Technical background Liquid crystals, widely used at present as electro- optical media in display devices, are organic materials with anisotropic physical properties. The operation of the liquid crystal displays is based on the changes of the optical appearance of the liquid crystal in the dis- play caused by an applied electric field.

One of the basic operational principle of liquid crystal displays and devices is the switching of the ori- entation of the liquid crystal molecules by an applied electric field that couples to the dielectric anisotropy of the liquid crystal (dielectric coupling). Such a cou- pling gives rise to an electro-optic response quadratic with the applied electric field, i. e. independent of the field polarity.

Another operational principle of practical impor- tance is the one utilizing linear coupling between an ap- plied electric field and a spontaneous polarization Ps present in ferroelectric liquid crystals.

There exist a number of different types of LCDs whose operation is based on dielectric coupling, espe- cially dynamic scattering displays, displays using defor-

mation of homeotropically aligned nematic liquid crystal, Schadt-Helfrich twisted nematic (TN) displays, super twisted nematic (STN) displays, and in-plane switching (IPS) nematic displays.

Another type of LCDs is the surface stabilized liq- uid crystal (SSFLC) display that operates on the basis of the linear coupling between the applied electric field and the spontaneous polarization Ps. The applied electric field could also give rise to a spontaneous polarization that linearly couples to the field. The deformed helix ferroelectric (DHF) liquid crystal displays and the anti- ferroelectric liquid (AFLC) displays are based on this principle.

For modern applications, an LCD should possess sev- eral important characteristics, such as a high contrast and brightness, a low power consumption, a low working voltage, short switching times, a low viewing angle de- pendence of the contrast, a grey scale or bistability, etc. The LCD should be cheap, easy to produce and to work with. Non of the prior-art LCDs is optimised concerning all the important characteristics.

In most of the conventional nematic liquid crystal displays, operating on the basis of the dielectric cou- pling, the electric field is applied normally to the liq- uid crystal bulk layer. These displays are usually slow, and nearly all suffer from non-satisfactory angular de- pendence of the contrast due to the switching of the liq- uid crystal molecules in a plane formed by the initial orientation of the molecules and the applied electric field (so-called out-of-plane switching).

There is also another type of LCDs with in-plane switching, in which the electric field is oriented paral- lel to the liquid crystal bulk layer. These displays ex- hibit a very small angular dependence of the image con- trast but the brightness and the switching times are not satisfactory.

Next, the other type of LCDs operating on the basis of the linear coupling between an applied electric field and the spontaneous polarisation in ferroelectric liquid crystals will be considered giving a short general de- scription of the smectic liquid crystals with emphasis on ferroelectric liquid crystals and their application in the surface stabilised ferroelectric liquid crystal (SSFLC) display.

In a smectic liquid crystal, the molecules are ar- ranged in adjacent smectic layers. The Smectic A and Smectic C phases are the two most important representa- tives of these"layered"phases of the liquid crystalline materials. In the Smectic A phase, the molecules are ori- ented along the smectic layer normal (0=0°), whereas in the Smectic C phase the molecules are tilted with respect to the smectic layers at angle typically in the order of 20°. Furthermore a smectic liquid crystal could be achiral (e. g. A or C) or chiral (e. g. A* or C*), where the term chirality means lack of mirror symmetry. It should be notified that the term"chiral"does not refer to the occurrence of a helical molecular ordering that may or may not appear as a secondary effect.

The chirality, i. e. the broken mirror symmetry of the constituent molecules, is a prerequisite for the ap- pearance of ferroelectricity, i. e. of spontaneous polari- sation Ps, in tilted smectic liquid crystals like Smectic C*. Ps is directed along the smectic layers, i. e. perpen- dicular to the molecular long axis, and it is sterically coupled to the molecules.

Ferroelectric phases exhibit not only the smectic liquid crystal materials built up of chiral molecules but also achiral smectic host materials doped with chiral dopants. Most of the available commercial ferroelectric liquid crystal materials are such mixtures. In these mixed systems it is much easier to combine the broad tem- perature range and low viscosity of the selected achiral smectic C host material with the large polarisation in-

duced by the selected chiral dopants. The proper combina- tion of these material parameters is of vital importance for the ferroelectric liquid crystal mixture.

In Smectic C*, the molecules, being free to rotate around a cone with apex angle ß=2O, adopt a helical or-' dering due to chirality with a helix axis along the smec- tic layer normal. However, the Ps is spiralling, thus re- sulting in self-cancelling of the local polarisation.

Therefore, in a bulk of Smectic C* there will be no mac- roscopic polarisation present (Ps = 0). However, if an electric field is applied along the smectic layers of the helical Smectic C* bulk, the field will couple to the permanent dipoles aligning them parallel to the field. As a result, the electric field will unwind the helical or- der inducing macroscopic polarisation of the bulk of Smectic C* liquid crystal.

There is another way of suppressing the helical mo- lecular order in order to obtain a spontaneous polarisa- tion Ps in the bulk of Smectic C*. This is done by using the solid surface/liquid crystal interactions instead of an external electric field. This is the so-called surface stabilised ferroelectric liquid crystal (SSFLC) concept according to which the Smectic C* bulk is aligned in book-shelf geometry, i. e. with Smectic layers perpendicu- lar to the confining surfaces.

In SSFLC displays, an external electric field ap- plied along the smectic layers, i. e. perpendicular to the substrates, will switch the molecules of the ferroelec- tric liquid crystal between two positions on the smectic cone. These two positions correspond to the directions of Ps at different field polarity. An important feature is that the"flipp-flopp"mechanism ("the Goldstone mode) is much faster than the rather slow dielectric coupling mechanism that switches liquid crystalline materials hav- ing no permanent polarisation, such as conventional twisted nematic liquid crystal displays. Moreover, with a proper surface treatment of the solid display substrates,

the switching of the liquid crystal molecules in SSFLC displays can be bistable. In the single component FLCs as well as in the FLC mixtures, the Ps present a homogeneous distribution in the bulk of these materials.

As an alternative to FLCs, the liquid crystal mate- rial could be in the so-called antiferroelectric (AFLC) liquid crystal phase, which means that in the absence of electric field, the molecules in adjacent smectic layers possess an opposite tilt. In an AFLC display, the dark state is obtained when placing the display between crossed polarizer set parallel and perpendicular, respec- tively, to the smectic layers. At this arrangement and when there is no applied electric field, the AFLC display is in the dark state. Under an applied electric field, both-E and +E give the same bright state. Thus, the AFLC display exhibits a three-state switching behaviour.

Drawbacks of the surface stabilized FLC and AFLC displays include difficulties in orienting the chiral smectic phases and maintaining their orientation. The FLC displays in addition have a sticking image problem due to building surface charged layers. The power consumption of these displays is also relatively high. Yet another draw- back is that the thickness of the liquid crystal layer in these devices must be of order 1-1.5 jj. m in order to ob- tain the unwound state of these materials. This require- ment for small thickness makes the production of FLC and AFLC displays complicated, delicate and very expensive.

In the displays discussed above, the desired align- ment of the liquid crystal layer is achieved by appropri- ate treatment of the confining solid surfaces like coat- ing with organic or inorganic layers as well as using a mechanical buffing. In the absence of external fields, the initial liquid crystal alignment is defined by solid surface/liquid crystal interactions.

The orientation of the liquid crystal molecules at the solid surface is transferred to liquid crystal mole- cules in the bulk via elastic forces. For instance, near

the substrate surface the liquid crystal molecules in general could be oriented perpendicular (homeotropic) or parallel (planar) to the substrate surface that impose the same alignment of the molecules in the bulk of the liquid crystal. Since the liquid crystals are strongly birefringent, any change in their alignment will result in a certain change in their optical performance as seen between suitable polarizers.

In the prior of art, there are in principal three different techniques for changing the optical performance of liquid crystals by accomplishing a new molecular ori- entation in the liquid crystals that differs from the initial alignment.

1. Reorientation by application of an external field The first, most widely used technique for reorien- tating the molecules is to apply an external electrical field over the entire bulk liquid crystal layer. Due to direct coupling between the electric field and some of the liquid crystal material parameters, such as dielec- tric anisotropy and spontaneous polarization, the field will directly reorient the liquid crystal molecules in a new direction if their initial alignment does not corre- spond to a minimum energy of interaction of the electric field with some of the liquid crystal material parame- ters. However, in some cases the liquid crystal molecules near the solid surfaces are difficult to reorient by an electric field, due to the above-mentioned surface liquid crystal interactions, whereas the"bulk molecules"more remote from the surfaces are fairly free and therefore easy to reorient by the field.

2. Reorientation by photo-controlled command surfaces The second known technique for reorienting the mole- cules of a liquid crystal layer is to design one or both of the confining alignment surfaces as a photo-controlled "command surface". Such a photo-controlled command sur- face is capable, when subjected to, for instance, UV light, to change the direction of alignment imposed by

the surface on the liquid crystal molecules in contact with the surface.

The concept of"photo commanded surface"has been described by K. Ichimura in a number of papers overviewed in Chemical Reviews, 100, p. 1847 (2000). More specifi- cally, an azobenzene monolayer is deposited onto the in- ner substrate surface of a sandwich cell containing a nematic liquid crystal layer. The azobenzene molecules change their conformation from"trans"to"cis"under il- lumination with UV light. The azobenzene molecules are anchored laterally to the substrate surface by the aid of triethoxysilyl groups. The trans-isomer of azobenzene moieties imposes a homeotropic alignment of the nematic liquid crystal (liquid crystal molecules oriented perpen- dicular to the substrate surface), whereas the cis-isomer gives a planar orientation of the liquid crystal mole- cules (parallel to the substrate surface). Hence, the conformational changes of the molecules in the alignment layer caused by the UV illumination will result in a change of the alignment of the nematic liquid crystal molecules. The relaxation to the initial alignment is ob- tained by illuminating the sample with VIS-light or sim- ply by heating it to the isotropic state.

A drawback of using photo commanded alignment sur- faces in order to switch a liquid crystal alignment be- tween two states is the low speed. In addition, so far the photo commanded alignment surfaces are shown to be effective only in nematic liquid crystal devices. Another drawback is that the life-time of a device having a photo commanded alignment surface is reduced by the degradation processes that take place due to W-irradiation. More- over, the use of light as external factor for switching the alignment of the liquid crystal is not convenient es- pecially for liquid crystal displays.

Therefore, the performance of liquid crystal dis- plays is chosen to be controlled by an external electric field instead of by light. The electric field directly

couples with the bulk liquid crystal and changes align- ment, thereby changing optical characteristics of the liquid crystal display, such as light transparency, light absorption at different wavelengths, light scattering, birefringence, optical activity, circular dichroism, etc.

3. Reorientation by Electrically Commanded Surfaces ECS The third known principle for reorientating liquid crystal molecules involves the use of so-called Electri- cally Commanded Surfaces (ECS). This principle is de- scribed in the published International patent application No. WO 00/03288.

The ECS principle, which is faster, is used to pri- marily control a ferroelectric liquid crystalline polymer layer. As mentioned above, LCDs based on the linear cou- pling between the spontaneous polarisation in ferroelec- tric liquid crystals and an applied electric field per- pendicular to the confining substrates has a number of advantages over LCDs based on a dielectric coupling. More specifically, ferroelectric LCDs are much faster, they allow an in-plane switching of the optic axis with image contrast less dependent on the viewing angle and, at proper anchoring conditions, ferroelectric LCDs makes it possible to achieve a bistable switching. However, as pointed out already, there are several problems related to the use of ferroelectric liquid crystals in displays and devices.

According to ECS principle, a separate thin ferro- electric liquid crystalline polymer layer is deposited on the inner surfaces of the glass substrates confining a liquid crystal bulk material in a conventional sandwich cell. The ferroelectric liquid crystalline polymer layer acts as a dynamic alignment layer imposing a planar alignment on the adjacent liquid crystal bulk material.

More specifically, when applying an external electric field across the cell-and thereby across the dynamic alignment layer-the molecules in the separate ferro- electric liquid crystalline polymer layer will switch.

This molecular switching in the separate polymeric layer will, in its turn, be transmitted into the bulk volume via elastic forces at the boundary between the separate alignment layer and the bulk layer, thus resulting in a relatively fast in-plane switching of the bulk volume molecules.

The ECS principle seems to have all advantages of the ferroelectric liquid crystals avoiding at the same time most of their serious problems. However, there are still several requirements that the ECS materials should meet that make the preparation of ECS somehow quite de- manding: - The ECS layer should be very thin (100-200nm).

- The ECS layer should preferably be well oriented in bookshelf geometry, i. e. with smectic layers normal to the confining substrates.

- In order to keep the ECS layer and it operation intact, the material of ECS layer must be insoluble in the liquid crystal bulk material.

In the light of the above-mentioned desired proper- ties of a liquid crystal device, and the above-mentioned different drawback of the known liquid crystal displays, a general object of the present invention is to accom- plish an improved liquid crystal device, an improved method for manufacturing a liquid crystal device, and an improved method of controlling a liquid crystal device.

The invention is not directed to displays only, but is useful in many other liquid crystal applications.

Description of the invention According to a first aspect of the invention, there is provided a liquid crystal device, comprising a liquid crystal bulk layer and chiral dopants being inhomogene- ously distributed in the bulk layer as a result of being permanently attached to at least one surface, termed chiral surface, said chiral dopants being soluble in the liquid crystal bulk layer.

This is in clear contrast with the prior-art solu- tion of having a separate, non-soluble ECS layer.

The invention makes it possible to induce a local variation of one or more chirality-related physical prop- erties (herein also referred to as effects of chirality), especially a local increase of one or more chirality- related physical properties, within a restricted volume (here termed"sub-volume"or"sub-region") of the bulk layer adjacent to said chiral surface. Thus, the induced variation or increase of an effect of chirality does not take place in the whole volume of the bulk layer, but only in one or more restricted regions, which in most im- plementations of the invention would be very thin. Con- sidering that each such sub-region is located near an as- sociated chiral surface, such a sub-region could also be termed"surface sub-region".

The dopant material and the liquid crystal bulk ma- terial may be selected such that the induced local variation/increase of an effect of chirality within the sub-volume causes the appearance of a spontaneous polari- sation increase at said sub-volume. Such an induced spon- taneous polarisation increase would normally present a non-homogenous distribution, being maximum at said chiral surface and dropping in a direction away from said chiral surface. Such a spontaneous polarisation may, in its turn, provide ferroelectric properties that allows direct electrical coupling to an external electrical field.

As used herein the term"soluble"means that the dopants are able to dissolve in the liquid crystal bulk layer.

According to the invention, the chiral dopants are permanently attached to a surface. This should be inter- preted as the dopants are bonded to the surface in such a way that they are prevented from freely moving out in the bulk volume. Thus, the inhomogeneous distribution of the dopants in the bulk volume is of a"permanent"nature, since a homogenous distribution will not take place. Ob-

viously, this differs from known techniques of temporar- ily repositioning dopants within a liquid crystal bulk volume by means of an applied external electrical field.

Although the dopants are"permanently attached"to the surface, they may still present a limited movability, especially a movability that allows the dopant molecules to reorientate at the surface, e. g. during switching by an applied external electrical field.

Although not homogeneously distributed in the bulk layer, the dopants may be referred to as (locally) dis- solved in the bulk layer.

Within the scope of the invention, the device may present a single chiral surface or, as an alternative, two or more such chiral surfaces each having its chiral dopants permanently attached thereto. More specifically, the device may comprise two or more essentially similar chiral surfaces, or two or more different chiral sur- faces, or combinations thereof. For instance, two chiral surfaces may result in induced spontaneous polarisations of different sign, they may present different twisting powers, different dopant material, different surface dis- tribution of the dopants, etc.

The term"surface"when referring to the above- mentioned"chiral surface"should be held to comprise not only a surface located at or defining the boundary of the bulk layer, but also a surface or a plane located inside the bulk layer. Thus, the term"surface"may comprise any surface physical or geometrical surface or plane having contact with the bulk layer material, directly or indi- rectly via the dopant material.

In those embodiments where the bulk layer is con- fined between two substrates, a chiral surface may be ar- ranged on one or both substrates. As an alternative, or as a complement, one or more chiral surfaces may be lo- cated inside the bulk layer, i. e. at a distance from the bulk layer boundaries. Embodiments having internal chiral

layers or planes may be implemented by e. g. a laser- activated polymerisation process as described below.

The dopant material or molecules permanently at- tached to the surface may be considered as defining a "chiral surface layer". Such a"chiral surface layer" would normally be very thin, e. g. in the order of 20 to 100 A. Especially, the thickness of the chiral surface layer (which extends into the bulk layer) may be formed as a"monolayer"comprising one layer of dopant molecules only.

In this context it should be noted that the princi- ple of attaching the chiral dopants to a surface gives a "bonus effect", considering that an ECS layer preferably should be very thin in order to achieve the aimed-at ef- fect. For instance, the chiral dopants may form a monolayer on said surface.

In the device according to the invention, the dopants, which are soluble in the liquid crystal bulk layer, are inhomogeneously distributed in the bulk volume as a result of being permanently attached to at least one surface. However, as to the distribution of the dopants over said surface (s), many possibilities exist. In one preferred embodiment, the dopants are evenly distributed over the surface or over each surface. However, it may also be possible to distribute the dopants according to a specific or predetermined pattern over said surface (s), e. g. within predetermined areas.

As an illustrative example only, the bulk layer may comprise an achiral (non-chiral) liquid crystal material, such as an achiral Smectic C material. In the sub-volume, or sub-region, the achiral Smectic C is doped by chiral dopants being soluble in the achiral Smectic C and perma- nently attached to the chiral surface. The dopants induce chirality, and thus variation of effect (s) of chirality, in the liquid crystal bulk material within the sub- volume. The induced chirality, in its turn, may give rise to a spontaneous polarisation Ps within the sub-volume

giving this sub-volume ferroelectric properties. The ac- tual volume of the sub-region being doped and the volume presenting chirality and an increased spontaneous polari- sation may differ slightly, since the dopant molecules will induce chirality also at a certain distance from the dopants. In this illustrative example, by applying an electrical field over the device (i. e. over the doped sub-volume as well as over the rest of the bulk volume), the molecules in the region presenting ferroelectric properties as result of the dopants may be switched very fast due to the direct ferroelectric response or coupling to the applied electrical field. This fast ferroelectric switching will, in its turn, result in a fast switching of the bulk molecules outside the sub-volume, because of the elastic coupling between the molecules in the surface sub-region and the adjacent molecules of the bulk volume.

In the example described above, the bulk volume com- prises an achiral liquid crystal material. However, it is also possible to use an initially chiral bulk material, presenting for instance a limited spontaneous polarisa- tion in its non-doped condition. In such an embodiment, the dopants may be selected to accomplish a variation of the spontaneous polarisation within the sub-volume that increases the (e. g. ferroelectric) response of the bulk material within the sub-volume. The initial chirality could be said to"bias"the induced increase of an effect of chirality.

The electrical field response of the bulk material- inside as well as outside the doped sub-volume-may be ferroelectric, antiferroelectric, flexoelectric and paraelectric or any combination thereof. The electrical field response of the undoped bulk material may also be dielectric only, i. e. no direct linear coupling with the electric field.

As to the initial chirality of the bulk layer mate- rial, the bulk layer may comprise an achiral liquid crys- tal material, such as an achiral nematic liquid crystal

material; a chiral liquid crystal material; or combina- tions thereof. If the bulk material is achiral, a chirality may be induced by the dopants. If the bulk is initially chiral, an increased effect of chirality may be induced by the dopants.

Thus, the liquid crystal material (s) forming the bulk layer may vary considerable within the scope of the invention. Depending on the field of use, the material (s) for the bulk layer may be selected from one or more of the following: nematic liquid crystal materials having a twisted or non-twisted configuration of a nematic direc- tor within said bulk layer; smectic liquid crystal mate- rials having a twisted or non-twisted configuration of a nematic director within said bulk layer; and discotic liquid crystals materials.

The bulk layer may present a surface director having a preferred orientation that is substantially parallel to said chiral surface. Especially, the bulk layer may com- prise a liquid crystal material being such that switching of said surface director of the bulk layer is not di- rectly controllable by an applied electric field. In such an embodiment, the doped area may present a direct re- sponse to an applied electrical field (primary switch- ing), whereas the rest of the bulk layer will response to an elastic coupling (secondary switching).

The device may further comprise a separate alignment layer arranged to provide a preferred molecule orienta- tion at said chiral surface, e. g. a non-dynamic alignment layer.

The chiral dopants may comprise photosensitive mole- cules, the photosensitivity being used during the manu- facture of the device for forming the chiral surface with the dopants attached thereto.

The chiral dopants may be produced from at least one of a polymer material, an oligomer material and a monomer material.

An liquid crystal device according to the invention may be structurally implemented in many different ways.

In embodiments with one chiral surface only, the bulk layer may be confined between, on the one hand, the sin- gle chiral surface arranged on a first side of the bulk layer and, on the other hand, a non-dynamic surface alignment layer arranged on an opposite side of the bulk layer. The non-dynamic surface alignment layer will es- tablish a predetermined preferred direction, which may be chosen depending on the intended use of the device.

The bulk layer may also be confined between two chiral surfaces, each having chiral dopants permanently attached thereto. However, the dopants would still be in- homogeneously distributed in the bulk layer in permanent manner, i. e. presenting a high"concentration"at the chiral surfaces. In such a double-sided embodiment, the two chiral surfaces may induce a spontaneous polarisation of equal or of opposite sign (Ps > 0 and Ps < 0). The first chiral surface may comprise a chiral material pre- senting a first handedness, whereas the second chiral surface may comprise a chiral material presenting a sec- ond, different handedness. Furthermore, multiple chiral surfaces may be made from chiral materials having differ- ent twisting powers.

As stated above, two chiral layers may differ from each other in respect of (i) the sign of the spontaneous polarisation, (ii) the sign of the handedness and/or (iii) the twisting powers. The same consideration applies between the chiral layer (s) on the one hand and the bulk properties on the other hand. Thus, as an example, the bulk material may present a property that may be counter- acted or even substantially cancelled by the dopants within the doped sub-region (s) The inventive liquid crystal device may comprise means, such as surface electrodes, for applying an elec- tric field over the sub-volume (s) at the chiral sur- face (s). Such field applying means may be arranged such

that the electric field is applied over said chiral sur- face (s) as well as over the bulk layer. Such field apply- ing means may comprise two intersecting electrode groups for producing a matrix addressed device, wherein each pixel of the device may contain a thin film transistor (TFT) for producing active addressing.

The inventive liquid crystal device may comprise means arranged to transform a directional switch of a di- rector within the bulk layer into a detectable effect. As an example, such means may comprise one or more of the following components: polarizers, retarders, reflectors and dyes doped into the bulk layer, and combinations thereof.

In one embodiment, the chiral dopants may induce in the sub-region of a nematic bulk layer a helical molecu- lar order with a helix axis perpendicular or parallel to the chiral surface. The pitch gradient may vary over the device.

According to another aspect of the invention, there is provided a method for manufacturing a liquid crystal device, comprising the step of distributing chiral dopants in a permanently inhomogeneous manner in a liquid crystal bulk layer, said chiral dopants being soluble in the liquid crystal bulk layer.

Preferably, the method comprises the step of perma- nently attaching the dopants to at least one surface, termed chiral surface, such as a confining surface (e. g. a surface on a substrate) or an internal surface (e. g. defining a plane within the bulk layer), or combinations thereof.

The material in the bulk layer and the dopants may be selected such that the chiral dopants induce a local variation of one or more chirality-related physical prop- erties, especially an increase of one or more chirality- related physical properties, within a sub-volume or sub- region of the bulk layer adjacent to said chiral surface.

The chiral surface may be produced by coating a sub- strate surface with a chiral material. In one embodiment, the chiral surface may be arranged on an alignment layer already deposited on a substrate surface. The chiral sur- face may be produced by applying light and/or temperature in order to obtain a polymerisation of a chiral surface layer.

In one embodiment, the dopants may be provided by the sub-steps of coating said surface with a dopant mate- rial that is chiral, polymerisable and photosensitive; aligning the molecules within the dopant material in a preferred direction by subjecting the dopant material to light; and subjecting the thus-aligned dopant material to a polymerisation in order to form a chiral surface layer.

In embodiments where one or more internal chiral surfaces or planes should be produced, in order to pro- vide associated sub-regions with an induced effect of chirality, a photoreactive chiral monomer may be dis- solved in a liquid crystal bulk layer. The thus dissolved photoreactive chiral monomer may then be light-activated in one or more planes in said bulk layer for producing such internal chiral planes. To this effect, a mask could be used, or an interference pattern.

The invention also provides a method of controlling a liquid crystal bulk layer, comprising the step of dop- ing a liquid crystal bulk layer with chiral dopants being soluble in the liquid crystal bulk layer in order to in- duce, in at least one sub-region of the bulk layer, a lo- cal, preferably permanent increase of an electrical field response property, and the step of applying an electrical field for controlling the bulk layer indirectly via said sub-region.

Specifically, the invention provides a method for accomplishing an in-plane switching in a liquid crystal bulk layer of a liquid crystal device, comprising the steps of providing, in contact with said bulk liquid crystal layer, at least one chiral surface layer which

induces in an associated sub-surface region of said bulk layer a phase having a director direction which is di- rectly controllable by an electric field and which inter- acts with said bulk layer, and applying an electric field over said sub-surface region in order to produce a direc- tional change in said sub-surface region and, thereby, an in-plane bulk switching within the bulk layer.

Thus, the invention makes it possible to achieve a local variation of one or more of the chirality-related physical properties (herein also referred to as effects of chirality) of a liquid crystal host material, like spontaneous polarisation, pitch, optical activity, etc, which in its turn may be used for controlling the bulk layer indirectly via the doped region.

The above and other features and embodiments of the invention are set out in the attached claims.

Brief description of drawings The invention will now be more fully described by way of non-limiting examples, with reference to the ac- companying drawings, in which: Fig. 1 schematically illustrates the principle of the invention.

Fig. 2A schematically illustrates a field-activated molecular reorientation in a single-sided embodiment of a liquid crystal device according to the invention.

Fig. 2B illustrates the operation of the device in Fig. 2A inserted between crossed polarizers.

Fig. 3 is a schematic cross section of a double- sided embodiment of a liquid crystal device according to the invention.

Fig. 4 is a schematic cross section of a single- sided embodiment of a liquid crystal device according to the invention.

Figs 5A and 5B are schematic perspective views il- lustrating embodiments of the invention having internal chiral planes in a bulk layer.

Fig. 6 is a schematic illustration of a set-up for analysing the electro-optic response of a liquid crystal device or cell according to the invention.

Figs 7 and 8 illustrate electro-optic responses of an embodiment of the invention.

Fig. 9A and 9B schematically illustrate a field- activated molecular reorientation in double-sided embodi- ments of a liquid crystal device according to the inven- tion.

Fig. 10 illustrates a grey-scale electro-optical ef- fect of an embodiment according to the invention.

Fig. 11 schematically illustrates a double-sided em- bodiment of a liquid crystal device according to the in- vention with opposite signs of a induced spontaneous po- larisation.

Fig. 12 schematically illustrates an embodiment of the invention presenting a gradient pitch.

Fig. 13 schematically illustrates the operation of the embodiment in Fig. 12.

Fig. 14A and 14B schematically illustrates an exam- ple of preparing a chiral layer.

Fig. 15 schematically illustrates a chiral surface presenting dendrimeric chiral molecules.

Description of embodiments of the invention Fig. 1 schematically illustrates a part of an em- bodiment of a liquid crystal device according to the in- vention. A bulk layer 10 containing liquid crystal bulk molecules 12 is confined between two opposite substrates 14 (of which only one is illustrated in Fig. 1), such as glass plates. As a non-restricted, illustrative example only, the bulk material 12 may be an achiral Smectic C material.

Chiral dopants 20 being soluble in the liquid crys- tal bulk layer are permanently attached (bonded) to the substrate 14, as schematically illustrated by bonds or links 16, e. g. implemented as chemical bonds. Thus, even though the dopants are soluble in the bulk layer, the

dopants 20 are not free to move out in the rest (24) of the bulk layer 10 but are maintained attached to a sur- face 18 of the substrate 14. However, the dopants 20 may still be able to reorientate in response to an applied electrical field.

The chiral dopants 20 induce an effect of chirality in a sub-volume or sub-region 22 of the bulk layer 10 close to the substrate 14. The thickness of this surface sub-region depends i. a. on the nature of the chiral layer covering the substrate surface 18, but would normally be very thin. As a consequence, the device will present a doped sub-volume or sub-region 22 of the bulk layer 10 presenting an increased effect of chirality, and a non- doped substantially larger bulk volume 24 with zero or low effect of chirality.

As schematically illustrated in Fig. 1, the dopants 20 may be located at such a distance from the surface 18 that allows bulk molecules 12 to be located close to the substrate 14 within the doped region. The surface 18 of the substrate 14 to which the dopants 20 are permanently attached may be referred to as a"chiral surface", indi- cating that it presents chiral dopants attached thereto.

The dopants attached to the surface may be referred to as a"chiral surface layer". However, it should be noted that the dopants as such may be located at a restricted distance from the surface via e. g. chemical bonds. Nor- mally, the sub-region will comprise both the dopant mate- rial/molecules and some of the bulk layer molecules.

Now, the induced effect of chirality in the sub- region 22 is an induced spontaneous polarisation Ps in this region 22. This, in its turn, will result in a ferroelectric electro-optic response of the liquid crys- tal material within the sub-region 22.

The operation of a device comprising the structure according to Fig. 1 will now be described with reference to Fig. 2A. When an electric field E is applied over the entire liquid crystal cell, the direction of the orienta-

tion of the liquid crystal molecules in the ferroelectric sub-region 22 will change, due to the presence of the spontaneous polarisation Ps in the region and, conse- quently, the direct electrical coupling between the liq- uid crystal material within the ferroelectric sub-region 22 and the applied electrical field E. In Fig. 2A, this molecule orientational change is schematically illus- trated in Fig. 2 by a dotted line DO (a possible initial direction at E = 0) and a solid line D2 (switched direc- tion; E w 0). This could be referred to as a"primary switching".

The primary switching DO- D2 of the liquid crystal molecules 12 in the sub-region 22 will, in its turn, penetrate via elastic forces into the bulk 24 of the achiral Smectic C liquid crystal material 12.

Since only one of the inner surfaces 18 of the two substrates 14 is made chiral, the molecular switching DO- D2 in the sub-region 22 will result in a field- induced twist state in the bulk, schematically illus- trated in Fig. 2A by the molecular in-plane switching M1 M2 (secondary switching). Depending on the sign of the dielectric anisotropy As of the bulk liquid crystal mate- rial 12, the dielectric coupling between the field E and the bulk material can stabilise (As < 0) or destabilise (As > 0) this in-plane switching Ml- M2.

Reference is now made to Fig. 2B, illustrating a cell according to Fig. 2A with only one of the substrates 14 being covered by a chiral layer 20.

In this embodiment, the bulk achiral smectic mate- rial 12 possesses a molecular tilt of about 45 degrees.

The surface of the other substrate 14 was covered with polyimide, unidirectionally rubbed, resulting in a non- dynamic surface alignment layer 32. The alignment layer 32 establishes a non-dynamic surface alignment direction D3 of the bulk molecules 12'located at this substrate 14. The alignment direction D3 coincides with the pre- ferred alignment direction D1 of the molecules (M1) at

the other substrate surface 14 for E w 0. It shall be noted that D1 represents one of the two switching states of the ferroelectric subregion.

In the"off-state" (E=0), the bulk liquid crystal material 12 is oriented homogeneously with the optic axis along direction M1 (parallel to Dl), as illustrated by dotted molecules in Fig. 2B. The cell is placed between crossed polarizers 34,36 oriented with the transmission direction of on polarizer 36 along the optic axis M1. In this arrangement, incident light will be extinguished un- der its the passage through the device. However, the ap- plication of an electric field of E of say about 5V/pm induces a direct, fast 90 degrees switching (D1-+ D2) of the molecules in the ferroelectric sub-region 22 at the chiral surface 18. This fast switching Dol-* D2 will, in its turn, result in an induced 90 degrees molecule twist M1-t M2 in the bulk liquid crystal material. In this switched"on-state", the device is light transmitting.

By another suitable setting of the polarizers 34, 36, the"off-state"can be made transmissive instead.

Fig. 3 illustrates a cross section of a liquid crys- tal device or cell including two chiral solid substrates 14, each presenting an inner surface 18 that have been made chiral. The cell comprises (starting from the top) the following components: a first glass or plastic (poly- mer) substrate 14, a first transparent conductive ITO (Indium Tin Oxide) film 30, a first passive (non-dynamic) alignment film or layer 32, a first relatively thin (preferably very thin) layer of chiral molecules 20, a bulk layer 24 of liquid crystal bulk material 12, a sec- ond relatively thin layer of chiral molecules 20, a sec- ond passive alignment film 32, a second transparent con- ductive ITO (Indium Tin Oxide) film 30, and a second glass or plastic substrate 14. The two substrates 14 are fixed at a predetermined mutual distance by spacers 17.

This spacing may be in the order of several pm.

The cell illustrated schematically in Fig. 3 is a double-sided embodiment of the invention including two thin films of chiral molecules 20 on opposite sides of the bulk layer 24. The cell illustrated in Fig. 4 is a single-sided embodiment including only one thin film of chiral molecules, i. e. according to the embodiment shown in Figs 2A and 2B.

Example 1 (Method of preparation) In one embodiment for the preparation of a chiral surface layer as illustrated in Fig. 3 or 4, the inner surface of the glass or plastic substrate 14 is first covered by transparent, conductive ITO film 30 and, thereafter, with an alignment layer 32 of e. g. a thin Au film, evaporated at normal incidence. The alignment layer 32 may be a passive alignment layer.

Then, the alignment layer 32 is covered by a thin film 20 comprising, in this example, a photosensitive or- ganic material including chiral molecules 20. This chiral material 20 may be of the side-on type of attachment of chiral moieties at ortho-or meta-position anchored on to Au film through sulphur-containing group. The molecular structure of the organic chiral material 20 may be the following: sx I R1 eNst vR2 Rl = alkyl or alkoxy goup Il = alkyl or alkoxy group X +CH2e0 Rl or 1t2 is chiral n= 2-20 R2 X = SH 91 < N X RS %,-S-S-R3 X-cHO

The substrate 14, which now has been covered by the thin layer of photosensitive chiral material 20, is ex- posed to linearly polarised UV light in order to obtain a unidirectional planar, predetermined alignment of the chiral molecules 20. This alignment of the chiral mole- cules 20 transfers, via elastic forces, to the liquid crystal bulk molecules 12 in contact. The light activated alignment make take place after as well as before the bulk layer 10 is applied.

Example 2 (Method of preparation) In a second example, the chiral surface may be pre- pared by a thin film of organic material containing a mixture of chiral molecules of the same kind as in the Example 1 but in this case they possess different length of the coupling group as shown schematically below: The length of the coupling group will define the maximum distance between the chiral dopant molecule and the surface. However, the chiral dopant molecule may be free to move in a direction towards the surface.

Example 3 (Method of preparation) According to Example 3, in order to prepare a chiral surface forming e. g. the first thin film of chiral mole-

cules 20 depicted in e. g. Fig. 4, the transparent conduc- tive ITO film 30 of the glass substrate 14 is covered by a passive alignment layer 32 of buffed SiOx film, evapo- rated at normal incidence, for instance. As well known, this passive alignment layer gives unidirectional planar alignment of most liquid crystal materials. The same kind of alignment could be obtained by evaporating the Sioux film 32 at angle of incidence a~60° with respect to the substrate surface 18. Thereafter, the passive alignment layer 32 is covered by a thin film of photosensitive or- ganic chiral material 20 of the kind described in example 1. The molecules of this material may be permanently at- tached to the silica surface by the use of e. g. silyla- tion.

Example 4 (Method of preparation) According to Example 4, the transparent conductive ITO film 30 of the substrate 14 is covered by a passive alignment layer 32 of SiOX film, evaporated at angle of incidence a equal to e. g. about 85° with respect to the substrate surface 18. This passive alignment layer 32, as known, provides a tilted alignment of most of liquid crystal materials. Thereafter, the passive alignment layer 32 is covered by a film of photosensitive organic chiral material 20 of the kind described in Example 1 permanently attached to the silica surface.

Example 5 (Method of preparation) According to Example 5, the contact surface 18 is made chiral by a deposition of a chiral side-chain liquid crystal polymer with structure that e. g. may be one of the following: Example 6 (Method of preparation) According Example 6, the chiral dopants 20 forming the chiral surface are of dendrimeric type having a structure that may be the following:

R* = Chiral end group The dendrimeric molecules are permanently attached to the passive alignment layer through either sulphur or silicon-containing bridging group. Fig. 15 illustrates schematically a chiral layer formed by dendrimeric mole- cules 20.

Example 7A (Method of preparation) According to Example 7A, a permanent inhomogeneous distribution of the chiral dopants 20 in the liquid crys- tal bulk layer 10 is implemented by forming at least one chiral plane internally within the liquid crystal bulk layer 24. This may be implemented by the use of a photo reactive chiral monomer and a crosslinker, the structures of which may be the following :

Initially, the photoreactive chiral monomer is dis- solved in the bulk liquid crystal host material. Then, a sandwich cell, comprising two glass substrates 14 pre- coated on their inner sides 18 with ITO film 32 on which is deposited a passive alignment layer 30, is filled with a liquid crystal bulk material 12 with the chiral monomer dissolved therein. Thereafter, the cell now containing the bulk 12 and the dissolved chiral monomer is exposed to light. The light triggers the photo-polymerisation of the photoreactive chiral monomeric molecules. The wave- length of the light is usually in the UV-light spectrum for promoting the photopolymerisation process. During this polymerisation process, the polymer is phase sepa- rated from the liquid crystal host 12.

By using the above principle, well-defined planes (i. e. surfaces) of the phase-separated chiral polymer 20 in the liquid crystal bulk 10 may be achieved through in- terference of two beams from e. g. Ar-laser, or by means of a masking technique.

One or more such chiral planes 40 may be oriented perpendicular or parallel to the solid substrates 14 as shown in Figs 5A and 5B, respectively. Combinations thereof is possible, as well as the combination of inter- nal chiral planes 40 with one or two chiral sub-surface layers at the substrates 14.

Example 7B (Method of preparation) According to Example 7B, as illustrated schemati- cally in Figs 14A and 14B, the substrate 14 is covered

with a thin layer 15 of a polymer containing functional groups that can give a chemical coupling to a chiral dopant or molecule 20. Prior to the attachment of the dopant 20 (Fig. 14A), the polymer 15 layer may be sub- jected to unidirectional mechanical rubbing to obtain a preferential direction of alignment of the subsequently applied dopant molecules 20 as well as the liquid crys- talline bulk material 12. Thus, said polymer layer 15 may play the role of a passive alignment layer.

The material of the polymer 15 may be photo respon- sive and thus make photo alignment possible through illu- mination with polarized UV light. In this case, the dopant material 20 may be applied before or after the alignment process.

As an example, the dopant 20 may be in the form of a chiral molecule or a chiral polymer or a chiral liquid crystalline polymer.

In-plane switching of bulk liquid crystals Several examples will now be given illustrating the use of the inventive principle for obtaining an induced in-plane switching.

The liquid crystal device with chiral surface (s) used in the following examples included two parallel solid substrates 14, forming a micron-size gap, whose in- ner surfaces 18 were covered with ITO electrodes. The in- ner surface of one or both electrode substrates was coated with a thin chiral film 20. The structures of the experimental devices were as those illustrated in Figs 3 and 4.

The electro-optic response of the experimental cells was detected by means of the set-up depicted schemati- cally in Fig. 6.

The sample S was inserted between two crossed polar- izers 34,36 (analyser and polarizer) with its optic axis oriented at angle 22.5 degrees with respect to the trans- mission direction of the one of the polarizers for achieving a maximum light modulation of the transmitted

light when an electric field E, supplied by a generator 38, is applied to the sample. If necessary the applied voltage can be amplified by a differential voltage multi- plier 42. The transmitted light intensity I was detected by means of a detector 44. An oscilloscope 46 was con- nected to the voltage supply 38 and to the light detector 44 for visualising the applied voltage and the corre- sponding electro-optic response of the cell S.

Example 8 (Single-side embodiment) In Example 8, one of the inner surfaces of the de- vice was pre-coated with a chiral organic layer according to Example 1. For producing the bulk liquid crystal layer 10, the device was filled with achiral liquid crystal ma- terial 20 Hoechst 908, possessing the following phase se- quence C 10°C SmC 63°C SmA 64°C I The cell was inserted in the set-up in Fig. 6 be- tween crossed polarizers 34,36 with its optic axis ori- ented at 22.5 degrees with respect to the one of the po- larizers. Here it should be noted that a"conventional'§ prior-art cell filled with this liquid crystal material but having no chiral layer deposited on the inner sub- strate surface would exhibit a dielectric response only to an applied external electric field E. However, in the inventive cell with one of the inner surfaces pre-coated with thin chiral layer there was found a distinct polar electro-optic response in the smectic phase on applying an electric field E of about E=5V/m to the cell with a cell gap of 3pm.

The electro-optic response for the cell according to Example 8 is depicted in Fig. 7, which clearly demon- strates that the response is polar. The optic axis of the sample S was proved to switch in the plane of the sample simply by rotating the sample S at 45 degrees which then

results in a 180 degrees phase shift of the electro-optic response depicted in Fig. 8.

Example 9 (Switching the handedness of circular polarised light) In this Example 9, an embodiment of a liquid crystal device is disclosed that may be used for switching the handedness of circular polarised light. The embodiment is schematically illustrated in Fig. 9A. In contrast to the single-sided embodiment illustrated in Fig. 2B, the em- bodiment in Fig. 9A presents a chiral surface of dopant material 22 at both substrates 14. Thus, a thin layer presenting induced ferroelectrical properties was formed on both sides of the LC bulk layer.

In Fig. 9A, the device parameters were chosen in such way to fulfil the B/4 wave plate conditions. The bulk achiral smectic liquid crystal material 20 was cho- sen to possess 45 degrees molecular tilt angle. The de- vice was inserted between crossed polarizers 34,36 with the switching states (D1/D2) being symmetrically disposed about the transmission direction of the polarizers 34,36 as shown.

The application of an electrical field E will pro- vide a fast (direct) switch in the ferroelectrical layers 22 and, via elastic coupling, a corresponding in-plane switching in the bulk layer 24. Since two primary switch- ing layers 22 are arranged, and these switches in the same direction in this embodiment, there will be essen- tially no helix twist of the bulk molecules 12 within the bulk layer. Thus, the handedness of circularly polarised light at the output of the device depends on the polarity of the applied electric field E. In this way, a fast switching of the handedness of the circularly polarised light can be implemented.

As pointed out above, the dielectric coupling can stabilise (As < 0) or destabilise (As > 0) the in-plane switching.

Example 10 (In-plane switching of bulk liquid crystals) In this Example 10, a device prepared according to Example 1 was filled with the racemic liquid crystal mix- ture WILC 48 (Hoechst) with a phase sequence: SmC 48°C SmA 57°C I The same kind of polar electro-optic response in the SmC phase as the one shown above was detected in a cell with gap of 3zm on applying an electric field of E~4 V/pm.

Example 11 (Double-sided embodiment-achiral bulk) In this Example 11 as illustrated in Fig. 9B, both substrates 14 of the liquid crystal device were precoated with a chiral organic layer 22 according to Example 1.

The device was filled with achiral liquid crystal mate- rial Hoechst 908.

Then, the cell was inserted between crossed polariz- ers 34,36 with its optic axis M1 (corresponding to one of the switching states) oriented parallel to the trans- mission direction of one of the polarizers. Applying an electrical field E across the liquid crystal cell results in an orientational change of the molecules in the two surface sub-regions 22, as indicated by the transition D1 D2 at both sub-regions 22. This primary switching DI D2 of the surface molecules in the sub-regions 22 with induced chirality will, in its turn, result via elastic forces in a switching (secondary switching) of the mole- cules 12 within the rest (24) of the bulk layer 10 of the achiral bulk molecules 12. This induced bulk molecule switching is schematically indicated by the initial mole- cule orientation M1 (dotted line) switching to a new di- rection M2 (solid line). The induced bulk switching of the optic axis of the bulk is in fact an in-plane switch- ing (as in Fig. 2B), e. g. the switching M1 to M2 takes place in a plane parallel to the substrates 14.

A similar polar response as above was found in the smectic phase on applying an electric field of about E~5V/pm to the cell with a cell gap of about 3pm.

Example 12 (Double-sided embodiment-chiral bulk) In this example similarly to the one described in Example 11, both substrates having a inner surface pre- coated with a thin chiral organic layer according to Ex- ample 1.

The device was filled with a ferroelectric liquid crystal with a very low spontaneous polarisation Ps. In this case, the chiral molecules attached to the substrate surface resulted in an increase of the magnitude of Ps in the sub-surface region. In this embodiment with an ini- tially limited chiral bulk material, an applied electric field E will also couple directly with the bulk liquid crystal material. The switching at the chiral surfaces will be more efficient, since the chirality is"biased" by the initial chirality of the bulk material.

Example 13 (greyscale) This example describes how the invention can be used to implement a grey scale function. A cell was prepared according to the embodiment shown in fig. 2B. A mechani- cal shear achieved by unidirectional displacement of the cell substrates with respect to each other results in a pretilt of the molecules in the surface sub-region as well as in the bulk of the liquid crystal. If the pretilt angle is equal to the molecular tilt in the Smectic C phase then a linear modulation of the transmitted light (grey scale) can be achieved. The grey scale capability of the cell in this example is illustrated in Fig. 10, wherein the y-axis shows the intensity of transmitted light in arbitrary units.

Example 14 (Induced Smectic C*) According to this Example 14, the cell gap is filled with a nematic liquid crystal possessing a negative di- electric anisotropy (As < 0) and underlying Smectic C phase. A liquid crystal material with As < 0 was chosen

in order to stabilise the planar alignment of the liquid crystal molecules by means of dielectric coupling. The chiral molecules 20 that are attached to the inner sur- faces 18 of the cell substrates 14 induce Smectic C* phase in the surface sub-regions 22. The induced Smec- tic C* phase is ferroelectric.

By the application of an external electrical field, the preferred direction of orientation of the molecules in the surface sub-region can be switched by direct cou- pling with the electrical field. The switching of the molecules in the surface sub-region is transferred, via elastic forces, to the molecules in the bulk of the liq- uid crystal.

Example 15 (Double-sided embodiment, field induced twist) Reference is now made to Fig. 11, schematically il- lustrating an embodiment of a liquid crystal device ac- cording to the invention, wherein a first chiral surface induces Smectic C* phase in the associated surface sub- region 22 with a positive spontaneous polarisation (Ps > 0), whereas a second chiral surface induces Smectic C* phase in the associated surface sub-region 22 with a negative spontaneous polarisation (Ps < 0). The molecular tilt in both surface sub-regions 22 was in the order of 22.5 degrees.

The device in Fig. 11 was filled with a nematic liq- uid crystal material possessing a negative dielectric anisotropy (As < 0) and underlying Smectic C phase.

Thereafter the device was placed between parallel polar- izers 34,36 with its optic axis (at E = O) oriented at 45 degrees with respect to the transmission direction of the polarizers. On reversing the sign of an applied elec- tric field, the device will be switched between a trans- missive non-twisted state and a non-transmissive (dark) twisted state, due to the opposite signs of Ps in the two surface sub-regions 22.

Example 16 (Photo-induced Ps changes) According to Example 16, a device was prepared ac- cording to Example 14. In this case, however, the chiral layer deposited onto the one of the inner substrates sur- faces is a mixture containing photosensitive molecules capable of changing the sign of the induced Ps in the surface sub-region upon light illumination.

It is also possible to provide a device where the chiral organic material covering both chiral surfaces is a mixture containing photosensitive molecules capable of changing the sign of the induced Ps in the surface sub- region under light illumination. The device may be filled with e. g. the achiral liquid crystal material Hoechst 908. The cell is illuminated with UV-light trough a mask.

On applying an electric field, the cell optic axis in the illuminated area of the cell will switch in opposite di- rection than in the light protected cell area.

Example 17 (Selective reflection) Reference is now made to Fig. 12. In this case, the chiral material 20 is chosen to have high twisting power.

The device was filled with the nematic liquid crystal mixture E7 (Merck) exhibiting AE>0. The two chiral sur- faces induce in the associated surface sub-regions 22 a helical molecular order with very short pitch, as illus- trated at reference numeral 50. This helical order pene- trate in the bulk 24 of the liquid crystal material 12 increasing continuously the pitch length on moving away from the substrate as illustrated at reference numeral 52.

Thus, in the cell there will be a helical molecular order with presenting a gradient pitch. By using this principle or effect, it will be possible to achieve a se- lective reflection of the incident light in a very broad spectrum. With a proper choice of chiral material and cell gap, it will be possible to obtain efficient selec- tive reflection of white light from the device and there- fore it will appear white in colour.

Now, placing the device on absorbing light black coloured background, the black background will be not visible through the cell. Further, applying an electric field across the cell will orient the liquid crystal molecules along the field, i. e. perpendicular to the sub- strates 14, and the cell will become transparent and the black background visible. Thus, in"field-on state"the cell will appears black whereas in"field-off state"it will be white, as schematically illustrated in Figs 13A and 13B, which illustrate selective reflection of light and absorption of light, respectively.