U.S. patent application number 09/043078 was filed with the patent office on 2001-08-09 for bistable liquid crystal display device.
Invention is credited to BARBERI, RICARDO, DOZOV, IVAN, DURAND, GEORGES, GIOCONDO, MICHELE, LELIDIS, IOANNIS, MARTINOT-LAGARDE, PHILIPPE, NOBILI, MAURIZIO, POLOSSAT, ERIC.
Application Number | 20010012080 09/043078 |
Document ID | / |
Family ID | 26232320 |
Filed Date | 2001-08-09 |
United States Patent
Application |
20010012080 |
Kind Code |
A1 |
BARBERI, RICARDO ; et
al. |
August 9, 2001 |
BISTABLE LIQUID CRYSTAL DISPLAY DEVICE
Abstract
A display device including two parallel transparent plates (10,
12) having transparent electrodes on the inner surfaces thereof and
containing a liquid crystal material (20). The device includes
means defining a monostable anchoring for each plate (10, 12),
means (40) controllable to break at least one of the anchorings,
and means for thereafter inducing volume bistability.
Inventors: |
BARBERI, RICARDO;
(ARCAVACATA DI RENDE, IT) ; DOZOV, IVAN;
(GIF-SUR-YVETTE, FR) ; DURAND, GEORGES; (ORSAY,
FR) ; MARTINOT-LAGARDE, PHILIPPE; (MARCOUSSIS,
FR) ; NOBILI, MAURIZIO; (PESSAC, FR) ;
POLOSSAT, ERIC; (MONTPELLIER, FR) ; LELIDIS,
IOANNIS; (LAUSANNE, CH) ; GIOCONDO, MICHELE;
(ARCAVACATA, IT) |
Correspondence
Address: |
BLAKELY SOKOLOFF TAYLOR & ZAFMAN
12400 WILSHIRE BLVD
7TH FLOOR
LOS ANGELES
CA
90025
|
Family ID: |
26232320 |
Appl. No.: |
09/043078 |
Filed: |
September 14, 1998 |
PCT Filed: |
November 8, 1996 |
PCT NO: |
PCT/FR96/01771 |
Current U.S.
Class: |
349/123 |
Current CPC
Class: |
G09G 2300/0486 20130101;
G02F 1/1391 20130101 |
Class at
Publication: |
349/123 |
International
Class: |
G02F 001/1337 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 8, 1995 |
FR |
95 13201 |
Apr 10, 1996 |
FR |
96 04447 |
Claims
1. Display device comprising two parallel transparent plates (10,
12) provided with electrodes on their internal surfaces and
containing a liquid-crystal material (20), characterized in that it
comprises: means defining a monostable anchoring on each plate (10,
12); means (40) breaking, on command, at least one of these
anchorings; and means inducing, after this breaking, a bistable
volume effect.
2. Device according to claim 1, characterized in that the anchoring
breaking means (40) are suitable for breaking the anchorings on
both the plates (10, 12).
3. Device according to either of claims 1 and 2, characterized in
that the monostable anchorings are planar.
4. Device according to either of claims 1 and 2, characterized in
that the monostable anchorings are homeotropic.
5. Device according to either of claims 1 and 2, characterized in
that at least one of the monostable anchorings is oblique with
respect to the plates (10, 12).
6. Device according to either of claims 1 and 2, characterized in
that one of the anchorings is homeotropic and the other planar.
7. Device according to either of claims 1 and 2, characterized in
that one of the anchorings is planar and the other oblique.
8. Device according to either of claims 1 and 2, characterized in
that one of the anchorings is homeotropic and the other
oblique.
9. Device according to one of claims 1 to 8, characterized in that
the anchoring breaking means comprise means capable of applying an
electric field.
10. Device according to claim 9, characterized in that the
anchoring breaking means comprise means capable of applying an
electric field perpendicular to the plates (10, 12).
11. Device according to claim 10, characterized in that the
anchoring is planar and the liquid crystal possesses a positive
dielectric anisotropy.
12. Device according to claim 10, characterized in that the
anchoring is homeotropic and the liquid crystal possesses a
negative dielectric anisotropy.
13. Device according to one of claims 1 to 12, characterized in
that the anchoring breaking means are suitable for placing the
liquid crystal in an unstable situation in which the elastic energy
of interaction of the liquid-crystal surface molecules with the
surface of the plates (10, 12) is maximum.
14. Device according to one of claims 1 to 13, characterized in
that the means inducing a bistable volume effect comprise means
capable of applying a lateral electric field to the device.
15. Device according to claim 14, characterized in that the means
inducing a bistable volume effect comprise interdigitated
electrodes (50, 52) on one of the plates (10), facing the plate
(12), the anchoring of which is broken, and means capable of
applying a drive voltage to at least one of these electrodes, this
being chosen alternately.
16. Device according to claim 14, characterized in that the means
inducing a bistable volume effect comprise at least one electrode
(60) possessing at least one edge (62) which is more conducting
that its central part (64).
17. Device according to claim 14, characterized in that the means
inducing a bistable volume effect comprise electrodes along the
edge of the device.
18. Device according to one of claims 1 to 13, characterized in
that the means inducing a bistable volume effect comprise means
capable of generating a hydrodynamic effect.
19. Device according to claim 18, characterized in that the means
inducing a bistable volume effect comprise means capable of
generating a shear on the liquid-crystal molecules close to the
plate whose anchoring is broken.
20. Device according to claim 19, characterized in that the means
inducing a bistable volume effect comprise means capable of
producing a mechanical displacement of at least one part of the
plate, for example by using a piezoelectric system or by using
sound waves.
21. Device according to claim 19, characterized in that the means
inducing a bistable volume effect comprise means capable of
ensuring a mechanical transverse stress to the plates.
22. Device according to claim 19, characterized in that the means
inducing a bistable volume effect comprise means which include an
auxiliary electrode (c) placed alongside an electrode (p) defining
a pixel.
23. Device according to claim 22, characterized in that the
anchoring is oblique on the auxiliary electrode (c) and is provided
with means selectively applying a drive field to the auxiliary
electrode at the moment when the electric field on the pixel
electrode (p) is cut off or a drive field to the auxiliary
electrode at the same time as the electric field on the pixel
electrode (p).
24. Device according to one of claims 18 or 19, characterized in
that the means inducing a bistable volume effect comprise means
defining a hydrodynamic coupling between the two plates (10,
12).
25. Device according to one of claims 1 to 24, characterized in
that the means for anchoring breaking and for bistable volume
switching are suitable for homogeneously driving the entire surface
of a pixel.
26. Device according to one of claims 1 to 24, characterized in
that at least one of the means for anchoring breaking and for
bistable volume switching is suitable for driving a variable part
of a pixel.
27. Device according to claim 19 or 24, characterized in that: the
plates (10, 12) define different anchoring thresholds; the
thickness of the device between the two plates (10, 12) is
sufficiently small to allow hydrodynamic coupling between the
internal surfaces of the plates; and means (40) are provided which
apply, between the electrodes of the two plates alternately, a
write electric-field pulse above a threshold capable of breaking
the anchorings on the two plates (10, 12) in order to define, after
interruption of this electric field, a twisted first stable state
resulting from hydrodynamic coupling between the two plates (10,
12) and a second electric field, below the said threshold capable
of breaking a single anchoring or having a falling edge which
varies very slowly in order to decouple the tilts on the two
plates, so as to define a homogeneous second stable state.
28. Device according to claim 27, characterized in that the
liquid-crystal material (20) is a nematic liquid crystal.
29. Device according to claim 27, characterized in that the
liquid-crystal material (20) is a cholesteric liquid crystal.
30. Device according to one of claims 27 to 29, characterized in
that the liquid-crystal material (20) possesses a positive
dielectric anisotropy.
31. Device according to one of claims 27 to 30, characterized in
that the thickness (d) of the liquid-crystal material is less than
l/.theta..sub.S, in which expression: l denotes the extrapolation
length defining the zenithal anchoring energy; and .theta..sub.S
denotes the angle of the surface molecules.
32. Device according to one of claims 27 to 31, characterized in
that the thickness (d) of the liquid-crystal material satisfies the
relationship: d/l<(.eta..sup.2/Kp).sup.1/3 in which: l denotes
the extrapolation length defining the zenithal anchoring energy;
.eta. denotes a viscosity; K is the elastic curvature constant; and
p is the density.
42. Device according to one of claims 27 to 41, characterized in
that the surface treatments on the two plates (10, 12) are suitable
for defining anchoring thresholds which differ by from 5 to
10%.
43. Device according to one of claims 27 to 42, characterized in
that the anchoring thresholds depend on the polarity of the applied
electric field.
44. Device according to one of claims 27 to 43, characterized in
that the liquid-crystal material (20) is doped by ions which make
it possible to modify the tilt thresholds of the molecules on at
least one of the plates.
45. Device according to claim 44, characterized in that the ions
are chosen from the group comprising sodium tetraphenylborate,
tetrabutylammonium chloride and cetyltributylammonium bromide.
46. Device according to one of claims 27 to 45, characterized in
that the fall time of the write drive voltage is less than 30
.mu.s.
47. Device according to one of claims 27 to 46, characterized in
that the fall time of the erasure drive voltage is greater than 30
.mu.s.
48. Device according to one of claims 27 to 47, characterized in
that the electrical drive means (40) are suitable for applying an
alternating electrical voltage.
49. Device according to one of claims 5 or 7, characterized in that
it comprises means applying write electric-field pulses with an
amplitude greater than the threshold for breaking the anchoring on
the plate (12) opposite the oblique anchoring master plate
(10).
50. Device according to claim 49, characterized in that it
comprises means applying an erasure electric field whose amplitude
decreases gradually in order to get over, by default, the threshold
for breaking the anchoring on the plate (12) opposite the oblique
anchoring master plate (10).
51. Device according to claim 49, characterized in that it
comprises means applying an erasure electric field consisting of
two successive steps: a first step markedly above the threshold for
breaking the anchoring on the plate (12) opposite the oblique
anchoring master plate (10) and the second step just slightly above
this anchoring breaking threshold in order to limit the
hydrodynamic effect during the cutoff of this second step.
52. Device according to claim 1, characterized in that the
anchorings are oblique on the two plates (10, 12).
53. Device according to claim 52, characterized in that the angle
of inclination of the anchoring is large on a master plate (10) and
lower for a slave plate (12).
54. Device according to either of claims 52 and 53, characterized
in that it comprises means applying write electric-field pulses
with an amplitude greater than the threshold for breaking the
anchoring on both the plates (10, 12).
55. Device according to one of claims 52 to 54, characterized in
that it comprises means applying an erasure electric field with an
amplitude greater than the threshold for breaking the anchoring on
the oblique anchoring slave plate (12).
56. Device according to one of claims 49 to 55, characterized in
that the anchorings on the two plates (10, 12) have a pretwist in
the off-state, favouring the hydrodynamic effect.
57. Device according to one of claims 1 to 56, characterized in
that parameters such as thickness of the cell, dimensions of the
pixel, chiral dopant and pretwist are suitable for defining a
desired spontaneous erasure time.
58. Device according to claim 57, characterized in that the
anchorings on the two plates (10, 12) are twisted through
90.degree..
59. Device according to either of claims 57 and 58, characterized
in that the liquid crystal comprises a nematic/cholesteric
mixture.
60. Device according to one of claims 1 to 59, characterized in
that, for a metastable operation, the anchorings on the plates (10,
12) define two textures in the absence of a field, one having an
elastic energy much greater than the other.
61. Device according to claim 60, characterized in that the two
textures correspond to an oblique anchoring on one plate, (10), and
a planar anchoring on the other, (12).
62. Device according to claim 60, characterized in that the two
textures correspond to oblique anchorings on each of the
plates.
63. Device according to one of claims 60 to 62, characterized in
that the two textures correspond one to a slightly splayed
non-twisted texture and the other to a half-turn twist texture.
64. Device according to one of claims 60 to 63, characterized in
that it comprises ball spacers between the plates (10, 12)
favouring the nucleation of defects.
65. Device according to claim 53, characterized in that the texture
of the LCD molecules includes a volume planar zone, and in that the
device includes means capable of applying electric-field pulses
with an amplitude greater than the irreversible breaking threshold
in order to convert the stable texture having a volume planar zone
into another stable texture.
Description
[0001] The present invention relates to the field of
liquid-crystal-based display devices.
[0002] More precisely, the present invention relates to the field
of display devices having a bistable effect.
[0003] Liquid-crystal-based display devices have already given rise
to a vast literature.
[0004] Mention may be made for example, in a non-limiting manner,
of the following documents:
[0005] (1) Europhysics Letters (25) (7), p 527-531 "Critical
Behaviour of a Nematic-Liquid-Crystal Anchoring at a
Monostable-Bistable Surface Transition" by M. Nobili et al.;
[0006] (2) J. Phys. II France 5 (1995), p 531-560 "Surface Walls on
a Bistable Anchoring of Nematic Liquid Crystals" by M. Nobil et
al.;
[0007] (3) Liquid Crystals 1992, vol. 12, No. 3, p 515-520
"Dynamics of surface anchoring breaking in a nematic liquid
crystal" by A. Gharbi et al.;
[0008] (4) Liquid Crystals 1991, vol. 10, No. 2, p 289-293 "Flow
induced bistable anchoring switching in nematic liquid crystals" by
R. Barberi et al., which describes bistable-anchoring devices;
[0009] (5) Appl. Phys. Letters 55 (24) "Electrically Controlled
surface bistability in nematic liquid crystals" by R. Barberi et
al., which describes bistable-anchoring devices;
[0010] (6) Appl. Phys. Letters 60 (9) "Flexoelectrically controlled
surface bistable switching in nematic liquid crystals" by R.
Barberi et al.;
[0011] (7) Appl. Phys. Letters (62) (25) "Intrinsic
multiplexability of surface bistable nematic displays" by R.
Barberi et al.;
[0012] (8) Appl. Phys. Letters 40 (11) "A Multiplexible bistable
nematic liquid crystal display using thermal erasure" by G. D. Boyd
et al.;
[0013] (9) Appl. Phys. Letters 37 (12) "Threshold and switching
characteristics of a bistable nematic liquid-crystal storage
display" by Julian Cheng et al.;
[0014] (10) Appl. Phys. Letters 36 (7) "Liquid-crystal
orientational bistability and nematic storage effects" by G. D.
Boyd et al.;
[0015] (11) J. Appl. Phys. 52 (4) "Boundary-layer model of field
effects in a bistable liquid-crystal geometry" by J. Cheng et
al.;
[0016] (12) J. Appl. Phys. 52 (4) "The propagation of disclinations
in bistable switching" by J. Cheng et al.;
[0017] (13) J. Appl. Phys. 52 (2) "Surface pinning of disclinations
and the stability of bistable nematic storage displays" by J. Cheng
et al.;
[0018] (14) Appl. Phys. Letters 40 (12) "A nematic liquid crystal
storage display based on bistable boundary layer configurations" by
J. Cheng et al.;
[0019] (15) App. Phys. Letters 43 (4) "Discovery of DC switching of
a bistable boundary layer liquid crystal display" by R. B. Meyer et
al.;
[0020] (16) J. Appl. Phys. 56 (2) "Physical mechanisms of DC
switching in a liquid-crystal bistable boundary layer display" by
R. N. Thurston et al.;
[0021] (17) J. Appl. Phys. 53 (6) "Optical properties of a new
bistable twisted nematic liquid crystal boundary layer display" by
R. N. Thurston et al.;
[0022] (18) J. Appl. Phys. 52 (4) "New bistable liquid-crystal
twist cell" by D. W. Berreman et al.;
[0023] (19) Appl. Phys. Letter 37 (1) "New bistable cholesteric
liquid-crystal display" by D. W. Berreman et al.;
[0024] (20) Asia Display 95 "A bistable Twisted Nematic (BTN) LCD
Driven by a Passive-Matrix Addressing" by T. Tanaka et al.;
[0025] (21) J. Appl. Phys. (59) (9) "Fast switching in a bistable
270.degree. twist display" by H. A. Van Sprang.
[0026] The abovementioned documents essentially concern studies
relating to the breaking of bistable anchorings, to anchoring
energies and to changes of state induced by the propagation of
defects.
[0027] The object of the present invention is to improve
liquid-crystal display devices in order to make it possible to
obtain a novel bistable effect.
[0028] This object is achieved according to the present invention
by virtue of a display device comprising two parallel transparent
plates provided with transparent electrodes on their internal
surfaces and containing a liquid-crystal material, characterized in
that the device comprises:
[0029] means defining a monostable anchoring on each plate;
[0030] means capable of breaking, on command, at least one of these
anchorings; and
[0031] means capable of inducing, after this breaking, a bistable
volume effect in the absence of an electric field.
[0032] These two volume textures, which maintain a stable state in
the absence of an external electric field, must be compatible with
the monostable anchorings on the plates.
[0033] According to one particular embodiment;
[0034] the plates define different anchoring thresholds (these
anchorings may be, for example, planar or homeotropic);
[0035] the thickness of the device between the two plates is
sufficiently small to allow hydrodynamic coupling between the
internal surfaces of these plates; and
[0036] means are provided which are capable of applying, between
the electrodes of the two plates, alternately a write electric
field pulse above a threshold capable of breaking the anchorings on
the two plates in order to define, after interruption of this
electric field, a twisted first stable state resulting from
hydrodynamic coupling between the two plates, and a second electric
field below the said threshold capable of breaking a single
anchoring or having a falling edge which varies slowly in order to
decouple the tilts on the two plates, so as to define a homogeneous
second stable state.
[0037] Other characteristics, objects and advantages of the present
invention will appear on reading the detailed description which
will follow, and with regard to the appended drawings given by way
of non-limiting example, in which:
[0038] FIGS. 1a and 1b diagrammatically illustrate two nematic
liquid-crystal textures obtained with planar anchorings;
[0039] FIG. 2 illustrates the forced alignment of liquid-crystal
molecules with a positive dielectric anisotropy in an applied
electric field;
[0040] FIG. 3 illustrates the curve relating the angle of the
molecules at the surface of the electrodes with respect to a normal
to the plates and the applied electric field;
[0041] FIG. 4 illustrates the curve relating the field threshold
for breaking the anchorings to the duration of the applied field
pulse;
[0042] FIGS. 5a, 5b, 5c and 5d diagrammatically illustrate four
textures obtained in succession when the applied electric field is
progressively decreased;
[0043] FIGS. 6a, 6b, 6c and 6d illustrate the textures obtained
when, in contrast, the electric field is suddenly cut off;
[0044] FIG. 7 shows diagrammatically a mass current obtained close
to a plate when the electric field is switched off;
[0045] FIG. 8 diagrammatically represents a localised flow shear
adjacent to a plate and spreading out as far as the other plate
upon cutting off the drive electric field;
[0046] FIG. 9 illustrates the effect of hydrodynamic coupling
between the two plates;
[0047] FIG. 10 illustrates a bend structure obtained by virtue of
the hydrodynamic coupling;
[0048] FIG. 11 illustrates a twist structure obtained after
relaxation of the bend structure in FIG. 10;
[0049] FIG. 12 illustrates the tilt of molecules on a second plate
due to the effect of hydrodynamic coupling;
[0050] FIGS. 13, 14 and 15 show diagrammatically the azimuthal
orientation of the molecules and the azimuthal moment obtained due
to the effect of the hydrodynamic coupling for various relative
orientations between the easy anchoring directions;
[0051] FIG. 16 shows diagrammatically the structure obtained when a
single anchoring is broken;
[0052] FIG. 17 shows diagrammatically two superposed plates
possessing easy orientation directions rotated with respect to each
other;
[0053] FIG. 18 diagrammatically illustrates a cell in accordance
with the present invention;
[0054] FIG. 19 diagrammatically represents a matrix-configured
screen in accordance with the present invention;
[0055] FIGS. 20, 21 and 22 diagrammatically represent three types
of electrical drive signals;
[0056] FIGS. 23 and 24 represent curves of switching voltage versus
duration of the electric field, respectively for pure 5 CB and for
doped 5 CB;
[0057] FIGS. 25, 26 and 27 diagrammatically illustrate three
possible orientations of the nematic director, in the vicinity of a
surface;
[0058] FIGS. 28, 29 and 30 diagrammatically illustrate three
possible textures for homeotropic anchorings;
[0059] FIGS. 31 and 32 diagrammatically illustrate two possible
textures for oblique anchorings;
[0060] FIG. 33 diagrammatically illustrates the switching caused by
an oblique field applied by interdigitated electrodes;
[0061] FIG. 34 shows diagrammatically another alternative form of
means making it possible to apply an oblique switching field, these
being based on the resistance of the electrodes, and FIG. 35
represents the equivalent diagram of these electrodes;
[0062] FIG. 36 diagrammatically illustrates the switching caused by
a hydrodynamic effect obtained by virtue of an auxiliary drive
electrode;
[0063] FIG. 37 diagrammatically represents four stages of a device
in accordance with the invention, comprising an oblique anchoring
master plate;
[0064] FIG. 38 diagrammatically illustrates the angle of the
surface molecules as a function of the static drive electric
field;
[0065] FIG. 39 illustrates the same angle as a function of time,
after stopping of the drive field in the absence of coupling
between the two surfaces;
[0066] FIG. 40 represents an example of a drive electric field for
erasing this device;
[0067] FIG. 41 diagrammatically represents three stages of the same
device, ending in erasure by virtue of the drive electric field
illustrated in FIG. 40;
[0068] FIG. 42 represents another example of a drive electric field
for erasure;
[0069] FIG. 43 represents four stages of the same device, leading
to erasure by virtue of the drive electric field illustrated in
FIG. 42;
[0070] FIG. 44 represents a diagram of the voltage U.sub.2 as a
function of time T.sub.2 and illustrates writing/erasure states as
a consequence;
[0071] FIG. 45 represents five steps of a device in accordance with
the invention, comprising an oblique anchoring slave plate and
leading to writing;
[0072] FIG. 46 represents the variation in the angle of the surface
molecules as a function of the static drive electric field;
[0073] FIG. 47 represents the orientation of the surface
molecules;
[0074] FIG. 48 represents five steps of the same oblique-anchoring
slave-plate device, leading to erasure;
[0075] FIG. 49 represents the angle of the surface molecules as a
function of the electric field;
[0076] FIG. 50 represents the spontaneous erasure time as a
function of the thickness of the cell;
[0077] FIG. 51 illustrates the optical behaviour of transparent
confinement plates 10, 12 of a display cell: for example, for
readily produced anchorings called monostable "planar" anchorings,
the two textures illustrated in FIGS. 1a and 1b may be obtained. In
the texture illustrated in FIG. 1a, the liquid-crystal molecules 20
are all parallel to each other in the volume, and at the surface on
the plates 10, 12. On the other hand, in the texture illustrated in
FIG. 1b, the liquid-crystal molecules 20 exhibit a 180.degree.
twist structure, that is to say that the molecules, while still
remaining parallel to the plates 10, 12, rotate progressively
through 180.degree. from one plate 10 to the other 12.
[0078] These two textures in FIGS. 1a and 1b have different optical
properties and could, in theory, be used to define two states,
white and black, for transmission of polarized light, by
maintaining the surface anchoring conditions on the plates 10, 12.
It is not possible to pass by continuous deformation from one
texture to the other (they are "topologically" different); it is
only possible to do so by creating defects which represent a high
energy barrier compared to thermal agitation: even if the energy of
the two textures a and b is very different, in the absence of
defects these states may be regarded as being stable forever. The
same is true if the defects become immobile, by adhering to the
surfaces. The simplest way of providing bistability of the two
different twist textures is well known to those skilled in the art:
it consists in cholesterising the nematic liquid crystal with
respect to a spontaneous twist intermediate between those of the
two textures.
[0079] The multiplicity of textures corresponding to defined
monostable anchorings is a general property of nematic or
cholesteric liquid crystals. Those skilled in the art know how to
choose, from these textures, two with similar energies but with
different optical properties.
[0080] The present invention aims to cause transition between these
two textures, in order to make it possible to produce stable pixels
and therefore bistable liquid-crystal displays.
[0081] The description will remain for the moment with planar
anchorings. It is known (see document [1]) that it is possible to
"break" the surface anchorings by using an electric field E normal
to the plates (see FIG. 2) and a nematic liquid crystal with
positive dielectric anisotropy, .epsilon..sub.a=.epsilon..sub./ /
=.epsilon..sub..perp.>0, which forces alignment along the field.
The critical field to break the anchoring is defined by the
condition:
.xi..sub.E=l
[0082] where .xi..sub.E is given by
K/.xi..sub.E.sup.2=(.epsilon..sub.a/4.- pi.)E.sup.2, K is the
elastic curvature constant (.about.10.sup.-6 cgs) and l is the
extrapolation length defining the zenithal anchoring energy. This
energy is written:
W.sub.s=(1/2)(K/l)cos.sup.2.theta..sub.S
[0083] where .theta..sub.S is the angle of the surface
molecules.
[0084] In the case of "strong" anchorings (l.about.1000 .ANG.),
E.sub.S.about.5 V/.mu.m and for "weak" anchorings (l.about.1
.mu.m), E.sub.S.about.0.5 V/.mu.m. For E increasing and approaching
E.sub.S, the surface angle .theta..sub.S goes rapidly from
90.degree. to 0. Above E.sub.S, the angle .theta..sub.S remains
zero and the surface is said to be "broken". The curve relating
.theta..sub.S to E is illustrated in FIG. 3. When the field E is
applied in the form of a pulse of length T, the threshold increases
when T decreases (see document [3]), but since the surface dynamics
are rapid it is possible to break the surface anchoring with
voltages which remain moderate: for example, about 30 V, for times
T.about.10 .mu.s, with the 5 CB liquid crystal at room temperature
(.epsilon..sub.a.about.10). The curve relating the threshold
E.sub.S to the duration T of the pulse is illustrated in FIG.
4.
[0085] When both surface anchorings are broken the texture of the
cell is uniform (as illustrated in FIG. 2) and there is no memory
of the initial state since the molecules 20 seen end-on cannot keep
any twist.
[0086] The effect used within the context of the invention to
initiate the textures is a dynamic effect. It relies on the
following studies and observations.
[0087] Let us first assume that the two anchorings of the plates 10
and 12 have been broken, as explained above; if the electric field
is decreased slowly, at every instant the system will choose its
lowest-energy state in order to define a slowly varying
texture.
[0088] Starting from the homeotropic orientation illustrated in
FIG. 2, in an electric field, these textures will always go, in a
zero field, towards the non-twisted state illustrated in FIG. 5d,
with a planar orientation, passing through an intermediate
situation illustrated in FIGS. 5b and 5c in which the molecules on
the two surfaces of the plates 10, 12 rotate in the same direction,
while remaining parallel. This arises from an elastic interaction
between the plates 10, 12 which minimizes the curvature and the
curvature energy of the system.
[0089] On the other hand, if the electric field is cut off
suddenly, the effect obtained is very different, as illustrated in
FIG. 6.
[0090] The dynamic effects are controlled by two characteristic
times: the volume characteristic time T.sub.v V and the surface
characteristic time T.sub.S.
[0091] T.sub.v is universally given by the curvature elasticity
over the thickness d of the specimen as:
[0092] 1/T.sub.v=K/d.sup.2.eta., where .eta. is a viscosity
(.eta.=0.1 or 1 poise).
[0093] T.sub.s is given by the same formula, in which d is replaced
by the surface extrapolation length l, namely
1/T.sub.s=K/l.sup.2.eta..
[0094] Since ld, T.sub.s is very much less than T.sub.v; typically,
for d=1.mu. and l.about.1000 .ANG., T.sub.v=1 ms and T.sub.s=10
.mu.s.
[0095] When the field E is released, the molecules on the two
surfaces rotate rapidly during their times T.sub.s, while the
volume molecules remain virtually immobile. On this timescale, the
elastic coupling between the plates 10 and 12 is negligible, but
there is hydrodynamic coupling. Associated with the rotation of the
molecules is a mass current (see document [22]) . This current
exists close to each plate, over a thickness of .about.1. Its
velocity V is approximately V=l/T.sub.s. Such a current is shown
diagrammatically in FIG. 7.
[0096] Let us assume that the plate 12 has a threshold E.sub.s12
greater than that E.sub.s10 of the plate 10. In this case, the
molecules 20 adjacent to the plate 12 tend to revert to the planar
state before the molecules adjacent to the plate 10. Moreover, the
return of the surface molecules adjacent to the plate 12 from the
.theta..sub.S=0 orientation to the stable .theta..sub.S=90.degree.
orientation (called the planar orientation) produces a localised
flow shear V over 1, as shown diagrammatically in FIG. 8.
[0097] This shear diffuses over the thickness d of the cell in a
time given classically by the relaxation of the vortices
(Navier-Stokes equation in hydrodynamics) by:
[0098] l/T.sub.D=.eta./d.sup.2p, where p is the density
(p.about.1). give a uniform final texture. The velocity gradient
shear v/d gives, within the volume, a moment density .eta.v/d to
the molecules. The sum of these volume moments is a surface moment,
(.eta.v/d)d.about..eta.v=.eta.Vl/d which will rotate the surface
molecules in the .omega..sub.H direction, as illustrated in FIG.
12.
[0099] In order to obtain the tilt which creates a twist structure
as illustrated in FIG. 11, the surface moment thus obtained must
therefore be greater than the anchoring moment which rotates in the
.omega..sub.a direction (FIG. 12).
[0100] This condition is:
[0101] (K/l) .theta..sub.S<.eta.Vl/d. Replacing V by 1/T.sub.s,
with 1/T.sub.s=K/l.sup.2.eta., gives .theta..sub.s<1/d.
[0102] .theta..sub.S is of the order of the variation in angle over
the time T.sub.D, and hence of the order of
T.sub.D/T.sub.s=(Kp/.eta..sup.2) (d.sup.2/l.sup.2). The condition
becomes: d/l<(.eta..sup.2/Kp)1/3; with .eta..about.0.1 poise,
this gives d<20l. If l.about.1000 .ANG., d must be less than 2
.mu.m. However, since d=2 .mu.m is a typical thickness of the
specimens, this condition is sometimes a little difficult to
achieve. It would be necessary to use weak anchorings, with a
longer response time.
[0103] Within the context of the invention, it will be considered
that, preferably, the thickness d of the cell must be less than 5
.mu.m.
[0104] Within the context of the invention, a method of
hydrodynamically coupling the anchorings is therefore proposed
which is more effective and which operates for strong
anchorings.
[0105] Hitherto, only the .theta..sub.S zenithal anchoring, which
is generally stronger, has been taken into account. However, there
is also a preferred azimuthal direction on the plates which adopt
"planar" orientations in a defined direction. Calling .o slashed.
the azimuthal angle of the molecules with respect to this
direction, the surface energy should be:
W.sub.S=1/2(K/l)cos.sup.2.theta..sub.S+1/2(K/L)sin.sup.2.theta..sub.Ssin.s-
up.2.o slashed.
[0106] where L is the extrapolation length defining the azimuthal
anchoring energy K/L.
[0107] In general, the azimuthal term has an amplitude an order of
magnitude smaller than the zenithal term (see document [1]): L is
an order of magnitude greater than 1. Looking at the lower plate 10
from above, it may be assumed that the surface molecules have been
inclined by an angle .theta..sub.S, after the time T.sub.D, as
illustrated in FIG. 13.
[0108] If the planar direction on the plate 10 is P, the molecules
may assume the two possible states P1 and P2 on it. In order to
force the molecules to drop down to the P2 state, which will give a
half-turn, and not the P1 state, it is sufficient to move the end m
of the molecule on the other side of yy', the mid-perpendicular of
P1, P2 (FIG. 13). To do this, instead of changing .theta..sub.S by
moving m along P1 P2, it is more effective to rotate m at constant
.theta..sub.S around the circle C (FIG. 13). To do this, it
suffices to rotate the easy anchoring direction of the upper plate
10 through an angle .alpha. with respect to P1 P2. The velocity v
is in the direction .alpha. and produces a final alignment f. Since
the moment exerted by the transient velocity gradient is now
balanced by the single reaction of the azimuthal anchoring energy,
the condition for the moments may be written here, for small
.theta..sub.S, as:
K/l .theta..sup.2.sub.S<K/d .theta..sub.S.
[0109] The condition to be fulfilled is now: .theta..sub.S<L/d.
Since L is an order of magnitude greater than 1, the coupling
condition is easier to fulfil. Thus, finally: d.sub..o
slashed.=d.sub..theta.(L/l)1/3>d.su- b..theta..
[0110] There exists an optimum rotation angle .alpha. of the two
plates. If .alpha. is very small, a tilt very close to P2 (through
180.degree.-.alpha..about.180.degree.) will occur, but it will be
difficult to exert the initial azimuthal rotation moment: the
system will prefer to change .theta..sub.S with less effectiveness,
as illustrated in FIG. 14. On the other hand, if .alpha. is close
to 90.degree. the strongest possible azimuthal moment will be
obtained, as illustrated in FIG. 15, but the rotation obtained will
be only 90.degree., which is ineffective for providing the tilt
since this rotation places the system just on the line of equal
energy between P1 and P2. There exists an optimum value, which may
be around 45.degree., or around 135.degree., if the anchorings have
a polarity defined in the plane, as is the case for evaporated SiO
or for a unidirectionally rubbed polymer.
[0111] In order to erase a "1/2-turn" twist, as shown
diagrammatically in FIG. 1b, it is sufficient to "break" only a
single surface anchoring, if carried out quickly, or to decrease
the applied field slowly in order to decouple over time the two
surface tilts, assumed to have different thresholds. In every case,
surface treatments will be chosen which will give different
anchoring thresholds on the two plates 10, 12.
[0112] The principle of effecting the 1/2-turn twist relies on the
following phenomenon. When only one of the two surfaces is broken,
as shown diagrammatically in FIG. 16, or when the two anchorings
are released in succession at a time interval >T.sub.s, there is
no longer a hydrodynamic coupling effect: the elastic couplings
dominate and the vertical orientation of one surface cannot
maintain the twist, which disappears. The 1/2-turn twist is
therefore erased.
[0113] On the basis of the above observations, the inventors
propose to produce a display (in fact a pixel) with the aid of two
plates 10, 12 treated in order to give planar anchorings A1 and A2
(or anchorings with a planar component) which are different. These
anchorings coupled to a nematic with .epsilon..sub.a>0 have
breaking thresholds E1 and E2 respectively. They are placed at
.alpha.=45.degree. to each other, as shown diagrammatically in FIG.
17, or at an angle .alpha. which is different from 0.degree.,
90.degree., 180.degree. or 270.degree. but which optimizes the
rotational hydrodynamic coupling.
[0114] This angle .alpha. is also chosen to give good contrast
between the initial texture, which is now twisted through the angle
.alpha., and the so-called "1/2-turn" final texture, which is now
twisted through an angle 180.degree.-.alpha.. In order to write, an
electric field pulse above the two thresholds, E>E1 and E>E2,
is applied. By abruptly cutting off the field, the
180.degree.-.alpha. state is still obtained, due to the effect of
the hydrodynamic coupling, irrespective of the initial state,
.alpha. or 180.degree.-.alpha.. In order to erase, a pulse E
between E1 and E2 is applied, rapidly cutting off the pulse, or a
pulse which has a level above the two thresholds E1 and E2, but the
amplitude of which is decreased slowly, is applied, in order to
decouple the tilts on the two plates 10, 12; the .alpha. state is
always obtained, whatever the initial state, .alpha. or
180.degree.-.alpha..
[0115] The supply means designed to apply such drive pulses are
shown diagrammatically by the reference 40 in FIG. 18.
[0116] The optical contrast between the two states of such a pixel
depends on the thickness of the specimen and on the orientation of
the polarizers 30 and analysers 32 used (see FIG. 18).
[0117] This optimization problem is known to specialists (see
document 23). In practical terms, for each liquid crystal and each
cell, instabilities of another type, for example the Freedericks
instability; v of about 1 volt, typically, for example, v.ltoreq.1
volt, will be chosen in order to do this. The higher threshold will
therefore have to be well-defined and uniform. The value of the
lower threshold is less constricting. It cannot be too low in order
for the system to remain rapid. In practical terms, anchorings will
therefore be chosen which give threshold values in volts, in the
region of about 1 volt. Since the typical thresholds are of the
order of 10 V/.mu.m (document 1), for a 2 .mu.m thick cell, the
thresholds must differ by from 5 to 10%.
[0118] In order to produce slightly different anchoring thresholds,
and therefore breaking voltages, on the two plates 10, 12, it may
be advantageous to use the same surface preparation technology
(oblique SiO evaporation or surface-rubbed polymer, for example),
but to vary the polarity of the thresholds. It is thus almost
possible to cancel out or to amplify an existing small threshold
difference. To do this, the flexoelectric effect or the ion
transport effect may be used.
[0119] The two anchorings on the two plates 10, 12 play
interchangeable roles in the proposed mechanism. Giving a
difference in threshold between the two anchorings, which is
related to the polarity of the applied field, is only meaningful if
the cell is initially unsymmetrical, with two different threshold
fields, E1#E2, and therefore two different threshold voltages V1
and V2.
[0120] A first way of varying the thresholds is to use the
flexoelectric effect which shifts the anchoring forces in
proportion to the applied field (see document 24). This effect has
a relative magnitude of e/(K)1/2.about.a few 10.sup.-1, that is to
say moderate or small.
[0121] A stronger polar effect may be obtained by ion doping. This
is because it is observed that the anchoring energy depends on the
polarity, as shown by the experiment below.
[0122] A 45.degree. twist cell is taken, with two planar anchorings
obtained by the same SiO evaporation. A cell
[0123] When the charged end of a chain adheres to the surface, the
chain induces a perpendicular orientation, which decreases the
planar anchoring force.
[0124] Those skilled in the art will understand that bringing the
thresholds E1, E2 closer together favours the writing procedure
while moving the thresholds E1 and E2 further apart favours the
erasure procedure.
[0125] Illustrative Embodiment
[0126] The inventors have produced a display with the
pentylcyanobiphenyl (5 CB) nematic liquid crystal which has a
nematic phase at room temperature and a high dielectric anisotropy
.epsilon..sub.a.about.10>- 0.
[0127] The display comprises glass plates 10, 12 treated with ITO
(Indium Tin Oxide) which gives transparent electrodes with a low
resistance (30 .OMEGA./square). These are treated by oblique SiO
evaporation with an almost grazing evaporation angle of 75.degree.,
and thicknesses of 25 .ANG. and 30 .ANG., these being known to give
a planar anchoring with slightly different anchoring force
(document 25). The cell has a thickness of d=1.5 .mu.m. with a
rotation of .alpha.=45.degree.. The geometry of the orientations of
the cell is shown in FIG. 18.
[0128] This cell makes it possible to obtain, for the 45.degree.
texture, a bright yellow colour with a high transmitted intensity.
For the 180.degree.-.alpha.=135.degree. state, a low transmitted
intensity, with a very dark blue, almost black, colour has been
obtained.
[0129] In order to test the model, the inventors applied square
pulses, of a fixed length of 300 .mu.s and of amplitude V varying
from 0 to 40 volts, to the system. The fall time was less than 1
.mu.s. Bright-to-dark (white to black) switching was obtained at
V=24.5 volts. Starting from a black state, the inventors always
obtained a black state with these same pulses. Next, the inventors
applied a pulse of the same polarity to this black state, but with
an amplitude of 21.5 volts. A black-to-white transition,
corresponding to erasure, was obtained. These same 21.5 volt pulses
leave an initial white state unchanged. The final state of the
system therefore depends, for the same polarity, only on the
amplitude of V. This behaviour is explained by the inventors by the
fact that one of the thresholds is slightly less than 24.5 V and
the other less than 21.5 V. system is controlled by just the final
decrease of the drive signal, the inventors performed the following
experiments.
[0130] In the first place, the inventors used pulses whose front
edge is linear in time, as illustrated in FIG. 20.
[0131] More specifically, the inventors chose a plateau time T=100
.mu.s and varied the rise time of the front edge T' from 0 to 300
.mu.s. With T'=0, the system undergoes the white-to-black
transition at V=25 volts (or black to black if the initial state is
black). Over the entire range of T' used, the behaviour does not
change, the threshold remaining at 25 volts .+-.0.5 volts. This
shows that only the amplitude and the fall of the pulse are
effective.
[0132] In the second place, the inventors used pulses whose falling
edge is linear in time, as illustrated in FIG. 21. For fall times
of 0<T'<30 .mu.s, the behaviour remains unchanged. Beyond
this, for 30<T'<300 .mu.s, a black-to-white erasure is
obtained on starting from black, and white goes to white on
starting from white. For T=100 .mu.s and T'=0, a threshold V=25
volts is found. This behaviour confirms that only the slow falling
edge of the pulse is effective for erasure. By falling linearly,
the triggering of the two thresholds is shifted in time. With the
21 volt and 25 volt values, the shift in time is
[(25-21)/25].times.30 .mu.s.about.5 .mu.s.
[0133] This value is the estimated value of the surface tilt
time.
[0134] In order to really isolate the two thresholds in the same
experiment, the inventors next used a pulse having a double-square
shape of amplitudes V and V' and of durations T and T', as
illustrated in FIG. 22.
[0135] The inventors chose T=1 ms in order to be sure that the
system switches only on the falling edge, without any memory of a
prior effect. With V'=0, a white-to-black or black-to-black switch
was obtained at V=22 volts.
[0136] Next, the inventors chose V=30 volts, in order to be well
above the threshold and, by taking T'=0.5 ms, they varied V'. For
30 volts>V'>20 volts, the black writing was preserved. On the
other hand, for 20 volts>V'>7 volts, the system becomes a
binary counter, that is to say that it produces white-to-black or
black-to-white switch-overs. For V' between zero and 7 volts, the
black writing is again well-defined.
[0137] The inventors next changed the polarity of V', keeping that
of V. The same behaviour was observed:
[0138] -30 volts<V'<-20 volts: writing in black
[0139] -20 volts<V'<-7 volts: "counter" regime
[0140] -7 volts<V'<0 volts: writing in black.
[0141] As it stands from their work, the inventors explain the
counter regime as an incomplete erasure the system remembers the
initial state.
[0142] The important result from this experiment is that, for
V'=-V, writing is obtained; this is also confirmed for T'=T =1 ms.
The inventors have therefore shown that AC driving is possible.
[0143] The inventors measured V(T) for the above specimen. On 5 CB
for the curve of V, they observed writing (T) as illustrated in
FIG. 23. This writing/erasure behaviour at a fixed polarity is
satisfactory down to T=150 .mu.s. For shorter times, a "counter"
regime is observed.
[0144] In order to improve this behaviour, the inventors then used
a 5 CB liquid crystal doped with 10.sup.-3 mol of Na.sup.+
T.PHI.B.sup.-, with a thickness of 1.5 .mu.m. They used positive
pulses for writing and negative pulses for erasure. In this case,
they obtained a controlled writing and erasure regime down to 10
.mu.s, at voltages of 30 and 38 volts as illustrated in FIG. 24. At
30 .mu.s, erasure and writing were obtained for 22 and 26
volts.
[0145] In white light, the inventors obtained a contrast of 20
between the two states.
[0146] Of course, the present invention is not limited to the
particular embodiment which has just been described, but extends to
any alternative form in accordance with its spirit.
[0147] In particular, the present invention is not limited to the
use of nematic liquid crystals. It also extends to the use of
liquid crystals of the cholesteric type.
[0148] Furthermore, the switching by hydrodynamic coupling is not
limited to the use of planar anchorings on the plates. It may
extend to homeotropic or even oblique anchorings.
[0149] Moreover, in a more general way, as explained above, the
invention is not limited to the use of switching by hydrodynamic
coupling but extends to any monostable-anchoring device, comprising
means capable of causing a break in at least one of the anchorings
and in subsequently inducing a bistable volume effect.
[0150] Furthermore, the invention applies to a large number of
possible textures.
[0151] It is known that the treatments applied to each of the
plates 10, 12 of a liquid-crystal cell may be designed to impose a
planar anchoring direction (nematic director parallel to the
plates, see FIG. 25), a homeotropic anchoring direction (nematic
director perpendicular to the plates, see FIG. 26) or an inclined
anchoring direction (nematic director which is oblique with respect
to the plates, see FIG. 27).
[0152] With both these plates arbitrary, it is possible to define
several textures with a single anchoring direction of the molecules
on each plate.
[0153] For example, for two planar anchorings it is possible to
produce a uniform planar texture, as illustrated in FIG. 1a, or
structures twisted to the left or to the right with a half-turn, as
illustrated in FIG. 1b, or indeed with several half-turns, the
nematic director in this case remaining parallel to the plates but
rotating progressively around an axis perpendicular to them, or
else bend structures, as shown diagrammatically in FIG. 10, for
which the nematic director does not remain parallel to the plates
but is progressively inclined with respect to them.
[0154] For two homeotropic anchorings, it is possible to obtain a
homeotropic uniform texture (FIG. 28) or bend textures with one
(FIG. 29) or more half-turns. These bend textures may, in addition,
be twisted (FIG. 30).
[0155] In general with two monostable surface anchorings in two
arbitrary directions, it is possible to obtain different textures:
a simple texture which connects, directly by a simple twist and a
simple bend, the two arbitrary anchoring directions, as illustrated
in FIG. 31, and textures which differ from this simple texture by
adding one or more half-turns on going from one surface to the
other, as shown diagrammatically in FIG. 32.
[0156] The nematic director has been shown diagrammatically in the
appended FIGS. 28 to 32 as an arrow.
[0157] By comparing FIGS. 28 and 29 or 30, 31 and 32, it may be
seen that the corresponding arrows on the two plates are in
opposite directions. Physically, since the interaction of the
nematic liquid crystal with the surface is not polar, the opposite
directions of the two arrows are equivalents with respect to the
surface. However, these arrows enable the differences in volume
textures, rotated for example through a half-turn between FIGS. 28
and 29, or 30, 31 and 32, to be clearly visualized. The same
applies to FIGS. 1a and 1b.
[0158] Moreover, these various textures, corresponding to the same
anchoring direction, possess different optical properties, which
allow them to be optically distinguished and to be used as one of
the two states of a black-and-white display pixel.
[0159] As indicated previously, the switching between the various
textures takes place, having broken the surface anchoring.
[0160] FIG. 3 illustrates the variation in the angle .theta. of a
surface molecule with planar anchoring as a function of an applied
electric field E.
[0161] Above E.sub.S, the surface molecules are in a situation in
which the elastic energy of the interaction with the surface is
maximum. If the field E is cut off, the surface molecules drop back
into the initial planar orientation, but they may choose two
different paths. In FIG. 3, these two paths correspond to the
bifurcation below E.sub.S between positive and negative angles. The
two final states, .theta.=.+-.90.degree., are identical for the
surface, as explained above. However, they give different volume
textures: the additional 180.degree. rotation corresponds to a
texture which is twisted through a half-turn with respect to the
initial texture. If the distortion remains in the plane of the
figure, a 180.degree. bend texture is obtained (FIG. 10). In
general, since twisting is easier than bending, the 180.degree.
bend is transformed continuously into a 180.degree. twist in FIG.
1b.
[0162] The function of the switching means capable of inducing a
bistable effect, after breaking the anchoring, is to control the
bifurcation of the orientation E.sub.S so as to obtain, as
required, one or the other of the two corresponding bistable
textures.
[0163] More generally, for any bistable texture mentioned above,
obtained by varying the angles, elastic constants and the twisting
power corresponding to the same anchoring on one plate and to two
anchorings differing by a half-turn on the other plate, there
exists a dividing line, similar to the bifurcation already
described, for the surface energy on the second plate.
[0164] The purpose of the anchoring breaking is to bring the
surface molecules into the vicinity of this dividing line by means
of a strong electric field.
[0165] In addition, the function of the switching means, through a
small external effect, is to control the movement of the system on
either side of the dividing line. The two resulting directions are
equivalent for the surface but lead to one or the other of the two
bistable textures.
[0166] In order to break the surfaces, appropriate means will be
chosen: if the field is perpendicular to the plates, it is
necessary, in order to break a planar anchoring, for the liquid
crystal to have positive dielectric anisotropy so that the
molecules are aligned parallel to the field; in order to break a
homeotropic anchoring, it is necessary for the material to have
negative dielectric anisotropy.
[0167] It will be noted that an important and general property of
surface breaking is their rapidity: the corresponding relaxation
times are in the microsecond range. They are independent of the
thickness of the nematic cells.
[0168] Various means will now be described which make it possible
to switch between the various possible textures, that is to say
making it possible to control the bifurcation of the orientation at
E.sub.S.
[0169] Let us assume that a planar anchoring has been broken above
its bifurcation point. The surface molecules are perpendicular to
the plates. When the field is cut off, the molecules drop back down
to one or the other of the two equilibrium states, +90.degree. and
-90.degree.. The function of the switching means is to control the
final direction of orientation between these two states. The
purpose of these means is to apply a small moment to the molecules
in order to make them tilt to one side or to the other. This moment
may be applied either at the same time as the breaking field or
just afterwards, but it must act for as long as the molecules
remain close to the dividing line.
[0170] A first way of generating such a moment consists in applying
a lateral electric field to the cell.
[0171] Such a lateral field may itself be obtained in several
ways.
[0172] According to a first variant, the lateral field may be
applied with the aid of interdigitated electrodes 50, 52 on one of
the plates 10, facing the plate 12 whose anchoring is broken as
shown diagrammatically in FIG. 33. The mean field remains applied
between the two plates 10, 12 at the top and bottom. The lateral
field gives a small oblique component to the resultant field.
Depending on its sign, the oblique fields E.sub.1 or E.sub.2 are
obtained.
[0173] The application of E.sub.1 or E.sub.2, which are shifted
through a small angle about the normal, makes it possible to
control drop-down onto the planar states 1 or 2, which are
identical for the surface but different for the texture.
[0174] According to a second variant, the lateral field may be
applied by means of electrodes provided along the edge of the
cell.
[0175] According to a third variant, the lateral field may result
from the resistance of the transparent electrodes provided on the
plates 10, 12. As illustrated in 34, one of the electrodes 60 in
this case possesses at least one edge 62, preferably two edges 62,
which are more conducting than its central part 64. The electrical
signal V necessary to break the anchoring is transmitted along the
RC circuit formed by the surface resistance R and the capacitance C
of the liquid crystal (see FIGS. 34 and 35). At high frequencies, a
signal is rapidly attenuated and the pixel appears as an electrode
edge, giving an oblique field. At low frequencies there is no
attenuation and the field is vertical. This mechanism is described
in document FR-A-86 06916. A field inclined in both directions is
obtained by using double-lateral-control pixels: the signal V.sub.1
or V.sub.2 is applied to one or the other of the conducting edges
of the semi-transparent electrode of the pixel. V.sub.1 gives the
right orientation and V.sub.2 the left orientation.
[0176] A second way of generating the aforementioned orienting
moment consists in exploiting a hydrodynamic effect.
[0177] In this case, at the time of returning to equilibrium, a
small shear v is generated between the plate with broken anchoring
and the nematic.
[0178] This may be achieved by a mechanical displacement of all or
part of the plate, due to the effect of a piezoelectric system for
example, or else due to the effect of sound waves.
[0179] The nematic is sensitive to the velocity gradient close to
the plate and drops, depending on the direction of v (v.sub.1 or
v.sub.2), on one side or on the other side of the bifurcation.
[0180] The shear v may also be produced by a flow between the two
plates, this being produced by any source whatsoever, for example
by simply pressing on the screen perpendicular to the plates.
[0181] The system then constitutes a pressure detector. It may be
used to write on a screen, by converting the pressure into an
electrical property associated with one of the two bistable
textures.
[0182] Another variant consists in exploiting the shear current
caused by the tilting of certain molecules. This effect is the
reverse of the previous effect, in which a shear controls a
tilt.
[0183] To do this, it is possible to use, for example, a linear
drive electrode c alongside a square pixel (FIG. 36).
[0184] At c the anchoring is, for example, oblique while at P it is
planar.
[0185] Due to the effect of a field applied to the pixel, the
anchoring P is broken.
[0186] If a drive field E' is applied to the lateral electrode, at
the time when the field E is cut off, the flow v' associated with
the reorientation of c due to the effect of E' switches the
orientation of the pixel P into state 1.
[0187] If on the other hand the field E' is applied at the same
time as E and if it is also cut off at the same time, the flow
caused by E' is in the opposite direction, -v', and the pixel
switches into state 2.
[0188] Another variant which exploits hydrodynamic coupling and
consists in breaking two face-to-face anchorings has been described
previously.
[0189] Of course, the invention is not limited to the bifurcation
control examples described previously in the case of planar
geometry which, after breaking, become homeotropic.
[0190] Indeed, as mentioned, the invention also applies to
controlling the bifurcations in geometries which are homeotropic or
inclined in the off state.
[0191] Furthermore, the tilting may be performed in two dimensions,
by involving not only the zenithal surface angle .theta. but also
the azimuthal angle .o slashed., as described previously in the
case of hydrodynamic coupling. Rotation of .theta. equal to
-90.degree. to +90.degree. may also be interpreted as a simple
180.degree. rotation of .o slashed.. This is important for the
couplings with the lateral electric fields or the lateral
hydrodynamic flows, which have a well-defined azimuthal
direction.
[0192] It will furthermore be noted that the switching may be
performed over the entire surface of a pixel at the same time, in
order to form a black-and-white display, or over a variable part of
this pixel, in order to form a grey-tone display.
[0193] Driving only a variable part of a pixel may be achieved
either by a non-uniform breaking field on the latter or by
non-uniform means for controlling the bifurcation.
[0194] Finally, it will be noted that, in some configurations, the
anchoring breaking means and the means capable of inducing a
bistable volume effect, which were described previously, may be
used with multistable, for example bistable, anchorings and not
just monostable anchorings.
[0195] Up to now, it has essentially been demonstrated in the
preceding description that the breaking of a planar anchoring makes
it possible to control a transition between two bistable volume
textures. This is, in this case, complete breaking-- in an electric
field the molecules on the surface orient precisely along the
field, passing through the point of bifurcation. The breaking of
oblique anchorings, also mentioned in the preceding description, is
different. This is a partial breaking: the molecules move towards
the direction of the field, without ever reaching it and without
passing through the point of bifurcation.
[0196] Within the scope of the invention, the use of this partial
breaking of the oblique anchorings will now be explained in
detail.
[0197] The oblique anchoring plate may fulfil two different
roles:
[0198] 1) Either an emitter role ("master plate")--in this case the
oblique anchoring plate serves to drive the
[0199] to its initial orientation (see FIG. 39). The angle
.theta..sub.S1 increases exponentially as
.theta..sub.S1.ident..theta..sub.01exp(t/T.sub- .s), from the high
initial value .theta..sub.01, and afterwards saturates to the value
imposed by the oblique anchoring. On the other hand, the planar
plate 12 is in unstable equilibrium at t=0 and tilts slowly:
.theta..sub.S2 also increases exponentially, but from a very low
angle .theta..sub.02, which is determined by fluctuations.
[0200] The shear produced by each of the plates 10, 12 is
proportional to the derivative of the angle with respect to time.
It is much greater for the oblique plate 10 (of the order of
.theta..sub.01/T.sub.s>>.thet- a..sub.02/T.sub.s). The latter
plate thus becomes a master plate: its shear current after
diffusion through the specimen 20 drives the planar slave plate 12
beyond the point of bifurcation (FIG. 37c). A bend half-turn is
therefore produced in the specimen 20, which transforms into a
twist half-turn (FIG. 37d).
[0201] Alternatively, in order to erase the half-turn, it is
necessary to prevent the hydrodynamic coupling between the two
plates 10, 12. One way of doing this is to decrease the voltage
across the threshold U.sub.C2 gradually, as illustrated in FIG.
40.
[0202] During the first part of the pulse (i.e. from t=0 to
T.sub.1), the anchoring on the planar plate 12 is broken and
irrespective of the initial texture the almost homeotropic texture
in FIG. 37b or 41a is obtained. The slow fall (T>>T.sub.s)
through the threshold renders the hydrodynamic effect induced by
the master plate 10 barely effective. The slave plate 12 is now
driven by the weak elastic static coupling (FIG. 41b), which always
favours the uniform final texture (FIG. 41c).
[0203] Another way of achieving the same effect-- that is to say of
erasing the previously obtained half-turn in the specimen 20--is to
use a two-step rectangular electrical signal (FIG. 42). Once again,
the initial texture is erased during the first part of the pulse
(FIG. 43a) (from 0 to T.sub.1 in FIG. 42). At t=T.sub.1, the
voltage abruptly falls to U.sub.2 slightly above the threshold
U.sub.c2. The master plate 10 produces a strong transient
hydrodynamic current (FIG. 43b), which gradually disappears. The
anchoring remains perpendicular in the broken position. The elastic
effect, which in this geometry is permanent, overcomes it and at
the end of the pulse the slave plate 12 has already chosen an
inventors have observed writing of half-turns. Lengthening
.tau..sub.2 without changing the voltage U, they always obtained a
uniform final texture.
[0204] These observations confirm the model explained above and
show that the use of an oblique anchoring master plate 10 is a
highly effective means for writing and erasing the half-turns.
Similar results were also obtained with cells having a pretwist
between 0.degree. and 90.degree., these being obtained by rotating
one of the plates with respect to the other (as already explained,
this geometry facilitates the hydrodynamic coupling).
[0205] We will now tackle the case of an oblique anchoring slave
plate.
[0206] To understand the utility of an oblique anchoring slave
plate 12, we will analyse the anchoring breaking in a cell in which
the molecules on both plates 10, 12 have oblique anchoring (FIG.
45): the angle of inclination .theta. is greater on the master
plate 10 and much smaller on the slave plate 12.
[0207] We start with the "uniform" (twist-free) texture in FIG.
45a. In an electric field perpendicular to the plates 10, 12, the
molecules on the two surfaces 10, 12 move towards the vertical,
without ever reaching it (FIG. 45b and 46): for both surfaces, the
point of bifurcation is on the other side of the vertical (FIG.
47). In FIG. 47, .GAMMA..sub.h depicts the hydrodynamic moment,
.GAMMA..sub.s the elastic moment and m the direction of the
anchoring energy maximum (bifurcation).
[0208] Let us now assume that the field is strong enough to orient
the molecules on both surfaces 10, 12 (FIG. 45b) so as to be almost
vertical (.theta..sub.s1.ident.0, .theta..sub.s2.ident.0). When the
field is cut off, a large elastic surface moment
.GAMMA..sub.s.ident.(k/l.sub.1).alpha- ..sub.1 acts on the master
plate 10. l.sub.1 represents the anchoring force on the plate 10.
The molecules on this plate 10 tilt towards their initial position,
emitting a high shear current. The hydrodynamic moment
.GAMMA..sub.b transmitted to the slave plate 12 is of the order of
K/d and it tries to tilt the molecules through the vertical (FIG.
45c). An elastic surface moment .GAMMA..sub.s, of the order of
K.sub..alpha.2/l.sub.2, opposes this. l.sub.2 represents the
anchoring force on the plate 12. The condition for writing
half-turns in this geometry is therefore obtained:
K/d>K.sub..alpha.2/l.sub.2, or .alpha..sub.2<l.sub.2/d, that
is to say that the oblique slave plate 12 can be driven effectively
if its anchoring is weak and its field-free inclination
.theta..sub.2 is very close to 90.degree. (almost planar). If this
is the case, the bend half-turn in FIG. 45d and, finally, the twist
half-turn in FIG. 45e are obtained.
[0209] For erasure, an electric field is once again applied to the
twist half-turn texture (FIG. 48a). In a field, it transforms to a
bend half-turn (FIG. 48b), which allows the molecules to orient
along the field almost throughout the specimen. However, close to
the slave plate 12, a thin region of almost planar orientation
remains. This region is topologically blocked in the texture: its
existence depends on the relative orientation of the two anchorings
and on the initial volume texture. A high (elastic and electric)
energy is stored in the planar region and the resultant moment
pulls on the molecules at the surface towards the plate 12 and no
longer towards the vertical: in this way, the planar region is
"expelled" from the specimen and the energy decreases (FIG. 48b and
48c).
[0210] The behaviour of the surface angle .theta..sub.S2 as a
function of the field is diagrammatically shown in FIG. 49,
assuming that, with no field, .theta..sub.2 is large and negative
(.vertline..theta..sub.2.vertl- ine.>90.degree. and
.theta..sub.2<0, the point A in FIG. 49). In a field,
.vertline..theta..sub.2.vertline. decreases (path ABC in FIG. 49)
and the molecules move towards the .theta..sub.2+90.degree.
direction, which corresponds to the anchoring energy maximum and
therefore to the zero anchoring moment. At the critical value
E.sub.c of the field, the anchoring moment can no longer balance
the electric moment and the surface becomes unstable:
.theta..sub.s2 switches to the path CD (FIG. 49) and the molecules
are now on the other side of the vertical (FIG. 48d). If the field
is now decreased gradually (in order to eliminate the hydrodynamic
coupling), .theta..sub.s2 follows the path DE (FIG. 49) and the
final state of the system is the non-twisted texture in FIG. 48e
the half-turn is erased.
[0211] This erasure mechanism, discovered by the inventors, uses a
breaking of the oblique anchoring on the slave plate 12 which is
induced by the elastic interaction with the oblique master plate 10
and the initial texture. An initial texture may be erased by this
mechanism only if it includes a planar region in the volume. On the
other hand, in order to write such a texture, it is necessary to
use other means, for example the hydrodynamic effect: the elastic
breaking of the anchoring of the slave plate 12 is a transient and
irreversible phenomenon, passage along the path CD (FIG. 49) being
a one-way passage.
[0212] On the other hand, if the field is cut off before exceeding
the point C, .theta..sub.s2 returns to A and the texture remains
twisted after cutting off a field. This is because, if the field is
too low to exceed the point C, rewriting occurs.
[0213] In order to demonstrate the utility of the oblique slave
plates 12, the inventors prepared several thin specimens
(d.ident.1.5 .mu.m) with the oblique master plates 10 (evaporated
SiO and an angle of inclination of the molecules with respect to
this surface of approximately 35.degree.)
(.theta..sub.2=90-35=55.degree.). The slave plates were prepared
with different rubbed polymers (.alpha..sub.2, angle of inclination
of between 2.degree. and 10.degree.).
[0214] In order to facilitate the writing of half-turns by means of
the hydrodynamic coupling, a pretwist angle .o slashed. (between
0.degree. and 90.degree.) was imposed by rotating one of the plates
with respect to the other. As already explained, this pretwist
helps the hydrodynamic effect. The writing of half-turns was
observed for .o slashed. close to 80.degree. and with a voltage of
40-50 V. This difficulty in writing is due to the fact that the
polymeric anchorings have a very high anchoring energy.
[0215] On the other hand, for all geometries, the inventors
observed erasure, by elastic transient breaking, of those initial
structures which include a planar region in the volume. The oblique
slave plates can therefore be used to write and erase bistable
volume textures on condition of having a low anchoring energy and a
low inclination.
[0216] The two volume textures used in the devices described above
are bistable: in the absence of an external field they cannot
undergo a transition to another, lower-energy texture except by
surface breaking or by defects. Therefore, in the absence of an
external field and of any defects, each of the textures is stable
for an infinitely long time by virtue of the topological
incompatibility of the two textures.
[0217] However, in practice the two textures may have very
different energies, especially in the case of nematic liquid
crystals. This may create defects which move with greater or lesser
rapidity, spontaneously erasing the high-energy texture and writing
the other. This property may be undesirable in some applications,
if a long memory time is required of the device.
[0218] The time for spontaneous erasure caused by movement of the
defects depends on several parameters: thickness of the cell,
dimensions of the pixel, chiral dopant, geometry (greater or lesser
pretwist), etc. Some of these parameters may be adjusted in order
to lengthen or shorten the spontaneous erasure time. For example,
FIG. 50 shows the dependence of the spontaneous erasure time
.tau..sub.e on the thickness of the cell (2.times.2 mm.sup.2 pixel,
undoped 5 CB nematic with a half-turn twist texture, which
transforms into a uniform texture with no pretwist). It may be seen
that the time .tau..sub.e changes with thickness over a wide range
(from 0.1 to 1 second) and may be adjusted depending on the
application. The spontaneous erasure time .tau..sub.e may also be
controlled by the pretwist .o slashed. of the cell. At .o
slashed.=90.degree. for example, the energies of the two textures
become equal and .tau..sub.e tends toward infinity.
[0219] However, the thickness and the pretwist are also important
parameters in the case of field-induced surface transitions. It is
therefore preferable to control .tau..sub.e by other means, in
particular by the spontaneous twist induced in the nematic by
adding a low concentration of cholesteric.
[0220] The inventors prepared several specimens with
nematic-cholesteric mixtures in order to control the spontaneous
erasure time .tau..sub.e . FIG. 51 shows the optical behaviour of
such a specimen when the half-turn (180.degree. twist) texture is
written and when it is erased (uniform texture, with no twist). The
energies of the two textures were made almost equal by adding a few
per cent of cholesteric. As a result, the time .tau..sub.e was
extended to several hours; it is infinitely long compared to the
scale of the figure.
[0221] The possibility of extending the spontaneous erasure time of
the bistable textures by equalizing their energies has been
described above. This makes it possible to use one of the main
advantages of the bistability: after cutting off the field, the
final texture is preserved indefinitely, or at least for a time
.tau..sub.e which is very long compared to the refresh time of the
device .tau..sub.r, specific for each application. For some
applications, .tau..sub.r is very long or ill-defined. For example,
be chosen (FIG. 52) one of which (FIG. 52A) has a field-free
elastic energy which is much greater than the other (FIG. 52B). As
a non-limiting example corresponding to an oblique anchoring on the
plate 10 and a planar anchoring on the plate 12, the low-energy
texture B in FIG. 52B is the twist-free slightly splayed texture
and the texture A in FIG. 52A is the half-turn twist texture.
[0222] In the absence of an external electric field, the texture A
is metastable: in a characteristic time .tau..sub.e it transforms
into B by nucleation and propagation of defects. The time
.tau..sub.e is adjusted to .tau..sub.e=.tau..sub.r, the refresh
frequency, by controlling, for example, the density of the defect
nucleation centres on the plates or by chiralization of the
nematic. In such a way, the texture A, once written, self-erases
after .tau..sub.r, that is to say at the end of the image. In order
to break the anchoring, only a single mechanism is now required,
that which writes the texture A. For this example, the hydrodynamic
coupling already described may be used.
[0223] A second non-limiting example of a metastable-mode device is
shown in FIG. 53. This time, opposite oblique anchoring is chosen
on the two plates 10, 12 and the texture A may be written by the
irreversible transient breaking of the anchoring on the slave plate
12. In a field E>E.sub.c, the anchoring on the plate 12 breaks
and .theta..sub.S2 jumps to the other side of the bifurcation, as
described already. In this specific case, the hydrodynamic coupling
and elastic coupling do not oppose each other but, on the other
hand, help each other to write the texture A. Writing becomes very
effective and rapid, and the threshold E.sub.c decreases.
[0224] The metastable anchoring-breaking displays described above
have, in particular, the following advantages.
[0225] The metastable display preserves all the advantages of
surface bistability, except the infinite memory, which in this case
is limited to the time .tau..sub.e. The write time .tau..sub.i is
very short, typically .ident.10 .mu.s for U.ident.20 volts.
[0226] A first advantage of the metastable display compared with
the conventional nematic displays is its abrupt threshold.
[0227] In a conventional display, a change in texture (and
therefore an optical response) is produced by applying a strong
short pulse. The optical response just after the pulse is plotted
in FIG. 54 (curve a) as a function of the applied voltage. At
U<U.sub.c, the threshold voltage, no change in texture occurs.
At U>U.sub.c, an optical response, the amplitude of which
increases with voltage, is obtained. The threshold is greatly
spread out, of the to the gradual change-over in the textures,
which pass, during and after the drive voltage, through a whole
continuum of intermediate states. This spread greatly limits the
multiplexing rate of continuous-response systems.
[0228] In the metastable displays in accordance with the present
invention, the threshold U.sub.c is very abrupt (curve b in FIG.
54): there is no longer a progressive B.fwdarw.A change-over, and
the display writes in "all or nothing" mode. No intermediate states
exist which would impart a gradual nature to the optical response.
Those skilled in the art know that such an abrupt threshold makes
infinite multiplexing possible. This is a very important advantage
compared with classical volume displays.
[0229] Another advantage of the invention compared with
nematic-liquid-crystal volume displays is the possibility of
adjusting the erasure time .tau..sub.e depending on the
application, without changing the duration and the voltage of the
drive pulses. It is possible, for example, to control .tau..sub.e
by the cholesteric doping or by the density of defect nucleation
centres, independently of the surface breaking threshold. In
contrast, in conventional nematic volume displays, a relationship
exists between the write time .tau..sub.i, the field-free erasure
time .tau..sub.e and the threshold voltage
U.sub.c:.tau..sub.e=.tau..sub.iU.sub.c.sup.2/U.sub.0.sup.2, or
U.sub.0.ident.1 V for nematics with a high dielectric anisotropy.
For rapid writing (.tau..sub.i.about.10 .mu.s) and video-rate
erasure (.tau..sub.e.about.40 ms), U.sub.c.ident.60 V is obtained
for a nematic volume display, compared to only U.sub.c.ident.15 V
for the metastable display of the invention (FIG. 55).
[0230] The inventors have produced a cell in the geometry of FIG.
52 which corresponds to the first example mentioned above, with the
undoped 5 CB liquid crystal. The master plate 10 is oblique (SiO,
grazing evaporation, .theta..sub.S1.apprxeq.55.degree.) and the
slave plate 12 is planar (SiO, .theta..sub.S2=90.degree.). The
thickness of the cell is defined by ball spacers (d=1.5 .mu.m)
placed on the planar plate 12. The texture A (twist half-turn) is
written using short rectangular pulses. The very low write
threshold in this cell (FIG. 55) confirms the effectiveness of the
hydrodynamic control brought about by the oblique master plate 10.
The spontaneous erasure occurs by nucleation of defects on the
spacer balls, the density of which was chosen to be quite high in
order to obtain a reasonably short erasure time
.tau..sub.e.about.300 ms (FIG. 56).
[0231] In a second cell, which corresponds to the second example
mentioned above, the inventors have tested the writing, using
elastic breaking, in the geometry of FIG. 53. This cell, filled
with pure 5 CB, is thicker (d=3.3 .mu.m) in order to demonstrate
that the elastic effect does not depend critically on the
thickness. The master plate 10 has a highly oblique anchoring (SiO,
grazing evaporation, .theta..sub.S1.apprxeq.55.de- gree.), while
the slave plate 12, prepared by deposition of a thin film of PVA on
the evaporated SiO, is slightly oblique (.theta..sub.S2.apprxeq.86-
.degree.). Once again, the write threshold for the half-turn state
is very low (E.sub.C=11 V/.mu.m for .tau..sub.i.about.10 .mu.s),
demonstrating the effectiveness of the elastic mechanism for
anchoring breaking. For this thick cell, .tau..sub.e.about.3 s is
measured.
[0232] The methods of excitation in an alternating field, in
accordance with the present invention, will now be explained in
detail.
[0233] For practical reasons, liquid-crystal displays must
preferably be driven by "alternating" signals such that the mean
value of the applied voltage is as low as possible. This makes it
possible to avoid the irreversible electrochemical effects which
would limit the lifetime of the display. The inventors have
demonstrated experimentally the equivalence of "polar" and
"alternating" signals for causing surface breaking. This arises
physically because of the fact that the volume moments transmitted
to the surfaces are mainly of a dielectric origin (.about.E.sup.2)
and does not depend on the sign of the electric field.
[0234] By way of example, the inventors have shown the equivalence,
in the case of writing, of a "polar" signal having an amplitude
V.sub.p=13 V and a duration .tau.=40 .mu.s, as illustrated in FIG.
57, with a square "alternating" signal having an amplitude very
close to V.sub.a=13.4 and the same duration, as illustrated in FIG.
58.
[0235] The same equivalence is observed for the erasure signals: a
polar voltage V'.sub.p=5 V and .tau.=240 .mu.s gives erasure while
an alternating voltage of approximately V'.sub.a=5.3 gives the same
erasure.
[0236] The inventors have observed a small difference between two
"alternating" signals of opposite phase, as illustrated in FIGS. 59
and 60. This difference arises from a small flexoelectric
contribution to surface anchoring breaking. They observe, for
example that the signal in FIG. 59, having V.sub.+=5.8 and
.tau.=240 .mu.s, writes the half-turn and that the signal V.sub.-,
having the same amplitude and same duration, in FIG. 60 causes
erasure. This may make it possible in practice to use only the
phase of alternating signals to cause surface breaking, as
explained previously, by using the sign of polar signals.
[0237] (22) P. G. de Gennes, "The Physics of Liquid Crystals",
Clarendon Press, Oxford, 1974.
[0238] (23) Journal of Applied Physics, vol. 64, No. 2, pp 614-628,
1988, "Origin and characteristics of the optical properties of
general twisted nematic liquid crystal displays" by H. L. Ong.
[0239] (24) Journal de Physique Lettres [Journal of Physics
Letters], vol. 46, pp L195-L200, 1985, "Linear flexoelectro-optic
effect in a hybrid aligned nematic liquid crystal cell" by N. V.
Madhusudana and G. Durand.
[0240] (25) Europhysics Letters, vol. 5, No. 8, pp 697-702, 1988,
"Order Electricity and Oblique Nematic Orientation on Rough Solid
Surfaces" by M. Monkade, M. Boix and G. Durand.
* * * * *