 to 5, it is preferred that the two
favoured azimuthal alignments at the first substrate are at
90.degree. to each other.
Embodiment 6
While embodiments 1 to 5 incorporate azimuthally bistable substrate
surfaces, this incorporates a zenithally bistable alignment layer
on substrate surface 1, generally of the type already illustrated
with respect to FIG. 1, and a second substrate 2 for homeotropic
alignment. By tailoring the grating pitch to depth ratio, and in
particular by using a relatively shallow grating, although this
would depend on other parameters of the device and liquid crystal
in question, it is possible to arrive at an energy profile as shown
in FIG. 8, in which the planar state is stable but with only a low
activation energy E for switching to the homeotropic state. Care
needs to be taken that the grating pitch to depth ratio is not so
low as to render the planar state totally unstable. The liquid
crystal material incorporates a dichroic dye.
Normally the device will rest with a homeotropic alignment as in
FIG. 9(a). An electric field or voltage pulse can be used to switch
the alignment at the surface of the bistable substrate 1 to planar,
FIG. 9(b). This planar alignment tends to relax to the homeotropic
alignment of FIG. 9(a) at a rate determined inter alia by the
activation energy E, and where this is sufficiently slow, selected
areas of the device may be forced to switch faster by the
application of polarised light L, FIG. 9(c), arranged to apply a
torque to the liquid crystal material. Thus it is possible to write
a temporary image on the device, which could be repeatedly
refreshed at an appropriate rate.
In the planar state the liquid crystal molecules lie perpendicular
to the grooves. Light incident on the liquid crystal and linearly
polarised perpendicular to the grooves will be absorbed by the
dichroic dye, which will then apply a torque to the liquid crystal
molecules, pushing them away from the polarisation direction. As a
consequence the liquid crystal so illuminated will tend to fall
into the energetically favoured homeotropic orientation.
In variations of this embodiment, the surface 2 can provide a
planar orientation with a high or low pretilt, and/or it could be
another grating surface. Any variation can also be operated with
non-linear, e.g. circular polarised, illumination.
Embodiment 7
This is similar to embodiment 6, but the grating is deeper, thereby
reversing the energy levels and making the planar state more
energetically favourable. A combination of electrical and optical
addressing is used to switch selected areas to the higher energy
homeotropic state, the electrical field being below the threshold
for switching in the absence of illumination. In this case it is
the homeotropic state which is metastable and relaxes to the planar
state. Forcing the transition to the planar state requires off-axis
illumination. The planar state can be induced across the whole
liquid crystal layer using a blanking electrical pulse.
Embodiment 8
Embodiments 6 and 7 represent extreme examples of zenithally
bistable devices, and as previously mentioned the planar state
sometimes involves a high tilt angle or defect state with line
defects in the nematic director. Other grating profiles may be used
which provide more distinct energy troughs for both types of
alignment, as shown in FIG. 10, which is generally similar to FIG.
1. In such a case conjoint use of electrical and optical addressing
is preferred, that is, the electric field may be used to switch
from a (planar) state A to, say, a state B, at which point
illuminated areas may pass over to (homeotropic) trough C (or the
process may occur in the opposite direction C to A). Unilluminated
areas will relax to state A on removal of the field and
illumination.
Embodiment 9
This is a variation of Embodiment 8 in which the final nudge over
the energy hump is provided by local heating rather than
illumination. Such local heating can occur through light absorption
by a dye or other light absorptive material, e.g. in the liquid
crystal material or the grating. Non-dichroic dyes provide a
polarisation insensitive device, whereas suitably aligned dichroic
dyes, as in a liquid crystal host, render the heating effect
dependent on the polarisation of incident illumination. The
switching direction of the device is determined by the polarity of
the applied electric field, and not by a light induced torque.
FIG. 11 shows the current invention being used to form a high
resolution display. A single panel of the display comprises a
liquid crystal display according to the present invention. Each
panel comprises substrates 1, 2 as disclosed in relation to FIG. 3
above, between which is a LC material. The plurality of panels are
arranged in a common plane. Any gap between adjacent panels is
preferably kept to a minimum to minimize the appearance of such
gaps to a viewer. Optical radiation may be directed towards one
side such as substrate 1 of each panel, and this radiation will
modify the properties of the LC as described in relation to the
embodiments mentioned above. Any such modification to the
properties of the LC material may be viewed as an image across all
panels of the display.
* * * * *