BACKGROUND OF THE INVENTION The present invention relates to a power efficient process for recording optical information. The present invention also relates to an optical recording medium useful in such a power efficient process. The laser power needed to record a bit of information in accordance with the present invention is relatively less than in typical deformation recording processes and phase change processes. The technology of digital data storage using optical memory devices has advanced rapidly since its inception. Commercial uses at present range from compact disks (CDs) which provide remarkably high- quality audio reproduction to computer memories which provide extremely large yet compact data storage capacity. As applied to the requirements of computer mass storage, optical disks have been categorized according to the ease with which digital data can be written onto them. Optical read-only memories (OROMs) such as compact disks (CD-ROMs) have data written onto them before they leave the factory; write-once read- many (WORM) disks allow the user to write data onto them once and read that data indefinitely; erasable optical disks permit the user to write and read data with the same flexibility as magnetic storage media such as floppy and hard disks.

One of the reasons that optical storage media have increased in popularity is the very large capacity they provide in a small volume. Information is usually stored on the disk in the form of pits in the recording film. The pits can consist of holes, which are arranged in tracks, in the recording layer. The holes are usually formed by an intense laser beam focused

onto the layer which is formed of a material which absorbs the laser light and ablates or melts as a result of heating caused by the energy absorbed. The intensity of the light reflected from the disk is modulated by the presence or absence of the pits. The recording layer may be composed of tellurium alloys, bubble-forming materials, multilayer optical cavities, colloids, icrotextured absorbers or organic materials, such as those described by Kuder, in the Journal of Imaging Technology. Vol. 12, No. 3, pp. 140-143. The pits are written by a relatively high power laser beam, for example 10-30 W, while they are read by a low power beam, typically 0.5 mW. Both reading and writing can be performed by a single semiconductor laser, such as those of gallium arsenide.

The light-sensitive material in the information layer normally comprises a material that exhibits a change in its physical characteristics, such as melting or evaporating to produce a microscopic hole, whenever a beam of light of sufficient intensity is focused thereon. An information signal is recorded in the optical disk by focusing onto the light- sensitive recording layer a beam of light, modulated in intensity in accordance with the information signal, as the disk is rotated in a prescribed fashion. The intensity of the beam is alternatively greater than and less than a predetermined threshold, at which melting or evaporation of the light sensitive layer occurs, whereby a sequence of spaced holes, representative of the information signal, is formed in the layer in a succession of substantially circular and concentrically arranged recording tracks. The recorded disk can then be read immediately, without any intermediate processing of the disk.

Included among the organic materials useful in optical recording media are the phthalocyanine and naphthalocyanine compounds. U.S. Patent No. 4,492,750 discloses the use of specific naphthalocyanine compounds in optical recording media. The film-coating properties of such dye materials, however, have been generally found to be poor, the read out Signal/Noise (S/N) ratio poor and tending to fluctuate depending on the particular portion of the layer, and the S/N ratio of the read-out deteriorating significantly after repeated irradiations with the read-out light.

The use of various substituted naphthalocyanine chromophores, and in particular silicon naphthalocyanine chromophores, in optical media is disclosed in U.S. Patent No. 4,725,525.

As the technology of optical recording media becomes more sophisticated, the need and desire for more sensitive recording media, thereby requiring less powerful and/or smaller lasers, will become greater. Media requiring less power and less time in which to record information, would be a great advance in the technology.

Accordingly, it is an object of the present invention to provide a system which can record information on an optical recording medium using relatively less power than would be required for typical deformation recording.

Another object of the present invention is to provide a recording medium useful in such a system. Still another object of the present invention is to provide a process for the recording of information on an optical recording medium, which process is energy efficient and energy wise, and which

process records information without any visually detectable deformation.

Still another object of the present invention is to provide an optical recording medium upon which information can be recorded at low energy requirements while providing an enhanced positive contrast in reflectivity.

These and other objects of the present invention will become apparent to the skilled artisan upon a review of the following specification, the Drawing and the claims appended hereto.

SUMMARY OF THE INVENTION In accordance with one aspect of the present invention, these and other objects and advantages are achieved by using an optical recording medium for the recording of optical information thereon, which medium comprises an information layer sufficiently sensitive to have its reflectivity changed without a visually or optically detectable change in the geometry or physical structure of the layer, with low power requirements, e.g. , 5 to 7 milliwatts of power or less. The information layer comprises an aza-annulene compound, and preferably a naphthalocyanine compound.

In another embodiment of the present invention, there is provided a process for recording optical information. The process comprises irradiating an optical information medium comprised of an information layer containing an aza-annulene compound, preferably a naphthalocyanine compound. The optical recording medium is irradiated by a focused light source, e.g., a laser, at a lower power level for a period of time insufficient to effect a visually detectable change in the information layer, yet at a

power level and for a time sufficient to change the reflectivity of those areas irradiated, with the change in reflectivity preferably being at least 20%. It has been surprisingly found that such a process using an optical recording medium comprised of an information layer containing an aza-annulene compound, and more preferably a naphthalocyanine compound, can provide an actual positive contrast in the reflectivity reading of the information layer as opposed to the normal negative contrast reflectivity reading observed with deformation recording of optical information. This process provides one with the advantage of having a much more sensitive and facile system requiring less power to effect the recording of optical information.

BRIEF DESCRIPTION OF THE DRAWING

The Figure of the Drawing graphically depicts the change in reflectivity in relation to laser power for a recording medium of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The recording medium of the present invention comprises an information layer supported by a substrate. The information layer contains a naphthalocyanine compound which will give a positive contrast reflectivity reading upon irradiation with a focused laser light source at low powers, i.e., about 5 to 7 milliwatts or less. By a positive contrast reflectivity reading, for the purposes of the present invention, is meant an increase in reflectivity of the information layer. This positive contrast reflectivity reading is also effected without any visually or optically detectable deformation being made in the information layer. In other words, the physical

geometry and structure of the information layer essentially remains the same to the visual eye using optical microscopy with a magnification up to about 800X. Aza-annulene compounds are the compounds used in the present invention as it has been surprisingly discovered that such compounds, and particularly naphthalocyanine compounds, do provide a positive contrast reflectivity reading even though no visually detectable surface deformation has been made in the information layer containing the aza-annulene compound. Generally, it has been known to record optical information in an information layer containing an aza- annulene compound where the bits of information are recorded using a deformation recording technique. The reflectivity reading in such an instance is of a negative contrast, i.e., the reflectivity of the information layer itself in those areas which have been irradiated decline in reflectivity vis-a-vis those areas which have not been irradiated. The present invention provides a positive contrast in reflectivity upon the recording of an information bit, with no visually detectable surface deformation.

In general, any aza-annulene compound, e.g., naphthalocyanine, phthalocyanine or porphyrin, is believed to be appropriate for the purposes of the present invention, as long as the compound observes a positive contrast reflectivity reading without having an optically detectable deformation made in the information layer. The more preferred compounds, however, are naphthalocyanines, particularly naphthalocyanines having central metallic atoms and silicon naphthalocyanines.

Among those naphthalocyanine compounds which are most preferred for the practice of the present invention are the silicon naphthalocyanines. Suitable silicon naphthalocyanines are described, for example, in U.S. Patent No. 4,725,525, and those described in copending U.S. Patent Application Serial No. 396,962.

The most preferred silicon naphthalocyanines are the following:

fOCH ₂ CH ₂ 0-^—^-OCH ₂ CH2-0-SiMe ₂ -0-(SiNc ⁾ - ⁰ - ^S iMe ₂ n

SiNc[0-Si(OBu)2(C ₁₈ H ₃₇ )] ₂

fOCH ₂ CH ₂ 0-Q>4- ^t Q-0-CH ₂ CH2-0-SiMe2-0-(SiNc)-0-SiMe ₂ 7n

where n = 2 or greater in the above formulae.

Of the foregoing compounds, the first and third are the most preferred compounds for use in the present invention. These compounds can be made from dihydroxy silicon naphthalocyanine and commercially available compounds. For example, the general procedure would be as follows:

SiNc(OH) ₂ + (ex) SiMe ₂ Cl ₂ Egbutylamine [SiNc(OSiM _e2 Cl ₂ ⁾ ., ^]

"Diol" Product

The diol in the foregoing general procedure is 4,4'-bis (hydroxyethoxy)-biphenyl if the first compound above is to be made, and bis(hydroxyethoxy)bisphenol-A if the third compound described above is to be made. While the products have been expressed in terms of polymers, it should be noted that the final product is generally a mixture of the polymer with some monomeric and oligomeric compounds.

In coating the information layer containing the naphthalocyanine or other aza-annulene compound onto the support, it is preferred that the chromophore be cast from a solution. Naphthalocyanines or the other aza-annulene chromophores can be used as a one component material, i.e., chromophore only material, or used in combination with a polymer. Thus, it is preferred to either cast a chromophore layer or a polymer/chromophore film. Conventional methods of casting may be utilized with the chromophores of the present invention, with spin coating techniques being the most preferred.

Since the read and write steps all require operating within a very narrow focus, the film, when applied, must provide a very flat surface in order to avoid errors and noise. In order to facilitate the coating procedure, it is also generally advantageous that the polymer and chromophore, if a polymer is used, be soluble in a readily available organic solvent such as an alcohol or ketone. In this regard, the polymer and chromophore should be compatible and mutually cosoluble. Also, upon evaporation of the solvent, the naphthalocyanine chromophore should not precipitate in a particulate form, which particulates would cause a scattering of light.

Any suitable coating technique may be used to achieve such a flat surface, with a conventional technique such as spin coating, which allows for a high degree of control of film thickness and flatness, being preferred. It is, of course, important that a thin film coating be formed.

The substrate which is used as a support should generally possess a surface of suitable smoothness. This may be imparted by appropriate molding or other forming techniques when the substrate is made. If the substrate has an inadequately smooth surface, a smoothing or subbing polymer layer may be used to attain the appropriate smoothness. Such smoothing or subbing layer should not, of course, interfere with the application or utilization of the recording layer which is subsequently applied thereto. The subbing layer can contain Preformatting information. A preferred subbing layer is a layer of polyvinyl alcohol or an acrylate formulation. The substrate may be optically featureless or may contain Preformatting information (e.g., tracking groove and/or encoded information in the form of readable marks) .

The material of which the substrate is comprised is generally a material exhibiting good mechanical strength and good structural integrity against warping. Examples of suitable materials include aluminum, glass, reinforced glass, ceramics, polymethacrylates, polyacrylates, polycarbonates, phenolic resins, epoxy resins, polyesters, polyimides, polyether sulfones, polyether ketones, polyolefins, polyphenylene sulfide and nylon. Polycarbonate is a preferred material for use as a substrate. Furthermore, the shape and size of the substrate, and hence the recording medium, can vary depending on the

application. The shape and format, for example, may be a disk, tape, belt or drum. A disk or tape format is most preferred.

The actual structure of the recording medium itself may also vary in that the recording layer may be coated on one side or both sides of the substrate. Or, two substrates having the recording layer on either side can be combined allowing the sides having the recording layers to face each other at a constant distance, the combined substrates being sealed to prevent dust contamination and scratches.

The medium of this invention may also have an undercoating layer such as a layer of various resins on the substrate if necessary, with the recording layer being coated over it. In addition, various thermoplastic resins, thermosetting resin, UV or electron beam cured resins, may be used as an undercoating material. In addition, guiding grooves may be installed on the substrate, and the recording layer may be installed on the extruded portions and/or intruded portions of the grooves. Furthermore, a suitable protective layer or cover, such as those known to the art, can also be used if desired to protect the recording layer from dirt, dust, scratches or abrasion. The sensitivity of the recording medium used in the present invention, and the low power requirements for writing, permit the use of a protective layer as a preferred embodiment.

In addition to the chromophore material and optional polymer, the recording layer may also contain other polymers or oligomers, various plasticizers, surfactants, antistatic agents, smoothening agents, flame retardants, stabilizers, dispersants, leveling agents, antibleeding agents, antioxidants, water

repellants, emulsifiers, etc. as may be desired. The effect the presence of such additives may have on the optical properties of the medium, however, should be taken into account. Practicing the process of the present invention, the optical recording medium is irradiated with a focused light source such as a laser at a wavelength at which the naphthalocyanine or other aza- annulene chromophore absorbs. The system design can be that of a conventional optical recording system employing a focused light source such as a laser. The power of the laser, however, need only be about 5 to 7 milliwatts or less for the writing to take place. It is preferred that the power of the writing laser be in the range of from about 2 to 6 milliwatts, with a range of from about 3 to 5 milliwatts being most preferred. This is in contrast to a laser power of about 10 milliwatts or more, which is generally necessary for deformation optical recording when using an aza- annulene optical information layer.

Using such limited power output in the writing mode will still effect a reflectivity change, which has been observed to range from about 20 to 40 percent or more, in those areas of the information layer which are irradiated. This reflectivity change is an increase in reflectivity, so that the reflectivity reading is one of a positive contrast with the areas that have not been irradiated. Furthermore, the irradiation is of insufficient power to create or effect any type of visually or optically detectable defconation in the surface of the information layer. Thus, the geometrical and physical structure of the information layer remains the same after the writing process.

Therefore, after the information has been recorded using the process of the present invention, the resulting medium is a medium having optical information recorded in its information layer such that upon passing a reading light beam over the information layer, the information shall be read as a positive contrast reflectivity reading. The reading laser power is generally about 0.5 milliwatts or less so no writing occurs. As an example of the positive contrast reflectivity reading observed in using the process of the present invention, the biphenyl silicon naphthalocyanine compound described above was cast as a film on a substrate and irradiated at different powers. Fig. 1 hereto demonstrates the various changes in reflectivity observed at the different powers of the laser radiation. As will be clear from Fig. 1, at powers of 7 milliwatts or less, a positive contrast reflectivity reading was observed, with the most significant changes in reflectivity occurring at a recording power of about from 3 to 5 milliwatts. At powers greater than 8 milliwatts, and in particular 10 and above, the more typical negative contrast reflectivity readings were observed due to deformation recording of information in the information layer. Although the invention has been described with preferred embodiments, it is to be understood that variations and modifications may be resorted to as will be apparent to those skilled in the art. Such variations and modifications are to be considered within the purview and the scope of the claims appended hereto.