GB2181263A - Optical storage medium - Google Patents
Optical storage medium Download PDFInfo
- Publication number
- GB2181263A GB2181263A GB08524289A GB8524289A GB2181263A GB 2181263 A GB2181263 A GB 2181263A GB 08524289 A GB08524289 A GB 08524289A GB 8524289 A GB8524289 A GB 8524289A GB 2181263 A GB2181263 A GB 2181263A
- Authority
- GB
- United Kingdom
- Prior art keywords
- liquid crystal
- layer
- ofthe
- crystal layer
- storage medium
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Granted
Links
- 230000003287 optical effect Effects 0.000 title claims description 10
- 239000004973 liquid crystal related substance Substances 0.000 claims abstract description 34
- 239000005262 ferroelectric liquid crystals (FLCs) Substances 0.000 claims abstract description 5
- 239000004990 Smectic liquid crystal Substances 0.000 claims description 13
- 238000000034 method Methods 0.000 claims description 5
- 125000006850 spacer group Chemical group 0.000 claims description 4
- 230000015572 biosynthetic process Effects 0.000 claims 1
- 230000005684 electric field Effects 0.000 claims 1
- 238000000926 separation method Methods 0.000 claims 1
- 239000000463 material Substances 0.000 description 4
- 238000001816 cooling Methods 0.000 description 3
- 239000004677 Nylon Substances 0.000 description 2
- 239000011521 glass Substances 0.000 description 2
- 229920001778 nylon Polymers 0.000 description 2
- 101100400378 Mus musculus Marveld2 gene Proteins 0.000 description 1
- 238000000149 argon plasma sintering Methods 0.000 description 1
- UHYPYGJEEGLRJD-UHFFFAOYSA-N cadmium(2+);selenium(2-) Chemical compound [Se-2].[Cd+2] UHYPYGJEEGLRJD-UHFFFAOYSA-N 0.000 description 1
- 238000013500 data storage Methods 0.000 description 1
- 230000000694 effects Effects 0.000 description 1
- 230000001747 exhibiting effect Effects 0.000 description 1
- 239000011554 ferrofluid Substances 0.000 description 1
- 238000010438 heat treatment Methods 0.000 description 1
- AMGQUBHHOARCQH-UHFFFAOYSA-N indium;oxotin Chemical compound [In].[Sn]=O AMGQUBHHOARCQH-UHFFFAOYSA-N 0.000 description 1
- 229910021421 monocrystalline silicon Inorganic materials 0.000 description 1
- 229920000642 polymer Polymers 0.000 description 1
- 230000001737 promoting effect Effects 0.000 description 1
- 230000000717 retained effect Effects 0.000 description 1
- 229910052710 silicon Inorganic materials 0.000 description 1
- 239000010703 silicon Substances 0.000 description 1
Classifications
-
- G—PHYSICS
- G02—OPTICS
- G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
- G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
- G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour
- G02F1/13—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour based on liquid crystals, e.g. single liquid crystal display cells
- G02F1/133—Constructional arrangements; Operation of liquid crystal cells; Circuit arrangements
- G02F1/135—Liquid crystal cells structurally associated with a photoconducting or a ferro-electric layer, the properties of which can be optically or electrically varied
Landscapes
- Physics & Mathematics (AREA)
- Nonlinear Science (AREA)
- Mathematical Physics (AREA)
- Chemical & Material Sciences (AREA)
- Crystallography & Structural Chemistry (AREA)
- General Physics & Mathematics (AREA)
- Optics & Photonics (AREA)
- Liquid Crystal (AREA)
Abstract
Electrical bias is applied across a potential divider formed by the series combination of a bistable ferroelectric liquid crystal layer 10 and a photoconductive layer 15 backing the liquid crystal layer. Data is written into the liquid crystal layer by means of a focussed laser beam which locally increases to above the switching threshold the bias appearing across the liquid crystal by locally reducing the resistance of the photoconductive layer. Data may be erased in the same manner, but with the electrical bias reversed. <IMAGE>
Description
SPECIFICATION
Optical storage medium
A known form of read-only binary, optical storage medium comprises a disc with etch pits formed in its surface. The disc is spun about its axis and a focussed light beam is tracked over its surface. The reflected light is analysed to distinguish the signal resulting when the light is incident upon an etch pit from that resulting when the light is incident upon a specularly reflecting portion ofthe disc. Such a disc incorporates some form of servo mechanism designed to ensure that the interrogating light beam tracks over the etch pits in a well defined matter.
Typically such a servo system may utilise an engraved or embossed groove structure on the disc between adjacent lines of etch pits. This use of etch pits provides a data storage system that is essentially read-only in operation. There is however, a requirementforan optical storage medium which is intrinsically capable of being addressed not only in a read mode but also in a write mode.
A positive dielectric anisotropy smectic A cell with surfaces promoting homeotropicalignmentofthe liquid crystal molecules can be arranged to exhibit bistability, being switchable between a homotropically aligned stable state in which the liquid crystal medium is optically clear and a focal conic stable state which scatters light. Such a cell is capable of being optically addressed using focussed laser irradiation to produce local heating ofthe liquid crystal layerto cause the heated portion to make an excursion from the smectic state into the isotropic state.If this takes place in the presence of a large enough el ectric field applied across the thickness ofthe liquid crystal layer, then, on cooling back into the smectic phasethe heated portion ofthelayerwill relax into the clear homotropicallyaligned state. On the other hand, if the cooling is relatively rapid and takes place in the absence of an applied field, the heated portion will relax into the focal conic state and hence into a state that scatters light. In principle therefore, such a cell can form the basisforan opticalstoragemedium with read and write capabilities. In practice however, its utility is severely restricted by the writing speed which is limited by the temperature excursion required forthe liquid crystal layer and the rate that it cools.
According to the present invention there is provided an optical storage medium forthe erasable recording of binary data in a bistableferroelectric smectic liquid crystal I I ayer switchable from afirstst- able state to an optically distinct second stable state bytheapplication ofanelectricfieldthroughthe thickness of the layer, in which storage medium the liquid crystal layer is confined between opposed front and rear plates, wherein the front plate is transparent and carries a transparent electrode on its rearfacing surface wherein the forward4acing surface of the rear plate is faced with a photoconductive layer whose dark resistance through the thickness ofthe layer is comparable with that of the liquid crystal layer, which rear plate is electrically conductive or carries an electrode layer immediately underthe photoconductive layer.
The present invention also provides a method of optically storing data in a bistableferroelectric liquid crystal Iayerwhich is switchable from a first stable state to an optically distinct second stable state which method comprises applying an electrical bias across the series combination of a bistable ferroelectric liquid crystal layer and a photoconductive layer backing the liquid crystal layer, and tracking a focussed light beam across the surface ofthe photoconductive layer to reduce locally the resistance of the photoconductive layer thereby effecting a local increase ofthe proportion ofthe bias locally developed across the liquid crystal layer from a value beneath its switching threshold to a value above that threshold.
There follows a description of an optical storage disc embodying the present invention in a preferred form. The description refers to the accompanying drawing which is a schematic cross-sectional representation of the disc.
A disc-shaped hermetically sealed envelope for a ferroelectric liquid crystal layer 10 is formed by securing togetherfrontand rear sheets 11 and 12 with a perimeter seal 13. Front sheet 11 is required to be transparent and is made of glass. On its rear-facing surface it carries a transparent electrode layer 14 of indium tin oxide. Rearsheet 12 carries a photo- conductive layer 15 on its forward facing surface, and does not have to be transparent. It does however, have to provide an electrically conductive backing for the photoconductive layer, and hence it must itself be electrically conductive or be faced with a conductive layer (not shown) immediately underthe photoconductive layer.
The photoconductive layer and the liquid crystal layerform a potential dividerforany potential applied between the transparent electrode layer 14 and the rear sheet 11 (or its electrode layer). More ofthis applied potential is developed across the liquid crystal layer when the photoconductive layer is illuminated than when it is not.The photoconductive layer is required to have a dark resistance, through the thickness ofthat layer, that is comparable with that ofthe liquid crystal layer so that the change in potential developed across the liquid crystal layer is of sufficient magnitude for it to be practical to apply a particular voltage between the electrode layer 14 and the rear sheet 11 that on the one hand, when the photoconductive layer is not illuminated, develops a voltage across the liquid crystal layer safely beneath its switching threshold value, and on the other hand when the photoconductive layer is illuminated, develops a voltage across the liquid crystal layer that is safely above threshold.
The rear sheet 11 may be provided by a slice of single crystal silicon, whose front facing surface can be processed to produce the photoconductive layer 15 in the forum of a high resistance layer with fast re- sponse both for rise and decay. Low doping is required to keep dark current low. Doping with golden- ablescarrierlifetimeto be reduced to about 0.1 to 1.0s. Alternatively the rear sheet may be provided by a sheet of glass faced with an electrode layer on its front surface and supporting an amorphous or poly crystalline photoconductive layer of for instance silicon or cadmium selenide.
The liquid crystal layer is required to be switchable fromafirststablestatetoan opticallydistinctsecond stable state. In the present example this property is provided by arranging for the molecules ofthe liquid crystal layerto be arranged in smectic layers extending at rightanglestothe majorsurfaces ofthe liquid crystal layer.
In the case of a ferroelectric existing in a C* phase the molecules of such a layer have only two stable orientations when the layer thickness is kept down to not morethan about 2.5 to 3 > m, and hence the layer exhibits bistability of operation. Atgreaterthicknesses other orientations become possible and the bistability is lost. However, as is disclosed in the
Specification of PatentApplication No.8426976, bistability is retained for somewhat greater thicknesses if the ferroelectric material is present in an l* or F* phase.
When bistability is exhibited the application of an electric bias of sufficient magnitude in one direction across the thickness of such a layer can be used to set into the layer into one stable state in which the elec tric dipole moments ofthe molecules are aligned with the applied field; while reversal ofthe direction ofthe bias can be used to set the layer into the other stable state by changing the molecular orientation so thattheir dipole moments become aligned with the new direction of applied field. These two states are optically distinct because they produce different changes because the state of polarisation (SOP) of lighttransmittedthroughthelayerwill undergo a change that depends upon the molecular orientation.
This difference in change of SOP can be revealed as an amplitude difference by directing light at the layer with an appropriate SOP, and then passing the transmitted lightthrough an appropriately oriented polarisation analyser.
In the case of a ferroelectric liquid crystal material that also exists in a smecticA (non-ferroelectric) phase the requisite alignment ofthe smectic layers is conveniently induced by using a rubbed alignment technique. The inward facing surfaces of the electrode layer 14 on the front sheet 10 and the photoconductive layer 15 on the rear sheet 1 are each covered, with in the area enclosed by the perimeter seal, with a polymer layer (not shown) such as nylon, which is provided for molecular alignment purposes.
Both nylon layers are rubbed in a single direction so that when a liquid crystal is brought into contact with them theywilltend to align the liquid crystal in its smecticA phase with the molecule layers extending at right angles to the rubbed surfaces. The cell is assembled with the rubbing directions aligned parallel with each other. The material forfilling the cell is heated into its isotropic phase to reduce its viscosity sufficiently to permitthe ready filling ofthe cell. As the material cools into a more ordered liquid crystal phase the rubbed surfaces promote molecular alignment in a single direction throughoutthe layer. On passing into the smectic A phase smectic layers are formed extending in planes at rightanglestothe molecular alignment direction.On further cooling into an inclined smectic phasethatis capable of exhi- biting ferroelectric effects the orientation ofthe smectic layers remains unchanged, butthe orientation ofthe molecules within those layers now changes to produce the requisite tilt exhibited by that phase.
The thickness ofthe liquid crystal layer 10 is determined by the thickness of the perimeter seal, and control overthe precision of this may be provided by a light scattering of short lengths ofglassfibre (not shown) of uniform diameter distributed through the materiaí ofthe perimeterseal.Thefrontsheet 11 may carry an engraved or embossed groove structure to enable servo tracking of the disc. Alternatively such a structure may be carried by the front face of the rear sheet. An embossed structure on the rear sheet can additionally be used to provide subsidiary spacers between the two sheets overthe area enclosed by the perimeter seal.Such spacers are located between adjacent data tracks so that they do not interfere with the storage of data. Typicallythey will be somewhat widerthanthe minimum spacing between adjacent data tracks, and hence, in the regions of such spacers, track spacing will normally need to be larger than elsewhere.
The assembled cell is mounted on an backing plate 16 provided with a spindle 17 for rotating the disc about its axis. The spindle also contains electrical connections 18 and 19 from contacts 20 and 21 respectivelyto the front sheet electrode 14 and to the rear sheet 11 or its electrode. Electrical connection with the contacts on the rotating spindle is then made by way of electrically conductive ferrofluid or electrical brushes (notshown).
In one mode of operation of the storage disc the whole switchable area of liquid crystal is switched into a particular one ofthe stable states by applying a suitable voltage of one polarity between electrical contacts 20 and 21 while illuminating the whole of the photoconductive layer 15 is illuminated. Then chosen areas ofthe liquid crystal are selectively switched to the other stable state by illuminating them with a focussed laser beam scanned overthe surface while the polarity of the potential difference applied across electrical contacts 20 and 21 is reversed. Selectivity may be achieved either by modulating the light during the scanning operation or by modulating the applied potential difference.
The disc is interrogated by examining the state of polarisation of light reflected as a focussed beam of polarised light is scanned over the surface ofthe photoconductor. In orderforthis light itself not to ef fect switching, it must either be significantly less intensethan the light used for addressing, or it must be of a different wavelength to which the photoconductive layer is significantly less sensitive, or the potential difference must be removed from across the cell, or at least significantly reduced in magnitude, during the interrogation period.
Claims (5)
1. An optical storage medium forthe erasable recording of binary data in a bistableferroelectric smectic liquid crystal layer switchable from a first st ablestateto an optically distinct second stable state by the application of an electric field through the thickness ofthe layer, in which storage medium the liquid crystal layer is confined between opposed front and rear plates, wherein the front plate is transparent and carries a transparent electrode on its rear facing surface wherein the forward4acing surface of the rear plate is faced with a photoconductive layer whose dark resistance through the thickness ofthe layer is comparable with that ofthe liquid crystal layer, which rear plate is electrically conductive or carries an electrode layer immediately underthe photoconductive layer.
2. An optical storage medium as claimed in claim 1,wherein spacers maintaining a predetermined separation between said rear-facing and forward facing surfaces are located between selected pairs of data recording tracks formed in the storage medium.
3. An optical storage medium as claimed in claim 1 or 2, wherein said rear facing and forward facing surfaces have molecular alignment properties that induce the formation in the smectic A phase of smectic layers in parallel planes at right angles to the plane ofthe liquid crystal layer.
4. An optical storage medium substantially as hereinbefore described with referencetotheac- companying drawing.
5. A method of optically storing data in a bistable ferroelectric liquid crystal layer which is switchable from a first stable state to an optically distinct second stable state which method comprises applying an electrical bias across the series combination of a bist ableferroelectric liquid crystal layer and a photoconductive layer backing the liquid crystal layer, and tracking a focussed light beam across the surface of the photoconductive layer to reduce locallythe re sistance ofthe photoconductive layer thereby effect- ing a local increase ofthe proportion ofthe bias locally developed across the liquid crystal layerfrom a value beneath its switching threshold to a value aboutthatthreshold.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB8524289A GB2181263B (en) | 1985-10-02 | 1985-10-02 | Optical storage medium |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB8524289A GB2181263B (en) | 1985-10-02 | 1985-10-02 | Optical storage medium |
Publications (3)
| Publication Number | Publication Date |
|---|---|
| GB8524289D0 GB8524289D0 (en) | 1985-11-06 |
| GB2181263A true GB2181263A (en) | 1987-04-15 |
| GB2181263B GB2181263B (en) | 1989-09-27 |
Family
ID=10586068
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| GB8524289A Expired GB2181263B (en) | 1985-10-02 | 1985-10-02 | Optical storage medium |
Country Status (1)
| Country | Link |
|---|---|
| GB (1) | GB2181263B (en) |
Cited By (13)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB2210703A (en) * | 1987-10-06 | 1989-06-14 | Gen Electric Co Plc | Optical storage device |
| GB2177228B (en) * | 1985-06-14 | 1989-12-28 | Semiconductor Energy Lab | Non-volatile liquid crystal optical storage device |
| FR2645322A1 (en) * | 1989-03-31 | 1990-10-05 | Univ Tokyo Agriculture | ELECTRO-OPTICAL MEMORY DEVICE WITH LIQUID CRYSTALS |
| EP0385346A3 (en) * | 1989-02-27 | 1991-11-21 | Nippon Telegraph And Telephone Corporation | Spatial light modulator and spatial modulating apparatus |
| EP0402944A3 (en) * | 1989-06-16 | 1992-05-27 | Seiko Instruments Inc. | Light addressed liquid crystal light valve |
| EP0405974A3 (en) * | 1989-06-28 | 1992-06-17 | Seiko Instruments Inc. | Holographic operating optical apparatus |
| EP0507242A1 (en) * | 1991-04-02 | 1992-10-07 | Yozan Inc. | Optical memory |
| US5187298A (en) * | 1990-05-18 | 1993-02-16 | Basf Aktiengesellschaft | Monomers and their use for the production of a laser-optical recording element which can be repeatedly erased and recorded on |
| US5187248A (en) * | 1989-05-26 | 1993-02-16 | Basf Aktiengesellschaft | Monomers and their use for the production of a laser-optical recording element which can be repeatedly erased and recorded on |
| US5212027A (en) * | 1990-05-18 | 1993-05-18 | Basf Aktiengesellschaft | Monomers and their use for the production of a laser-optical recording element which can be repeatedly erased and recorded on |
| US5312703A (en) * | 1989-09-14 | 1994-05-17 | Basf Aktiengesellschaft | Reversible or irreversible production of an image |
| US5359565A (en) * | 1991-04-12 | 1994-10-25 | Yozan Inc. | Optical memory |
| US5538824A (en) * | 1991-02-28 | 1996-07-23 | Basf Aktiengesellschaft | Reversible or irreversible production of an image |
-
1985
- 1985-10-02 GB GB8524289A patent/GB2181263B/en not_active Expired
Cited By (18)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB2177228B (en) * | 1985-06-14 | 1989-12-28 | Semiconductor Energy Lab | Non-volatile liquid crystal optical storage device |
| GB2177227B (en) * | 1985-06-14 | 1989-12-28 | Semiconductor Energy Lab | Optical non-volatile liquid crystal memory |
| US5157528A (en) * | 1987-10-06 | 1992-10-20 | The General Electric Company, P.L.C. | Devices and process for producing microfilm and microfiche records |
| GB2210703B (en) * | 1987-10-06 | 1991-03-20 | Gen Electric Co Plc | Apparatus and process for producing microfilm and microfiche records. |
| GB2210703A (en) * | 1987-10-06 | 1989-06-14 | Gen Electric Co Plc | Optical storage device |
| EP0385346A3 (en) * | 1989-02-27 | 1991-11-21 | Nippon Telegraph And Telephone Corporation | Spatial light modulator and spatial modulating apparatus |
| US5130830A (en) * | 1989-02-27 | 1992-07-14 | Nippon Telegraph And Telephone Corporation | Spatial light modulator and spatial light modulating apparatus with alignment film having particular conductance |
| FR2645322A1 (en) * | 1989-03-31 | 1990-10-05 | Univ Tokyo Agriculture | ELECTRO-OPTICAL MEMORY DEVICE WITH LIQUID CRYSTALS |
| NL9000757A (en) * | 1989-03-31 | 1990-10-16 | Univ Tokyo Agriculture | ELECTRO-OPTICAL MEMORY CRYSTAL MEMORY. |
| US5187248A (en) * | 1989-05-26 | 1993-02-16 | Basf Aktiengesellschaft | Monomers and their use for the production of a laser-optical recording element which can be repeatedly erased and recorded on |
| EP0402944A3 (en) * | 1989-06-16 | 1992-05-27 | Seiko Instruments Inc. | Light addressed liquid crystal light valve |
| EP0405974A3 (en) * | 1989-06-28 | 1992-06-17 | Seiko Instruments Inc. | Holographic operating optical apparatus |
| US5312703A (en) * | 1989-09-14 | 1994-05-17 | Basf Aktiengesellschaft | Reversible or irreversible production of an image |
| US5187298A (en) * | 1990-05-18 | 1993-02-16 | Basf Aktiengesellschaft | Monomers and their use for the production of a laser-optical recording element which can be repeatedly erased and recorded on |
| US5212027A (en) * | 1990-05-18 | 1993-05-18 | Basf Aktiengesellschaft | Monomers and their use for the production of a laser-optical recording element which can be repeatedly erased and recorded on |
| US5538824A (en) * | 1991-02-28 | 1996-07-23 | Basf Aktiengesellschaft | Reversible or irreversible production of an image |
| EP0507242A1 (en) * | 1991-04-02 | 1992-10-07 | Yozan Inc. | Optical memory |
| US5359565A (en) * | 1991-04-12 | 1994-10-25 | Yozan Inc. | Optical memory |
Also Published As
| Publication number | Publication date |
|---|---|
| GB8524289D0 (en) | 1985-11-06 |
| GB2181263B (en) | 1989-09-27 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| 732E | Amendments to the register in respect of changes of name or changes affecting rights (sect. 32/1977) | ||
| PCNP | Patent ceased through non-payment of renewal fee |
Effective date: 19931002 |