Optical high density recording mediums, method for making same and method for recording optical information in the medium

Abstract: An optical recording medium which comprises a substrate and a recording layer of at least two J-aggregates of dyes formed on the substrate is described. The dyes may be photochromic dyes or other types of dyes capable of forming the aggregates which have narrower absorption spectral ranges than original dyes. A plurality of information bits can be written in the recording layer by irradiation of one laser beam spot. The recording layer is formed by an LB film method or a method capable of fluidizing a dye solution in a direction horizontal to the substrate surface. A recording method using the medium is also described.

Description: This invention relates to optical high density recording mediums according to the preamble of claim 1. The invention also relates to a method for making such mediums as mentioned above and to a method for recording optical information in the recording mediums. The optical recording mediums of the above type may be broadly classified into two groups, one group being a rewritable medium and the other group being a write-once medium. A typical rewritable optical recording medium comprises an optically transparent disc substrate and a recording layer of a photochromic dye formed on the substrate. For information recording, while the optical recording medium is being rotated for tracking, a laser beam having a wavelength of λ1 in spotted on one point of the disc surface. The photochromic dye on the spot undergoes the photochromic reaction and thus, the colorless dye on the spot is colored as having a visible absorption spectrum at λ2. By this, single bit information is recorded on the spot. On reading, a weak laser beam having a wavelength of λ2 is applied to the disc surface. The absorption of the beam in the colored portion can be detected from a change in intensity of transmitted light. On the other hand, when an intense laser beam having a wavelength of λ2 is applied to the colored portion in the same manner as in the writing operation, the photochromic dye in the portion is turned colorless and thus the information can be erased. If dyes having no photochromic properties are used as the recording layer, and the erasing operation is omitted from the above procedure for the photochromic dye medium, write-once optical recording is effected. However, in view of the principle of the known systems, only one bit is recorded by application of one spot of a laser beam. This places a limitation on the increase of a recording density. JP-A-59-152 528 which is reflected in the preamble of claim 1 relates to a heat mode process using a rewritable optical recording medium comprising an optically transparent substrate and a recording layer formed on said substrate and comprising at least two dyes which are formed as separate sub-layers of the said recording layer, the at least two dyes having different absorption spectrum ranges optically distinguishable from each other. According to this heat mode process pits are formed in the respective layers by laser beams with inherent wavelength. JP-A 59-223 957 relates to an optical information recording medium which is used for a process wherein information is recorded in the form of irregularities by means of a laser beam and is reproduced from variations of the reflected light. To provide a high S/N ratio and resolving power and to improve shelf life an aggregate of an organic dye - so called J-aggregate - is incorporated as at least a part of the constituting components of the recording layer. The dyes used are cyanine dyes which undergo decomposition by application of light and heat finally leading to information recording in the form of irregularities. The document "Thin solid Films" [133, 21 - 28 (1985)] describes the preparation of J-aggregates of photochromic spiropyrans in Langmuir-Blodgett films. Specifically the use of two spiropyrans (so called SP 16 and SP 1822) is disclosed. It is the object of the present invention to provide a rewritable optical recording medium and a method for recording optical information on said medium which shows improved properties, in particular an increased sensitivity together with decreased background reflections leading to a further increase of the S/N ratio. According to the invention this object is achieved by an recording medium according to claim 1 and the method according to claim 4. The invention is advantageously developed by the measures mentioned in the subclaims. The recording layer is preferably formed on the substate by a method in which a solution or solutions of a plurality of dyes are coated onto the substrate while permitting the solution to be fluidized in a direction parallel to the substrate surface such as by spin coating. Alternatively, the recording layer may be formed by the Langmuir-Blodgett film method (hereinafter referred to simply as LB film method). The resultant recording layer is heated to a temperature sufficient to allow the respective dyes to form aggregates without causing the change of the molecular structure of the dyes. As a matter of course, the temperature depends on the type of dye. The formation of the J-aggregate is advantageous in that the aggregate has a narrower absorption spectrum range than the dye used, thus enabling one to make a recording layer or layers of higher sensitivity in response to laser beams having different wavelengths by using different types of dye aggregates. When light sources and photosensors are utilized corresponding to the respective aggregates having inherent absorption spectra, a plurality of bit signals can be written in one spot of laser beams having different wavelengths corresponding to the respective aggregates. For recording of optical information, the optical recording medium having an optical transparent substrate and a layer of at least two dyes capable of forming corresponding aggregates and formed as separate sub-layers is irradiated with an UV ray to form photomerocyanines of the at least two dyes, then heated to a temperature sufficient to cause the respective photomerocyanines to form aggregates, and subjecting the resultant medium to optical information recording. Fig. 1 is a schematic view, partially in section, illustrating a principle of a recording apparatus using an optical recording medium of the invention; Fig. 2 is an absorption spectrum chart of a double-layer recording film of an optical recording medium obtained in Example 1 prior to formation of aggregates of the respective dyes in the layer; Fig. 3 is an absorption spectrum chart of the recording layer of Fig. 2 after formation of the aggregates of the respective dyes; and The recording layer or film of the invention comprises at least two types of organic dyes which are, respectively, converted into aggregates by association. As is well known in the art, organic dyes are able to form an aggregate several dye molecules under certain conditions. In some cases, the aggregate has physical properties, such as stability, spectral characteristics and the like, completely different from an original dye. The aggregate used is a J-aggregate. The term 'J-aggregate' used herein is intended to mean an aggregate of a plurality of dye molecules without involving any change in chemical structure which has a sharper visible absorption spectrum range than the dye while the spectrum is shifted to a longer wavelength side. This property of the J-aggregate is utilized in the optical recording medium. A plurality of dyes having different absorption spectra within a narrow range can be optically distinguished and thus used for optical recording service. The dyes capable of forming J-aggregates include photochromic dyes. Typical photochromic dyes include spiropyrans, azobenzenes, fulgides, indigoes, thioindigioes, triarylmethanes and the like. All these dyes do not form the aggregates but have the possibility of forming J-aggregates if properly substituted or under proper treating conditions. In the practice of the invention, as photochromic dyes capable of forming J-aggregates the spiropyrans of the following general formula are used because of the good formation of J-aggregates in which R₁ and R₂ independently represent an alkyl group having from 1 to 5 carbon atoms or an aromatic group, R₃ represents an alkyl group having at least 10 carbon atoms and preferably up to 30 carbon atoms, X is a single bond or a group selected from the following formulas -C n H 2n OCO-, -C n H 2n COO-, -C n H 2n NHCO-, -C n H 2n CONH-, and - C n H 2n O-, wherein n ≧ 0, R₄ represents an alkyl group having from 1 to 30 carbon atoms. In the above formula, R₁ and R₂ are each preferably methyl, ethyl, propyl and the like, or phenyl. Likewise, R₃ is preferably decyl, undecyl or the like. X is preferably - CH₂OCO- or -O-. Examples of typical spiropyrans include 6-nitro-1'-octadecyl-8-docosanoyloxy-3',3'-dimethylspiro[2H-1-benzothiopyran-2,2'-indoline], and the like . Needless to say, these spiropyrans are used in combination of at least two compounds. Spiropyrans are generally colorless but upon irradiation of UV rays, they are converted by photoisomerization into purple photomerocyanines having maximal absorption leaks at approximately 400 to 700 nm. In turn, when visible light is irradiated on the photomerocyanine, it is reversibly converted into a corresponding spiropyran. However, photomerocyanines are unstable in nature and are naturally faded in color even in the dark, thereby forming original spiropyrans. It has been found that when spiropyrans which are able to form J-aggregates are formed as a layer and subjected to UV irradiation to form corresponding photomerocyanines, the photomerocyanines can be converted into more stable J-aggregates by thermal reaction. In addition, the J-aggregate is found to have a significantly narrower absorption spectrum range than an original dye. The spiropyrans defined by the general formula may be used in combination by proper selection in consideration of absorption spectra. Typical and, in fact, preferable combinations of spiropyrans include 6-nitro-1'-octadecyl-8-docosanoyloxy-3',3'-dimethylspiro[2H-1-benzothiopyran-2,2'-indoline] capable of conversion to an aggregate predominantly absorbing light of 653 nm. It should be noted that dyes which are used in combination should preferably have absorption peaks, which are separate from each other at least by a wavelength value corresponding to the half-value width (whichever larger) or at least about 20 to 30 nm, when converted into corresponding aggregates. The dyes useful in the practice of the invention are formed as a recording layer or layers by a variety of methods. Such methods include, for example, an LB film method, or methods in which a solvent used is fluidized in a direction horizontal to the surface of a substrate, e.g. a spin coating method, an air doctor coating method, a blade coating method, a rod coating method a knife coating method and a squeeze coating method. Of these, the LB film method or the spin coating method is preferably used because of the good film properties. In these methods, spiropyrans or the like dyes are dissolved in inert solvents such as chloroform, aromatic hydrocarbons, e.g. benzene, toluene and the like, tetrahydrofuran, acetonitrile, acetone, ethanol and the like. A plurality of dyes is separately dissolved in the respective solvents and applied onto a substrate one by one. The dyes are each dissolved in a solvent at a concentration of 0.1 to 50 wt% of the solution. In the spin coating method, a 5 to 15wt% dye solution is applied onto an optically transparent substrate, such as glass, quartz or synthetic resins including polystyrene, which is preferably surface treated with an alkylsilanes, such as trimethylchlorosilane, in order to render the surface hydrophobic, followed by spinning first at 50 to 600 r.p.m. for 10 to 100 seconds and then at 1000 to 2000 r.p.m. for 100 to 300 seconds for drying, thereby forming a dye layer. The LB film method may be effected as follows: a solution of a dye in a suitable solvent is dropped over a water surface of an LB film-forming apparatus under conditions of water of a pH of 3 to 11 and a temperature of 10 to 30 o C. Thereafter, the solution is compressed by means of a moving metal bar to such an extent of a surface pressure of 0 to 80 dynes/cm. This film is applied onto a surface-treated substrate. The above procedure is repeated to form a multilayer film on the substrate to a desired thickness of 0.0025 to 0.25 µm. In these method, the dyes are preferably used in combination with an aid such as a semi-solid or solid alkane, in order to form a uniform dye layer or stabilize the layer. The alkane aid may be n-octadecane or n-hexadecane and is used at a molar ratio, to the dye, of 1:3 to 3:1. The recording layer or layers are then subjected to UV irradiation to convert the dye or dyes into a corresponding photomerocyanines. Thereafter, the photomerocyanine layer is heated to a temperature of 30 to 45 o C for 0.1 to 20 minutes although these temperature and time conditions may vary depending on the type of dye. As a result, J-aggregates of the photomerocyanines are formed. As described before, the LB film method and the spin coating method are preferred for the formation of the J-aggregate. The reason for this is not clearly known, but it is considered that these methods are effective in orientation of dye molecules in the recording layer. This oriented state of dye molecules may be called 'preaggregate'. It will be noted that the heating should be effected to such an extent that the molecular structure of a dye does not undergo an irreversible change. In this sense, exact heating conditions should be determined on the respective dyes. When the respective dyes are formed into separate layers, each layer should preferably have a thickness of from 0.2 to 2 µm. For ensuring good optical reading and writing operations, absorption spectra of different types of dye aggregates should preferably have a difference in half-value width of at least about 30 nm as indicated before. The optically transparent substrates used in the present invention may take any forms of a disc, plate, sheet or the like and are generally made of glass, quartz, synthetic resins such as polystyrene, polyacrylates and the like. The present invention is more particularly described by way of examples. Example 1 In this example, spiropyrans of the following chemical formulae (I) and (II) were used to make an optical recording medium by the LB film method. A Langmuir-type LB film-forming apparatus having a 1000 mm x 500 mm trough was provided, in which a subphase of an aqueous phosphate buffer solution adjusted to a pH of 7.2 and a temperature of 18 o C was filled. One mmol/l of compound (I) and 2 mmol/l of n-octadecane were dissolved in chloroform. Seven hundreds microliters of the chloroform solution was developed over the surface of the subphase to form a monomolecular film. The film pressure was automatically controlled to be 20 mN/m. A glass disc having a diameter of 3.5 inches was provided as a substrate. The substrate was rendered hydrophobic on the surface thereof by immersion in a 10 vol% trimethylchlorosilane solution in toluene for 30 minutes. This substrate was vertically moved toward the monomolecular film of the compound (I) to form on LB film (20 layers of Y film) on the substrate as a layer. The above procedure was repeated using compound (II) and n-octadecane, thereby forming another layer on the first-mentioned layer. The thus fabricated double-layer recording medium is optically recorded and reproduced by an optical recording system as schematically shown in Fig. 1. In the figure, there is shown in optical recording medium 1 which has a glass substrate 11 and a recording layer 12 consisting of a sub-layer 13 of compound (I) and a sub-layer 14 of compound (II). Indicated by 2 is a lamp for erasure capable of emitting a UV ray of 350 nm and by 3 is an IR lamp for formation of aggregates of compounds (I) and (II). In front of the recording layer 12, there are provided laser devices 4, 5 capable of emitting two laser beams of 618 nm and 653 nm. These laser devices 4,5 are suitably arranged to give one optical axis and are able to emit the laser beams while carrying out tracking of the medium. The laser devices 4,5 are capable of changing output power from 500 mJ/cm² to 50 mJ/cm². Lasers used in the invention may be CO₂ laser, Xe laser, dye lasers or the like. Indicated by 6,7 are reading sensors having optical filters therein and capable of sensing light rays of 618 and 653 nm, respectively. The sensors 6,7 detect the intensities of beams from the laser devices 4, 5 passed through the optical recording medium 1. Recording of the medium is described. (a) Initialization The recording layer obtained just after the fabrication is colorless. The medium 1 is exposed over the entire surface thereof with light from the erasure lamp 2, for example, for 10 minutes. As a result, colored photomerocyanines are formed in the respective sub-layers 13, 14. The absorption spectra of the sub-layers 13, 14 are shown in Fig. 2 as curves I and II, respectively. From the figure, it will be seen that the spectral curves are superposed in most parts. In this state, the IR ray from the lamp 3 is irradiated on the medium 1 and the recording layer 11 is maintained at a temperature of 35 to 40 o C for about 15 minutes, with the result that the photomerocyanines in the recording layer are converted into J-aggregates with different colors. The absorption spectra of the sub-layers 13, 14 after conversion into the J-aggregates are shown in Fig. 3 as curves I and II, respectively. As will be seen from the figure, the spectra are shifted to a longer wavelength side by about 35 to 50 nm with half-value widths as sharp as about 30 nm. From this fact, the formation of the J-aggregates is confirmed. By the above procedure, the optical recording medium is initialized. (b) Writing While tracking, the laser device is set at output power of 500 mJ/cm² and the medium is irradiated with a spot of the laser beam. By the irradiation, the sub-layer 13 alone is bleached on the spot and thus recorded. Moreover, when the optical recording medium 1 is irradiated with the laser device 5 alone at the same output level, the sub-layer 14 alone is bleached on a portion or spot where irradiated. In this manner, two bits can be independently written in one spot. (c) Reading-out While tracking in the same manner as in the writing operation, laser beams from the laser devices 4,5 set at 5 mJ/cm² are simultaneously irradiated on the medium 1 and the intensities of the transmitted beams are detected by the sensors 6, 7. The written spots are higher in the intensity of transmitted light than the other portions, from which information can be read out. The information signals of the the sub-layers 13, 14 can be independently and simultaneously read out by the sensors 6,7. (d) Erasure and Rewriting The information of the written optical recording medium can be erased by repeating the initialization procedure (a). Information can be again written in the medium 1 by repetition of the procedure (b). In the above procedure, the initialized state is determined as a colored state, and the color is bleached on writing, but the colorless state may be regarded as the initialized state and turned into a colored state by writing. Example 2 A solution of 5 wt% of compound (I) in toluene was spin coated at 500 r.p.m. on such a substrate as used in Example 1 to form the sub-layer 13. On the sub-layer 13 was further formed the sub-layer 14 by spin coating a solution of 5 wt% of compound (II) in toluene. The resultant optical recording medium could be used in the same manner as in Example 1, i.e. two bit signals per spot could be written, read, and erased.

Claims: A rewritable optical recording medium comprising: an optically transparent substrate and a recording layer formed on said substrate and comprising at least two dyes which are formed as separate sub-layers of the said recording layer, the said at least two dyes having different absorption spectrum ranges optically distinguishable from each other characterized in that the at least two dyes are J-aggregates of photochromic spiropyran dyes having the general formula in which R₁ and R₂ independently represent an alkyl group having from 1 to 5 carbon atoms or an aromatic group, R₃ represents an alkyl group having at least 10 carbon atoms, X is a single bond or a group selected from the following formulas -C n H 2n OCO-, -C n H 2n COO-, -C n H 2n NHCO-, -C n H 2n CONH-, and -C n H 2n O-, wherein n ≧ 0, R₄ represents an alkyl group having from 1 to 30 carbon atoms and each of the dyes being capable of undergoing a color change on irradiation of a laser beam having a wavelength inherent to the respecitve J-aggregate. An optical recording medium according to claim 1, wherein the spiropyran dye is used in combination with a semi-solid or solid alkane. An optical recording medium according to claim 1, wherein said photochromic spiropyrane dyes are dissolved in a solvent to form corresponding J-aggregates and gelled. A method for recording optical information on an optical recording medium according to anyone of claims 1 to 3, which comprises forming solutions of the respective dyes, applying the respective solutions as separate sub-layers to form a recording layer, irradiating the layer with an UV ray to form photomerocyanines of the at least two dyes, heating the layer to a temperature sufficient to cause the respective photomerocyanines to form aggregates, and subjecting the resultant medium to optical information recording. A method according to claim 4, further comprising heating the recorded medium to erase the optical information.

Inventors: ANDO EIJI, MIYAZAKI JINSEI, MORIMOTO KAZUHISA, YOSHINO KIMIAKI, ANDO, EIJI, MIYAZAKI, JINSEI, MORIMOTO, KAZUHISA, YOSHINO, KIMIAKI
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1986
Main classification: G11B7
IPC codes: G11B7