CRYSTAL DEVICES

BACKGROUND TO THE INVENTION

Field of the invention

The present invention relates to liquid crystal devices in which the liquid crystal has a uniform lying helix (ULH) structure. The invention also relates to methods for

manufacturing such liquid crystal devices.

Related art

In 1969, Meyer first discussed theoretically the splay-bend deformation of liquid crystals in association with an electric polarization, illustrated schematically in Fig. 17. He further derived the linear electro-optic equation in liquid crystals assuming negligible dielectric anisotropy [Ref. 1 ]. In the case of a cholesteric liquid crystal, the rotation of the optics axis has been predicted giving rise to a tilt angle when an electric field is applied normal to the optics axis. The optics axis is parallel to the helix axis of a cholesteric liquid crystal in the absence of an electric field. For small rotation, this tilt angle is linearly dependent on the magnitude of electric field. In addition, the sign of the tilt angle follows the polarity of the electric field. Interestingly, the response time does not depend on the electric field in a small rotation approximation. The rotation of the optics axis on the application of an electric field is commonly referred to as the flexoelectric effect. This effect has been observed in nematic, cholesteric and smectic phases, in particular in wedge shape or bend core liquid crystal molecules.

In the case of small dielectric anisotropy, a field-induced domain pattern containing regions of splay and bend can theoretically be formed in nematic liquid crystals. The domain width was predicted to be inversely proportional to the magnitude of electric field and the e/Ki ratio. The mean flexoelectric coefficient and the splay elastic constant are denoted by e and Ki respectively. Therefore, the domain width can be large since e/Ki is usually small. This is conditioned on the opposite sign of flexoelectric coefficients, one elastic constant approximation and negligible current effects. It was further discussed by Patel and Meyer that the formation of a continuously rotating director structure in alternating bands of splay and bend deformation could be observed in cholesteric liquid crystal [Ref. 2]. This is considered to be because the director is already continuously rotating in the cholesteric in a pure twist fashion. The formation is manifested readily as the flexoelectric effect becomes non-negligible. Later in 1989, Patel and Lee [Ref. 3] measured about 10Ομβ for the response time of ULH structure in cholesteric liquid crystals. This agreed well with the predicted value of Ι Ο-Ι ΟΟμβ. This fast response time which was also predicted to be independent of the electric field for small rotations was shown experimentally. The problem to obtain a stable and homogeneous ULH texture has been reported since the late 1980s. For some liquid crystals, it considered necessary to cool the liquid crystal from the isotropic phase in the presence of an electric field in order to obtain the ULH texture. Mechanical shearing has been considered in some circumstances to be necessary for providing a homogeneous ULH texture. To minimize the elastic energy associated with the distortion, the ULH texture in homogeneous or homeotropic boundary condition has been shown to be not stable. It slowly relaxes back to the Grandjean texture after the removal of electric field. Some attempts have been made to address this problem. For example, it is known to include a polymer network in the bulk of liquid crystal or at the substrate surface in order to stabilize the ULH texture [Refs. 4-5]. The polymer-stabilized ULH texture was restored in the thermal cycling from the isotropic phase to the cholesteric phase. It was also stable for electric field greater than the helix unwinding field. There were, however, issues such as hysteresis problems and retarded response time when using a polymer network in order to stabilize the ULH texture. Also reported in the literature are stabilization of the ULH texture by: grating structures with periodic anchoring conditions [Ref. 6]; periodic microchannels [Ref. 7]; surface relief structures generated by laser [Ref. 8] and by moulding [Ref. 9]. The issues associated with these techniques include the fact that the liquid crystal device has a low filling factor since the grating structure occupies part of a footprint for the optical modulation. Other techniques such as shearing [Ref. 10], treatment by plasma beam [Ref. 1 1 ],

incorporation of a cholesteric alignment layer [Ref. 12], inhomogeneity induced by in- plane field [Ref. 13-14] and dynamic state switching [Ref. 15] were found favourable to the formation of meta-stable ULH textures.

Stabilization of the ULH structure by shearing [Ref. 10] would not be compatible with present LCD manufacturing processes.

Treatment by plasma beam [Ref. 1 1] provides questionable homogeneity over a large area, and is considered to be incompatible with present LCD manufacturing processes.

Stabilization of the ULH structure by the incorporation of a cholesteric alignment layer [Ref. 12] has the disadvantage that a sizeable thickness of cholesteric alignment layer is required. This causes problems such as voltage drop, a voltage holding ratio issue, and residual birefringence.

Inhomogeneity induced by inplane field [Refs. 13 and 14] has the possibility of stabilizing the ULH structure. However, this provides a limited fill factor due to the requirement for interdigitated electrodes.

Stabilization of the ULH structure by dynamic state switching [Ref 15] has the drawback that a periodic reset is required to refresh the ULH structure.

Relevant patent documents in this field of technology include

US 8264639 B2 US 7038743 B2

US 7777850 B2

US 8330931 B2

US 2012/0140133 A1

US 2012/0038842 A1

US 8848156 B2

US 4917475

SUMMARY OF THE INVENTION

The present invention has been devised in order to address at least one of the above problems. Preferably, the present invention reduces, ameliorates, avoids or overcomes at least one of the above problems. Following from the work presented in Ref. 1 , theory predicts that the ULH structure having a splay and bend deformation can be formed in the presence of an electric field. However, maintaining such a ULH structure in stable condition after removal of the electric has not been possible, according to the knowledge of the present inventors at the time of writing. Without wishing to be bound by theory, it is possible that this problem is attributed to minimization of the elastic energy which is in favour of forming Grandjean texture instead of the ULH structure. Various approaches have been proposed in order to stabilize the ULH structure after the removal of an electric field, as discussed above, but these all have drawbacks. The present invention is based on the inventors' insight that the ULH structure can be stabilized after the removal of an electric field by aligned domains of splay and bend deformation.

Accordingly, in a first preferred aspect, the present invention provides a liquid crystal device comprising a first substrate;

a second substrate;

a chiral nematic liquid crystal layer sandwiched between the first substrate and the second substrate and in contact with at least one alignment layer, the chiral nematic liquid crystal layer further comprising a uniform lying helix structure having a helical axis substantially perpendicular to the first and/or second substrate normal and stabilised under zero applied electric field by aligned domains of splay and bend deformation.

In a second preferred aspect, the present invention provides a method of manufacturing a liquid crystal device, the method comprising providing a first substrate and a second substrate and a chiral nematic liquid crystal layer sandwiched between the first substrate and the second substrate and in contact with at least one alignment layer, the method further comprising applying an electric field to the liquid crystal layer and cooling it through the isotropic to chiral nematic phase transition temperature while maintaining the application of the electric field to the liquid crystal layer to develop elongate aligned domains of splay and bend deformation in the liquid crystal layer, the liquid crystal layer being formed in a uniform lying helix structure.

The first and/or second aspect of the invention may have any one or, to the extent that they are compatible, any combination of the following optional features.

The alignment layer is provided typically in the form of a polymer layer. This has a rubbed surface, the rubbing inducing a preferred orientation of the LC material. A suitable alignment layer can be chosen based on the LC material selected.

Preferably, the anchoring energy provided by the alignment layer to the liquid crystal at least 5nJ/cm ² . Preferably, the anchoring energy provided by the alignment layer to the liquid crystal at most 1000nJ/cm ² . Preferably, the liquid crystal layer is substantially free of polymer. This avoids the disadvantages associated with known approaches of forming a polymer network in the LC layer in order to try to stabilize the ULH structure. Such a polymer network tends to reduce the response time of the device.

Similarly, it is preferred that the liquid crystal layer, at the interface with the alignment layer, is substantially free of polymer. This avoids the corresponding disadvantages (again in terms of the response time of the device) associated with the provision of a polymer network only at the interface region of the LC layer.

Preferably, the interface between the liquid crystal layer and the alignment layer is substantially free of anchoring protrusions and/or anchoring recesses. This allows the device to be manufactured efficiently. Furthermore, it ensures that the device thickness can be maintained to a desired level, without part of the thickness being taken up with anchoring protrusions and/or anchoring recesses.

The alignment layer may comprise a photo-alignment material. The alignment layer may give rise to a preferred orientation of the chiral nematic liquid crystal on irradiation of the alignment layer with UV light.

In some embodiments, the first and/or second substrates are provided with switching electrodes for switching the optics axis of the liquid crystal layer in order to affect the polarisation direction of light transmitted through the liquid crystal layer via the flexoelectric effect, the switching electrodes having a configuration selected from:

a transverse-field switching electrode configuration;

an in-plane switching electrode configuration; and

a fringe-field switching electrode configuration. In some embodiments, the boundary condition of the alignment layer is substantially planar for the chiral nematic liquid crystal, when the chiral nematic liquid crystal has a positive dielectric anisotropy. In some embodiments, the boundary condition of the alignment layer is substantially homeotropic for the chiral nematic liquid crystal, when the chiral nematic liquid crystal has a negative dielectric anisotropy.

Preferably, the domains have an average width of at least 1 μηη. The average width of the domains may be at least 2μηι, at least 3μηι, at least 4μηι, or at least 5μηη.

Preferably, the domains have an average width of at most 100μηι. The average width of the domains may be at most 80μηι, at most 60μηι, at most 40μηι, or at most 20μηη. The domains may have an average length of at least 10 times the width of the domains. More preferably, the domains have an average length of at least 20 times the width of the domains.

Preferably, the domains have an average length of at least 100μηι. More preferably, the domains have an average length of at least 150μηι, or at least 200μηι, or at least 250μηη.

Preferably, the liquid crystal device has a response time at 22°C of at most ΘΟμβ.

Preferably, the liquid crystal device has a root mean square error of phase modulation of not more than 1 % at room temperature. This allows the LC device to be used for 8 bit phase modulation.

Preferably, the liquid crystal device has a light transmission rate of not less than 75% at 780nm at room temperature. The chiral nematic liquid crystal may be selected from the group consisting of:

a) Ether-linked non-symmetric bimesogens FFOnOCB, FF'OnOCB, F30nOCB, F3FOnOCB where n indicates the number of carbon atoms in the spacer chains, CB representing the cyanobiphenyl unit, FF or FF' the

difluorobiphenyl unit, F3 the trifluorobipenyl unit and F3F the tetrafluorobiphenyl unit;

b) Ester-linked non-symmetric bimesogens FFEnECB, FF'EnECB, F3EnECB,

F3FEnECB where n indicates the number of carbon atoms in the spacer chains, CB representing the cyanobiphenyl unit, FF or FF' the

difluorobiphenyl unit, F3 the trifluorobipenyl unit and F3F the tetrafluorobiphenyl unit;

c) Ether-linked symmetric bimesogens FFOnOFF and ester-linked symmetric

bimesogens FFEnEFF where n indicates the number of carbon atoms in the spacer chains, FF representing the difluorobiphenyl unit;

d) Ether-linked symmetric bimesogens CBOnOCB where n indicates the number of carbon atoms in the spacer chains, CB representing the cyanobiphenyl unit; e) Ether-linked symmetric bimesogens CBFOnOFCB where n indicates the number of carbon atoms in the spacer chains, FCB representing the monofluoro- cyanobiphenyl unit;

f) Ether-linked non-symmetric nitrostilbene bimesogen NSOnOFF where n

indicates the number of carbon atoms in the spacer chains, NS representing the nitrostilbene unit and FF the difluorobiphenyl unit;

g) Ether-linked non-symmetric bent-core bimesogen NSOnOphOnOCB where n indicates the number of carbon atoms in the spacer chains, NS representing the nitrostilbene unit, ph the phenyl unit, and CB the cyanobiphenyl unit;

h) Vinyl-terminated estradiol-cyanobiphenyl bimesogen mEsnCB and

hydrogenated estradiol-cyanobiphenyl bimesogen (H)mEsnCB, where m and n indicate the number of carbon atoms in the terminal and spacer chains respectively, Es representing the estradiol unit and CB the cyanobiphenyl unit, and mixtures thereof. The uniform lying helix structure is formed between 0.8 and 1.2 of the isotropic to chiral nematic phase transition temperature, expressed in °C. In the method, preferably the liquid crystal layer is cooled at a rate of not greater than 1 °C/min. More preferably, the liquid crystal layer is cooled at a rate of not greater than 0.8°C/min, or not greater than 0.6°C/min, or not greater than 0.4°C/min, or not greater than 0.2°C/min. For example, the liquid crystal layer may be cooled at a rate of about 0.1 °C/min. This allows development of a suitable aligned domain structure to stabilize the ULH structure.

The electric field strength applied to the liquid crystal layer during cooling through the phase transition temperature may be at least ΐν/μηη. For example, the electric field strength applied to the liquid crystal layer during cooling through the phase transition temperature may be about 4ν/μηι.

Where the alignment layer comprises a photo-alignment material, the method may include irradiation of the alignment layer with UV light to give rise to a preferred orientation of the chiral nematic liquid crystal. In this case, the alignment layer may be exposed to linearly polarized UV light at least one time.

Further optional features of the invention are set out below.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which:

Fig. 1 shows a schematic cross sectional view of a liquid crystal device according to an embodiment of the invention. Fig. 2 shows a schematic plan view of the electrode configuration for a first substrate for use with an embodiment of the invention employing a transverse field switching electrode pair.

Fig. 3 shows a schematic plan view of the electrode configuration for a second substrate for use with the same embodiment of the invention as in Fig. 2.

Fig. 4 shows a schematic plan view of the electrode configuration for a substrate for use with another embodiment of the invention, employing an in-plane switching electrode pair.

Fig. 5 shows a schematic plan view of the electrode configuration for a substrate for use with another embodiment of the invention employing a fringe field switching electrode pair.

Fig. 6 shows optical images at different magnifications in (a) to (e) and demonstrates the ULH structure in a liquid crystal device according to an embodiment of the invention having aligned domains of splay and bend deformation in the absence of electric field. Note: 1 mm scale bar for (a)-(b) and 80μηι scale bar for (c)-(f); (e) with tint plate at 137nm; (f) with tint plate at 530nm.

Fig. 7 shows an optical image of a liquid crystal device according to an embodiment of the invention having a transverse-field switching electrode configuration, with azimuthal angle Ψ=0°. The scale bar represents 1 mm.

Fig. 8 shows an optical image of the liquid crystal device shown in Fig. 7 but with azimuthal angle Ψ=45°. The scale bar represents 1 mm.

Fig. 9 shows an optical image of a liquid crystal device according to an embodiment of the invention having an in-plane switching electrode configuration, with azimuthal angle Ψ=0°. The scale bar represents 20μηι in the main image and 200μηι in the inserted image.

Fig. 10 shows an optical image of the liquid crystal device shown in Fig. 9 but with azimuthal angle Ψ=45°. The scale bar represents 20μηι in the main image and 200μηι in the inserted image.

Fig. 1 1 shows an optical image of a liquid crystal device according to an embodiment of the invention having a fringe-field switching electrode configuration, with azimuthal angle Ψ=0°. The scale bar represents 20μηη in the main image and 200μηη in the inserted image.

Fig. 12 shows an optical image of the liquid crystal device shown in Fig. 1 1 but with azimuthal angle Ψ=45°. The scale bar represents 20μηη in the main image and 200μηη in the inserted image.

Fig. 13 shows a plot of tilt angle as a function of electric field at different temperature for a liquid crystal device according to an embodiment of the invention having a transverse- field switching electrode configuration. Note that the different data point markers show the data for different measurement temperatures: 22°C (Triangle), 30°C (Circle) and 40°C (Square).

Fig. 14 shows a plot of response time as a function of electric field at different temperature for a liquid crystal device according to an embodiment of the invention having a transverse-field switching electrode configuration. Note that the different data point markers show the data for different measurement temperatures: 22°C (Triangle), 30°C (Circle) and 40°C (Square). Fig. 15 shows a plot of rectangular flexoelectric response at different applied electric field strengths, for a liquid crystal device according to an embodiment of the invention having a transverse-field switching electrode configuration. Fig. 16 shows a plot of saw-tooth flexoelectric response at 5ν/μηι electric field for a liquid crystal device according to an embodiment of the invention having a transverse-field switching electrode configuration.

Fig. 17, from Ref. 1 , shows a schematic cross sectional view of a field-induced domain pattern, during the application of an electric field across the LC layer, containing alternating regional of splay (S) and bend (B). The local symmetry axis is parallel to the lines indicated in the LC layer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS. AND FURTHER OPTIONAL FEATURES OF THE INVENTION

In the preferred embodiments of the invention, the liquid crystal device is useful for controlling the polarization state of light transmitted through the liquid crystal. Fig. 1 shows a schematic cross sectional view of an embodiment of the invention. First substrate 221 and second substrate 214 are in the form of panels. The substrates can be formed of any suitable materials, such as glass or silicon. First substrate 221 has electrode 220 formed on its upper surface, the electrode 220 being covered with alignment layer 219. Similarly, second substrate 214 has electrode 215 formed on its lower surface, the electrode 215 being covered with alignment layer 216.

Suitable electrodes can be formed by sputtering, evaporation or printing of a conductive film. The contact resistance between the electrode and an adjacent conductive film applied for measurement of the contact resistance is preferably in the range from 0.1 micro-Ohm to 100 Ohm. In Fig. 1 , only one electrode for each substrate is shown. However, as will be understood, a plurality of electrodes may be provided on each substrate, for example to establish individual pixels. In the schematic arrangement of Fig. 1 , the electrodes 215, 220 are arranged as a transverse field switching pair of electrodes.

A liquid crystal is filled into space 217 to be sandwiched between the alignment layers 216, 219 formed on the substrates. The space 217 is maintained by spacers 218. As explained in more detail below, the alignment layer is irradiated by a UV light source to cause a preferred orientation of the liquid crystal. The electrodes are formed of a conductive film having a low contact resistance. The liquid crystal layer comprises a polymer-free chiral nematic liquid crystal mixture, which may also be referred to as a polymer-free cholesteric mixture.

The a uniform lying helix (ULH) texture (also referred to here as ULH structure)is obtained by applying an electric field to the liquid crystal in the vicinity of the chiral nematic to isotropic phase transition temperature, and cooling the liquid crystal from that temperature. The helical axis of ULH structure is orientated substantially parallel to the plane of substrate. In use, it is intended that the direction of light is propagating normal to the substrate through the liquid crystal layer. The device may be operated in transmission mode or in reflection mode.

To the knowledge of the inventors, there has been no published work on the uniform lying helix texture comprising aligned domains of splay and bend deformation in the absence of an electric field. The present inventors have found, in embodiments of the invention, that domains of mean width of 7.2μηη and typical length of about 300μηη have stable and reversible characteristics against the thermal and electrical stresses. In addition, the ULH texture is found to have a very good homogeneity over a large area. For example, suitable homogeneity is demonstrated for a circular pixel of 6mm in diameter. The inventors also demonstrate here that the ULH texture can be obtained in different electrode configurations such as transverse field switching, in-plane switching and fringe field switching. For embodiments of the invention, the average flexoelectric response time was measured as 40μβ at 22°C, this being reduced to 28μ8 at 40°C. This compared to ' 00[\s at room temperature reported by Chien ef a/ in Ref. 5 and in US 8,264,639. It is considered that the longer response time reported by Chien et al was likely due to a polymer network deliberately formed at the substrate surface for the purpose of stabilization of the ULH texture. In view of the fast response times possible with embodiments of the present invention, these have particular applicability for example in a high speed grey-scale phase modulator.

In a microscopic view, small domains were observed growing as the liquid crystal was cooling from the isotropic temperature in the presence of an applied electric field. These small domains coalesced and became large domains having a structure of long thin strips. These large domains further grew to a ULH structure comprising aligned domains of splay and bend deformation. The ULH structure was aligned in a preferred orientation induced by the alignment layer. A photo-alignment material was used for the alignment layer in this case.

In Figs. 6(c)-(f), the mean width of the long thin domains was measured to be 7.2μηη and the typical length was about 300μηη. The dimensions can be measured and averaged over a field of view using image processing techniques. Specifically, the domain dimensions can be measured image processing software from Nikon and Media

Cybernetics. The mean width and typical length were obtained.

The ULH texture comprising these aligned domains were found to have stable and reversible characteristics even though it was stressed using an electric field of 10ν/μηι and a temperature up to 0.8 Tc. In addition, the ULH texture exhibited a very good homogeneity in a viewing area of 3mm x 4mm as shown in Fig. 1 (a)-(b). It was noted that the tint plates of 137nm (Fig. 6(e)) and 530nm (Fig. 6(f)) were used to help visualize the domain structure. The azimuthal angle of optics axis relative to the polarizer axis was denoted by Ψ whereas P, A and E denoted the polarizer axis, analyzer axis and ease-axis respectively.

For an embodiment of the invention, the first 203 and second 206 substrates are shown in Figs. 2 and 3 respectively. The arrangement of features on these substrates will now be explained. A transverse field switching electrode pair 201 and 205 is provided on the respective substrates. A metallic electrode pair 202 and 204 is also provided. The liquid crystal layer is disposed in the space arranged between the first 203 and second 206 substrates. Suitable alignment layers are were coated on top of the electrodes.

Another embodiment of the invention is shown in Fig. 4, in which the substrate 209 has an in-plane switching electrode pair 208a and 208b in addition to a metallic electrode 207.

Another embodiment of the invention is shown in Fig. 5, in which the substrate 213 has a fringe field switching electrode pair 21 1 a and 21 1 b in addition to a metallic electrode 210. In this case, an insulating layer 212 was used to prevent the electrode 21 1 a being short- circuited with the electrode 21 1 b.

Figs. 7 and 8 demonstrate the operation of a device according to an embodiment of the invention with the liquid crystal layer having the ULH texture and the electrode

configuration being a transverse field switching electrode configuration. Very good homogeneity at the azimuthal angle of 0° and 45° was observed in a viewing area of about 6mm in diameter. It was noted that the scattering of the non-illuminated

background area was due to the formation of a focal-conic texture. The preparation of the device having this ULH texture is described in Example 1 , below. Figs. 9 and 10 demonstrate the operation of a device according to an embodiment of the invention with the liquid crystal layer having the ULH texture and the electrode

configuration being an in-plane switching electrode configuration. The homogeneity was very good in a viewing area of 600μηι x 760μηι as shown in the inserted images. The in- plane electrodes were viewed dark at the azimuthal angle of 0° and 45° as shown in Figs. 9 and 10 respectively, since the photo was taken in the transmission mode. The preparation of the device having this ULH texture is described in Example 2.

Figs. 1 1 and 12 demonstrate the operation of a device according to an embodiment of the invention with the liquid crystal layer having the ULH texture and the electrode configuration being a fringe field switching electrode configuration. The homogeneity was also very good in a viewing area of 600μηη x 760μηη as shown in the inserted images. The fringe field electrode was viewed dark at the azimuthal angle of 0° and 45° as shown in Figs. 1 1 and 12 respectively, since the photo was taken in the transmission mode. It was noted that the ease-axis was 45° relative to the polarizer axis and there was fringe field induced inhomogeneity at the perimeter of electrode. The preparation of the device having this ULH structure is described in Example 3.

For the flexoelectric effect in cholesteric liquid crystals, we recall the equations derived by Patel and Lee [Ref. 3] and use the same notations here. In the presence of electric field, the ULH structure of cholesteric liquid crystal is found rotated by a tilt angle φ about the x-axis. It is assumed that the helix axis of ULH structure is initially along the z- direction and the electric field E is applied in the x-direction while the director of cholesteric liquid crystal is parallel to the x-y plane. The equilibrium helical distortion is determined by the balance between the elastic energy and the field energy due to the flexoelectric coupling with the electric field, under the assumption of non-negligible dielectric contribution. Therefore, taking the one elastic constant approximation and for small rotations, the tilt angle is given by

tan (q>)=eE/toKi eq. (1 ) where Ki is the splay elastic constant, to is the equilibrium twist of cholesteric liquid crystal and e is the mean flexoelectric coefficient.

In addition, according to the balance of torque equation that governs the dynamics of the helix rotation, the characteristic time for small rotations is given by

where γι is the viscosity of pure rotation. It is independent of electric field and it is measured as the average of rise time and fall time. This is so called the response time. To measure the tilt angle and the characteristic time, the cholesteric liquid crystal device having the ULH structure is placed between the cross polarizers. The probing light beam is propagating normal to the substrate surface. The relative change in the transmitted intensity is given by

Al/2l=sin (4cp)/tan (2β) eq. (3) where β is the angle of optic axis relative to the polarizer axis in the absence of electric field. At β=22.5 ⁰ , the tilt angle can be obtained by eq. (1 ) and eq. (3). The relative change in transmitted intensity at low fields becomes proportional to the electric field.

For the measurements of tilt angle and response time, the sample in Example 1 was used. The pitch and the phase transition temperature were measured as 309nm and 48.6°C respectively. The tilt angle at different temperature was plotted in Fig. 13. For 22°C and 30°C, the tilt angles were linearly dependent on the electric field although there was a small deviation at an electric field higher than 8ν/μηι for the case of 40°C. Taking the elastic constant 10.3pN, the mean flexoelectric coefficient was determined to be about 1 pC/m according to eq. (1 ) and eq. (3) and the ratio e/Ki was equal to 0.1 C/Nm. This is in good agreement with other published data.

In Fig. 14, the rise time and the fall time were measured at different temperatures. The rise time and the fall time were approximately equal and the response time was calculated as an average. The response time was found weakly dependent on the electric field since the ratio of elastic constants K3/K1 was 1 .54 in this case. Taking the average without the data of maximum value and the data of minimum value, the average response time at 22°C, 30°C and 40°C were 40μ8, 34μβ and 28μ8 respectively. The response time at 22°C reasonably agreed with the predicted value of 47 s using eq. (2). It was assumed that the viscosity γι was 0.22 Pa-s.

The electro-optic response on a rectangular input signal of 400Hz is shown in Fig. 15. The waveforms show a clean and very low noise response even down to the low electric field of 1 ν/μηη. The step in the increase of intensity was apparently equal as the electric field was ramping up. The waveforms had flat-top and sharp transition edge at both polarities, in addition to the change of sign when the polarity was reversed. Furthermore, the electro-optic response on a saw-tooth input signal of 400Hz and 5ν/μηι is shown in Fig. 16. The waveform was approximately linear in shape to the input signal. The linearity was good for the positive and negative ramping voltages. Therefore, we were able to conclude that the cholesteric liquid crystal device in the present invention exhibited the flexoelectric effect.

It is possible to measure the anchoring energy provided by the alignment layer to the liquid crystal using the approach set out in Ref. 17. The anchoring energy is preferably in the range 5nJ/cm ² to 1000nJ/cm ² . It is considered, without wishing to be bound by theory, that operating in this range allows the suppression of the formation of

disclinations in the liquid crystal layer.

Suitable LC materials for use in embodiments of the present invention include known liquid crystal mixtures and bimesogens. Symmetric and non-symmetric bimesogens having fluorinated ether-linked and ester-linked groups can be used, and chiral bimesogenic compounds can be used.

The equipment we used in these measurements were Olympus optical microscope BX 60, Hewlett Packard UV-visible spectroscopy system 8453, Agilent Technologies digital storage oscilloscope DSO5034A, Thurlby Thandar Instruments programmable function generator TG 1304, Linkam hot stage LTS 350 with controller TMS 94, Thorlabs photo- diode PDA 55 and a calibrated high-voltage amplifier. It was noted that the measurements and the photos were taken at 22°C unless it was mentioned otherwise.

Example 1

In Example 1 , the transverse field switching electrodes were fabricated using an indium tin oxide (ITO) film. Azo-dye film was spin-coated on top of the transverse field electrode and it was baked at 80°C for 15min. The azo-dye material was laboratory synthesized and it was a variant of AD1 reported by Yip et al [Ref. 16]. It was exposed in proximity to a linearly polarized UV light at 5J/cm ² . A quartz photo-mask was used to transfer the image to the illuminated region. A polymer-free chiral nematic liquid crystal mixture comprising E7 and 4.5wt% R501 1 from Merck was disposed between the first and second substrates. Both substrates were made of glass material in this example. The spacers were 3μηη in size and the perimeter was sealed with Norland NOA 68 UV adhesives. An alternating electric field was applied while the liquid crystal was cooling from the isotropic phase. In this case, the electric field was homogeneous and substantially normal to the substrate surface. The ULH texture was observed to grow and became stable in the absence of electric field.

Example 2

In Example 2, the in-plane switching electrodes were fabricated using the aluminium on chromium film. Negative photoresist AZ nLOF 2020 from MicroChemicals was spin- coated at 4000rpm for 30 second before it was baked at 100°C for 1 min. It was then exposed to UV light and it was post baked at 1 10°C for 1 min before it was developed in AZ 726 MIF for 90 second. A layer of about 20nm chromium and 100nm aluminium was deposited in sequence by sputtering. Lift-off in 1-methyl-2-pyrrolidone (NMP) and resist stripping by oxygen plasma were followed. At the end of the processes, the substrate having the electrodes for in-plane switching was obtained and it was substituted for the first substrate in Example 1. The in-plane switching electrodes were short-circuited externally by a connection wire. These electrodes therefore were able to act as the counter electrode when the electric field was applied in this case. Therefore, the electric field was not homogeneous spatially. A stable ULH texture was formed after the application of electric field was stopped. For normal operation, the connection wire can be removed and the ITO electrode on the second substrate can be left disconnected. Example 3

In Example 3, the fringe field switching electrodes were fabricated using the materials of aluminium on chromium, silicon dioxide and ITO. To fabricate the first electrode, the ITO film was patterned by lithography and was subsequently etched in diluted hydrochloric acid. A layer of 400nm silicon dioxide was then deposited by PECVD on top of the first electrode. The lithography, film deposition and lift-off steps for the second electrode were repeated according to Example 2. At the end of these processes, the substrate having the electrodes for fringe-field switching was obtained and it was substituted for the first substrate in Example 1. The electric field was also not homogeneous spatially. To compromise the low operation voltage and the high breakdown voltage, high quality PECVD silicon oxide was critical. Stable ULH structure was formed as in Example 2 although there were fringe field induced inhomogeneity at the perimeter of electrode. For normal operation, the ITO electrode on the second substrate can be left disconnected.

In summary, the ULH texture in all of these electrode configurations was found stable under thermal and electrical stresses. There was no noticeable change over a long period of time after the ULH texture was subjected to the stresses. Nevertheless, it was classified as meta-stable since it required an electric field at the initial stage of forming the ULH texture. In commercial applications, a feedback control circuit comprising a temperature sensor, a heater and an electrical oscillating source can help to keep the ULH texture at the normal operation conditions.

The ability to develop and maintain the uniform lying helix (ULH) texture when using a polymer-free chiral nematic liquid crystal mixture is considered to be very significant, because this permits the response time of the device to be very short. The device is also robust in the sense that the ULH texture can be maintained even when the LC layer is stressed at an electric field of 10ν/μηι and a temperature up to 0.8Tc. The device therefore is of particular utility as a high speed grey-scale phase modulator, such as for holographic video projection and/or corrected wave-front free space optical

communication.

Suitable liquid crystal materials for use with the preferred embodiments of the present invention include the following homologous series of bimesogenic compounds, which are known for studies of the flexoelectric effect:

a) Ether-linked non-symmetric bimesogens FFOnOCB, FF'OnOCB, F30nOCB, F3FOnOCB where n indicates the number of carbon atoms in the spacer chains, CB representing the cyanobiphenyl unit, FF or FF' the

difluorobiphenyl unit, F3 the trifluorobipenyl unit and F3F the tetrafluorobiphenyl unit.

b) Ester-linked non-symmetric bimesogens FFEnECB, FF'EnECB, F3EnECB,

F3FEnECB where n indicates the number of carbon atoms in the spacer chains, CB representing the cyanobiphenyl unit, FF or FF' the

difluorobiphenyl unit, F3 the trifluorobipenyl unit and F3F the tetrafluorobiphenyl unit.

c) Ether-linked symmetric bimesogens FFOnOFF and ester-linked symmetric

bimesogens FFEnEFF where n indicates the number of carbon atoms in the spacer chains, FF representing the difluorobiphenyl unit.

d) Ether-linked symmetric bimesogens CBOnOCB where n indicates the number of carbon atoms in the spacer chains, CB representing the cyanobiphenyl unit. e) Ether-linked symmetric bimesogens CBFOnOFCB where n indicates the number of carbon atoms in the spacer chains, FCB representing the monofluoro- cyanobiphenyl unit.

f) Ether-linked non-symmetric nitrostilbene bimesogen NSOnOFF where n

indicates the number of carbon atoms in the spacer chains, NS representing the nitrostilbene unit and FF the difluorobiphenyl unit.

g) Ether-linked non-symmetric bent-core bimesogen NSOnOphOnOCB where n indicates the number of carbon atoms in the spacer chains, NS representing the nitrostilbene unit, ph the phenyl unit, and CB the cyanobiphenyl unit. h) Vinyl-terminated estradiol-cyanobiphenyl bimesogen mEsnCB and

hydrogenated estradiol-cyanobiphenyl bimesogen (H)mEsnCB, where m and n indicate the number of carbon atoms in the terminal and spacer chains respectively, Es representing the estradiol unit and CB the cyanobiphenyl unit. These liquid crystal materials are discussed in detail in a series of PhD dissertations listed as Refs. 18-26.

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While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.

All references referred to above and/or listed below are hereby incorporated by reference.

Non-patent document references

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[Ref. 18] Bronje Musgrave, PhD Dissertation, University of Southampton, 2000.

[Ref. 19] Matthew J. Clarke, PhD Dissertation, University of Southampton, 2004.

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[Ref. 24] Flynn Castles, PhD Dissertation, University of Cambridge, 2010. [Ref. 25] Katie L. Atkinson, PhD Dissertation, University of Cambridge, 2014. [Ref. 26] Rachel Garsed, PhD Dissertation, University of Cambridge, 2015.