In response to the demand for more reliable and higher capacity data storage and retrieval systems, there is considerable activity in the research and development of so-called optical disk recording systems. These systems utilize a highly focused modulated beams of light, such as a laser beam, which is directed onto a recording layer which is capable of absorbing a substantial amount of the light. The heat thusly produced causes the light-absorbing material in the areas struck by the highly focused laser beam to change chemically and/or physically, thus producing a concomitant change in optical properties, e.g., transmissivity or reflectivity in the affected area. For readout, the contrast between the amount of light transmitted or reflected from the unaffected parts of the absorbing layer and from the marked areas of the layer is measured. Examples of such recording systems are disclosed in U.S. patents throughout the literature and in numerous U.S. patents such as U.S. Pat. Nos. 3,314,073 and 3,474,457. In recording data, a rotating disk having a light-absorptive recording layer is exposed to modulated radiation from a laser source. This radiation is passed through a modulator and appropriate optics and the highly focused laser beam is directed onto the disk which forms by chemical and/or physical reaction of the light-absorbing layer a series of very small marks along a circular path within the light-absorptive layer. The frequency of the marks is determined by the modulator inputs. Using laser beams with a diameter of 1 .mu.m or less, data can be stored at a density of 10.sup.8 bits/cm.sup.2 or higher.
The simplest optical disk medium consists merely of a dimensionally stable solid substrate on which is coated a thin layer of light-absorptive material such as a metal layer. When the light-absorptive layer is struck by an intense beam of coherent light, such as from a laser source, the light-absorptive material is vaporized, thermally degraded, and/or otherwise physically and chemically modified, thereby producing a very small marked area which exhibits different transmissivity or reflectivity than the adjacent unmarked layer.
The desired properties of optical recording media are (1) high sensitivity, (2) high signal-to-noise ratio (SNR), (3) high tolerance to material variation, contaminants and other defects, and (4) high archival stability after extended storage and/or recording and readout (see Bartolini, J. Vac. Sci. Technology, Vol. 18, No. 1, Jan/Feb 1981, p. 70). Based upon these criteria, a considerable amount of research has been and continues to be carried out directed to obtaining the best possible disk materials. In particular, a majority of the work done up to this time on materials for the light-absorptive or recording layer has been directed to thin films of metals and chalcogenides such as tellurium, tellurium alloys, rhodium, bismuth, indium, lead, aluminum, platinum, nickel, titanium and silver. Of these, by far the greatest amount of work has been directed to the use of tellurium and its alloys.
In addition, considerable effort has been directed to finding suitable organic-based light-absorptive materials. These have been largedly metal/polymer composites or dye/polymer composites. In the former case, finely divided metal particles are dispersed in an organic polymer medium. In the latter case, a dye is dissolved in, or finely divided pigment particles are dispersed in, an organic polymer medium.
The many issued patents which are directed to various dye and dye/polymer dispersions are indicative of the high level of interest in such materials. Several patents disclose the idea of using as an absorptive medium a thin layer of dye. e.g., U.S. Pat. Nos. 4,023,185, 4,097,895, 4,101,907, 4,190,843, 4,218,689, 4,219,826, 4,241,355, 4,242,689 and 4,315,269. Other patents disclose the use of dispersions of dye in an organic polymer medium. For example, U.S. Pat. No. 3,314,073 to Becker discloses the use of dyed gelatin or India ink and U.S. Pat. No. 4,360,908 to Howe et al. discloses the use of (dialkylaminobenzylidene) ketone dyes dispersed in a cellulose nitrate binder. In a similar manner, U.S. Pat. No. 3,723,121 to Hauser discloses a process for laser beam recording using colored thermochromic materials which, when heated with the laser beam, change to a color which transmits the laser beam. The materials are used either by themselves or dispersed in finely divided form in a film-forming organic polymer such as polyvinyl alcohol and/or gelatin.
In a different vein, U.S. Pat. No. 4,360,583 to Engler et al. is directed to a process for producing an optical record by UV exposure through a photomask. The light-absorptive layer is a film comprising functionally substituted tetraheterofulvalene and liquid halocarbon which co-react upon exposure to light. The photoreacted film is then solvent developed to produce contrasting light-absorptive image areas, which can be read by a laser beam.
In still another vein, polymeric materials having electrical conductivity in the semiconductive region are known. These are generally used in a doped form whereby the electrical properties are modified. Such materials are disclosed in U.S. Pat. No. 4,222,903.
Despite the great amount of research and development in this area of technology and the great number of materials tested, none of these has exhibited the capability of being formed into optically suitable imaging layer with both low cost of manufacture and with high performance reliability. In particular, the goals of economically achieving good sensitivity, high signal-to-noise ratio and exceptionally smooth surface characteristics have heretofore not been achieved.
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