US 3861914 A
A holographic recording medium comprising a conductive substrate, a photoconductive layer and a room temperature vulcanizable silicone elastomer layer, which elastomer cross-links to a thermoset condition upon application of an electric field from a corona discharge.
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Description (OCR text may contain errors)
I United States Patent [1 1 [111 3,861,914
Gange Jan. 21, 1975  PERMANENT HOLOGRAPIIIC RECORDING 3,716,359 2/1973 Sheridon i. 96/15 MEDIUM  Inventor: Robert Allen Gange, Belle Mead, Primary EXami'1erDa\/id Klein N J Assistant Examiner-John L. Goodrow Attorney, Agent, or Firm-Glenn H. Bruestle; Birgit E.  Assrgnee: RCA Corporation, New York, NY. Morris  Filed: Jan. 15, 1973  Appl. No.: 323,747  ABSTRACT A holographic recording medium comprising a con- U-S- .1, H ductive ubstrate a photoconductivc layer and a room temperature vulcanizable ilicone elastomer layer Fleld of Search H elastomer cross links to a thermoset ondition upon application of an electric field from a corona dis-  References Cited charge UNITED STATES PATENTS 2/l97l Urbach 96/l.l
8 Claims, 2 Drawing Figures PATENTEBJANZI 191s lie. I
PERMANENT IIOLOGRAPHIC RECORDING MEDIUM The invention described herein was made in the performance of work under a NASA contract and is subject to the provision of section 305 of the National Aeronautics and Space Act of 1958, Public Law 85-568 (72 Stat. 435; 42 U.S.C. 2457).
This invention relates to a holographic recording medium. More particularly, this invention relates to a recording medium which forms permanent phase holograms after exposure to a corona discharge.
BACKGROUND OF THE INVENTION Phase holograms can be formed on a heat softened thermoplastic surface which selectively deforms during exposure to an applied charge pattern, as has been disclosed by Urbach in U.S. Pat. No. 3,560,205. According to this system, a corona discharge device connected to a suitable recording medium ionizes the air near the surface of the thermoplastic, whereupon positive ions are deposited uniformly on the surface of the thermoplastic. This surface is now exposed to an image by means of coherent light split into an object beam and a reference beam in known manner. The light interacts with a photoconductor which causes a redistribution of the charge in the areas where it impinges on the thermoplastic. When the thermoplastic is charged again, the electric field increases in the previously illuminated areas. The thermoplastic is then exposed to a temperature sufficient to soften the surface which deforms according to the electric field, becoming thinner or forming valleys in the areas of high field intensity. When cooled to room temperature, a hologram is recorded as a thickness variation or relief pattern in the thermoplastic. This process produces phase holograms of excellent quality and resolution.
Such holograms can be erased in the absence of exposure and corona discharge by heating the thermoplastic above its softening point to a temperature sufficient to allow the surface tension of the thermoplastic to revert to its undeformed state.
The use of known thermoplastic recording media has several drawbacks when a permanent holographic record is desired. The materials which are easily deformed during recording often have low melting points, and are tacky, soft materials which are readily damaged in handling or by contact with dust particles. Further, inadvertent exposure to elevated temperatures will, of course, erase the holograms as noted above. Folger et al., in U.S. Pat. No. 3,565,978, have disclosed a method of replicating holograms to form permanent recordings, but this method involves several steps and is cumbersome and expensive.
SUMMARY OF THE INVENTION I have found that by proper choice of the recording surface, deformation phase holograms can be recorded in permanent form. Such recording surfaces deform to form the holographic image pattern and simultaneously cross link to a thermoset condition upon exposure to an electric field from a corona discharge.
BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional view of one embodiment of a recording medium of the invention and FIG. 2 is a cross-sectional view of another embodiment of a recording medium of the invention.
DETAILED DESCRIPTION OF THE INVENTION The recording medium most suitable for use in the present invention comprises a conductive substrate, a photoconductive insulating layer over the conductive substrate and a room temperature vulcanizable silicone resin over the photoconductive layer which silicone cross-links in the presence of a corona discharge.
The conductive substrate is conventional and can be flexible or rigid. It can be made of a conductive metal. such as aluminum, brass, copper and the like; or of a non-conductive substrate coated with a thin conductive layer. Suitable non-conductive substrates include glass, quartz, plastics and the like, which can be coated with a conductor such as tin oxide, copper iodide, indium oxide and the like. Preferably, the conductive substrate is transparent. Such substrates are well known and include glass coated with a thin transparent tin oxide or indium oxide film or like-coated polymeric films of polyethylene terephthalate, polycarbonates, acrylics, polyurethanes and the like.
The insulating photoconductor is also conventional and can be inorganic, such as a layer of armorphous selenium, or pigments such as cadmium sulfide, cadmium selenide, zinc sulfide, zinc selenide, zinc oxide, lead oxide, lead sulfide, mercuric sulfide, antimony sulfide, mercuric oxide, indium trisulfide, titanium dioxide, arsenic sulfide, gallium selenide, lead iodide, lead selenide, lead telluride, gallium telluride, mercuric selenide, and the like. Alternatively, the photoconductor can be organic, such as anthracene-3- benzylideneaminocarbazole, poly-N-vinylcarbazole, 2,5-bis(p-aminophenyl-l )-l ,3,4-oxadiazole, 1,4- dicyanonaphthalene, 2,4-diphenylquinazolin, lmethyl-2-(3,4-dihydroxymethylenephenol)benzimidazole and the like. These and other organic photoconductors can be complexed with Lewis acids, such as 2,4,7-trinitrofiuorenone, as is known. High sensitivity photoconductors such as poly-N-vinylcarbazole containing 2,4,7-trinitrofluorenone are preferred.
The room temperature vulcanizable (RTV) silicones useful in the recording layer are elastomers which cross-link to a thermoset condition upon exposure to a corona discharge. The preferred silicones are transparent, colorless resins of dimethyl silicones or dimethyldiphenyl silicones. Suitable commercial products include RTV 619A and RTV 602 silicones of General Electric, which require the addition of a curing agent; and 236 dispersions of Dow Corning Corporation and SFl 154 silicone of General Electric which do not require the addition of a curing agent. Resins having a very low viscosity can be employed as is, such as the SFll54 resin. Higher viscosity resins can be diluted with a suitable organic solvent.
An alternate recording medium can be provided whereby a silicone resin as described above contains a photoconductive material dissolved therein. This composition can be applied directly to the conductive substrate. Soluble photoconductors include polyhalogenated aliphatic hydrocarbons such as iodoform, carbon tetrabromide, methylene iodide, tetraiodoethylene and the like.
The silicone resins can be applied in any conventional manner, such as by spraying, dip coating, brushing and the like but is preferably applied by spin coating. The silicone layer must be thick enough so that well defined holographic patterns may be formed in it upon exposure, but if the silicone layer is too thick, exposure will not extend through the entire layer, leaving a soft, gel-like portion adjacent to the photoconductor layer. The preferred thickness is about 1 micron, but optimum thickness can vary depending on the strength of the corona discharge.
In the case where a reactive curing agent must be added to the silicone resin to effect curing, the resultant mixture may adversely affect the photoconductive layer and a barrier layer may be required between the photoconductive layer and the recording layer. A thin layer of a transparent polymeric material can be applied over the photoconductive layer. An acrylic resin commercially availabel as Elvacite 20l3 resin from du Pont de Nemours & Company has been found to be excellent for this purpose. This resin is a low molecular weight methyl/n-butylmethacrylate copolymer having an inherent viscosity of 0.2 determined from a solution containing 0.25 gram of polymer in 50 ml of chloroform at 25C.
A defect free, thin film of the barrier polymer can be applied by dipping the photoconductor coated substrate into a solution of the acrylic resin or by spin coating in known manner. The thickness of the barrier layer, although not critical, should be regulated so that an excessive amount of applied electric field will not be lost across this layer. Suitably, the barrier layer can be up to about 0.25 micron in thickness.
Alternatively, a cross-linked polystyrene layer applied by glow discharge technique can be employed as the barrier layer.
Referring now to the drawings, in FIG. 1 there is shown a cross-sectional view of a recording medium of the invention. The recording medium comprises a conductive layer 1 having a photoconductive layer 2 thereon and a cross-linkable silicone layer 3 over the layer 2.
FIG. 2 is a cross-sectional view of another embodiment of a recording medium. This recording medium has a conductive layer 4, a photoconductive layer 5 thereon, a thin barrier layer 6 on the layer 5 and a cross-linkable silicone layer 7 on the barrier layer 6.
The recording media described herein have high write sensitivity and high readout efficiency for the storage of information useful when permanent holographic information is to be recorded. For example, the recording media can be employed to record identifying information for security purposes in permanent form. The recording media can, of course, be coated with a protective finish to prevent abrasion with consequent distortion or loss of the recorded holographic information if desired.
The invention will be further illustrated by the following examples but it is to be understood that the invention is not meant to be limited to the details described therein.
ln the examples, parts and percentages are by weight unless otherwise noted.
EXAMPLE 1 A glass substrate coated with a thin, transparent indium oxide layer was dipped into a solution of poly-N- vinylcarbazole: trinitrofluorenone (10:1) in 1:1 pdioxanemethylene chloride so as to apply a layer about 1-2 microns thick.
Another solution was prepared containing 0.2 part of Elvacite 2013 in 5 parts by volume of acetone warmed to 60C. to which 50 parts by volume of warm ethanol was then added. The coated glass substrate was immersed in the above solution and withdrawn at a rate of 2 inches/sec. A uniform barrier layer about 1,000A thick was deposited onto the photoconductive layer.
A third solution was prepared by mixing 3.14 parts of a room temperature vulcanizable silicone resin commercially available as RTV 619A with 0.31 part of hardener for the silicone, and immediately adding 40 parts by volume of n-hexane. This solution was spin coated at 1,800 rpm using a Preco whirler. The resultant layer was about 1 micron in thickness.
Holograms were formed in the recording medium prepared as above following the general procedure of U.S. Pat. No. 3,560,205, with a helium-neon laser using a continuous corona discharge. The recording medium cross linked during discharge to form permanent holograms about one half micron deep.
EXAMPLE 2 The procedure of Example 1 was followed except using a different silicone layer as follows: 2.5 parts of a silicone resin available commercially as RTV 602 was mixed with 0.25 part of catalyst and then 50 parts by volume of n-hexane immediately stirred in. The resultant layer applied to the barrier layer was about 1 micron in thickness.
Permanent holograms were formed in the exposed recording medium.
EXAMPLE 3 The procedure of Example 1 was followed except that the barrier layer was omitted and a different silicone layer was applied as follows: a mixture was prepared containing 1 part by volume of silicone resin 236 dispersion commercially available from Dow Corning Company and 5 parts by volumeof n-hexane. This mixture was spun onto the coated substrate at 1,000 rpm to form a layer about 1 micron thick.
Permanent holograms were formed in the exposed recording medium.
EXAMPLE 4 The procedure of Example 3 was followed except that a different silicone layer was applied as follows: a dimethyldiphenyl silicone fluid commercially available as SF-1154 from General Electric was applied dropwise to a coated substrate so as to form a layer about 1 micron thick.
A high quality permanent hologram was formed in the exposed recording medium.
1. A medium for recording permanent phase holograms in the form of a surface relief pattern comprising in sequence an electrically conductive substrate, a photoconductive layer and an electrically alterable storage layer, wherein the storage layer comprises a room temperature vulcanizable silicone elastomer which crosslinks upon application of a corona discharge to a thermoset condition.
2. A medium according to claim 1 wherein the silicone layer is about 1 micron thick.
3. A medium according to claim 1 wherein the conductive substrate is transparent.
4. A medium according to claim 1 wherein the photoconductive layer is poly-N-vinyl carbazole.
8. A medium for recording permanent phase holograms in the form of a surface relief pattern comprising in sequence an electrically conductive substrate and a recording layer, the recording layer comprising a room temperature vulcanizable silicone elastomer containing dissolved therein a polyhalogenated aliphatic hydrocarbon photoconductor which cross-links upon application of a corona discharge to a thermoset condition.