GB2354834A

GB2354834A - Read/write liquid crystal

Info

Publication number: GB2354834A
Application number: GB9923167A
Authority: GB
Inventors: Ian James Forster
Original assignee: Marconi Caswell Ltd; Marconi Electronic Systems Ltd
Current assignee: Marconi Caswell Ltd; BAE Systems Electronics Ltd
Priority date: 1999-09-30
Filing date: 1999-09-30
Publication date: 2001-04-04
Anticipated expiration: 2019-09-30
Also published as: GB9923167D0; GB2354834B

Abstract

The bias potential for a liquid crystal device is provided by radiation falling on a bias potential generating region 20. The region may be pyroelectric, photovoltaic or include a radio antennae. Information can be written on the liquid crystal 120 using an infra-red laser beam. The liquid crystal can be one that is normally crystalline but can be heated to an isotropic state by the laser beam. Alternatively a bistatic cholesteric material can be used. The liquid crystal can be incorporated in a PDLC structure.

Description

2354834 READ/WRITE OPTICAL STRUCTURE The present invention relates; to a

read/write optical structure, in particular, but not exclusively, to a structure which is capable of being read and written upon using laser 5 radiation.

Structures for conveying information which can be attached to or formed onto products are well known. Such structures include various types of labels and tags fabricated from paper or plastics materials, and direct markings such as non-contact ink-jet printed text. Often, for example in a manufacturing organisation, products have labels attached thereto indicative of when the product!s have passed through various manufacturing and quality control stages, thereby enabling identification of the status of the products during manufacture. Such labels often remain attached when the products are supplied to customers as a certification of quality and origin of the products.

It is known that lasers can be used for writing onto surfaces, for example for individually labelling wires in wiring looms for use in complex electronic systems such as civil aircraft. Such writing corresponds to ablation or discolouration of insulation of the wires using focussed laser radiation to uniquely identify each wire with a code reference; it corresponds to an irreversible process such that erasure of written information on the insulation cannot be achieved by using laser radiation.

These structures suffer from one or more of the following problems:

(a) they are not adapted to be, overwritten or erased, for example bar codes printed onto or affixed to items, or reference codes applied to wire insulation; (b) they provide relatively limited information bearing capacity, for example ink-jet printed text providing best-by date information for perishable products; and (c) they are tirne-consuming to replace, thereby potentially adding to manufacturing or handling cost if information provided thereby associated with the products needs 4- to be changed frequently.

The inventor has appreciated that it is feasible to use an enhanced read/write optical structure for conveying information, the structure being remotely readable and remotely writeable to using beams of radiation.

According to a first aspect of the present invention, there is provided a read/write optical structure susceptible to being written upon and read from using electromagnetic radiation, the structure including information bearing means comprising a liquid crystal material for recording information, and bias voltage generating means included within the structure for generating a switchable bias potential for use in controlling information recordal in the information bearing means, the generating means being operative to generate the bias potential in response to radiation received at the structure.

The invention provides the advantage that inclusion of the generating means within the structure is capable of circumventing a requirement to bring external electrical connections to the infon-nation bearing means for controlling the state of the liquid crystal material, thereby enabling remote setting of the state to be achieved.

A prior art laser writeable liquid crystal device is described in a publication "Mermotropic Liquid Crysta&' by GW Gray ISBN 047191504 -1 pages 99 to 100. The device described includes a liquid crystal layer typically 12-14 [im thick which is initially substantially transparent. The layer comprises a mixture of nematic and cholesteric liquid crystal materials. The layer is susceptible to texture change is response to a focussed laser radiation beam incident thereupon. The layer becomes locally heated in a region where the beam is incident, the region experiencing sufficient heating for its liquid crystal layer to become an isotropic fluid. On cooling, the isotropic fluid appears opaque on account of it exhibiting light- scattering properties. Writing speeds of several centimetres per second are reported as typically achievable on the device with relatively high spatial defmition.

Erasure of laser written featues in the layer is effected by heating the device so that its layer becomes entirely isotropi; such heating is achievable by applying a short d.c. pulsed electric field across the layer and then allowing the layer to cool relatively slowly whilst maintaining a relatively small field in the order of 2000 Vim in the layer so as to produce a non-light scattering homeotropic alignment of liquid crystal molecules in the layer. As an alternative to applying the d.c. pulse, a relatively larger alternating electric field in the order of 7000 to 10000 V,./m at a frequency of 1 kHz to 1.5 kHz can be employed to erase written features in the layer.

Selective erasure of features in specific areas of the layer is achievable by passing a focussed laser beam over the areas whilst maintaining a small electric field in the order of 2000 VJm. across the layer which causes its liquid crystal in the vicinity of the areas to align homeotropically.

The prior art device differs from the structure according to the invention described above in that it does not incorporate bias voltage generating means but relies on externally applied voltages to erase it. Furthermore, the externally applied voltages are not generated in response to radiation received at the prior art device.

In the structure according to the invention, the generating means advantageously comprises a pyroelectric material for generating the bias potential in response to the radiation received. The pyroelectric material can be addressed using convention techniques such as laser bearn interrogation. Incorporation of such pyroelectric material counteracts a need for complex equipment for reading from and writing to the structure, A first embodiment of the invention will now be considered.

In the first embodiment where a continuous film. of liquid crystal material is employed in the information bearing means, the liquid crystal requiring to be heated to its isotropic state for information recordal purposes, the generating means is conveniently substantially separate from the information bearing means, the generating means and the information bearing means being mutually electrically connected.

In order to make the structure relatively inexpensive and susceptible to mass production, 5 the generating means berieficially incorporates a pyroelectric polymer material polyvinylidene fluoride (PVDF) for generating the bias potential.

Information is more reliably recorded in the structure when heating is required to store the information in the liquid crystal material of the information bearing means. Thus, advantageously, the liquid crystal material is operative to be read in a solid crystalline state and to be written to by selectively heating it to its isotropic state.

Conveniently, the liquid crystal material is selectively switchable between a transparent state in which it substantially transmits infta-red radiation and an opaque state in which it substantially scatters infta-red radiatiom These states provide a simple manner of recording information in the information bearing means which, for example, are susceptible to laser beam interrogation. The liquid crystal material can also be made thereby amenable to naked eye inspection.

In order to fabricate the structure, certain types of mass produced liquid crystal materials are especially appropriate. Thus, conveniently, the liquid crystal material comprises one or more substances from a list: 4-cyano-4'-octylbenzyhdeaniline, 4-alkyl-4'-cyanbiphenyls, 4-cyano-4'decylbiphenyl, 4-alkoxy-4"- cyano-p-terphenyls, 4-alkylphenyl-4-alkoxy3cyanobenzoates.

When fabricating the structure, features for conveying the bias potential from the generating means to the information bearing means must be included for the structure to function when written to. Thus, conveniently, the structure includes conductive regions for connecting the information bearing means to the generating means, the conducting regions configured to be capable of generating an electric field in the information bearing means.

In order to make the structure inexpensive to fabricate, the conducting regions beneficially comprise films of indium tin oxide. For convenience of mass production and wide utilisation, the films can be operative to be substantially transmissive to infra-red radiation to the information bearing means. To achieve such transmissiveness, the films are of a thickness in a range of 2 to 20 nin. The films are also transmissive to visible radiation in of wavelength in a range from 350 nm to 1000 nTn thereby allowing naked eye inspection of the information bearing means therethrough.

1 In order to assist information readout from the structure, especially where laser beam 10 interrogation is employed, one of the filins is advantageously operative to substantially reflect infta-red radiation transmitted through the information bearing means. This film is preferably of a thickness in a range of 50 to 500 rim, It is desirable that the structure is robust in use. Thus, advantageously, the structure 15 comprises outer layers of a piastics material for physically protecting the inforrnation bearing means and the generating means from an environment surrounding the structure, the layers comprising one of a polycarbonate, acrylic, polyvinyl chloride or polyethylene plastics material. Such plastic materials are inexpensive and robust, for example against physical flexing of the structure and many types of chemical solvents.

A second embodin-&nt of the invention will now be considered.

In order to circumvent the need to handle liquid crystal niaterials when fabricating the structure, it is desirable -that the information bearing means incorporates the liquid crystal material in the form of a polymer dispersed liquid crystal (PDLQ device. Such a device is robust and inexpensive to manufacture in large quantities, thereby assisting to make the structure inexpensive.

Conventionally, PDLC devices do not incorporate a pyroelectric polymer matrix to include 30 droplets of liquid crystal material. The inventor has appreciated in contradistinction to convention that the PDLC device advantageously comprises a pyroelectric polymer fihn including voids filled with the liquid crystal material.

Advantageously, the liquid crystal material is a bistatic cholesteric liquid crystal material 5 when reduced levels of radiation are desirable for writing information onto the structure. Use of such a bistatic material enables information to be written without a need to heat the material from a crystalline to an isotropic state. Incorporation of a bistatic cholesteric liquid crystal potentially enhances information writing rate on the structure.

Conveniently, the liquid crystal material is derived from a chemical group 4-alkyl-4'cyanobiphenyls. These are readily available substances suitable for use in mass production of the structure.

When bistatic liquid crystal materials are used, the pyroelectric polymer film in the 15 structure is conveniently fabricated from PVDF. Bistatic liquid crystal materials require an electric field to be applied to them to change their state; PVDF is an appropriate pyroelectric polymer for use in generating such an electric field.

The PDLC device referred to above conveniently incorporates mutually electrically 20 connected films on its major faces operable to counteract build-up of static charges on the faces which are, for example, capable of causing spontaneous erasure of information conveyed by the device. It is highly desirable to prevent build of such static charges when bistatic liquid crystal materials are employed because the presence of such static charge can result in spontaneous erasure of information from the information bearing means.

Conveniently, to reduce structure fabrication costs, the conductive films are fabricated from indium. tin oxide. In practice, it is desirable that one of the films is of a thickness in a range of 2 to 20 mn and is operative to substantially transmit infta-red radiation. Such a range of thickness also allows naked eye inspection of the device.

In order to assist laser interrogation of the structure when reading information therefrom, one of the films is preferably of a thickness in a range of 50 to 500 nm. and is operative to substantially reflect infra-red radiation transmitted through the device. Such reflection provides the advantage that the structure is efficient at returning interrogating radiation, thereby improving signal to noise ratio at a detector used for reading information from ffie 5 structure.

As an alternative to employing indium tin oxide conductive layers, the mutually connected films advantageously include a conductive polymer film operable to substantially transmit infra-red radiation. Use of a conductive polymer film also potentially allows naked eye inspection of the device. Such a conductive polymer film. is susceptible to silk screen printing and potentially provides enhanced radiation transmission relative to indium. tin oxide conductive films.

Conveniently, the structure comprises outer layers fabricated from plastics materials for 15 physically protecting the information bearing means and the generating means from an environment surrounding the structure, the layer comprising one of a polycarbonate, acrylic, polyvinyl chloride or polyethylene plastics material. The outer layers makes the structure robust and more reliable.

Advantageously, when more reliable information recordal in the structure is desired, the liquid crystal material is operative to be read in a solid crystalline state and to be written to by selectively heating it to its isotropic state. Use of such a liquid crystal material counteracts spontaneous erasure which can occur when bistatic. liquid crystal materials are employed.

An alternative embodiment of the invention will now be considered.

In the alternative embodiment of the structure, the generating means comprises a pyroelectric polymer fihn partitioned into a plurality of pixel regions, the information bearing means distributed to each of the pixel regions and corresponding at each of the pixel regions to a cavity including a liquid crystal material, each pixel region comprising mutually oppositely polarised regions mutually electrically connected in parallel and operable to selectively generate a bias potential across the cavity when the structure is being written to, thereby selectively setting each pixel to an opaque state or a substantially transparent state to record the information. Inclusion of the oppositely polarised regions assists to counteract spontaneous erasure of information stored in the information bearing means.

Conveniently, in the alternative embodiment, the pyroelectric polymer fihn comprises PVDF. Use of PVDF provides the advantage of low cost and physical robustness.

Advantageously, in order to be able to use lower intensities of radiation when writing infonmfion. to the structure, the liquid crystal material is a cholesteric bistatic liquid crystal material. The bistatic liquid crystal is switchable between substantially transparent and opaque optical states without there being a need to heat the liquid crystal from its crystalline state to its isotropic state.

In order to be able to write information onto the structure at pixel resolution, each pixel region beneficially includes an associated isolated connection layer for electrically connecting the mutually oppositely polarised regions to the cavity. Conveniently, the isolated connection layer is fabricated from indium tin oxide and is preferably in a range of 2 to 20 nm thick. 'Me connection layer is thereby able to substantially transmit infra-red radiation and also capable of allowing naked eye inspection of the optical state of the liquid crystal material for viewing information recorded therein. As an alternative to employing indium tin oxide for the connection layer, the isolated connection layer is advantageously fabricated from a conductive polymer.

When constructing the structure, it is convenient to form the cavity of each pixel region into the pyroelectric polymer film. Advantageously, for developing an electric field across the liquid crystal material, the cavity of each pixel region is in a range of 20% to 60% deep relative to the thickness of the pyroelectric polymer film i In some situations, it is undesirable, for example for reasons of cost and convenience, to form the cavity of each pixel in the pyroelectric polymer film. An alternative solution involves fonning the cavity of each pixel region into an outer fihn layer of a plastics material included within the structure and operable to protect the pyroelectric film, from an environment surrounding the structure. Conveniently, the outer layer is fabricated from one of a polycarbonate, acrylic, poly vinyl chloride or polyethylene plastics material.

In a further alternative embodiment of the invention, the information bearing means comprises a PDLC device and the generating means comprises a layer of pyroelectric plastic, the layer partitioned into pixel regions, each pixel region including mutually oppositely polarised regions mutually electrically connected through an isolated connection layer associated with each pixel region, the connection layer operable to selectively generate an electric field across t be PDLC device when writing information thereto, thereby individually setting each pixel region to a substantially transparent optical state or an optically scattering state for information recordal purposes. This ffirther alternative embodiment provides the benefit that the PDLC device does not need to be fabricated from a pyroelectric polymer, thereby reducing manufacturing cost because standardly available PDLC devices can be employed in the structure.

In yet another alternative embodiment of the invention, the generating means comprises an antenna for receiving radio radiation and generating a corresponding signal for use in generating the bias potential. Use of an antenna provides an advantage that pyroelectric techniques do not need to be used for information recordal on the structure.

I In the context of describing the invention, radio radiation is herein defined as electromagnetic radiation having a carrier frequency substantially in a range of 50 kHz to GHz. Moreover, visible radiation to which the naked eye responds is herein defines as electromagnetic radiation having a wavelength substantially in a range of 350 nm to 1000 Mn Conveniently, the antenna is a loop antenna providing first and second connections, the first connection connected to an earth plane of the structure and the second connection connected to an electrode region of the structure, the electrode region operable to cooperate with the earth plane to generate an electric field in the information bearing means for purposes of recordal of information therein.

In order to enhance sensitivity of the structure and generate an enhanced magnitude of electric field in the information bearing region, the antenna is advantageously operable to resonate with a capacitance provided between the electrode region and the earth plane at a frequency of the radio radiation received at the structure.

When fabricating the structure, it is beneficial to employ a configuration wherein the information bearing means corresponds to a PDLC device, the earthing plane abutting onto a first major face of the device, and the antenna and the electrode region abutting onto a second major face of the device.

In yet another embodiment of the invention, the bias generating means is implemented in the form of photodiodes. Using photodiodes provides the advantage of potentially increased writing speeds in comparison to the structures of the invention which utilise the pyroelectric. effect to generate the switchable bias potential.

In another aspect, the invention provides a method of writing information to a structure according to the first aspect of the invention, the method including the steps of- (a) selectively irradiating the generating means, thereby selectively generating the bias potential; (b) selectively irradiating a portion of the information bearing means from its crystalline state to its isotropic i state; (c) allowing the portion to cool to its crystalline state whilst selectively controlling the bias potential to render the portion opaque or transparent as required for purpose 5 of information recordal in the information bearing means; and (d) repeating steps (a) to (c) until the information is stored in the information bearing means.

Conveniently, the method is such that hifta-red laser beams are used for irradiating the structure. Finely focussed beams can be generated inexpensively using standard solid state i infra-red lasers, for example as employed in commercial CD players.

Where the generating means is not spatially separated from the infonnation bearing means, it is practicable to use a single beam of radiation for irradiation purposes instead of the individually irradiating the ge'neratmg means and information bearing means with respective beams. As a consequence, steps (a) and (b) of the method can thereby be perfonned simultaneously.

In order to ensure reliable recordal of information using the method, it is advantageous to further detect whether or not the portion of the infon-nation bearing means has been correctly written and to repeat steps (a) to (c) until the portion is correctly written.

In a further aspect of the invention, the invention provides a method of writing information to a structure according to the first aspect, the structure including a bistatic. cholesteric i liquid crystal material in the inf I onnation bearing means, the generating means distributed throughout the information bearing means, the method including the steps of (a) applying a pulse of radiation to a portion of the information bearing means to generate thereat the bias potential and thereby an electric field across the liquid crystal material; (b) terminating the pulse after a selected duration and pulse power, to render the portion transparent or opaque as required for information repordal therein; and 5 (c) repeating steps (a) and (b) until the information is stored in the information bearing means.

Embodiments of the invention will now be described, by way of example only, with 10 reference to the following diagrams in which:

Figure I is a schematic plan view of a structure according to a first embodiment of the invention, Figure 2 is a schematic cross-sectional view of the structure in Figure 1; Figure 3 is a schematic cross-sectional view of a structure according to a second embodiment of the invention employing liquid crystal droplets dispersed in a pyroelectric polymer matrix; Figure 4 is a schematic cross-sectional view of a structure according to a third embodiment of the invention incorporating complementary pyroelectric elements and recesses to accommodate liquid crystal material; Figure 5 is a schematic diagram of a fabrication process for the structure shown in Figure 4.

Figure 6 is a schematic cross-sectional view of a structure according to a fourth embodiment of the invention fabricated by thermal welding; i Figure 7 is a schematic cross-sectional view of a structure according to a fifth embodiment of e invention comprising multiple stacked layers including a pyroelectric polymer layer; 1 Figure 8 is a plan view! of a structure according to a sixth embodiment of the invention including a loop antenna for receiving inductively coupled radiation; Figure 9 is a cross-sectional view of the structure shown in Figure 8; Figure 10 is a cross-sectional view of a structure according to a seventh embodiment of the invention, including photovoltaic devices; and Figure 11 is a cross-sectional view of a structure according to an eighth embodiment of the invention including mutually compensating photovoltaic devices.

Refening now to Figure 1, there! is shown a read/write optical structure according to a first embodin&nt of the invention; the structure is indicated by 10. The structure 10 includes a first elongate rectangular upper film layer 12 fabricated from a substantially transparent plastics material. Beneath the layer 12, the structure 10 further includes a rectangular electrode region 14, a circular electrode region 16 and an interlinking conductive track 18 electrically connecting the regions 14, 16 together. Concentrically aligned to the circular electrode 16 and beneath it is a pyroelectric rectangular region 20 of pyroelectric plastic which is shown included within a dotted hue 22. In Figure 1, there is shown an axis A-B aligned to an elongate axis of the structure 10 and passing through the circular electrode 16.

In order to describe the structure 10 niore fully, there is provided a cross-sectional view along the axis A-B in Figure 2; the view is indicated by 100. The structure 10 is of a multilayer form and includes in sequence the first layer 12 fabricated from a plastics material, the regions 14, 16 and the associated track 18, the region 20 in a similar plane to a liquid crystal region 120, a conductive electrode region 130, and finally a second film layer 140 fabricated from a plastics material together with end regions 160, 170.

The first and second film layers 12, 140 are of a thickness in a range of 50 to 200 lam and comprise a substantially transparent polycarbonate, acrylic, polyvinyl cl-Aoride (PVC) or polyethylene plastic material. Moreover, the layers 12, 140 have major faces which are 55 mrn long and 40 nim wide. 10 The electrode regions 14, 16 and the conductive track 18 are a patterned continuous indium tin oxide metallic film having a thickness in a range of 2 to 20 nm. The film is vacuum deposited onto the layer 12 during fabrication of the structure 10. Moreover, the film is sufficiently thick to provide a conductive electrical path to equalise potentials of the regions 14, 16 and also sufficiently thin to substantially transmit infrared radiation received 15 on the layer 12 through to the liquid crystal region 120, and allow naked eye inspection of the region 120. The region 20 comprises a pyroelectric plastic film, the film is fabricated from a pyroelectric: plastics material such as polyvinylidene fluoride (PVDF), although other types of pyroelectric polymers can alternatively be used. The region 20 has a thickness in a range of 15 to 50 pm which is arranged to match a corresponding thickness 20 of the liquid crystal region 120. The region 120 spatially substantially coincides with the region 14. The film in the region 20 is polarised in a direction normal to its major surfaces prior to incorporation into the structure 10; such polarisation is achievable, for example, by 25 applying an electrical field across it to cause permanent molecular polarisation therein; electric field strengths in the order of 1C V/m are required to achieve polarisation of the film, When the film is exposed after polarisation to iufra-red radiation, positive charge is generated at the region 16 and a corresponding negative charge at the region 130. Because the regions 14, 16 are mutually electrically connected, the positive charge propagates from 30 the region 16 to the region 14 to provide an electric field across the region 120.

f The electrode region 130 comprises a continuous indium tin oxide metallic film having a thickness in a range of 50 to 500 nm to impart reflective properties thereto. It provides a conductive film which is coincident with the regions 14, 16. Moreover, it is fabricated by vacuum depositing indium. and tin metals in the presence of trace oxygen onto the second 5 layer 140.

The end regions 160, 170 of the structure 10 form. an edge around an outside perimeter of the layers 12, 140. The regions 160, 170 are slightly conductive to prevent accumulation of charge in the regions 14, 16 relative to the region 130 when the structure 10 is exposed to a steady level of ambient infra-red radiation, for example strong sun light. The regions 160, 170 are formed from a flexible epoxy adhesive loaded withtrace quantities of carbon particles or nicbrome particles.: Moreover, the layers 12, 140 and the regions 160, 170 in combination protect the regions 20, 120 from influences such as environmental contamination and humidity which could adversely affect the structure 10 in operation.

The region 120 comprises a cholesteric liquid crystal material having a melting point intermediate between a maximum temperature at which the structure 10 is to be used and retain infon-nation, and a melting point of the layers 12, 140 which is substantially around 130 'C depending upon the type of plastics material used. The liquid crystal material is in a crystalline state below its melting point and in an isotropic state above its melting point. Moreover, the material incorporates polarised molecules which give the material:

(a) a transparent appearance when the molecules are mutually aligned, namely in a horneotropic state; or (b) an opaque appearance when the molecules are mutually misaligned when they scatter and reflect incident radiation in a range of directions.

When the liquid crystal material is in the isotropic state, an electric field can be applied to the molecules to align them to achieve a transparent appearance; this transparent appearance is maintained if the electric field is maintained whilst the molecules are permitted to cool relatively slowly to their crystalline state. If a reduced electric field is applied to the molecules when in a isotropic state and the molecules are permitted to cool relatively rapidly, the molecules return to their crystalline state but retain their opaque appearance due to relative misalignment of the molecules.

The material can comprise one or more of the following types of liquid crystal, alone or in combination, as provided in Table 1; approximate melting points of the liquid crystals where they attain their isotropic state are also indicated.

TABLE 1

OC F4-cyano-4'-octylbenzylideanilin 50 OC 4-alkyl-4'-cyanobiphenyls 4-cyano-4'-decylbiphenyl 50 OC 4-alkoxy-4"-cyano-p-terphenyls - 4- 80 OC alkylphenyl-4-alkoxy-3-cyanobenzoates Operation of the structure 10 will now be described with reference to Figures 1 and 2.

When the structure 10 is to be read, a first laser (LASER A) 200 providing a first beam 2 10 of inka-red radiation is employed to selectively interrogate a portion of the region 120; infra-red radiation is defined as radiation having a wavelength in a range of substantially I to 10 Rm. When reading the structure 10, the beam 2 10 is substantially normal to a plane parallel to the region 14 and focussed at a plane intermediate between the region 14 and the region 130.

Where the liquid crystal material in the region 120 has been made selectively opaque when written to, the beam 2 10 becomes scattered and gives rise to scattered radiation 220 which is detected at an infta-red detector (DETECTOR A) 230 mounted off-axis with respect to I the beam 210. Where the liquid crystal material has been made selectively transparent when written to, namely in 4 homeotropic molecularly aligned state, the beam 210 L propagates in a forward direct ion through the region 120 to the region 130 whereat it is reflected and propagates in a reverse direction through the region 120 back towards the laser 200 and thereby giving rise to reduced scattered radiation at the detector 230. When reading the structure 10, intra-red radiation power in the beam 2 10 is maintained at below a level which would cause heating of the liquid crystal material to change its relative molecular aligninent from that iinparted to it when the structure 10 was written to.

When the structure 10 is to be written to, a second laser (LASER B) 240 providing a second laser beam 250 of infra-red radiation is employed to selectively irradiate the region 16 and hence the pyroelectric region 20. The first laser 200 is operated so that its beam 2 10 is of sufficient energy to heat localised parts of the region 120 so that its liquid crystal material is heated from its crystalline state to its isotropic state.

When writing an opaque feature 260, represented by a black circle in Figure 1, into the region 120, the second laser 240, is not activated so that the beam 250 is not generated. As a consequence, the regions 14, 1'6 are at a substantially identical potential to the region 130 and a zero electric field is generated across the region 120. The beam 210 from the first laser 200 is then directed to illuminate the region 120 in a part thereof where the feature 260 is required. The beam 2 10 heats the liquid crystal material to an isotropic state at the feature 260. The beam 2 10 is then promptly switched off to allow the feature 260 to cool rapidly; such rapid cooling gives rise to opacity at the feature 260 which is detectable using the detector 230. If a sufficient degree of opacity has not been obtained, the laser 200 can be used to reheat the region of the feature 260 and then allow it to cool rapidly again; this process can be repeated until a desired degree of opacity is achieved as detected using the detector 230.

When writing a clear feature 270, represented by a clear circle in Figure 1, into the region 120, the second laser 240 is activated to generate the beam 250 which illuminates the pyroelectric region 20, thereby generating a potential difference in the order of volts between the regions 14, 16 relative to the region 130. The potential difference results in an electric field in the order of 4 x 10' Win being generated across the region 120 in a direction normal to a plane parallel to the regions 14, 130. Those parts of the region 120 which are in a crystalline state are unaffected by the electric field because orientation of their liquid crystal molecules are set in a crystalline state. However, the beam 2 10 heats the region 120 at the feature 270 to an isotropic state whereat liquid crystal molecules associated with the feature 270 become aligned by the electric field into an homeotropic state in which they appear substantially transparent. The first beam 210 is then gradually reduced in intensity whilst focussed at the feature 270 and the liquid crystal molecules allowed to cool relatively slowly; whilst the molecules cool, the second beam 250 is maintained to provide the electric field across the region 120. When the molecules have cooled to their crystalline state, the first laser 200 is employed at reduced power in its reading mode of operation to illuminate the feature 270 and the detector 230 is used to check that the feature is transparent as intended. If the feature 270 is not sufficiently transparent, the lasers 200, 240 are re-activated to rewrite to the feature 270 as described above. If the feature is sufficiently transparent, the lasers 200, 240 are then de-activated.

The lasers 200, 240 and the detector 230 are linked to a computer control unit (not shown) which coordinates reading and writing to the structure 10. Moreover, the laser 200 incorporates optical components (not shown) for actuating the beam 210 over the region as indicated by an arrow 280. Moreover, the lasers 200, 240 are operated in a pulse mode so that charge generated by the region 20 has insufficient time to dissipate through the regions 160, 170 whilst writing of features onto the structure 10 is occurring.

Thus, the control unit, the lasers 200, 240 and the detector 230 provide an apparatus for use for reading and writing to the structure 10; the apparatus is capable of monitoring to check whether or not a feature has been correctly written on the structure 10 thereby enabling a degree of checking and quality control to be achieved. Although use of infta-red radiation is used for reading from and writing to the structure 10, the region 120 is also susceptible to naked eye inspection at visible radiation wavelengths for reading information thereon.

Polymer dispersed liquid crystals (PDLCs) are known. They comprise miniature droplets of a liquid crystal material dispersed within a polymer matrix. PDLCs can be fabricated using a number of alternative methods. One method is to make a mixture of a prepolymer, a liquid crystal material and a solvent. The mixture is then processed by allowing the solvent to evaporate, thereby resulting in the liquid crystal material forming into small droplets surrounded by polymerised prepolymer; the droplets have a diameter which can vary in a range of 0.2 to 4 prn Liquid crystal droplet size in the PDLCs is determined by the rate of polymerisation which, in turn, depends upon the rate of solvent evaporation. Chloroform is frequently empl. yed for the solvent.

PDLCs function by altering their light scattering characteristics depending upon electric field applied thereto. In the absence of the field, a director associated with orientation of liquid crystal molecules included within the droplets is randon-Ay orientated, thus leading to a strong scattering of incident infra-red radiation. When an electric field is applied, the director orientates itself along a direction of the electric field due to the Frederiks effect; in this state the refractive index of the polymer matches the refractive index of the liquid crystal material, thereby rendering the PDLCs substantially transparent to infra-red radiation.

- The inventor has appreciated that it is possible to fabricate a PDLC incorporating a pyroelectric polymer to provide a plastic matrix to spatially suspend droplets of a bistatic cholesteric liquid crystal material which can be switched between a transparent state and an opaque state depending upon duration of an electric field applied thereto, the electric field generated by the pyroelectric polymer in response to irradiating it with infra-red radiation.

A PDLC structure incorporating a pyroelectric polymer matrix. including bistatic cholesteric 30 liquid crystal droplets is shown in Figure 3. The structure is indicated by 300 and comprises in sequence from its front surface through which it is interrogated to it rear surface: a plastic film layer 3 10, a first indium. tin oxide layer 320 vacuum deposited onto the layer 3 10, a PDLC region 330, a second indiurn tin oxide layer 340 vacuum deposited onto a fihn layer 350 fabricated from a plastics material. At a peripheral edge of the structure 300, there is a peripheral layer 370 providing mechanical protection to counteract delamination of the structure 300, to counteract ingress of contaminants and also to provide a discharge path to prevent charge accumulation on the layers 320, 340 as a consequence of exposure to direct sunlight which could cause a steady electric field across the PDLC region 330 and thereby spontaneous changes in the optical characteristics of liquid crystal material in the droplets.

The layers 310, 350 are fabricated from a polyvinyl chloride (PVC), polyethylene, polycarbonate or an acrylic plastics material. The layer 340 is operative to function as a reflective backplane and has a thickness in a range of 50 to 500 Mn The. layer 320 is in a range of 2 to 20 nm thick to:

(a) provide a discharge path for steady state charge accumulation on the layer 320 through the peripheral layer 370 to the layer 340; and (b) provide for a substantial transmission of infra-red radiation to the PDLC region 20 330, and also allow naked eye inspection of the region 330.

The PDLC region 330 comprises a PVDF polymer matrix incorporating droplets of a cholesteric bistatic liquid crystal material, for example a material in a chemical group 4alkyl-4'-cyanobiphenyls. When fabricating the structure 300, the region 330 is subjected to a relatively high electric field in the order of W Win to polarise it in a direction normal to its exposed major surfaces prior to bonding onto it the layers 310, 350 bearing the layers 320, 340 respectively.

Operation of the structure 300 will now be described with reference to Figure 3.

I When writing information into the PDLC region 330, a laser (LASER Q 375 generates an infra-red radiation beam 37 8 which is in a direction substantially normal to the layer 3 10 and focussed to provide a focussed spot at a plane midway between the layers 320, 340. When the beam 378 passes through the region 330, it locally heats up the PVDF polymer 5 which develops locally a positive charge on the layer 320 and a negative charge on the layer 340; this is represented by.... and ---- symbols in Figure 3. Tlie charges give rise to local electric field in the PDLC region 330 which is capable of changing droplets between transparent homeotropic state and a focal conic state where the droplets appear opaque.

Droplets which are in a homeotropic state are represented by white circles, for example a 10droplet 360; droplets which have been altered by the beam 378 from a honleotropic state to a focal conic state are represented by black circles in Figure 3, for example a droplet 380. Droplets in the focal conic state appear opaque because they scatter incident radiation as a consequence of their refractive index being different from that of the PVDF polymer surroundiniz them is The laser 375 functions in a pulsed mode when writing to the structure 300. Switching droplets from the homeotropic state to the focal conic state requires a relatively low electric field to be generated locally across the PDLC region 330. In contrast, switching from the focal conic state to the homeotropic state requires a relatively higher voltage pulse which 20 drives the droplets through a transient planar texture to the eventual horneotropic state.

Although charges generated locally in the region 330 give rise to instantaneous electric fields capable of changing locally states of the droplets, the charge dissipates over a longer period through the edge region 370. Imus, when writing to the structure 300, it is necessary for the laser 375 to be operated in a pulsed mode.

When reading information from the PDLC region 330, the laser 375 operates with reduced radiation power in the beam 378,to that the beam is incapable of generating charges at the layers 320, 340 which can alter the state of droplets in the region 330; the laser 375 can, for example, be operated in continuous non-pulsed mode when reading information from the structure 300. When the bearn 378 passes through the region 330 where droplets are transparent, namely in a homeotropic state, the beam 378 is reflected by the layer 340 and returns substantially back at the laser 375. However, when the beam 378 passes through the region 330 where droplets are opaque, namely in a focal conic state, the beam 378 is scattered and gives rise to a scattered radiation 390 which is detectable at an infta-red detector 395 mounted off-axis relative to the beam 378.

In a first modified version of the structure 300, the layers 320, 340 can be replaced by conductive polymer films which provide the advantage of a higher electrical resistance and greater transrnissivity of infra-red radiation compared to indium tin oxide vacuum 10 deposited layers.

Moreover, in a second modified version of the structure 300, the bistatic: cholesteric liquid crystal material included within the droplets can be substituted by a liquid crystal material as employed in the device 10 having a relatively high temperature associated with its isotropic state. In the second version, the beam 378 heats the droplets to their isotropic state during writing, the electric field being used during cooling to control orientation of liquid crystal molecules present in the droplets, thereby selectively rendering the droplets transparent or opaque when cooled to their crystalline state.

In some applications, it may be prohibitively costly to use a PDLC including a PVDF polymer. Moreover, especially in the case of the structure 300 incorporating bistatic liquid crystal materials, spurious charge generation due to ambient radiation received at the structure 300 can cause spontaneous erasure of information written into the region 330. In order to counteract such spontaneous erasure, the inventor has devised an alternative structure indicated by 400 in Figure 4.

Referringnow to Figure 4, the structure incorporates a pyroelectric polymer region 410, and first and second layers 420, 430 fabricated from a plastics material and providing protection for the region 410 included therebetween. The region 410 incorporates an array of pixels, each pixel comprising a cavity 450 filled with a cholesteric bistatic liquid crystal I material as used in the structure 300, a vacuum deposited conductive layer 440 isolated I from that of other pixels, and mutually oppositely polarised regions 470, 480 of the region 410. The structure 400 farther comprises a conductive reflecting layer 460 vacuum deposited onto the layer 430 and incorporated in the structure between the region 410 and 5 the layer 430.

For each pixel, the conductive layer 440 is interposed between the layer 420 and the regions 470, 480 of the pixel, and between the layer 420 and the cavity 450. Moreover, the conductive layer 440 of each pixel is operative to electrically connect the cavity 450 of the I pixel to the regions 470, 480 of the pixel.

The structure 400 provides the' a dvantage that, for each pixel, ambient radiation received at the pixel generates opposing pyroelectric charges in the regions 470, 480 resulting in a net zero charge being generated in the layer 440 and hence a substantially zero electric field, thereby counteracting spontaneous erasure of information recorded in the region 410.

When writing information to the structure 400 pixel by pixel to record information, a laser beam is selectively guided onto the region 480 and the cavity 450 associated with a pixel and not onto the region 460 thereof, thereby generating a potential difference between the layer 440 and the reflecting layer 460 and hence providing an electric field across the cavity

450. The electric field aligns molecules of the material to either a homeotropic state thereby rendering the cavity 450 transparent or a focal corlic state thereby rendering the cavity 450 opaque depending upon the intensity of the laser beam used and its pulse duration.

When reading information from the structure 400, a laser beam of reduced power is used which illuminates whole pixel areas, thereby counteracting generation of an electric field across the cavity 450 during reading. Alternatively, the structure 400 can be inspected by naked eye by viewing the pixels through the layer 420.

The layer 460 comprises a vacuum deposited indium tin oxide metallic film having a thickness in a range of 50 to 500 nrr.L It is operative to reflect infra- red radiation incident thereupon. The layer 440 comprises a vacuum deposited indiurn tin oxide metallic film having a thickness in a range of 2 to 20 mi; it is sufficiently thick to conduct charge generated at the regions 470, 480, and sufficiently thin the substantially transmit infra-red radiation and allow naked eye inspection of the cavities 450.

The polymer region 410 is in a range of 20 to 50 [Lm thick. PVDF is employed as the pyroelectric polymer for the region 410. Moreover, the layers 420, 430 are in a range of 50 to 100 tm thick and comprise an infra-red transmissive polycarbonate, acrylic, polyethylene or polyvinyl chloride (PVC) plastics material. Furthermore, the cavities 450 are of a depth in a direction normal to the layer 460 in a range of 20 to 60 % of the thickness of the region 410.

A method of fabricating the structure 400 wiU now be described with reference to Figure 5. Steps of the method are indicated by 500.

In step 1, there is provided a sheet of a polymer PVDF for fabricating the region 410.

In step 2, the sheet from step I is interposed between a planar backing plate 530 and a moulding plate 5 10 incorporating an array of projections such as a projection 520. Pressure and heat are then applied to the plates 510, 530 to compress the sheet and thereby form cavities in it which correspond to the cavities 450 in the structure 400.

In step 3, the sheet is removed from the plates 510, 530 to provide a preform for the region 410. In step 4, the sheet incorporating the cavities from step 3 is mounted onto a machine comprising a planar conductive platter 550, a series of pointed probes 560 such as a probe 30 565, an array of switches such as a switch 570, and sources of relatively high positive and negative potentials 580, 590 respectively relative to the platter 550. The probes 560 can be individually selectively switched to have a positive or negative potential relative to the platter 550. A con4:)uter control unit (not shown) controls the positions of the probes 560 relative to the sheet and their, potential relative to the platter 550. The control unit is operative to move the probesl over the sheet selectively polarising pixel regions of it to generate the regions 470, 480 of the structure 400.

In step 5, the polarised sheet from step 4 is removed from the platter 550.

In step 6, there is provided the layer 420 in the forin of a sheet of plastic.

In step 7, a layer of indium tin,oxide is vacuum deposited onto the layer 420 to provide a uniform metallic film thereon. The film is then covered in a coating of photoresist, the coating then selectively exposed through a mask to delineate the pixels of the device 400, developed to provide etch windows to the fih-n and the film then etched through the windows to delineate the pixels. When the etching is completed, the photoresist is removed using an organic solvent such as acetone or etched away using an oxygen plasma.

In step 8, the region 410 with its cavities 450 facing upwards is exposed to liquid crystal material to fill the cavities 450 with the material. Then the layer 420 together with the layer 430 and its associated vacuum deposited indiurn tin oxide layer 460 are thermally fused or adhesively bonded onto major fares of the region 4 10 to provide the structure 400.

The method 500 provides the advantage that it enables the structure 400 to be fabricated without a need to form a PDLC using PVDR In fabricating the structure 300, the liquid crystal material contained in the droplets is subjected to a relatively high electric field necessary for polarising the PVDF polymer; such a relatively high field can be potentially damaging to the liquid crystal material. When fabricating the structure 400, addition of liquid crystal material occurs in step 8 after polarisation in step 4, thereby counteracting such potential damage.

Referring now to Figure 6, there is shown a structure indicated by 600 which functions in an identical manner to the structure 400. During fabrication of the structure 600, the region 410 is covered in a thin surface fihn of a liquid crystal material and the layer 420 then offered up to and thermally welded to form domed cavities 6 10 into which the liquid crystal material is squeezed; such fabrication avoids a need to perform steps 2 and 3 as shown in Figure 5. Thermal welding can be performed by pressing a heated grid onto the layer 420 when in contact with the region 410, the liquid crystal material being squeezed towards hole regions of the grid corresponding to the cavities; the grid is arranged to have a pitch corresponding to spacing of pixels within the structure 600.

Figure 7 illustrates an alternative structure indicated by 700. The structure 700 comprises in sequence a first outer layer 7 10 fabricated from a plastics material, a first indium. tin oxide metallic layer 720, a PDLC region 730, a plurality of pixel conductive electrodes such as an electrode 740, a layer 750 of PVDF pyroelectric plastics material, a second indium tin oxide metallic layer 780 and a second outer layer 790 fabricated from a plastics material. At a peripheral edge of the structure 700 is incorporated an edge region 795 comprising an electrically conductive polymer material which functions to seal peripheral edges of the structure 700, thereby counteracting delamination of the structure 700 and ingress of contamination therein. The layer 795 is also operative to mutually electrically connect the layers 720, 780.

The pixel conductive electrodes 740 are spatially arranged in a regular pattern within the structure 700. Moreover, the layer 750 is polarised locally in regions corresponding to pixel electrodes 740, for example polarised regions 760c, 770c corresponding to a pixel electrode 740c. The regions 760, 770 are polarised in mutually opposite directions in a similar manner to the regions 470, 480 in the structure 400.

The PDLC region 730 comprises a non-pyroelectric plastics material and liquid crystal material droplets included within cavities formed within the plastics material. The liquid crystal material can be a bistatic cholesteric. liquid crystal as in the structure 400 or a relatively higher melting point liquid crystal material as employed in the structure 10. The PDLC region 730 is in a rangeof 20 to 50 [Lm thick.

I The first metallic layer 720 is in a range of 2 to 20 nm thick, namely sufficiently thick to provide an electrically conducting plane, and sufficiently thin to substantially transmit infra-red radiation and to allow inaked eye inspection of the region 730 at visible radiation wavelengths. The second rnetallic layer is in a range of 50 to 500 nm thick and is operative to function as a reflecting surface. The first and second outer layers 710, 790 are in a range of 100 to 200 [Lm thick and are fabricated from an acrylic, polycarbonate, PVC or 10 polyethylene plastics material.

The pixel electrodes 740 are fabricated from vacuum deposited indium tin oxide having a dickness in a range of 2 to 20 nfit They can alternatively be fabricated from an electrically conductive polymer.

Operation of the structure 700 Iill now be described with reference to Figure 7.

An infta-red laser beam (not shown) operating at a relatively low power illuminates the region 730 to read information therefrom The beam is directed normally to the layer 7 10.

It becomes scattered as scatter radiation where the liquid crystal material in the region 730 has been switched to a focal conic state; such scattered radiation is detectable at a radiation detector mounted offaxis relative to the bearn Conversely, the beam is transrnitted tbrough to the layer 780 and is reflected thereat to propagate in a direction back to the laser when the material in the region 730 is in a homeotropic: state in which it appears substantially transparent.

Writing to the structure 700 will be described where the region 730 incorporates a bistatic liquid crystal material capable of actively switching its molecular alignment state at room teirperature. When writing infortnation to the region 730 using the laser, the beam of the laser is directed to a pixel to be written to and illuminates either the region 760 or the region 770 of the pixel but not both. Such selective illumination of the regions 760, 770 generates a pyroelectric charge on the associated electrode 740 of the pixel and thereby generates a potential difference between the electrode 740 and the layer 780. Because the layer 780 is electrically connected through the region 795 to the layer 720, an electric field is generated across the PDLC region 730 in the vicinity of the pixel. Depending upon the duration and amplitude of pulses in the bearn, the pixel can be selectively switched between a homeotropic state in which it appears substantially transparent and a focal conic state in which it appears opaque.

Writing to the structure 700 will. now be described wbere the region 730 incorporates liquid crystal material as in the structure 10 which is crystalline at room temperature and melts to an isotropic state at an elevated temperature.

When writing infon-nation to a pixel of the region 730 using the laser to make the pixel substantially transparent, the beam is directed to the pixel to illuminate either the region 760 or the region 770 of the pixel but not both. This generates a pyroelectric charge which flows to the electrode 740 of the pixel and thereby results in an electric field being established in a part of the region 730 corresponding to the pixel. The beam also heats the liquid crystal material in the region 730 associatedwith the pixel causing it to reach its isotropic state. Because an electric field is present across the region 730, molecules of the liquid crystal material of the pixel mutually align in a homeotropic state. The laser beam is then reduced in power to aRow the pixel to cool whilst maintaining the electric field so that the liquid crystal material returns to its crystalline state with its molecules mutually aligned in a horneotropic state.

When writing information to a pixel of the region 730 using the laser to make the pixel substantially opaque, the beam is directed to the pixel to illurinate both regions 760, 770. This ensures that no charge is transferred to the electrode 740 of the pixel, thereby resulting in an absence of electric field in a part of the region 730 corresponding to the pixel. The beam also heats the liquid crystal material in the region 730 associated with the pixel causing it to reach its isotropic state. Because no electric field is present across the region 730, molecules of the liquid crystal material are mutually misaligned rendering the pixel opaque. The laser beam is then deactivated to allow the pixel to cool relatively rapidly whilst maintaining a substantially zero electric field so that the liquid crystal material returns to its crystalline state with its molecules in a misaligned opaque state.

The structure 700 provides the advantage that it is not necessary to form cavities, for example by steps 2 and 3 in the method 500, and that a PDLC device can be used which 10 does not require its polymer to be pyroelectric and polarised.

In the structures 10, 200, 300, 400, 600, 700 described above, electric fields are generated by pyroelectric techniques. Other techniques can be used for generating electric fields for controlling liquid crystal molecular alignment during writing information, for example by using externally applied magnetic fields. A structure employing coupled magnetic fields to generate electric fields within the structure during writing information thereto is shown in Figure 8; the structure is indicated by 800.

The structure 800 incorporates a first plastic layer 8 10 under which is formed a conductive coil 820 connected to a central electrode region 830 at its first end and to a connection region 840 at its second end. T'he first layer 8 10 is of elongate rectangular form having an elongate axis C-D. A cross-section view of the structure 800 along the axis C-D is shown in Figure 9 and indicated by 900. - Referring to Figure 9, the structure 800 is a multilayer structure comprising in sequence the first layer 810 fabricated from a plastics material, the coil 820 and the central region 830, a PDLC region 910, a conducting layer 920 and a second layer 930 fabricated from a plastics material The layer 920 is electrically connected through the coil 820 to the central region 830 via the connection region 840 formed at a peripheral edge of the structure 800.

The PDLC region 910 comprises droplets of a liquid crystal material as employed in the device 10 suspended in a matrix of non-pyro electric polymer. Moreover, the region 910 is in a range of 20 to 50 [tin thick. The central region 830 comprises a layer of vacuum deposited indium. tin oxide having a thickness in a range of 2 to 20 rini so that the region 830 is substantially transparent to infra-red radiation, allows naked eye inspection of the region 910 and is also electrically conductive. The first and second layers 810, 930 are in a range of 100 to 200 ltrn thick and comprise an acrylic, PVC, polyethylene or polycarbonate plastics material.

The structure 800 is read from by projecting an infra-red bearn from a laser in a direction normal to the layer 920 through the first layer 8 10 and the region 830 to the region 9 10. When liquid crystal droplets in the region 910 are in a homeotropic state, they appear transparent so that the beam propagates from the laser to the layer 920 at which it is reflected and passes substantially unscattered back through the regions 910, 830 and the layer 810 and finally back to the laser. When molecules of the liquid crystal in the droplets in the region 9 10 are mutually misaligned, they appear opaque so that the beam propagates through the layer 810 and the region 830 to the region 910 where the beam becomes scattered and propagates in a variety of directions through the region 830 and the layer 8 10 to detectors (not shown) mounted off-axis with respect to the beam The amount of scattered radiation can be used to determine information recorded where the beam has interacted with the region 9 10.

The structure 800 is written to by exposing it to an alternating magnetic field arranged to couple to the coil 820 and thereby generate an alternating potential difference between the region 830 and the layer 920. This alternating potential difference creates an alternating electric field through the thickness of the region 9 10 in a direction normal to the layer 930.

When writing to the region, the laser is operated in pulsed mode with timing of the pulses arranged to coincide with an instantaneous potential difference between the region 830 and the layer 920. In a first mode, the laser is pulsed when the potential difference is substantially zero. In a second mode, the laser is pulsed when the potential difference is at a maximum in its alternating cycle. In order to achieve these modes, the beam is pulsed with a duration which is considerable shorter than the period of the alternating potential difference.

When writing to apart of the region 910 to make it transparent, the beam is pulsed in the second mode and is of sufficient power to heat the liquid crystal material in the part from its crystalline state to its isotro Pic state in the duration of one pulse. Because an electric field is present across the region 9 10 in the second mode, the liquid crystal material attains a transparent homeotropic state and cools to a horneotropic crystalline state before the 10 potential difference alters significantly.

When writing to a part of the region 9 10 to make it opaque, the beam is pulsed in the first mode and again of sufficient power to heat the liquid crystal material in the part from its crystalline state to its isotropic state in the duration of one pulse. Because there is an absence of electric field in the region 9 10 when the liquid crystal material in the part attains its isotropic state, molecular alignment does not occur rendering the part opaque. When the pulse ends, the part cools to its focal conic scattering state before a significant potential difference develops between the region 830 and the layer 920.

It will be appreciated that it is only feasible to write to the region 830. In practice, the structure will have the antenna 820 implemented as a relatively narrow peripheral boarder such that the region 830 corresponds to a majority of a major surface area presented by the structure 800.

The structure 800 provides the advantage that it does not rely on pyroelectric: techniques to generate electric fields and is hence more robust against spontaneous erasure due to anbient thermal effects, for exaniple exposure to strong sunlight, which can occur with the structure 300.

The region 830 in conjunction with the layer 920 forms a capacitor with the region 910 functioning as a dielectric. The capacitor can be tuned with the coil 820 to exhibit a resonance corresponding to the frequency of the magnetic field applied to the device 800. This improves efficiency of the structure 800, thereby making it more sensitive to the magnetic field.

Although the structures 10, 300, 400, 600, 700, 800 can be read using infra-red laser beams, they can also be inspected using visible light radiation having wavelengths in a range of 350 rim to 1000 nm, for example inspected using the naked eye, because the liquid crystal materials are operative also to scatter radiation in this range of radiation wavelengths.

The inventor has appreciated that it is also practicable to fabricate a read/write structure incorporating liquid crystal material where photodiodes included therein are employed to generate electric fields to assist with switching the material between a relatively more opaque state and a relatively more transparent state in selected regions of the structure for information recordal purposes. Such a structure is distinguished from the structures 10, 300, 400, 600, 700 which function by exploiting the pyroelectric effect to generate an electric field therein and the structure 800 which relies on coupling of electromagnetic radiation to generating an electric field therein.

Referring now to Figure 10, there is shown a read/write structure indicated generally by 1000. The structure 1000 is in the form of a planar sheet which can be used to make tags, cards and related information bearing products. The structure 1000 comprises a stainless steel backing plate 1010 having a thickness in a range of 50 [im to 300 [im. An array of pixel regions are fonned onto the plate 1010, for example a pixel region 1020. Each pixel region includes a n-doped amorphous silicon electrode layer 1030 abutting onto a major surface of the plate 1010, a substantially intrinsic amorphous silicon electrode layer 1040 abutting onto the n-doped layer 1030 and a p-doped amorphous silicon electrode layer 1050 abutting onto the intrinsic electrode layer 1040. The doped layers 1030, 1050 in combination with the intrinsic layer 1040 constitute a photovoltaic diode at each pixel region. Each pixel region ftu-ther comprises a substantially transparent tin oxide conductive electrode 1060 abutting onto the p-doped electrode layer 1050 and extending beyond the diode over a neighbouring region 1070 to the diode, the neighbouring region 1070 being filled with a liquid crystal material. The structure 1000 further comprises a polyamide 5 passivation layer 1080 which provides a substantially transparent protective cover over the conductive electrode 1060, the passivation. layer being in a range of 10 [Lm. to 100 4m. thick.

The n-doped layers 1030 of the pixel regions are electrically mutually connected together through the backing plate 10M The conductive electrode 1060 of each pixel region is restricted in area so as to be isolated from corresponding electrodes of pixel regions neighbouring thereto.

The structure 1000 is fabricated by a method comprising a sequence of steps as follows:

(a) providing a partially evacuated first chamber having associated therewith an oscillator connected to electrodes in the charnber for generating a gas plasma within the chamber; (b) inserting the backing plate 1010 into the chamber; (c) injecting trace quantities of silane and diborane into the first chamber and creating a plasma of these gasses therein, thereby depositing a first layer of p- doped amorphous silicon onto a major surface of the backing plate 1010; (d) providing a partially evacuated second chamber having associated therewith an oscillator connected to electrodes in the chamber for generating a gas plasma within the chamber; (e) transferring the backing plate 1010 into the second chamber; (f) injecting trace quantities of only silane into the second chamber and creating a plasma of this gas therein, thereby depositing a second layer of substantially intrinsic amorphous silicon onto the first layer; (g) providing a partially evacuated third chamber having associated therewith an oscillator connected to electrodes in the chamber for generating a gas plasina within the chamber; (h) transferring the backing plate 1010 into the third chamber; (i) injecting trace quantities of silane and phosphine into the third chamber and creating a plasma of these gasses therein, thereby depositing a third layer of n-doped amorphous silicon onto the second layer; covering the backing plate 1010 and its three layers with photoresist, exposing the photoresist through a lithography mask and then developing the resists to leave islands of resist corresponding to the diodes in the structure 1000; (k) selectively reactively ion etching or argon sputtering the layers to leave the diodes remaining where they are protected by the resist islands; (1) stripping the resist using an oxygen plasma; (m) fiffling cavities formed adjacent to the diodes with liquid crystal material and then cooling the backing plate to solidify the crystal material; (n) transferring the backing plate and its associated diodes and liquid crystal material into a metal sputter coating chamber and selectively depositing isolated tin oxide electrodes, each electrode straggling and connecting to an associated diode and neighbouring: liquid crystal material filled region; and (o) coating the backing plate 101.0 with polyamide to cover the isolated tin oxide electrodes and any exposed liquid crystal material surfaces.

- Operation of the structure 1000 will now be described with reference to Figure 10. When writmg information onto the structure to obtain a transparent optical effect, an intense laser beam is directed at a specific pixel region whose optical properties are to be modified The 25 beam generates a potential difference in the order of 0. 7 volts between the p-doped and -n- doped electrodes of the diode of the specific region, the potential difference resulting in an electric field being formed between the conductive electrode 1060 of the specific region and the backing plate 1010. The laser beam also heats the liquid crystal material of the specific region to a molten state whereat the electric field aligns molecules of the material 30 to render it substantially transparent. The laser beam is then deflected to the diode only so that the liquid crystal material cools with an electric field applied thereto to a transparent crystalline state. The beam is then switched off rendering the specific pixel region substantiaRy transparent.

When writing information onto the structure to obtain an opaque optical effect, the laser beam is directed at the specific pixel region substantially avoiding illumination of the diode but heating the hquid crystal material of the specific region to its melting point. By not illuminating the diode, only a residual electric field is generated across the liquid crystal material which is insufficient to align molecules of the material, thereby rendering the material opaque. The bearn is then switched off allowing the liquid crystal material to cool to its crystalline state, the specific pixel region being substantially opaque.

Information recorded on the structure is readable using the naked eye, the backing plate 1010 functioning as a reflector to aid viewing. The structure 1000 is also readable using a laser beam of reduced power to illuminate pixel regions, the beam power being insufficient to melt the liquid crystal material in the regions.

The structure 1000 suffers a disadvantage that information recorded therein can be erased if the structure 1000 is given a blanket exposure to intense illun-fination or heated to melt the liquid crystal material therein. In order to address this problem, the inventor has a devised a modified version of the structure 1000, namely a structure indicated by 1200 in Figure 11.

Refening now to Figure: 11, the structure 1200 incorporates an array of pixel regions, for 25 example a pixel region 12 10. Each pixel region incorporates a first stack of amorphous semiconductor electrode layers, for example a stack indicated by 1220, and a second stack of amorphous silicon electrode layers, for example a second stack indicated by 1230. Each second stack includes a cavity, for example a cavity 1240, filled with a bistatic liquid crystal material. The first and second stacks are mutually laterally isolated by insulating regions, for example insulating regions 1250, 1260. During fabrication of the structure 1200, the insulating regions and the stacks are deposited onto the packing plate 1010 using deposition processes similar to those used to fabricate the structure 1000.

Each first stack is connected to its corresponding second stack through the backing plate 5 1010. Each first stack is electrically connected to its associated upper tin oxide electrode, for example an electrode 1270, which extends over the pixel region's associated cavity. On an opposite major face of the structure 1200 to the backing plate 1010, the structure 1200 is coated in an overall passivating layer 1280 of polyanlide polymer.

Each first stack comprises a series of three photovoltaic PIN diodes, each diode comprising an amorphous n-doped electrode layer ("N"), an amorphous intrinsic electrode layer ("I") and an amorphous p-doped electrode layer ("P"). The layers are illustrated in Figure 11 by direction of cross-hatching used, for exarnple a layer 1330 is an ri- doped layer, a layer 13 10 is an intrinsic layer, and a layer 1320 is a p- doped layer. By intrinsic, it is meant that dopant concentrations in the intrinsic layer ("I") are several orders of magnitude less than in the p-doped and n-doped layers ("N", CVI).

In a similar manner, each second stack corresponds to a series of two photovoltaic PIN diodes similar to those included in the first stack. Each second stack is operable in combination with its associated surface electrode, for example the surface electrode 1270, to develop an electric field across its associated cavity.

In operation, flood illumination results in each first stack generating a potential in the order of 2 volts between its associated surface electrode and the backing plate 1010, and also results in each second stack generating a potential in the order of 1.35 volts, thereby developing a potential difference of 0.65 volts across the cavity associated with the stacks, the first and second stacks operable to generate mutually opposing potentials. This potential difference of 0. 65 volts generates a field in the cavity which is insufficient to change the optical state of the material in the cavity. As a consequence, the structure 1200 is less susceptible to erasure by flood illumination than the structure 1100.

When writing to the structure 1200, a focussed laser beam is employed to illuminate selectively pixel regions of the structure 1200 onto which information is to be recorded. The laser beam is directed to a pixel region to be modified and selectively illuminates either the first stack or the econd stack of the pixel, thereby generating potential 5 differences of +2 volts or -L volts respectively across the cavity of the pixel region. These potential differences are capable of switching the liquid crystal material in the cavity between an opaque state and a transparent state.

The structure 1200 is not drawn to scale. In practice, the cavity of each pixel region 10 occupies a larger proportion thereof than illustrated in Figure 11.

I Fabrication of the structure 12 0 involves the following fabrication steps:

(a) depositing nine layers of selectively undoped and doped amorphous silicon onto the backing plate 1010 using plasma chambers; (b) etching trenches into the layers to define first and second stacks; (c) filling the trenches with insulating material; (d) selectively etching three layers of the second stacks away to define cavities; (e) filling the cavities with a bistatic liquid crystal material; (f) cooling the structure to solidify the material and then sputter depositing or screen printing using conductive inks the surface electrodes; and (g) coating the structure 1200 with the layer 1280 of polyamide polymer.

Incorporation of photodiodes into the structures 1000, 1200 provides the benefit of increased writing speed compared to that possible in the structures 10, 300, 400, 600, 700 which rely on the pyroelectric effect for generating a bias potential.

It will be appreciated that modifications can be made to the embodiments of the invention described above without departing from the scope of the invention. For example, tin oxide layers through which laser bean-is are transmitted can be replaced with conductive polymer layers. Moreover, alternative mixtures of liquid crystal materials can be used compared with those described above provided that they function in a mamer described above.

The structures 10, 300, 400, 600, 700, 800, 1000, 1200 can be fabricated as relatively large 5 sheets, for example to A3 sheet size, and used for recording drawings, maps and similar which need to be periodically updated. The structures 10, 300, 400, 600, 700, 800, 1000, 1200 can also be adapted for use as a data storage medium, for example in read/writeable compact discs (CDs).

Being largely constructed of plastics or metallic materials, the structures 10, 300, 400, 600, 700, 800, 1000, 1200 can be made waterproof and used in rugged environments, for example in image projection apparatus where relatively high levels of illuminating radiation are employed to illuminate the structures and water cooling is necessary to prevent the structures from overheating. Moreover, the structures can be incorporated as an integral externally visible part of products, the structures bearing information relating to the products such as serial numbers, model numbers, manufacturing dates and reference numbers of patents protecting the products.

It will ftu-ther be appreciated that the structures 10, 300, 400, 600, 700, 800 can be operated 20 to be read from transrnission of radiation therethrough if respective reflective layers are thinned so as to become transmissive to infra-red and visible radiation.

Claims

1. A read/write optical structure susceptible to being written upon and read from using electromagnetic radiation, the structure including information bearing means comprising a liquid crystal material for recording information, and bias voltage generating means included within the structure' for generating a switchable bias potential for use in controlling information recordal in the information bearing means, the generating means being operative to generate the bias potential in response to radiation received at the structure.

2. A structure according to Claim 1 wherein the generating means comprises a pyroelectric -material for generating the bias potential in response to the radiation received.

3. A structure according to Claim 1 or 2 wherein the generating means is substantially I separate from the information bearing means, the generating means and the information i bearing means being mutually electrically connected.

4. A structure according to Claim 2 or 3 wherein the pyroelectric polymer material comprises a pyroelectric polymer polyvinylidene fluoride (PVDF).

1

5. A structure according to Claim 1, 2, 3 or 4 wherein the liquid crystal material is operative to be read in a solid crystalline state and to be written to by selectively heating it to its isotropic state.

6. A structure according to any preceding claim wherein the liquid crystal material is selectively switchable between a transparent state in which it substantially transmits inftared radiation and an opaque state in which it substantially scatters infra-red radiation.

7. A structure according to any preceding claim wherein the liquid crystal material con-4)rises one or more substances from a list: 4-cyano-4'octylbenzyhdeauiline, 4-alkyl-4'cyanbiphenyls, 4-cyano-4'-decylbiphenyl, 4-alkoxy-4"- cyano-p-terphenyls, 4-alkylphenyl4-alkoxy-3-cyanobenzoates.

8. A structure according to any preceding claim including conductive regions for connecting the infort-nation bearing means to the generating means, the conductive regions configured to be capable of generating an electric field in the information bearing means.

9. A structure according to Claim 8 wherein the conducting regions comprise films of indium. tin oxide.

10. A structure according to Claim 9 wherein one of the films is substantially transmissive to infra-red radiation.

11. A structure according to Claim 10 wherein said one of the filins is transmissive to visible radiation for allowing naked eye inspection of the information bearing means therethrough.

12. A structure according to Claim 10 or 11 wherein said one of the films is of a thickness in a range of 2 to 20 nm.

13. A structure according to Claim 9 wherein one of the films is substantially reflect infra-red radiation transmitted through the information bearing means.

14. A structure according to Claim 13 wherein said one of the filins is of a thickness in a range of 50 to 500 nm.

15. A structure according to any preceding claim comprising outer plastic layers fabricated from plastics materials for physically protecting the information bearing means and the generating means from an environment surrounding the structure, the layers comprising one of a polycarbonate, acrylic, polyvinyl chloride or polyethylene plastics material.

16. A structure according, to Claim 1 or 2, wherein the information bearing means I incorporates the liquid crystal material in the form of a polymer dispersed liquid crystal (PDLC) device.

17. A structure according to Clairn 16 wherein the PDLC device comprises a pyroelectric polymer film including voids filled with the liquid crystal material.

18. A structure according to Claim 17 wherein the material is a bistatic cholesteric liquid crystal material.

19. A structure according to Claim 18 wherein the material is derived from a chemical group 4-alkyl-4'-cyanobiphenyls.

20. A structure according to Claim 17, 18 or 19 wherein the pyroelectric polymer film is fabricated from a pyroelectric polymer material polyvinylidene fluoride (PVDF).

21. A structure according to: Clairn 16, 17, 18, 19 or 20 wherein the device incorporates mutually electrically connected films on its major faces operable to counteract build-up of static charge of the faces.

22. A structure according to Claim 21 wherein the conductive films are fabricated from indium tin oxide.

23. A structure according to Claim 22 wherein one of the fihns is operative to substantially transmit infta-red radiation.

24. A structure according to Claim 23 wherein said one of the films is operable to transmit visible radiation.

25. A structure according to Claim 23 or 24 wherein said one of the films is of a thickness in a range of 2 to 20 nin

26. A structure according to Claim 22 wherein one of the films is operative to substantially reflect infra-red radiation transmitted through the device.

27. A structure according to Claim 23 where said one of the films is of a thickness in a range of 50 to 500 nm.

28. A structure according to Claim 21 wherein the mutually connected films include a conductive polymer fihn operable to substantially transrnit infra-red radiation and also transmit visible radiation.

29. A structure according to any one of Claims 17 to 28 comprising outer layers fabricated from plastics materials for physically protecting the information bearing means and the generating means from an environment surrounding the structure, the layers comprising one of a polycarbonate, acrylic, polyvinyl chloride or polyethylene plastics material.

30. A structure according to Claim 17 wherein the liquid crystal material is operative to be read in a solid crystalline state and to be written to by selectively heating it to its isotropic state.

31. A structure according to Claim 1 or 2 wherein the generating means comprises a pyroelectric polymer film partitioned into a plurality of pixel regions, the information bearing means distributed to each of the pixel regions and corresponding at each of the pixel regions to a cavity including a liquid crystal material, each pixel region comprising mutually oppositely polarised regions mutually electrically connected in parallel and operable to selectively generate a bias potential across the cavity when the structure is being written to, thereby selectively setting each pixel to an opaque state or a substantially transparent state to record the information.

32. A structure according to Claim 31 wherein the pyroelectric polyrner film comprises polyvinylidene. fluoride (PVDF) material.

33. A structure according to Claim 31 or 32 wherein the liquid crystal material is a bistatic cholesteric liquid crystal material.

34. A structure according to: Claim 31, 32 or 33 wherein each pixel region includes an I associated isolated connection jayer for electrically connecting the mutually oppositely polarised regions of the pixel region to the cavity of the pixel region.

35. A structure according to Claim 28 wherein the isolated connection layer is operable to be substantially transrnissive to infra-red radiation

36. A structure according to Claim 35 wherein the isolated connection layer is fabricated from indiurn tin oxide and is in a range of 2 to 20 nm thick.

37. A structure according to Claim 35 wherein the isolated connection layer is fabricated from a conductive polymer.

38. A structure according to any one of Claims 31 to 37 wherein the cavity of each pixel region is fornied into the pyroelectric polymer film 1

39. A structure according to Claim 38 wherein the cavity of each pixel region is in a range of 20% to 60% deep relative to the thickness of the pyroelectric polymer film.

40. A structure according to any one of Claims 31 to 37 wherein the cavity of each pixel region is formed into an outer film layer included within the structure to protect the pyroelectric fihn from an environment surrounding the structure.

41. A structure according to Claim 40 wherein the outer layer is fabricated from one of a polycarbonate, acrylic, polyvinyl cl-Aoride (PVC) or polyethylene plastics material.

42. A structure according to Claim 1 or 2 wherein the information bearing means comprises a polymer dispersed liquid crystal (PDLC) device and the generating means comprises a layer of pyroelectric plastics material, the layer partitioned into pixel regions, each pixel region including mutually oppositely polarised regions mutually electrically connected through an isolated connection layer associated with each pixel region, the connection layer operable to selectively generate an electric field across the PDLC device when writing information thereto, thereby individually setting each pixel region to a substantially transparent optical state or an optically scattering state for information recordal purposes.

43. A structure according to Claim 42 wherein the PDLC device incorporates a nonpyroelectric: plastic for including the liquid crystal material.

44. A structure according to Claim 1 wherein the generating means cornprises photodiodes for generating the bias potential.

45. A structure according to Claim 44 wherein the photodiodes are fabricated from amorphous silicon.

46. A structure according to Claim 44 or 45 including an array of pixel regions, each region incorporating a first stack of photodiodes connected in series for generating the bias potential.

47. A structure according to Claim 46 wherein each pixel region includes a further second compensating stack of photodiodes, the compensating stack operable to oppose a potential generated by the first stack when the structure is flood illuminated with radiation, the first and second stacks being individually selectively illuminable using a focussed beam of illurrinating radiation to generate the bias potential for infon- nation recordal on the information bearing means.

48. A structure according to any one of Claims 44 to 47 including a metallic backing plate onto which the generating means and information bearing means are fabricated.

49. A structure according to Claim 48 wherein the backing plate is fabricated from stainless steel.

50. A structure according to Claim 1 wherein the generating means comprises an antenna for receiving radio radiation and generating a corresponding signal for use in generating the bias potential.

51. A structure according to Claim 50 wherein the antenna is a loop antenna providing first and second connections, the first connection connected to an earth plane of the structure and the second connection connected to an electrode region of the structure, the electrode region operable to cooperate with the earth plane to generate an electric field in the information bearing means for purpose of recordal of information therein.

52. A structure according to Claim 51 wherein the antenna is operable to resonate with a capacitance provided between the electrode region and the earth plane at a frequency of the radio radiation received at the antenna.

53. A structure according to Claim 52 wherein the information bearing means corresponds to a polymer dispersed liquid crystal (PDLC) device, the earthing plane abutting onto a first major face of the device, and the antenna and the electrode region abutting onto a second major face of the device.

54. A method of writing information to a structure according to Claim 1 to 15, the method including the steps of (a) selectively irradiating the generating means, thereby selectively generating the bias potential; (b) selectively irradiating a portion of the information bearing means to heat said portion from its crystalline state to its isotropic state; (c) altowing the portion to cool to its crystalline state whilst selectively controlling the bias potential to render the portion opaque or transparent as required for purpose of information recordal in the information bearing means; and (d) repeating steps (a) to (c) until the information is stored in the infon-nation bearing means.

55. A method according to Claim 54 wherein infra-red laser beams are used to irradiate the structure.

56. A method according to Claim 54 or 55 wherein a single beam is used to irradiate the structure for performing steps (a) and (b) simultaneously.

57. A method ac cording to any one of Claims 54 to 56 wherein detecting means are used in the method to identify whether or not the portion of the information bearing means has been correctly written and repeating steps (a) to (c) until the portion is correctly written.

58. A method of writing information to a structure according to any one of Claims 1, 15 and 16, the structure including a bistatic cholesteric liquid crystal material in the information bearing means, the generating means distributed throughout the information bearing means, the method including the steps of- (a) applying a pulse of radiation to a portion of the information bearing means to generate thereat the bias potential and thereby an electric field across the liquid crystal material; (b) terminating the pulse after a selected duration and pulse power, to render the portion transparent or opaque as required for information recordal therein; and (c) repeating steps (a) and (b) until the information is stored in the information bearing means.

59. A method according to Claim 58 wherein the an infra- red laser is used to provide the pulse of radiation.

60. A method according to Claiin 58 or 59 wherein the method uses detecting means to identify whether or not the potion of the information bearing means has been correctly written and steps (a) and (b) are repeated until the portion is correctly written.

61. A read/write optical structure substantially as hereinbefore described or substantially as illustrated in one or more of Figures 1 to 9.

62. An information bearing tag including a structure according any one of Claims 1 to 53 for bearing the infon-nation.

63. A read/writeable compact disc (CD) including a structure according to any one of Claims 1 to 53.

64. An apparatus for reading or writing to a structure according to any one of Claims 1 to 53.