Continuing advances in the technologies supporting wideband communications and information handling are leading to extremely large volume, wide bandwidth digital data systems. Until recently conventional storage techniques, through evolutionary product improvements, have been able to keep pace with the growth. However, a point has been reached where moderate technical improvements are becoming economically impractical. Recognizing these and other problems inherent to this specialized application of wideband, high density recording, RADC has undertaken through its exploratory and advanced development programs to satisfy two specific areas: First, to improve and expand upon the technology base that exists in conventional and nonconventional magnetic recording to overcome several of its deficiencies; second, in those instances where magnetic recording is not feasible due to bandwidth or packing density limitations, RADC has exploited alternative approaches. Techniques ranging from laser and electron beam on film to laser holography and optical digital disk are under investigation or in various stages of development.
In the course of time we have investigated different kinds of materials which change their optical properties after exposure to laser light. Our main effort, however, has been concentrated on films of crystalline Te and its amorphous compounds. These materials were also used for experiments on the mechanism of hole opening.
There are many high density optical recording systems that require storage media capable of high recording and playback performance. In this paper, the general requirements which govern the selection, design, and optimization of the recording media are described. The use of a high power diode laser as the recording and playback source is also described.
The change in the ablative writing sensitivity of Te films due to SiO2 and polymethyl methacrylate overcoats of various thicknesses were measured. These experimental data were compared to results of thermal calculations. We found that thermal losses of the absorbed laser energy to the overcoat layer is usually not negligible even for very thin (≤5500 Å thick) overcoat layers. In addition to thermal effects which degrade the writing sensitivity of Te films, we found that rigid overcoats combined with rigid substrates can constrain the ablative writing process of Te and hence cause further degradation in the writing sensitivity and in the read-during-write signal.
We discuss here some of the basic issues involved in the development of an optical storage medium based on the laser micromachining of thin tellurium films. Results are presented in the areas of recording sensitivity, materials resolution, error statistics, and archival stability. It is shown that a medium using 20-30 nm of tellurium deposited on polymethyl methacrylate substrates is an extremely attractive choice.
The objective of this paper is to present the concept, description, and characteristics of a new class of direct-read-after-write (DRAW) reflective laser recording material which has been given the tradename DREXONTM. Information is recorded by thermal melting of the media surface. The material consists of an organic film containing dispersed metal particles which have such a high volume concentration that the surface has a mirrorlike appearance. However, the particles do not touch one another; and, therefore, the surface is electrically nonconductive. When recording, the laser-beam energy is absorbed by the metal particles, which rise in temperature and cause melting of the organic film at temperatures in the range of 200°C. This melting creates spots of low reflectivity in a field of high reflectivity. The metal particles do not melt since their melting temperature is much highter than that of the underlying organic film. It is possible that one version of the media could achieve archival life of a hundred years. This version comprises three ingredients, all of which have rated archival lives of hundreds of years: (1) cross-linked, photographic-quality gelatin, (2) filamentary silver crystals, and (3) spheroidal silver crystals. This archival version of the media is produced from special, fine-grained silver-halide emulsions.
Bilayer structures consisting of organic dye/polymer binder systems solvent-coated on reflective substrates are useful for real-time high-density optical recording. Information is recorded by using focused actinic light that is highly absorbed in the vicinity of the surface of the dye/binder layer. Recorded marks consist of steep-walled flat-bottomed depressions or "pits" that have depth designed to impart a phase shift of π /2 to a focused readout light beam having a wavelength at which the dye/binder layer is essentially transparent. The optical and physical characteristics of these structures that lead to good recording sensitivity and high levels of playback performance are discussed.
In this review, the utility of polymers as disk substrates, surfacing/subbing layers, protective overcoatings, and ablative imaging layers in optical recording media is exemplified. Each of these applications may require specific different polymers. Amorphous polymers such as poly(methylmethacrylate) and poly(4-methylpentene-1) may be particularly advantageous as disk substrates. Specialty polymers may find application in surfacing/subbing layers which are used to perfect the disk surface and to provide an appropriate surface for the deposition of the imaging layer. Crosslinked siloxane elastomers have been widely used as protective overcoatings for ablative imaging layers. Depolymerizable polymers, polymers which solublize dyes, and polymers which are effective in the formation of homogeneous colloidal pigment or metal dispersions are essential in a promising new class of optical recording media, polymer/absorber composites.
There is current interest in developing optical storage materials that can be written with GaAlAs lasers. Dyes which absorb strongly at those wavelengths are potential candidates for this application due to their attractive thermal properties. Through optical and thermal modelling, the properties that are necessary if they are to be writeable at energies of ≤1 nJ are examined. A specific class of infrared absorbing dyes, squarylium, is discussed and preliminary data on optical characteristics, writing energy, and stability are presented.