FR2624985A1 - LIQUID CRYSTAL OPTICAL DEVICES WITH A SURFACE ORDER CONTROLLED GRADIENT - Google Patents

Abstract

The present invention concerns the field of liquid crystal devices. According to the invention, a roughness of the order of magnitude of the liquid crystal molecules is defined on at least one of the plates. This roughness results in an order gradient which induces an ordoelectric polarization associated with a depolarizing field which acts on the liquid crystal molecules according to an orientation oblique with respect to the plate. By controlling the thickness and preferential direction of roughness, the oblique orientation psi and the azimuthal orientation phi of the molecules can also be controlled in a continuous range of variation. The invention also makes it possible to obtain nonpolar interfaces for ferroelectric smectic C* displays.

Description

 The present invention relates to optical devices using liquid crystals.

The present invention was made in the Laboratory of
Solid State Physics of the University of Paris Sud, laboratory associated with
NATIONAL CENTER FOR SCIENTIFIC RESEARCH number 04 0002.

Numerous research studies have been carried out for at least fifteen years on liquid crystals.
Different results of research carried out at
Laboratory of Solid State Physics of the University of Paris Sud are described in the French patent application filed on April 28, 1982 under number 82 07309 and published under number 2 526 177, the French patent application filed on October 23, 1984 under the number 84 16192 and published under the number 2 572 210, the French patent application filed on June 18, 1985 under the number 85 09224 and published under the number 2 587 506, or the French patent application filed on May 14, 1986 under number 86 06916 and published under number 2 598 827.

 In addition, work on liquid crystals has given rise to numerous publications.

We will cite for example the documents: I) Applied Physics
Letters volume 25, N. 9, November 1974, Urbach and AI: "Alignment of nematics and smectics on evaporated films", pages 479 to 481, 2) Letters in Applied and Engineering Sciences, volume 1, 1973, pages 19-24, Guyon et Al: 'tOn different boundary conditions of nematic films deposited on obliquely evaporated plates ", and 3) Physical Review Letters ,. volume 56,
N. 19, May 1986, Durand et aI: "Order electricity and surface orientation in nematic liquid crystal, pages 2056 to 2059".

 Document 1) Applied Physics Letters, volume 25, N. 9, November 1974, reports experimental tests carried out on a liquid crystal device comprising a plate on which a deposit of gold or SiO, with a thickness of d ' order of 50 to 1000 A, was previously carried out by evaporation under oblique incidence. According to the results recorded in this document, it is possible to obtain either a planar orientation of the liquid crystal, that is to say molecules of liquid crystal oriented parallel to the plates of the device, or an oblique orientation of the liquid crystal, that is to say ie liquid crystal molecules tilted relative to the plates. According to this document I) page 480, lines 43 to 46, the oblique inclination of the molecules relative to the plates is practically independent of the thickness of the coating deposited by evaporation.

Document 2) Letters in Applied and Engineering
Sciences, volume 1, pages' 19-24, 1973, proposes a modeling of the results recorded in the document 1) This modeling concerns the theoretical case of a deposit by evaporation having a thickness of 100 to 6000.

Document 3) Physical Review Letters, volume 56, N. 19,
May 1986, relates a theoretical analysis concerning the influence of the disturbance of the nematic order, at the level of a nematic / isotropic liquid interface. I1 emerges from this analysis that the order disturbance at the interface is accompanied by an energy which forces the molecules at an angle called "magic angle" defined by cos2 e = 1/3.

 The present invention now aims to provide new means for defining, simply, reliably and economically, an oblique inclination of the liquid crystal molecules relative to a plate of the device.

 The present invention, which is based on numerous theoretical studies and experimental observations, proposes for this purpose to define on a plate of the device, a roughness of the order of molecular size of the liquid crystal considered.

 Preferably, the roughness thus defined on the plate of the device will be of the order of magnitude from 20 to 40 A.

 The inventors have determined that such roughness results in an order parameter gradient which induces an ordoelectric polarization, associated with a depolarizing field which tends to force the molecules of the crystal in an oblique orientation relative to the plate.

 More precisely still, by extending their research, the inventors determined that, by defining a preferential direction of roughness, for example by depositing on the plate a rough coating by evaporation in a determined direction, it was possible to obtain both an oblique orientation and a controlled azimuthal orientation of the liquid crystal, relative to the plates. More precisely still, the oblique orientation and the azimuthal orientation of the liquid crystal evolve continuously, in an oblique plane with respect to the plates as a function of the thickness of the roughness and of its preferential direction, that is to say in the case of an evaporation as a function of the direction of incidence.

 This arrangement is likely to give rise to numerous applications. In fact, it suffices to control the thickness of roughness and its preferential direction to impose both the oblique orientation and the azimuthal orientation of the liquid crystal.

 Other characteristics, objects and advantages of the present invention will appear on reading the detailed description which follows, and with reference to the appended drawings, given by way of nonlimiting examples and in which - FIGS. 1 to 4 represent, for four experiments, the azimuthal orientation tp and the oblique bearing (8 of the liquid crystal as a function of the angle of evaporation and the thickness of a deposit made on a plate ,, - FIG. 5 represents geometrically the oblique and azimuthal orientation of a liquid crystal molecule relative to the direction of evaporation, - Figure 6 represents two curves illustrating the azimuthal orientation and the oblique orientation Y as a function of a thickness of deposit, for an angle determined evaporation, - Figures 7, 8 and 9A, 9B schematically illustrate three applications of the invention to nematic liquid crystal devices, and - Figure 10 shows curves illustrating the evolution of the orientation n oblique of a smectic liquid crystal, on each of the two faces of the device, according to a thickness of SiO deposited.

CENERAL ANALYSIS
The present invention is based on the following theoretical analysis.

Nematic liquid crystals can be considered as liquids of oriented electric quadrupoles for which one can write
Q = -e3 / 2 S (- T / 3) relation in which - -e represents the density of quadrupoles (-el0 cgs), - S represents the order parameter of the liquid crystal (S is between
O and 1), - n is the unit vector representing the orientation of the molecules.

In the presence of spatial variations, we obtain the electrical polarization P = -V. = Q = 3/2 eV.S (-I / 3). ~~
We know that if, working at constant S, V acts on n, we obtain the flexoelectric effect and the polarization P becomes proportional to the curvature (V.).

If on the other hand we work at n constant and S variable, we obtain the ordoelectric polarization
P = 3/2 eV S. [- 1/3.

 This polarization is generally oblique with respect to #S.

With this polarization is associated a depolarizing field defined by Dz = O = 4TC Pz + e Ez (where z 'is the direction of
The energy associated with a density
1/2 (E / 4R) EZ2 = 1/2 (4Tt /) pZ2-
This energy is minimum for pz = O, that is to say for an oblique orientation e with respect to 7 defined by cos # = 1/3.

BASIS OF THE INVENTION AND FIRST TEST RESULTS
Starting from the general analysis above, according to which a gradient of the order parameter must lead to a stress on the liquid crystal in an oblique orientation relative to the plates, the inventors proposed the idea of controlling the order parameter S, on the surface of a liquid crystal device, by deposition on a plate of the device of small grains whose size is of the order of magnitude of the molecular size of the liquid crystal considered, that is to say of l '' from 20 to 40 A.

 The experiments were carried out using a nematic liquid crystal having an orientation parallel to the surface of the plate, but degenerate in the presence of a polished plate, that is to say of variable direction in a plane parallel to the surface of the plaque.

 More precisely, the inventors have controlled the orientation of the liquid crystal for different pairs of evaporation angle and thickness of the deposited grains. The inventors have found that achieving a rough surface state by evaporation in a determined direction makes it possible to obtain not only an oblique orientation of the liquid crystal molecules relative to the plates, but also an azimuthal orientation which is controlled and variable with respect to to the direction of evaporation.

 The first results obtained are recorded in Figures 1 to 4 and 6 attached.

 These results are illustrated with reference to the geometric model represented in FIG. 5.

 This geometric model includes a coordinate system with three axes x, y and z, orthogonal to each other.

The axes x, y extend parallel to the plate P considered. The z axis extends perpendicular to this plate P, in the direction of the volume of the liquid crystal. The direction of evaporation arbitrarily coincides with the plane defined by the x and z axes. The incidence of the direction of evaporation, which corresponds to the inclination between the direction of evaporation and the z axis is defined by the angle d
The oblique orientation of the liquid crystal, which corresponds to the inclination between the z axis and the longitudinal axis of the liquid crystal molecules is illustrated by the angle e in FIG. 5. The complement of this angle e is referenced tp
Finally, the azimuthal orientation of the liquid crystal, which corresponds to the inclination between the x axis and the projection onto the xy plane of the longitudinal axis of the molecules is illustrated by the angle referenced k4 in FIG. 5.

 Illustrated in Figure 1 attached is the evolution of the azimuthal orientation 9 for MBBA liquid crystal as a function of the evaporation angle and the thickness of a deposit of SiO on ordinary glass.

 For a deposit thickness less than 5, it can be seen that the liquid crystal molecules are oriented parallel to the plates, but in random directions.

 When the thickness of the deposit reaches 5, the degeneration is lifted, that is to say that the molecules orient themselves parallel to themselves and perpendicular to the plane xz defining the direction of evaporation. At this stage the azimuthal orientation 9 of the molecules equals 90, and the oblique orientation 43 '= 90 ".

 This initial position of the molecules is illustrated diagrammatically in FIG. 5 under the reference M '.

 When the thickness of the deposit increases further, a progressive azimuthal rotation of the molecules in an oblique plane with respect to the plate P is obtained as a function of the evaporation angle i. The liquid crystal then passes from a domain for which l the azimuthal orientation tp equal 900 to an azimuthal domain for which) can reach 00, that is to say a domain for which the molecules are oriented substantially parallel to the plane xz containing the direction of evaporation. The azimuthal rotation is accompanied by a raising of the molecules; when gradually goes from 90 0 to 0 ", - goes from 0 to about 20-30.

 The hatched range in FIG. 1 corresponds to the range of values of the angle of evaporation i and of the thickness of evaporation for which the azimuthal orientation varies progressively between 90 and 0 and the oblique orientation # varies progressively between 0 and about 20-300.

This progressive variation of the azimuthal orientation 9 and of the oblique orientation 8), controllable as a function of the roughness, constitutes an essential characteristic of the invention, original compared to the prior art according to which only the terminals 9 = 90 or O and # = O or 20-300 were known.

 Comparable results are illustrated in Figure 2 attached. This FIG. 2 relates to tests carried out under the same operating conditions, that is to say deposition by evaporation of SiO on ordinary glass, for different values of evaporation angle and thickness of deposition, but with the difference of Figure 1, using liquid crystal 5 CB instead of MBBA.

 Figures 3 and 4 similarly represent the value of the azimuthal orientation # as a function of the evaporation angle od and the thickness of deposition of -SiO, on an ITO plate, respectively for MBBA liquid crystal. on the one hand, and 5CB on the other.

 Figures 2, 3 and 4, like Figure I above, reveal a range (hatched in the figures) of pairs of values, angle of evaporation d / thickness of deposit, for which the azimuthal orientation X passes progressively from 90 " at Oc.

 FIG. 6 represents the simultaneous evolution of the azimuthal orientation kp and the oblique orientation4> as a function of the thickness of a deposit of SiO evaporated on an ITO plate with an incidence Dg of 74 ", with the use of a 5CB liquid crystal.

 In agreement with the results illustrated in FIG. 4, it is recognized that the azimuthal orientation 9 progressively passes from 90 "to 0" for a thickness of deposit comprised substantially between 30 and 40.

 In parallel, we can see that the oblique orientation 9) gradually rises from 0 to 20 for an evaporation thickness ranging from 30 to 40 A.

 In conclusion, the tests carried out by the inventors have made it possible to observe that the bare substrate, before evaporation, promotes a planar orientation, that is to say 4 parallel to the surface, but degenerate.

When the average evaporated amount (of SiO) reaches 5 AC, the azimuthal degeneration is lifted, but the anchoring remains planar. Above 10A of average evaporation, there is an increase in the nematic, which rotates in an oblique plane from its initial planar position, perpendicular to the direction of evaporation, to an oblique position towards the direction of evaporation.

 Of course, the realization of a rough state on the plates, capable of generating an order gradient, and from there thanks to the induced ordoelectric polarization, to request a raising of the liquid crystal in an oblique orientation relative to the plates, can be operated by means other than evaporation, such as for example, by chemical attack, spraying, or even ion bombardment.

 We will now discuss in the following description different examples of applications of the present invention, without limitation.

APPLICATION TO THE PRODUCTION OF DISPLAYS
SUPER TWIST NEMATICS
We are trying to develop nowadays nematic displays having anchoring directions on 'the two plates, inclined relatively by an angle 2r greater than 90, with oblique orientation ç relative to at least one of the plates, as illustrated schematically in Figure 7 attached, to allow multiplexing of the device and obtain a satisfactory contrast. The rough surface state in accordance with the present invention makes it possible to easily obtain an oblique orientation YJ of 10 to 20 relative to the plates, and a controlled azimuthal orientation.

APPLICATION TO THE PRODUCTION OF ANCHORAGES
BISTABLE NEMATICS.

 The inventors have also determined that the molecules of the liquid crystal are capable of taking two symmetrical azimuthal orientations with respect to the plane xz containing the direction of evaporation.

 There have thus been shown diagrammatically in FIG. 8 two azimuthal orientations symmetrical with respect to the plane xz defined by the direction of evaporation. By choosing the angle of evaporation Q (and the thickness of evaporation on a plate so that the azimuthal orientation (p with respect to the plane xz is of the order of + 45 ", we can easily, by application of an external electric field cause the controlled tilting of the molecules from one azimuthal orientation to the other on said plate. This gives a modification of the azimuthal orientation of the molecules of the order of 90, making it possible to obtain a maximum contrast, if the device is placed between polarizers.

 In the absence of an electric field, the positions obtained from the liquid crystal remain stable.

 If an electrode or plate is provided with a rough surface state, two domains + tP are obtained.

 If the two electrodes or plates are provided with a rough surface state, four domains can be obtained: + a ++ b; + a 9 a + Pb; a -, considering that the indices a and b denote the upper and lower plates respectively.

APPLICATION TO CRYSTAL DEVICES
NEMATIC LIQUID AT THE INSTAESILITE THRESHOLD
In the context of the third envisaged application of the invention to the design of nematic liquid crystal devices, a planar and parallel anchoring is defined on the two plates of the device, as illustrated in FIG. 9A. However, on one of the plates, the angle of evaporation d and the thickness of evaporation of a deposit are controlled to place the liquid crystal in the vicinity of the hatched transition range in FIGS. 1 to 4. The liquid crystal is thus placed in the vicinity of an instability threshold, that is to say in the vicinity of an azimuthal tilting threshold. This arrangement facilitates the tilting of the liquid crystal towards a helical formation, by application of a external electric field, as is known per se and as schematically illustrated in FIG. 9B.

 As a variant, it is possible initially to define a helical arrangement of the liquid crystal, close to an instability threshold, and to transform this helical arrangement into a parallel orientation of the molecules by application of an external electric field.

APPLICATION OF THE INVENTION TO THE POLARITY CONTROL OF
SOLID-SMECTIC C-FERROELECTRIC INTERFACES.

 Ferroelectric smectic C displays, when they have a degenerate surface anchoring, theoretically allow two positions of the symmetrical molecules with respect to the smectic layers.

In one of these positions, the electric dipole of the molecules is oriented towards the electrode, in the other position, it is oriented towards the volume, by an external applied field. In theory, these two positions should be equivalent and allow the realization of bistable displays.

 However in practice, it is found that instead of a bistable display between two symmetrical states, one obtains monostable displays on distorted textures whose contrast is poor and which cannot be multiplexed. The inventors have determined that this phenomenon is due to the fact that the electrode / liquid crystal interface is polar. This polarization seems to be chemical in nature (difference in affinity between the surface and part of the molecules).

 The present invention makes it possible to cancel the chemical polarization by virtue of an ordoelectric polarization induced by creation of a gradient of nematic order; in the smectic phase, of suitable sign and amplitude.

 To obtain a good compensation for the chemical polarity, it is possible to take highly polished interfaces, on which the nematic order is greater than the volume value, or rough interfaces on which the surface order is lower.

 Illustrated in FIG. 10 is the orientation 9 S of the liquid crystal thus obtained on the lower and upper plates of the device for different thickness values of a deposit of SiO operated by evaporation at an incidence t of 81 "on plates. ITO separated by 3um, respectively for temperatures of 550C and 48.0 C.

 Of course, the present invention is not limited to the above embodiments and applications. It extends to any variant that conforms to its spirit.

Claims (12)

 1. A method of preparing a liquid crystal device, characterized in that it comprises the step of defining a roughness of the order of molecular size of the liquid crystal used on at least one of the plates.  2. Method according to claim 1, characterized in that the roughness produced on at least one of the plates of the device has a thickness between 20 and 40 A.  3. Method according to one of claims 1 and 2, characterized in that the roughness is produced on at least one of the plates of the device using one of the following techniques: deposition of a coating by evaporation, deposition of a coating by spraying, chemical attack or ion bombardment.  4. Method according to one of claims 1 to 3, characterized in that the roughness is carried out on at least one of the plates with control of a preferential direction of roughness, for example by control of a direction of evaporation, to obtain both an orientation of the molecules of the oblique liquid crystal with respect to at least one plate, and an azimuthal orientation of the molecules with respect to the preferential direction of roughness.  5. A liquid crystal device obtained by implementing the method according to one of claims 1 to 4 of the type comprising two parallel plates and a material comprising liquid crystal molecules placed between the two plates, characterized in that at least one of the plates has on its surface adjacent to the liquid crystal a roughness of the order of molecular magnitude of the liquid crystal considered.  6. Device according to claim 5, characterized in that the roughness produced on at least one of the plates has a thickness of between 20 and 40.  7. Device according to Itune of claims 5 and 8, characterized in that a roughness is produced on each of the two plates of the device.  8. Device according to one of claims 5 to 7, characterized in that the liquid crystal is a nematic liquid crystal.  9. Device according to one of claims .5 to 7, characterized in that the thickness and the orientation of the roughness on at least one of the plates are suitable for defining an oblique orientation Y / of the liquid crystal by with respect to at least one of the plates and of the directions of anchoring on the two plates inclined relatively by an angle ss greater than 90, for the production of nematic displays with superforms.  10. Device according to one of claims 5 to 8, characterized in that the roughness has a preferred direction, defined for example by a direction of evaporation, that the thickness and the orientation of the roughness on one at least plates are adapted to define two possible azimuthal orientations Ap of the liquid crystal molecules symmetrical with respect to the preferential direction of roughness and of the order of 45C with respect to this direction, and that it comprises means making it possible to apply an external electric field to the device to cause the controlled tilting of molecules from one azimuth orientation to the other, in order to achieve bistable nematic anchors.  11. Device according to one of claims 5 to 8, characterized in that the thickness and the orientation of the roughness on at least one of the plates are adapted to place the liquid crystal in the vicinity of a threshold of azimuth tilting to facilitate this tilting when applying an external electric field.  12. Device according to one of claims 5 to 7, characterized in that the liquid crystal is a ferroelectric smectic C and that the thickness and the orientation of the roughness on at least one of the plates are adapted to define an ordoelectric polarization canceling the overall surface polarization.