Optical disc memory with liquid crystal

Abstract: An improved magneto-optical disc memory with a fer­ roelectric liquid crystal (FLC) is shown. As FLC a chiral smectic liquid crystal is employed to exhibit a bistability between a pair of substrates which have easy polarization axes normal to each other. Informations to be recorded are written on the disc in terms of a binary system by use of the bistability.

Description: The invention relates to a memory system comprising: (a) a rotating optical disc memory including a pair of substrates, at least one of which is light transparent, a liquid crystal layer having a plurality of laminated molecular layers and interposed between said pair of substrates, said liquid crystal layer having a bistability of the electrical dipole moment directions of the molecules, and means for keeping the attitude of each molecule; (b) a data writing means including a light source for emitting a light to a particular portion of said liquid crystal layer under an existence of an electric field in order that the condition of said liquid crystal film on said portion is switched to a predetermined one of said stable dipole moment directions; (c) a data reading means including a light source for emitting a light to a portion of said liquid crystal layer, a photosensor with polarising means for sensing the light polarisation of light passing through said liquid crystal layer for detecting the condition of said liquid crystal layer at said portion. As an optical disc there is known a non-rewritable digital disc memory device that utilises the reflection conditions of laser light from an uneven reflecting surface, as represented by the compact disc. Applications of this kind are being considered that will become very promising in the future, not only for audio and video uses but also for an optical disc memory for information processing. However, a disadvantage of the disc memory is that it is not possible to be rewritten. For this reason, systems that permit rewriting had been in demand, and as such a product there is known the magneto-optic memory device. In addition, there is also known an optical disc memory device of an amorphous semiconductor type that employs a chalcogen series (tellurium series). However, disc devices that employ magneto-optic memory use rare materials and are extremely expensive so that people are apprehensive of their mass production in the future. Moreover, the light control for the method that employs a chalcogen series amorphous semiconductor is extremely delicate. Because of these, there have been sought means that possess such features as the use of materials that can be mass produced, facility in turning on and off of light, possession of nonvolatility, and nonconsumption of external energy in storing the memory. A memory device which widely fulfills this requirement is known from GB-A-2 090 673. The disc memory comprises a smectic liquid crystal layer as a rotating liquid crystal information storage medium confined by two substrates. Various thin films on the two layers are used to establish proper conditions within the liquid crystal medium which, in combination with incident radiation and applied voltages, accomplish read, write and erase functions. This system provides a locally erasable optical disc memory with high density storage, long term information stability under a wide range of temperature and read, write and erase times compatible with modern high speed computers. Transparent electrodes are provided within the disc memory. From US-A-3,775,757, a liquid crystal memory is known which uses properties of the smectic C-phase of a liquid crystal to store information represented by the bistabile orientation directions of molecular axis. The information state of storage elements is set by causing the molecular axis within the element to assume either a first or a second stable orientation of the axis. It is the object of the present invention to provide a memory system of the type as mentioned at the beginning of the description which is improved over the prior art. A memory system according to the invention is contained in claim 1. In the memory system according to the present invention, electric field applying means are provided which includes a pair of electrodes disposed outside of and proximate to the optical disc on its opposite sides. By this inventive feature, the reliability of the memory system can be greatly improved because there is no electric contact in the liquid crystal cell as in the prior art so that problems which may be generally caused by electrical contact do not occur. In a preferred embodiment the electrodes may have a length equal to the radius of the disc memory. If such an elecrode assembly is used, the entire surface of the disc can be erased by one rotation of the disc. Reference is also made to EP-A-205 188 containing a method corresponding to the memory system according to the invention. Referring to Fig. 1 a disc memory having inner electrodes is shown. A first system 100 is for "reading" information, a second system 101 is for "writing" information and a third system 103 is for "erasing" information. Reference 10 designates an optical disc. The optical disc has two disc substrates 3 and 7 opposed to each other. At least one of the substrates, the substrate 3 in the figure, is light transmitting. On the insides of the two, there are provided electrodes 4 and 6 respectively. Here, the electrode 4 provided on the substrate 3 must be light transmitting and the electrode 6 on the substrate 7 must be reflective. Moreover, at least one of the substrates 3 and 7 is formed with the oriented inside. The substrates 3 and 7 are separated from each other by a 4µm distance with spacers (not shown). Between the substrates 3 and 7, a ferroelectric liquid crystal (FLC hereafter), such as smectic C*, is disposed. The disposing process is carried out with the hot liquid crystal in a smectic A phase. In so doing, the FLC molecules are aligned and formed into a plurality of molecular layers perpendicular to the substrates at room temperature, according to the oriented inner surface of the substrate, and get a stability. The inner and outer peripheries of the optical disc are sealed off by sealing members 30 and 30' to shut out FLC from making contact with air. On the inner peiphery side of the optical disc 10 there are provided contacts 32 and 32' that are formed extending from a pair of electrode 4 and 6 for applying voltage between the electrodes. The optical disc will be described in more detail in the following: Namely, for the substrate 7 use was made of plastic or forming 7059 glass. On this substrate there was formed a reflective electrode 6 of Alminium by vapor deposition in vacuum. The oppsing electrode 4 was prepared by forming a light-transmitting conductive film on the transparent substrate 3 of plastic or glass. As the light transmitting conductive film use was made of ITO (indium-tin oxide). Then, on the inside of electrode 4, there are provided oriented film (not shown) of PAN (polyacryl-nitryl) and PVA (polyvinyl-alcohol) by spinning method to a thickness of 0.1µ. FLC 5, e.g. S8 (octyl-oxy-benzylidene-amyno-methyle-butyl-benzoate) of thickness 1.5µ, was sandwiched by the electrodes. A number of spacers are interposed between the pair of substrates to reinforce the disc memory. To the inner surface a known rubbing treatment is given. For example, a cylinder covered with nylon film is rotated in a rubbing device at 900 RPM and the inner surface of the film to be oriented undergoes the treatment of the rubbing device by moving at a speed of 2m/min. In this embodiment, DOBAMBC or a blending of a plurality of FLCs may also be used as FLC to be filled in the disc instead of S8. Some examples are shown by J.W. Goodby et al., "Ferroelectrics Switching in the Tilted Smectic Phase of R-C-3-4-n-Hexyloxydenzylidene 4'-Am'no-(2-Chloropropyl) (innamate(HOBACPC))" , Ferroelectrics 1984 Vol. 59 pp. 126-136, Japanese Patent Published Applications Nos. sho 59 - 98051 and 118744. As an FLC confined in the disc, a chiral smectic C liquid crystal is used. Between the two substrates 3 and 7 FLC has a molecular structure composed of a plurality of laminated layers each perpendicular to the substrates 3 and 7. In each layer the molecules of the liquid cystal tend to be arranged in the same attitude having a ferroelectric polarization P (electric dipole moment) normal to the long axis of the liquid crystal molecule and laying on the layer. The long axes spontaneously make a certain tilt angle with the layer normal. FIG. 2 (A) shows a light transmissivity versus a voltage across the liquid crystal in reference to duly polarized light incident on FLC between the substrates 3 and 7. By means of the contiguous surface of the substrate to FLC, FLC exhibits hysteresis, Namely when +15V ( or 3 x 10⁴V/cm) is applied between the electrodes, FLC appears transparent having the electric polarization (referred to as having the "up" dipoles hereinafter). On releasing application of the voltage, the transparency of FLC remains as it was. On the other hand, an application of reverse voltage, e.g. -15, transforms the condition of FLC into an opaque condition having the opposite electric polarization (the "down" dipoles, hereinafter). This opaque condition can be observed even after the applied voltage is increased to a forward voltage providing the forward voltage is less than the coercive voltage Ec. Having increased from - 15v to a certain forward level less than the coersive voltage Ec, the applied voltage becomes to favor the "up" dipole although FLC has yet held the "down" dipoles in virtue of the contiguous surfaces of the substrates. Some suitable external disturbance can inverse the dipoles of the molecules from "down" to "up" as shown in the figure with a broken line. In this embodiment, some domains of FLC are irradiated with light beam, especially of infrared ray, to be reversed selectively by external inpact. In the following, the condition with the "up" dipole is referred to as "0" state and the condition with the "down" dipole as "1" state. Fig. 2 (B) shows the characteristics of FLC with respect to light incident thereon polarized on the plane of polarization normal to that of Fig. 2 (A). Now the explanation of how information is written on or erased from the above described disc memory is in order. In the erasing system 103 the electrodes for external contacts 32 and 32' are connected to the terminals 31 and 31' of the leads 13 and 13' that are taken out from a controller 25 to carry out "erasing". A sufficient voltage is applied between the opposing electrodes 32 and 32' to arrange FLC molecules in one direction, referred to "0" state, entirely throughout the disc. With this condition of the disc, "writing" of information will be carried out for the disc by the use of the system 101. Namely, "writing" is carried out by irradiating 25 a prescribed address of FLC whose molecules are arranged in one direction throughout its surface, with a light beam under application of weak voltage less than Vc. If wavelength is 1 - 3µ, FLC can absorb a large part of the light energy. The appropriate wavelength depends on the applied voltage. For example, 10V versus 1.2µm or 6V versus 2µm are suitable. In so doing, information is being recorded on the disc in terms of binary system. The light beam is radiated from a light source 23 through a half-mirror 22 and a system 21 such as a collecting optics and an autotracking device. After reflected from the disc, this light beam further reaches a photosensor 9 via the mirror 22 and a light intensity controllor 24. That writing is in progress is monitored there. Next reading process will be described. The light beam from a semiconductor laser 12 impinges, after passing through a half-mirror 2 and a system 10 such as a collecting optics and an autotracking device, upon the optical disc 10 as a beam 16. Then, the light is reflected from the disc 10 as a beam 16', its path is split by the half-mirror 2, and reaches the light-receiving sensor 9 via a polarizing plate 8. According to the sense of the liquid crystal on which the light is incident, the reflected light is polarized either on parallel plane or unparallel plane to the plane of polarization of the plate 8. Although, if the tilt angle of the liquid crystal is 45°, the light incident on the plate 8 has been polarized on parallel or normal plane to the polarizing plane of the plate 8, the polarization by the liquid crystal is occasionally not satisfactory because the liquid crystal, even with complete regular arrangement of molecules with the same attitude, is necessarily to polarize light wave completely. To enhance the polarizing power of such a liquid crystal, some additive is available so that the molecules of the additive are interposed and oriented between the molecules of the liquid crystal. The additive functions as a polarizer in cooperation with the liquid crystal while the liquid crystal determines the sense of attitude to be taken by molecules of itself and the additive. For example, as an additive, dichromatic substance such as anthraquinone dye or azo-compound are effective. The polarizing power is attributed to an absorption anisotropy. An embodiment of a memory system according to the present invention is shown in Fig. 3. In the figure, the construction is the same as in the foregoing embodiment except for the electric field applying method, therefore redundant explanations are not repeated. In this case, a voltage is applied, for the optical disc 10, directly to a pair of electrodes 31 and 31' from a high voltage source 25 on the outside, in order to impress a prescribed electric field on FLC. The electrodes 31 and 31' are proximately close to the optical disc 10. Although, in the figure, the electrode 31 and 31' have a length which is only a fraction of the radius of the disc, they may have a length equal to the radius. In the former case, electric power consumption is reduced. In the latter case, dispensed with is a means for radially scanning the disc 10 with the electrodes 31 and 31' from outside toward inside, or vice versa, of the disc 10, and it is possible to erase the entire surface of the disc with one rotation of the disc 10. According to this another embodiment, the optical disc 10 comprises the pair of substrates 31 and 31', one of which is transmissive and other of which has reflective inside, and FLC between the substrates. In the figure a reflective plate 6 is provided on the glass plate 7. Although the reflective plate 6 must not be conductive, use of conductive plate makes it possible to place the electrode 31' apart from the disc. A polarizing plate 8' can be placed between the half-mirror 2 and the laser 12 instead of the plate 8. In this modification, the light incident on the disc is polarized in advance in favor of one of the bistate conditions.

Claims: A memory system comprising: (a) a rotating optical disc memory including a pair of substrates (3,7), at least one (3) of which is light transparent, a liquid crystal layer (5) having a plurality of laminated molecular layers and interposed between said pair of substrates, said liquid crystal layer having a bistability of the electric dipole moment direction of the molecules, and means for keeping the attitude of each molecule; (b) a data writing means (101) including a light source (23) for emitting a light to a particular portion of said liquid crystal layer under an existence of an electric field in order that the condition of said liquid crystal layer on said portion is switched to a predetermined one of said stable dipole moment directions; (c) a data reading means (100) including a light source (12) for emitting a light to a portion of said liquid crystal layer, a photosensor (9) with polarising means (8) for sensing the polarisation of light passing through said liquid crystal layer for detecting the condition of said liquid crystal layer at said portion; and (d) electric field applying means (103) including a pair of electrodes (30,31) disposed outside of and proximate to the optical disc on its opposite sides. The system of claim 1, wherein said electrodes substantially extend over the length of said radius of the disc memory. The system of claim 1 or 2, wherein said optical disc memory is rotatable around its centre. The system according to one of the claims 1 to 3, wherein said data writing means includes a focusing device through which said light is directed to said particular portion of said liquid crystal layer.

Inventors: SHUNPEI YAMAZAKI, SHUNPEI, YAMAZAKI
Applicants: SEMICONDUCTOR ENERGY LAB, SEL SEMICONDUCTOR ENERGY LABORATORY CO., LTD.
Application year: 1986
Main classification: G11B7
IPC codes: G11B7, G11B13