GB2218533A - Information storage - Google Patents

Abstract

A liquid crystal storage device comprises a nematic liquid crystal polymer contained between two transparent plates the inside surfaces of which are coated to provide electrodes for applying an electric field across the nematic polymer for erasure purposes. A laser scans the device and applies heat to address regions of the nematic polymer, the direction of orientation of the molecules of the polymer being controllably variable over a range of directions dependent on the direction of scanning, such that information is stored as the angle of orientation imparted to the molecules of the nematic polymer by the laser. The stored information is viewed by passing plane polarised light through the storage device and hence through a pleochroic analyser.

Description

Title: Information Storage Field of the Invention This invention relates to means for storing information and to a method of storing information.

Background to the Invention There is a great deal of interest in the storage of pictorial and digital information both for immediate viewing and for archiving and access at a later time. In particular, there is interest in information which is recorded serially onto materials by building up the complete picture a bit at a time, rather than by a parallel or 'flash' transfer as in a photographic exposure. Examples are laser or thermal printers, facsimile machines, active matrix displays or laser plotters, which take time to build up an analogue display.

There are also optical disc systems which use lasers to write and read binary information such as in the form of a series of tiny 'pits' or bubbles left on a rotating disc.

The information storage and retrieval process is optical since it is performed using electromagnetic radiation in the visible, infrared or ultra violet parts of the spectrum. The laser-sensitive material can either be a thin metallic or organic layer.

Liquid crystalline materials have found widespread use, mainly in displays, although they have been used to demonstrate digital storage. They are especially interesting due to their strong optical anisotropy and their ability to be switched efficiently between two stable states. This switching is achieved by either the application of electric fields, as in twisted nematic displays, or by the additional application of other energy sources such as a scanning laser spot as in smectic A storage displays. Unlike twisted nematic devices, smectic A materials (with a smectic to isotropic phase transition above room temperature) support indefinite stable storage at ambient temperature in the absence of electrical fields. Whereas smectic A devices are used for developing scattering textures, nematic materials are associated with birefringent, polarization-sensitive devices.

The behaviour of the liquid crystal materials when subjected to electric fields and when they are bulk or locally heated is well understood. Both low molar mass and polymeric liquid crystal materials can be synthesised to produce the desired optical texture for display or storage.

Recent work has revealed an interesting and unusual birefringent behaviour of several liquid crystal polymer materials when they are scanned with a focused laser beam.

Under the right circumstances the material, which is configured as a thin layer between two flat and transparent plates, can be used to record and display colour images. The origin of this property is reasonably well understood and polymer samples have been used to give colour demonstrations. Reference 1 gives the patent number in which this material, and other similar ones which were produced on the project, are to be found. The work also produced a paper describing the laser addressing of these liquid crystal polymers, and this is given in reference Further investigation has produced a list of other classifications of similar liquid crystal materials, which show promise and might be used as alternative active layers.It is the manipulation of the liquid crystal material's local alignment by a focused laser spot,in addition to the usual electrical fields and aligning agents, which is the key to this invention. Within the uniform background, local birefringent textures carl be laser-written and stored indefInitely at room temperature without the ald of external fields. By control of the laser during the writing process, under suitable vIewing conditions the material can be used to demonstrate a colour storage display. Other appllations may be possible, eg optical processing components.

Other research on liquid crystal polymer materials (see reference 3) describes laser and field addressing to produce clear, scattering and birefringent textures on a smectic side chain polymer called PG296. This work concerns the promotion of the interesting and potentially useful properties of the polymer. Reference 4 is the related patent describing a liquid crystal information storage device.

Another recent publication (reference 5) reports investigations into glassy nematic liquid crystal (non polymeric) materials which could possibly be used as an alternative active material in the present application. In addition to the application of heat and electric fields, experImentation 15 with static point-wise heating from a laser rather than with a scanned laser beam. The action of scanning is crItical In the observation of the effects later descrIbed. Three patents relate to thermo-electrooptic storage displays using glassy nematic lIquId crystal materials (see references 6, 7 and 8), but are principally concerned with material synthesis.

For the present invention, the scanning technology has been used with low molar mass smectic A liquid crystals In the construction of several prototype laser-addressed displays. A focused laser is scanned over a clear layer (eg an optically unlaxial or homeotropic condition) of the active material, which Is contained between two ITO (indium tin oxIde) coated transparent plates, to produce an easily distinguishable scattering texture. Reference 9 describes the configuration of such a display.

A large amount of research has been done into the generation of colour by using novel types of electrically addressed birefringent devices. PrevIous papers described twisted nematic cells which are electrically switched and whlch can only store data while the electric field is kept on, although a bistable twist cell filled with cholesterlc material has been reported to give two stable configurations of the liquid crystal director (see reference 10).

Devices have also been constructed where the degree of birefringence can be made continuously variable by controlling the field strength that is applied to an aligned nematic liquid crystal cell (see references 11).

A method of providing storage of the liquid crystal director orientation In a low molar mass smectic liquid crystal has also been described. This was studIed In a devIce used to generate different colours through the action of an electric field (see reference 12).

In colour display applications, fast colour switching has been achieved at R.S.R.E.in a field sequential system using 'two-frequency' materials to provide a colour display (see reference 13). Tektronix Inc. manufactured a commercial liquid crystal full colour display based on 'pi-cells' and a monochrome CRT (see reference 14).

Papers which have been written about these devices describe the use of nematic cells in conjunction with birefringent films, colour selective and neutral polarizers. The theory which is used could be adapted to describe the generation of colour by liquid crystal polymers, and glassy nematics, although laser-written lines are used to control the colour generation instead of electric fields.

Ferroelectric liquid crystal materials are also generating interest for use in displays.

Therefore, to summarise the present state of knowledge three broad areas of liquid crystal materials research can be loosely defined, which relate to laser scanning, ie: (i) Field addressed optically responsive displays; devices where the switching of the orientation of the liquid crystal director is achieved with an electric field and alignment layers. This gives a limited range of director movement that can often be described with respect to a cross-sectional plane through the active material between the containing surfaces. The information is usually lost when the electrical power is removed.

(ii) Laser-addressed smectic liquid crystal storage displays; these use lasers to produce written lines with a (random) scattering texture on a pre-aligried background.

The pre-alignment is achieved by applIcation of a large electric field across the material. In the presence of a smaller field the laser can be used selectively to erase the lines by over-writing. Alternatively reverse contrast may be produced whereby clear lines, with a homeotropic texture, are written on scattering background texture.

This is achIeved by writing In the presence of an applied field. Using the laser allows great freedom, with the ability to access randomly all of the actIve material which can be placed In the focal plane of a suitable scanning device.

(iii) Laser-addresed birefringent storage dIsplays; this combines features of (i) and (ii) for materials where an electric field and a laser are used to manipulate locally the liquid crystal director orientation. This gives the greatest range of orientations which are stored indefinitely at ambient temperature, and which can be achIeved In a random access manner.

Areas (i) and (ii) are well established, but it is area (iii) which could provide interesting research in the future and which is the area of interest in this document.

The right type of materials are becoming available, but hitherto no one has serIously attempted any laseraddressing of these materials. A thermal print head may also be an alternative method of addressing these types of material. One potential application is for the generation and storage of colour images. This involves a similar theoretIcal treatment to that used for electrically- switched birefringerit cells.

An investigation Into the unusual birefringent behaviour of one of the polymers and its potential for use as a laser-addressed colour display, has been carried out by the applicants. This work has enabled the applicants to define the properties of an ideal recording material and propose the device structure, the ideal material properties, suitable for laser-scanning systems for information recording, and the method for perception of colour in the retrieval or display process. The recording layer could be used either for storage alone or in a project ion display.

This work on the project therefore describes the experiments with a material which can be used to give actual demonstrations of colour from laser-wrItten lines This gives a good basis on whIch to buIld a more general descriptIon, which Includes other interesting materials and possible alternative non-laser scannIng systems.

Summary of the Invention According to one aspect, the invention provides means for storing information, comprising an active medium, addressing means which scans an area of the active medlum to impart the information to the active medium by applying heat to address regions of the actIve medIum, the direction of scanning being representative of the information to be stored, the direction of orientation of the molecules of the active medium being controllably variable over a range of directions dependent on the direction of scanning, such that said information Is stored as the angle of orientation Imparted to the molecules of the active medium by the addressing means.

According to another aspect, the inventIon provIdes a method of storing information, comprising addressing an area of active medium so as to impart heat to address regions of the active medium, controlling the direction of scan of the addressing In accordance with the information to be stored, and employing as the active medium a material in which the direction of orientation of the molecules is controllably variable over the range of directions dependent on the direction of scanning, such that the information is stored as the angle of orientation imparted to the molecules by the addressing.

There are two dlstinct areas of the InventIon for consideration (i) a localised source of energy (eg 'heat pen' or scanned laser source) may be used to manipulate and define the molecular configuration of a devIce containing a suitable active material; (ii) the active medium may be a liquid crystal polymer material. One material, a nematic polymer known as GN2/8 has been used between glass plates and as a coating on glass. Other materials have been identified which could be Important.

Preferred Embodiments of the Invention In a preferred embodiment of the invention the information storage device comprises an active material contaIned between two transparent plates (eg glass or rigid or flexible non-birefringent plastic) as shown In Fig 1. The inside surfaces of the plates can be coated with transparent indium tin oxide electrode layers which are used to apply an electric field across the active material, and can provide heating for the devIce.

Other layers may be deposited on the inside surfaces. For example, absorbing layers required for localising the energy input, dielectric layers to enhance the configuration of transmissive or reflective modes of operation, etc. Normal or off-axis projection may be used, possibly with Schlieren filtering techniques. Alignment layers are considered to be unimportant since theIr influence on the bulk material is much weaker than the localized forces acting within the polymer materIal itself.

For very viscous materials a coated layer applied to a suitable backing strip could be constructed. The backing strip could be configured with any of the layers described above using commonly avialable methods. The exposed surface of the layer may be coated with a protectIve coating. For best results, the thickness of the layer should be less than ten microns.

A localised and moving source of energy (eg heat, light etc) Is used locally to define the molecular orientation of a suitable active material. Assume that the active material has an initial bulk homeotropic aligned texture which is optically clear. In its wake, the moving source leaves a narrow trail of aligned material which Is produced by the thermal stresses from a temperature gradient as the material rapidly cools from a melt state.

This rapid cooling preserves the flow alignment in a stable storage state and defines an optic axis or a component of the optic axis along or transverse to the writing direction. Therefore, the optic axis can be defined throughout the full 360 degrees of angle and can be selected at will by the control of the scanning system.

This precise and predictable control of the molecular orientation, and hence optic axis, is unlike the influence of external aligning fields or layers which have a single preset (unadjustable) alignment. This control is also unlike the random scattering features which are often produced in similar circumstances.

This procedure can be called the writing process. The new orientation is retained and may be stored for as long as the material resides in its mesophase or glass phase after the energy source has been removed. The material is capable of storing very small features, with dImensIons of the order of microns.

In the storage device where the polymer is contained between two plates, application of switchable alignment field such as an electric, magnetic or pressure field may be used to restore the initial molecular alignment. This is called total erasure and the active materIal can remaIn indefinitely in this state if required. It can be rewritten again using the same procedure as described above.

Alternatively a re-writing of the active material in the presence of a weaker alignment field can be used to restore only locally the initial molecular alignment.

This process is called selective erasure.

Both of the field erasure processes are only possible in the sandwich type cell arrangement. For the coated product, total erasure by simple bulk heating is also possIble sInce In this case the initial state of the thin polymer layer need not be homeotropic.

An example of an arrangement for realising these processes is one using a focused laser spot to scan over the active material. This produces flow alignment where the moving laser leaves a localized planar alignment In the written line. The molecular orientation is transverse to the writing direction. It may also be possible to utilise a moving 'heat pen', or thermal print head.

Only certain types of birefringent material are suitable for the active layer. These materials must also have storage properties, and they should be able to support many wrlte-erase cycles. Materials wlll tend to be ImmobIle and viscous in order to support a temperature gradient and produce the required thermal stress for flow alignment.For example, any liquid crystalline materials and mixtures which exhibit storage properties in the absence of an electric field are relevant, ie: (i) liquid crystalline polymer materials such as polymer GN2/8; (ii) immobile low molecular mass (LMM) liquid crystal materials such as glassy nematic liquid crystal materials or low molecular mass liquid crystal materials dispersed in polymer binders, (iii) any suitable material which demonstrates flow alignment.

The active material may include the addition of a dyestuff to couple the energy into the recording medium. This may not be necessary in the case where a metal absorbing layer is used, eg with a Nd-YAG laser. In the case of liquid crystal polymers this dyestuff may be Incorporated into the macromolecular structure.

This section describes a laser-addressed optical storage display based on a device containing a suitable birefringent material which has storage properties. For example a storage device which contains one of the suitable materials listed above. A scanning system is used to control the movement of a focused laser beam.

A summary of the write and erase processes is as follows.

Operational Properties of the Material: (i) Bulk Erasure A high voltage (approx 100V) ac field applied to produce d homeotropic texture with zero birefringence (optically uniaxial).

(ii) Writing Local heating with a moving focused laser spot is used to produce planar (homogeneous) flow alignment transverse to, or with a component transverse to the writing direction.

This produces a highly birefringent line with the optic axis determined by the writing direction.

(iii) Selective Erasure Local heating, by re-writing with the laser in the presence of a lower voltage ac field is used to restore the homeotropic texture and the zero birefringence in selected areas.

If the material is kept at ambient temperature the information is stored indefinitely; such that it remains in its mesophase or glass phase.

If the device is illuminated with plane polarized light then the regions of the material which have been written upon produce a rotation of the plane of polarization. The behaviour of the lines resembles that of common polarizing optical elements, in particular a half-wave plate. This rotation depends on the angle of the written lines.

Lines written on the material have very little scattering effect and so are virtually indistinguishable from the background when viewed with the unaIded eye and without polarising optical elements. To reveal a coloured Image on the material it is necessary to view or project the image, with plane polarized light, through a three colour pleochroic analyser. Plane polarized white light passing through the analyser becomes coloured depending on the angle of the incident plane of polarization. Lines written by the laser on the material in different directions can therefore be used to switch this plane of polarization and hence control the colour observed through the colour analyser. The arrangement required to view or read the coloured Image is shown in Fig 2 of the accompanying drawings, using the configuration of red, green and blue polarizers as shown for the complete colour analyser.

To utilise the colour properties of ths material it is necessary for lines to be drawn in a specific direction.

This is accommodated in the construction of the laser scanning hardware and its control system.

The materials given above are for use in a device which could be described as a non-volatile RAM storage device (see reference 15). This has the facility for random access reading and writing made possible by selective erase. If selective erase is not availalbe to the user due to the absence of the required electrical connection o electrodes, then a re-usable ROM would result.

Some materials may be possible which allow only the writing process to take place, thus creating an updatable device. ie a write-once ROM.

References 1. G.W. Gray, G. Nestor, D. Lacey, C.B. McArdle, "Liquid Crystal Polymers", U.K. Patent, 1986, Number 8615527.

2. C.B. McArdle, M.G. Clark, C.M. Haws, M.C.K.

Wiltshire, A. Parker, G. Nestor, G.W. Gray, D. Lacey and K.J. Toyne, "Laser Addressed Thermo-Optic Effect in a Novel Dyed Liquid-crystalline Polysiloxane", Liquid Crystals, 1987, Vol 2, No. 5, 573-584.

3. H.J. Coles and R. Simon, "High-resolution Laseraddressed Liquid Crystal Polymer Storage Displays", Polymer, 1985, 26, 1801.

4. H.J. Coles and R. Simon, "A Liquid Crystal Information Storage Device", U.K. Patent Application, 1987, GB 2 14e 787 B.

5. D. Demus, G. Pelzl and W. Wedler, "Thermo-electrooptic Storage Displays using Glassy Nematic Liquid Crystals", Eurodisplay Proceedings, 1987, 71.

6. D. Demus, G. Pelzl, "Thermo-electrooptic Storage Displays", East German Patent, 1987, DD 242 624 A 1.

7. D. Demus, H. Dehne, A. Roger, W. Weissflog, A.

Wiegeleben, G. Pelzl, "Applications of Glassy Nematic Liquid Crystals", East German Patent, 1987, DD 242 627 A 1.

8. D. Demus, W. Weissflog, G. Pelzl, W. Wedler, H.

Dehne, A. Roger, A. Wiegeleben, "Glassy Nematic Liquid Crystals", West German Patent, 1987, DE 36 35 584 A 1.

9. J. Harrold and C. Steele, "High-Resolution Vector Addressed Liquid-Crystal Light Valve", Proc. SID, 1985, Vol 26/2, 141.

10. D.W. Berreman and W.R. Heffner, "Fast LCDs with Low Power AddressIng and Permanent Memory", SID 82 Digest, (1982), 242-243.

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Claims (7)

1. Means for storing information, comprising an active medium, addressing means whch scans an area of the active medium to impart the information to the active medium by applying heat to address regions of the active medium, the direction of scanning being representative of the informaton to be stored, the directon of orientation of the molecules of the active medium being controllably variable over a range of directions dependent on the direction of scannng, such that said information is stored as the angle of orientation imparted to the molecules of the active medium by the addressing means.

2. Means for storing information according to Claim 1, wherein the active medium is contained between two transparent plates the inside surfaces of which are coated to provide electrodes for applyng an electric field across the active medium, for erasure purposes.

3. Means for storing information according to Claim 1 or 2, wherein the addressing means is a laser.

4. Means for storing information according to any of the preceding claims, wherein the active medium is a nematic polymer.

5. A method of storing information, comprising addressing an area of active medium so as to impart heat to address regions of the active medium, controlling the direction of scan of the addressing in accordance with the information to be stored, and employing as the active medium a material in which the direction of orientation of the molecules is controllably variable over the range of directions dependent on the direction of scanning, such that the information is stored as the angle of orientation imparted to the molecules by the addressing.

6. A method according to Claim 5, wherein the stored information is viewed by passing plane polarised light through the medium and thence through a pleochroic analyser.

7. Means for storing information, or a method of storing information, substantially as been particularly described with reference to the accompanying drawings.