GB2377275A

GB2377275A - Fast switching nematic or chiral nematic liquid crystal shutter

Info

Publication number: GB2377275A
Application number: GB0210600A
Authority: GB
Inventors: Michael John Towler
Original assignee: Individual
Current assignee: Individual
Priority date: 2001-05-15
Filing date: 2002-05-09
Publication date: 2003-01-08
Anticipated expiration: 2022-05-09
Also published as: GB0111821D0; GB0210600D0; GB2377275B

Abstract

A nematic liquid crystal cell or pixel is in a global energy minimum configuration with low applied voltage, but is not in a global energy minimum configuration with high applied voltage. The configuration present at high applied voltage 42 is continuously accessible from the configuration present at low applied voltage. Preferably the configuration at high voltage may be a symmetric H-state. Means to isolate or stabilise the symmetric H-state to prohibit its replacement by otherwise lower energy states may be included.

Description

Fast Switching Nematic or Chiral Nematic Liquid Crystal Shutter BACKGROUND OF THE INVENTION.

1. Field of the Invention.

The present invention relates to fast switching nematic or chiral nematic liquid crystal based optical modulators that are suitable for use in devices such as liquid crystal televisions, liquid crystal projection devices and many other such devices 2. Description of the Prior Art Liquid crystal device based electro-optical shutters and displays are well known.

These involve the insertion of a nematic or chiral nematic liquid crystal layer between two bounding glass or plastic surfaces that have been coated with a conducting layer and a thin alignment layer. This structure is known as a cell, or in the case of an array of such structures constituting a display, a pixel. Boundary surfaces are also known as substrates. The conducting layer, for example indium tin oxide, allows various voltages to be applied across the liquid crystal. The alignment layer acts to impart an initial configuration to the liquid crystal molecules throughout the cell or pixel and applied voltages act to modify this configuration. On removal or reduction of an applied voltage the liquid crystal relaxes to a new equilibrium state under the influence of the alignment layers. Such liquid crystal cells or pixels are used in combination with two polarising elements, a polarising element and a reflector, a dye dopant and a reflector or solely a dye dopant and many other techniques known to those skilled in the art to achieve an optical shutter or display.

There are two broad classes of nematic or chiral nematic liquid crystal devices. The first consists of devices that have one stable configuration of the liquid crystal layer under zero applied voltage and give greyscale modulation as a function of voltage level applied across the liquid crystal. In these devices optical modulation is obtained by distorting the initial configuration of the liquid crystal layer within the cell or pixel, but typically not by modifying it to a topologically different configuration, over the range of voltages under which the device is operated. Typical examples of these are the super twisted nematic liquid crystal device and the twisted nematic liquid crystal device. These devices are typically used in commercial displays such as those in mobile telephones or laptop computers.

The second class consists of nematic or chiral nematic liquid crystal devices that utilise more than one stable or metastable configuration of the liquid crystal layer at zero applied voltage. An example of such a device is the bistable twisted nematic (US 4239345 D. W. Berreman and W. R. Heffner) device. In these devices a voltage is applied across the liquid crystal layer to switch it into one of the configurations that is stable under zero applied voltage. Such devices can be used to display a constant image indefinitely without dissipating more energy than that required to initially write the display (array of pixels).

The present invention relates to liquid crystal devices of the first type, that is the greyscale modulation of the display is a function of the voltage applied across the liquid crystal layer. The problem with such devices is the response time of the liquid

crystal layer configuration of molecules to an applied voltage. (The liquid crystal layer configuration of molecules is also known as the director configuration, director profile, director field or liquid crystal profile by those skilled in the art. ) Typically, at common operating temperatures, twisted nematic and super twisted nematic liquid crystal devices take between 10 and 100 milliseconds to reach an equilibrium state once the applied voltage level is changed from a higher to a lower level. Such switching speeds have been found to be adequate for mobile telephone and laptop displays but are inadequate for good quality moving image displays such as liquid crystal televisions and projectors. They particularly fail when fast moving images with well defined edges need to be displayed, such as ticker-tape scrolling text or computer generated images as used in games.

One partial solution to this problem has been the use of the pi-cell combined with a flashed backlight (see for example the article'Improving the Moving-Image Quality of TFT-LCDs'by K. Sueoka, H. Nakamura and Y. Taira 1997 Society for Information Displays Proceedings of the International Display Research Conference pages 203-206). Following a change from a higher to a lower applied voltage, the picell typically reaches an equilibrium liquid crystal layer configuration of molecules in approximately 5 milliseconds. This is typical for a transmissive device in which the optical thickness of the device corresponds to approximately a half-wave plate at a low voltage and zero at a high voltage. Those skilled in the art consider the pi-cell as the state of the art in fast responding nematic or chiral nematic liquid crystal devices.

Although this leads to some improvement for moving image quality further improvement will require a reduction of this response time to 1 to 2 milliseconds or less. This requirement will become evermore necessary as new display technologies in which the colour information is presented within the time dimension rather than the space dimension are used, so called time sequential colour, and also as stereoscopic information is displayed in the time dimension.

(Fast switching nematic or chiral nematic liquid crystal devices) US 3,854, 751 W. E. Hass and J. E. Adams describes a liquid crystal device with interdigitated electrodes allowing two approximately orthogonal field directions to be applied to a liquid crystal device. This allows one field direction to drive the liquid crystal device to an on state and another field direction to drive the liquid crystal to an off state. This technique allows some improvement in response times of nematic or chiral nematic devices but is difficult to implement in an array of liquid crystal pixels and gives rise to device structures of reduced brightness and contrast ratio.

US 3,857, 629 M. J. Freiser, US 4,009, 934 R. M. Goodwin et al, US 4,386, 836 K. Aoki et al all disclose liquid crystal devices that have dual frequency operation. This allows an applied voltage of one frequency to drive the liquid crystal device to an on state and an applied voltage at another frequency to drive the liquid crystal to an off state.

This method requires liquid crystal materials that have a positive dielectric anisotropy over a frequency range and a negative dielectric anisotropy over a second frequency range. The frequency at which the dielectric anisotropy is zero is termed the crossover frequency. This technique allows some improvement in the response times of nematic or chiral nematic devices but is unfortunately both very sensitive to

temperature variations, due to the sensitivity of the cross-over frequency to temperature, and difficult to implement in an array of liquid crystal pixels.

US 4,595, 259 A. E. Peregaux describes a technique for achieving a millisecond transient response time in nematic liquid crystal devices. In this technique an optically thick nematic cell is held at 50 volts rms in a dark state (between crossed polarisers) and the voltage is reduced to 0 volts, a half-wave plate optical thickness and hence bright state is reached transiently within a millisecond, the high voltage is then reapplied to return to the dark state. The technique of fast transient switching is applied to printer bars. Unfortunately this technique is not appropriate for devices such as liquid crystal televisions, firstly the voltages are about 5-10 times higher than those that can be used with typical thin film transistor matrices used in such displays and secondly a non-transient bright state is often required.

US 5 347 382 S. Rumbaugh teaches the use of multiple cell configurations (that is more than one liquid crystal cell are consecutively placed in the incident light path) to obtain a fast switching liquid crystal optical modulator.

US 4 917 475 R. Meyer and J. Patel teach the use of coupling between the applied voltage and the flexoelectric polarisation in short-pitch chiral nematics in which the helical axis is orthogonal to the applied electric field, and the applied electric field is parallel to the normal of the cell substrates. This chiral nematic device can produce short response times but requires either high voltages or a high flexoelectric polarisation to be present; it is therefore not compatible with typical thin film transistor matrices. It has additional problems associated with the interaction between ionic impurities in the liquid crystal and the flexoelectric polarisation. It also has problems of obtaining and maintaining the required configuration of the chiral nematic liquid crystal.

The articles"The pi-Cell: A Fast Liquid Crystal Optical Switching Device"by P. J.

Bos and K. R. Koehler/Beran in Molecular Crystals and Liquid Crystals 1984 volume 114 page 329-339,"A LC/CRT Field Sequential Colour Display"by R. Vatne et al in Society for Information Display, Digest of Technical Papers, May 1983 pages 28-29 and"A Liquid Crystal Optical Switching Device (pi-Cell)"by P. J. Bos et al in Society for Information Display, Digest of Technical Papers, May 1983 pages 30-31 all describe the use of the pi-cell as a fast switching liquid crystal device. Those skilled in the art consider this device to be the state of the art in fast switching nematic or chiral nematic devices that are suitable for arrays of pixels as used in for example liquid crystal televisions. Unfortunately this device typically switches in approximately 5 milliseconds for voltages compatible with thin film transistor matrices used with arrays of liquid crystal pixels. This device has the added difficulty that the liquid crystal configuration in which the device is operated (known as the V-state or bend state) must be first nucleated from the state that is stable at zero applied voltages, there can be some difficulty in accomplishing this without error over a large array of pixels. This problem is compounded by the state that is first stable persisting in interelectrode (inter-pixel) gap regions unless some special treatment is carried out. The device is then typically operated between two voltage levels at which the V-state (in which molecules near to each of the alignment layers are rotated from the normal in opposite senses to each other and the molecules near to the cell centre are

substantially parallel to the normal of the cell substrates) is the global energy minimum.

The article"Dynamic Bend Mode in a pi-Cell"by H. Nakamura in Society for Information Display, Proceedings of the International Display Workshop 1999 pages 37-40 describes a pi-cell device driven between a higher applied voltage in which the V-state configuration is the global energy minimum and a lower or zero applied voltage in which the V-state is not the global energy minimum configuration.

Although a slight improvement in the characteristics of the pi-cell are obtained these are not sufficiently significant to bring about a large improvement in switching speed.

The pi-cell utilises a liquid crystal material with positive dielectric anisotropy.

(Stabilisation of non-global minimum liquid crystal configurations) EP 0768562 M. D. Tillin, E. P. Raynes and M. J. Towler describes a pi-cell which has first been brought into a V-state configuration and is then stabilised by the generation of a polymer network throughout the pi-cell. The V-state configuration then remains at zero and low voltages.

US 5668651 N. Yamada et al first nucleates the V-state in a pi-cell and then utilises insitu polymerised walls to stabilise and isolate the V-state configuration so that at low voltages the V-state remains present in pixels even though it is not the global energy minimum configuration.

The article"A Multi-Dimensional Alignment (MDA) Structure and its Application for the Stabilization of the Bistable Twist Cell"by C. D. Hoke and P. J. Bos in Society for Information Display, Proceedings of the International Display Workshop 1998 pages 201-204 use in-situ polymerised walls to stabilise non-global energy minima configurations in another type of liquid crystal device.

"A High Contrast and Rugged FLCD with Stripe-Type BCB and Sticky Spacers"by S. R. Lee et al in Society for Information Display, Symposium Digest 1997 pages 1051 to 1054 describes the fabrication of photolithographically defined walls.

(The symmetric H state) The symmetric H-state is a nematic or chiral nematic liquid configuration that can be used in the present invention.

EP 0614107 P. J. Bos describes a technique for optimising the appearance of liquid crystal cells as a function of observer viewing angle, in particular it focuses on nematic or chiral nematic liquid crystal configurations that are described as symmetric. Two such configurations are shown therein as Figures 12a and 12b, each of these configurations represent a symmetric H-state, such a configuration can be used within the present invention. The symmetric H-state has molecules near to each of the alignment layers rotated from the normal in opposite senses to each other and the molecules near to the cell centre are substantially parallel to the cell substrates. EP 0614107 does not describe how to use such a state to obtain fast switching by using as a high applied voltage level a voltage at which the symmetric H-state is not a global energy minimum state.

US 6061116 S. Nishida, H. Takahasi, H. Saito and S. Murata, US 6067142 J. Anderson, C. Cai and S. A. Lien, and the article"Nematic Director Configurations in High Pretilt, Parallel Aligned Layers"by H. G. Walton and M. J. Towler in Liquid Crystals 2000, volume 27 page 157-161 describe a device. using the symmetric Hstate liquid crystal configuration, in this device though the symmetric H-state is a global energy minimum at high voltages. The device is slower switching than the state of the art pi-cell. The device uses a liquid crystal material with negative dielectric anisotropy and an alignment layer such that liquid crystal molecules near to the boundary surfaces are at an angle greater than 45 degrees to the plane of the boundary surfaces when zero voltage is applied across the liquid crystal layer.

The article"The Role of Surface Tilt in the Operation of Pi-Cell Liquid Crystal Devices"by E. J. Acosta, M. J. Towler and H. G. Walton in Liquid Crystals 2000 volume 27 pages 977-984 includes a description of the symmetric H-state (Figure 1 therein) with liquid crystal molecules near to each of the alignment layers rotated from the normal in opposite senses to each other and the molecules near to the cell centre being substantially parallel to the cell substrates. The article also describes how as the applied voltage is increased, with the given alignment and nematic material parameters, the symmetric H-state is progressively replaced with an asymmetric Hstate (in which the molecules near to each of the alignment layers remain rotated from the normal in opposite senses to each other, molecules near to the cell centre are substantially not parallel to the cell substrates and molecules a short distance from one substrate become substantially parallel with it) and the V-state as these in turn become the global energy minimum state. The article does not however show utilising the symmetric H-state as the high voltage operating state in a nematic or chiral nematic liquid crystal based electro-optical modulator.

SUMMARY OF THE INVENTION It is the object of the present invention to overcome the limits of response time of existing nematic and chiral nematic liquid crystal devices and achieve response times below 2 milliseconds in device structures that can be easily and cost effectively used as electro-optical shutters or displays in products such as liquid crystal televisions, time sequential colour displays and projectors.

In the present invention the nematic or chiral nematic liquid crystal cell or pixel is in a global energy minimum configuration at zero or a low applied voltage but is in a nonglobal energy minimum configuration for example a metastable or local minimum energy configuration at a higher applied voltage. The liquid crystal molecules can evolve continuously between configurations as the voltage level is changed. The liquid crystal cell or array of pixels is combined with crossed polarisers or other imaging means to allow modulation of transmitted or reflected light. The device is switched between a highest applied voltage amplitude and at least one lower applied voltage amplitude which may be zero. Intermediate greylevel modulation is obtained by applying intermediate voltages.

It is an additional object of this invention that the non-global energy minimum configuration at least present under application of the highest applied voltage corresponds to a fast growing mode under the application of high voltage to the

initially stable low voltage configuration. (A fast growing mode means a liquid crystal layer configuration of molecules that evolves more quickly from the initial configuration than do alternative configurations. ) One such suitable configuration is the symmetric H-state. The liquid crystal layer configuration known as a symmetric H-state is characterised by the molecules near to the cell centre (that is the midpoint between the bounding surfaces) being substantially parallel to the bounding surfaces and molecules above the cell centre being rotated from the normal in opposite sense to molecules below the cell centre.

It is a further object of this invention to apply stabilisation and/or isolation means to maintain the presence of the non-global energy minimum configuration under high applied voltage that is to prohibit its replacement in the cell or pixel by a lower energy or global energy minimum configuration. Alternatively or complementarily the higher voltages can be applied for only a fraction of the frame time and zero for the remainder to maintain the presence of the non-global energy minimum configuration under higher applied voltage.

It is still a further object of this invention to avoid the requirement for nucleation, a problem of some existing fast switching nematic or chiral nematic devices such as the pi-cell by operating in configurations that are continuously accessible from the zero applied voltage global energy minimum configuration. Continuously accessible means that one liquid crystal layer configuration may switch to the other configuration without the formation of a defect.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows a basic liquid crystal cell construction.

Figure 2 is prior art and shows the operation of the pi-cell device that is the state of the art for fast responding nematic or chiral nematic devices.

Figure 3 is prior art and shows the operation of the Fredericks cell, another fast responding nematic or chiral nematic device.

Figure 4 shows the operation of an embodiment of the current invention.

Figure 5a shows a schematic voltage waveform applied across devices shown in Figures 2,3 or 4. This waveform could also be the amplitude envelope of an AC waveform that is the root mean square value. Figure 5b shows a schematic of the resulting optical response achievable with such a waveform and devices.

Figure 6 shows a schematic of an alternative voltage waveform that may be used as part of this invention.

Figure 7a shows a schematic cross-section of a device structure used for the present invention. Figure 7b shows a schematic plan view of the same device structure. Figure 7c shows a schematic cross-section including a slight modification to the structure of Figure 7a.

Figures 8a and 8b show schematic cross-sections of another possible device structure used for the present invention. Figure 8c shows a schematic plan view of the same device structure.

Figures 9a and 9b show schematic cross-sections of yet another possible device structure used for the present invention. Figure 9c shows a schematic plan view of the same device structure.

Figure 10a and Figure 10b show a liquid crystal volume in series with an interelectrode gap in a different liquid crystal layer configuration of molecules to liquid crystal volumes not in series with the inter-electrode gap.

DESCRIPTION OF EMBODIMENTS.

Embodiment 1.

Referring to Figures 1 and 4. Two parallel transparent glass substrates 11 having transparent electrodes 12 on the inner surfaces are each coated with a thin polymer layer 13. The thin polymer layers are treated so as to be able to align the molecules of a positive dielectric anisotropy nematic liquid crystal 15 in a preferred direction and at a small angle (typically less than 45 degrees, more typically less than or equal to5 degrees) to the plane of the substrates. The two substrates are arranged to face each other in a parallel fashion and are preferably spaced between 0.5 and 6 microns apart, for example 2.2 microns apart. This structure constitutes a cell, the spacing of the two substrates is termed the cell gap, d. The cell gap can be set by the use of plastic or glass spacer beads 14, alternatively the cell gap may be set using photolithographically defined pillars or walls. The substrates are additionally arranged so that molecules of a positive dielectric anisotropy nematic liquid crystal near to each of the alignment layers would rotate from the normal in opposite senses to each other.

The cell is filled by capillary action with a positive dielectric anisotropy liquid crystal of birefringence An such that the product An. d-0. 2 to 0.35 microns. The nematic liquid crystal will form a symmetric H-state configuration similar to that of Figure 4a in which the molecules near to each of the alignment layers are rotated from the normal in opposite senses to each other, molecules near to the cell centre are substantially parallel to the cell substrates. This is a symmetric H-state with zero twist. The cell is positioned between two polarisers with mutually perpendicular absorption axis so as to give a transmissive state. An additional subtracting retarder may be positioned between the cell and one of the polarisers. A low rms voltage 41 is applied across the nematic liquid crystal, this low voltage is such that the symmetric H-state structure remains the global minimum energy state and may typically be between 0.0 and 1.0 volts inclusive depending upon the positive dielectric anisotropy liquid crystal used. A higher voltage 42, under the application of which the symmetric H-state is no longer a global energy minimum, is then applied such that the liquid crystal switches into the non-global energy minimum symmetric H-state. This high voltage is typically between a maximum level which results in a dark state (that is a maximum attenuation of transmitted light), typically 3 to 10 volts and a minimum level which results in a near maximum transmission grey level (that is a near minimum attenuation of transmitted light). This minimum level is typically approximately two times the rms voltage level at which the symmetric H-state would first become a non-global energy minimum state. Following the application of the

higher voltage 42 for some time the applied voltage level is reduced back to the low level 41 and the liquid crystal relaxes into its original configuration.

Those skilled in the art will recognise that alternatives using parallel polarisers or single polarisers and a reflector (with An. d-0. 1 to 0.175 microns) could be used.

(Example 1) At zero and low applied voltages a nematic liquid crystal for example ZLI 1132 or MLC 6100-100 (both available from Merck KGaA, Darmstadt) in a cell of approximately 2. 2u. m cell gap with suitable alignment layer gives a liquid crystal layer configuration similar to that of Figure 4a or Figure 4b. The liquid crystal cell is positioned at approximately 45 degrees between crossed polarisers together with an additional subtracting uniaxial optical retarder of between 10nm and 150nm but typically approximately 60nm. This configuration gives a bright transmitting state at low voltages and a dark state under the application of 6 volts. The voltage waveform of Figure 5a is applied to the cell, the vertical axis 51 indicates the voltage applied, the horizontal axis 52 indicates the time for which the voltage is applied. Firstly a low rms voltage 41 is applied, then a high rms voltage 42, and then a low rms voltage 41.

The resulting optical response 54 is shown in Figure 5b. The vertical axis 53 indicates the transmission of the device. The response time 55 corresponding to a change from 100% to 10% transmission and the response time 56 corresponding to a change in transmission from 0% to 95% are shown in Table 1. Under the application of the high voltage the nematic liquid crystal is in a high voltage symmetric H-state configuration similar to that of Figure 4c, this is a non-global energy minimum configuration (Comparative Example la Prior Art Device) A positive dielectric anisotropy nematic liquid crystal material for example ZLI 1132 or MLC 6100-100 in a cell of approximately 6um cell gap with parallel alignment (in which molecules 21 near to each of the alignment layers 13 are rotated from the normal in opposite senses to each other) is positioned at approximately 45 degrees between crossed polarisers together with an additional subtracting uniaxial retarder of approximately 60nm. The liquid crystal is brought into the V-state configuration of the prior art pi-cell device for means of comparison. At low voltages the liquid crystal layer configuration is similar to Figure 2b, at high voltages the liquid crystal cell is in a global energy minimum configuration similar to Figure 2c. Under the application of an applied waveform similar to Figure 5a the liquid crystal switches between these states with an optical response 54 similar to that shown in Figure 5b. The response time 55 corresponding to a change from 100% to 10% transmission and the response time 56 corresponding to a change in transmission from 0% to 95% are shown in Table 1.

(Comparative Example 1 b Prior Art Device) At zero and low applied voltages a nematic liquid crystal for example ZLI 1132 or MLC 6100-100 in a cell of approximately 2. 2um cell gap with suitable alignment gives a liquid crystal configuration similar to that of Figure 3a (in which molecules 21 near to each of the alignment layers 13 are rotated from the normal in the same sense

as each other), at high voltages the liquid crystal is in a global energy minimum configuration similar to Figure 3b. The liquid crystal cell is positioned at approximately 45 degrees between crossed polarisers together with an additional subtracting uniaxial optical retarder to give a bright transmitting state at a low voltage and a dark state under a high voltage of 3 volts. Under the application of the applied waveform similar to Figure 5a the liquid crystal switches between these states with an optical response 54 similar to that shown in Figure 5b. The response time 55 corresponding to a change from 100% to 10% transmission and the response time 56 corresponding to a change in transmission from 0% to 95% are shown in Table 1.

Device Type 100%-5% Response Time 0%-90% Response Time This invention example 1 0.6 milliseconds 1. 0 milliseconds Prior art comparative 0.8 milliseconds 4.0 milliseconds example la Prior art comparative 3.0 milliseconds 9 milliseconds example 1 b Table 1. A comparison between approximate response times of the device of the present invention and those of devices of the prior art.

Embodiment 2.

A cell structure and liquid crystal material and configuration similar to that of Embodiment 1 are used. However when the cell is switched from a low or zero to intermediate voltage, for example to obtain a greylevel corresponding to the intermediate voltage, the applied voltage follows a sequence similar to that of Figure 6. Firstly a low or zero voltage 41 is applied, then for a short time a high voltage 42 is applied and then the intermediate voltage 61 is applied.

Embodiment 3.

Referring to Figures 7a and 7b a cell structure and liquid crystal material and configuration substantially similar to that of Embodiment 1 are used. (Figure 7a shows a schematic cross-sectional view along the perpendicular to the preferred alignment direction of the liquid crystal. Figure 7b shows a schematic of a corresponding plan view). However prior to capillary filling of the cell a polymer precursor for example a reactive liquid crystal monomer is mixed with the nematic liquid crystal material. After the cell is filled with the mixture, in-situ polymer walls 71 are formed while the nematic liquid crystal 15 is in a global energy minimum symmetric H-state that is a zero or a low voltage only is applied. The in-situ walls are formed by polymerisation of the polymer precursor in a volume of the cell. The walls are formed by a similar technique to that described in the article by C. D. Hoke and P. J. Bos (Society for Information Display, Proceedings of the International Display Workshop 1998 pages 201-204). The walls 71 are formed to isolate regions of nematic liquid crystal from each other. In the case of an array of pixels addressed by a thin film transistor matrix as in a liquid crystal display, these isolated areas may be coincident with each of the individual pixels or colour subpixels defined by the electrode 12 pad at each pixel. As shown in Figure 7c an additional ultra-violet

blocking layer 72 may be incorporated between an electrode 12 and an alignment layer 13 to help prohibit polymerisation of the reactive liquid crystal monomer or other polymer precursor in the pixel area.

Limited polymerisation in the pixel area such that the liquid crystal volume remains switchable into a high voltage symmetric H-state can also be advantageous to stabilise the symmetric H-state, but a large amount of such polymerisation can increase the response times and/or the voltage required to switch between minimum and maximum attenuation. Those skilled in the art can optimise the effect, amount and distribution of polymerisation in the switchable volume of the liquid crystal layer.

Methods for changing the distribution of polymerisation in the switchable volume include; the use of ultra-violet absorbing liquid crystals, the use of ultra-violet transparent liquid crystals, the use of anisotropic photoinitiator with polarised ultraviolet light together with the inclusion of twist in the cell and/or polymerisation under applied voltage.

Embodiment 4.

Referring to Figures 8a, 8b and 8c. A cell structure and liquid crystal material and configuration similar to that of Embodiment 3 are used. (Figure 8a shows a schematic cross-sectional view along the preferred alignment direction of the liquid crystal.

Figure 8b shows a schematic cross-sectional view along the perpendicular to the preferred alignment direction of the liquid crystal. Figure 8c shows a schematic of a corresponding plan view). However in-situ polymer walls 71 are only formed orthogonal to the preferred alignment direction of the liquid crystal. Isolation of regions of nematic liquid crystal is achieved by fabricating walls 81 onto one of the substrates prior to coating with the alignment layer 13 on that substrate, and prior to cell assembly. The walls 81 are parallel to the preferred alignment direction of the liquid crystal 15 and can be fabricated using photolithographic techniques known to those skilled in the art. An example technique is given in"A High Contrast and Rugged FLCD with Stripe-Type BCB and Sticky Spacers"by S. R. Lee et al in Society for Information Display, Symposium Digest 1997 pages 1051 to 1054.

Embodiment 5.

Referring to Figures 9a, 9b and 9c a cell structure and liquid crystal material and configuration similar to that of Embodiment 3 are used. However in-situ polymer walls 71 are only formed parallel to the preferred alignment direction of the liquid crystal. Isolation of regions of nematic liquid crystal 15 is achieved by fabricating walls 81 onto one of the substrates after coating with the alignment layer 13 on that substrate, and prior to cell assembly. The walls, which may be fabricated using photolithographic techniques, are made from a material that favours the positive dielectric anisotropic nematic liquid crystal molecules to lie perpendicular to its surface.

Embodiment 6.

A cell structure and liquid crystal material and configuration similar to that of Embodiment 1 are used. However, as shown in Figures 10A and 10B the electrodes 12 on at least one of the substrates are patterned in such a way that the symmetric Hstate is a global energy minimum or low voltage configuration in the volume of liquid crystal 101 in series with the inter-electrode gaps when the volume of liquid crystal 102 in series with the electrodes is in a non-global energy minimum or high voltage symmetric H-state.

Embodiment 7.

Referring to Figures 1 and 4. Two parallel transparent glass substrates 11 having transparent electrodes 12 on the inner surfaces are each coated with a thin polymer layer 13. The thin polymer layers are treated so as to be able to align the molecules of a positive dielectric anisotropy nematic liquid crystal 15 in a preferred direction and at a small angle (-5 degrees) to the plane of the substrates. The two substrates are arranged to face each other in a parallel fashion and are preferably spaced between 0.5 and 6 microns apart, for example 2.2 microns apart. The substrates are additionally arranged so that molecules of a positive dielectric anisotropy nematic liquid crystal near to each of the alignment layers would rotate from the normal in opposite senses to each other. The cell is filled by capillary action with a positive dielectric anisotropy liquid crystal of birefringence An such that the product An. d-0. 2 to 0.35 microns.

The nematic liquid crystal will form a symmetric H-state configuration similar to that of Figure 4a in which the molecules near to each of the alignment layers are rotated from the normal in opposite senses to each other, molecules near to the cell centre are substantially parallel to the cell substrates. The cell is positioned between two polarisers with mutually perpendicular absorption axis so as to give a transmissive state. An additional subtracting retarder may be positioned between the cell and one of the polarisers. Means are included to illuminate the cell with a set of switchable coloured lights. A voltage waveform that consists of six periods per frame is applied.

Each period is approximately 2.78 milliseconds. During the first period the voltage level of the waveform corresponds to the required greylevel modulation of a simultaneously emitting red light. During the second period the voltage level of the waveform corresponds to the required greylevel modulation of a simultaneously emitting green light. During the third period the voltage level of the waveform corresponds to the required greylevel modulation of a simultaneously emitting blue light. During the subsequent fourth, fifth and sixth periods the voltage level of the waveform is zero or low, and no light is emitting. A viewer will observe an intended time averaged colour.

Embodiment 8.

An array of pixels of similar structure and liquid crystal material to the prior embodiments is used, however each pixel has associated with it a circuit for detecting whether the liquid crystal in said pixel has left the symmetric H-state (for example by detecting capacitance). If this is the case the voltage applied across that pixel is reduced to zero until the H-symmetric state is restored.

Embodiment 9.

A cell or an array of pixels of similar structure and liquid crystal material to the prior embodiments is used. However in addition to means to apply a voltage 42 across the liquid crystal layer to give a transverse electric field orthogonal to the bounding surface, additional means are provided to apply a lateral electric field with direction parallel to the bounding surfaces. The lateral electric field changes polarity at a higher frequency than the transverse electric field. For one polarity of the lateral electric field an up asymmetric H-state is favoured, for the opposite polarity of electric field a down asymmetric H-state is favoured. The alternation of the lateral electric field maintains the symmetric H-state.

Embodiment 10.

A cell or an array of pixels of similar structure to the prior embodiments is used.

However the liquid crystal used has the additional characteristic of significant flexoelectric polarisation. The voltage applied across the liquid crystal layer changes polarity at a higher frequency than it changes magnitude. For one polarity of the voltage an up asymmetric H-state is favoured, for the opposite polarity of voltage a down asymmetric H-state is favoured. The alternation of the polarity of the voltage maintains the symmetric H-state.

Many variations and modifications are apparent from the foregoing description of the invention and all such variations and modifications are intended to be within the scope of the present invention. These include the use of different preferred alignment directions, different non-global energy minimum high voltage configurations, different isolation patterns and means, different optical configurations, different applied voltage waveforms and such other variations and modifications familiar to those skilled in the art.

Claims

CLAIMS: 1. A liquid crystal device based electro-optical shutter or display comprising: a liquid crystal layer disposed between first and second bounding surfaces to form a cell or pixel; and means for applying a voltage across the liquid crystal layer; wherein, when zero voltage is applied across the liquid crystal layer the liquid crystal cell or pixel is in a global energy minimum configuration; wherein, when at least one higher voltage is applied across the liquid crystal layer the liquid crystal cell or pixel is in a non- global energy minimum configuration; and wherein the non-global energy minimum configuration is continuously accessible from the global energy minimum configuration.
2. A liquid crystal device based electro-optical shutter or display as claimed in claim 1, wherein for at least one applied voltage the non-global energy minimum configuration corresponds to maximum or minimum attenuation of transmitted or reflected light.
3. A liquid crystal device based electro-optical shutter or display as claimed in claim 1 or 2, wherein the non-global energy minimum configuration is a metastable energy configuration.
4. A liquid crystal device based electro-optical shutter or display as claimed in claim 1 or 2, wherein the non-global energy minimum configuration is a local minimum energy configuration.
5. A liquid crystal device based electro-optical shutter or display comprising: a nematic or chiral nematic liquid crystal layer disposed between first and second bounding surfaces to form a cell or pixel ; and means for applying a voltage across the liquid crystal layer ; wherein, when zero voltage is applied across the liquid crystal layer the liquid crystal cell or pixel is in a symmetric H-state; wherein, the liquid crystal material has positive dielectric anisotropy; and wherein the liquid crystal cell or pixel remains substantially in a symmetric H-state when non-zero voltage is applied across the liquid crystal layer.

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6. A liquid crystal device based electro-optical shutter or display as claimed in claim 5; wherein when a first voltage level is applied across the liquid crystal layer the symmetric H-state corresponds to maximum attenuation of transmitted or reflected light; and wherein when a second voltage level is applied across the liquid crystal layer the symmetric H-state corresponds to minimum attenuation of transmitted or reflected light.
7. A liquid crystal device based electro-optical shutter or display as claimed in claim 5 or 6, wherein liquid crystal molecules near to the boundary surfaces are at an angle of less than 45 degrees to the plane of the boundary surfaces when zero voltage is applied across the liquid crystal layer.
8. A liquid crystal device based electro-optical shutter or display as claimed in claim 6 or 7, wherein at least one of the first voltage level or the second voltage level has magnitude greater than 3 volts.
9. A liquid crystal device based electro-optical shutter or display as claimed in claim 5,6, 7 or 8 wherein a lateral electric-field component changes polarity at a higher frequency than the transverse electric-field component due to the voltage applied across the liquid crystal layer.
10. A liquid crystal device based electro-optical shutter or display as claimed in claim 5,6, 7 or 8 wherein the voltage applied across the liquid crystal layer changes polarity at a higher frequency than it changes magnitude.
11. A liquid crystal device based electro-optical shutter or display as claimed in any of the preceding claims, wherein voltages are applied across the liquid crystal layer in the sequence low voltage, high voltage, intermediate voltage.
12. A liquid crystal device based electro-optical shutter or display as claimed in any of the preceding claims, wherein the liquid crystal layer includes a polymer precursor which has undergone polymerisation in at least one volume of the cell or pixel array.
13. A liquid crystal device based electro-optical shutter or display as claimed in claim 12, wherein the polymer precursor has undergone polymerisation to form at least one in-situ polymer wall.
14. A liquid crystal device based electro-optical shutter or display as claimed in claim 13, wherein at least one wall is fabricated onto at least one of the bounding surfaces prior to assembly of the cell or array of pixels.

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15. A liquid crystal device based electro-optical shutter or display as claimed in claim 14, wherein at least one wall is orthogonal to at least one in-situ polymer wall.
16. A liquid crystal device based electro-optical shutter or display as claimed in claim 12, wherein the polymer precursor has undergone polymerisation in at least one switchable volume.
17. A liquid crystal device based electro-optical shutter or display as claimed in claim 12,13, 14,15 or 16, wherein the polymer precursor is a reactive liquid crystal monomer.
18. A liquid crystal device based electro-optical shutter or display as claimed in claim 1,2, 3 or 4, wherein at least one electrode on at least one of the bounding surfaces is patterned such that the liquid crystal volume in series with the inter-electrode gap is in a global energy minimum state when the liquid crystal volume not in series with the inter-electrode gap is in a non- global energy minimum.
19. A liquid crystal device based electro-optical shutter or display as claimed in claim 5,6, 7,8, 9 or 10 wherein at least one electrode on at least one of the bounding surfaces is patterned such that the liquid crystal volume in series with the inter-electrode gap is in a low voltage symmetric H-state when the liquid crystal volume not in series with the inter-electrode gap is in a high voltage symmetric H-state.
20. A liquid crystal device based electro-optical shutter or display as claimed in any of the preceding claims; wherein the applied voltage waveform comprises six periods per frame; wherein the cell or array is illuminated during three consecutive periods of the six periods; and wherein one of the three periods is illuminated with red light, one of the three periods is illuminated with blue light and one of the three periods is illuminated with green light.
21. A method of driving a liquid crystal layer within the symmetric H-state which comprises the steps: applying a voltage across the liquid crystal layer which has magnitude less than the magnitude at which the symmetric H- state would first become a non-global energy minimum state, and applying a voltage across the liquid crystal layer that is greater than twice the magnitude of the voltage at which the symmetric H-state would first become a non-global energy minimum state.

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22. A liquid crystal cell or pixel substantially as herein described and illustrated in the accompanying drawings.