WO2001037266A1 - Three dimensional data storage device and method for reading - Google Patents

Abstract

A method for reading a three-dimensional data storage device, including a) providing a data storage medium constituting a three-dimensional matrix and a plurality of dye molecules dispersed therein, wherein the dye molecules are capable of a fluorescence change induced by multiple-photon excitation; b) inducing a fluorescence change of the dye by multiple-photon excitation under conditions effective to write an information code in a selected portion of the medium; c) inducing one-photon excitation in the fluorescence-changed dye; d) detecting a fluorescence emission in the one-photon excited dye portion; and e) correlating the fluorescence with the dye molecules contained in the selected portion that are detectably altered effective to retrieve the information code is disclosed. The process can be repeated to write multiple layers of information. The data storage methods and media are particularly useful for storing or archiving a series of three-dimensional images or information in the form of barcodes, medical bracelets, and identification tags. Methods for reading data stored in the data storage media using confocal microscopy are also disclosed.

Description

THREE DIMENSIONAL DATA STORAGE DEVICE AND METHOD FOR READING

This application claims the benefit of U.S. Provisional Patent Application Serial No. 60/165,953, filed November 17, 1999.

This invention was made through the support of the U.S. Air Force Office of Scientific Research/BMDO (Grant No. F49620-97-10529). The Federal Government may retain certain rights in the invention.

FIELD OF THE INVENTION

The present invention relates to methods and media for storing and reading data generally and, more particularly, for reading and storing data in three dimensions.

BACKGROUND OF THE INVENTION

The need for data storage and processing has been increasing at a high rate. Moreover, the need to store, access and rapidly retrieve mass amounts of medical and military information is well recognized. As the threats against military personnel become more and more diverse in today's changing world, there has been an exponential increase in the need to carry this out. A more efficient way to carry mass amounts of information in a convenient, durable device not affected by chemical or environmental exposure is highly desirable. Such a device must also be secure and encoded; readable only by authorized personnel.

In medicine and the military, the potential to store mass data on medical bracelets and dogtags, which could be secure and instantly accessible would be highly desirable. This new generation of dogtags could be a tremendous advance in the field of emergency medical care, allowing authorized medical personnel immediate access to an individuals entire health history in critical field situations.

In response to this need, significant advances in memory design have been made. Two major considerations which impact the desirability and utility of various memory devices are cost per bit of information stored and access time. For example, conventional magnetic tape storage costs 10"⁵ eVbit and has an access time of 100 seconds. Disk, drum, and core storage have considerably faster access times (300 msec, 10 msec, and 1 μsec, respectively) and considerably higher costs (0.05, 0.01, and 2 /bit, respectively). Semiconductor storage devices offer yet faster access times (100 nsec) but at still higher cost (20 /bit).

Optical data storage systems have access times of 10 nsec and costs which range from 10"⁴ to 10^-3 eVbit. Conventional two-dimensional optical data storage can register information at about 10⁸ bits per square centimeter using visible or infrared wavelengths at the diffraction limit. In view of the increasing need for still less expensive data storage systems with still faster access times, and recognizing that cost and access time is governed in large measure by the density of the stored data, efforts have focused on increasing data storage density.

It has been proposed that by writing and reading data in a three- dimensional format, data storage densities of greater that 10¹² bits per cubic centimeter could be achieved. U.S. Patent Nos. 4,466,080 and 4,471,470 to Swainson et al. (collectively "Swainson"), for example, disclose the use of two intersecting beams of radiation which are matched to selected optical properties of an active medium to form and to detect inhomogeneities. In such a system, a stack of two-dimensional planar bit arrays effectively multiplies data density by the number of planes in the third dimension. In media which are linearly photoactive, the primary difficulty with such a scheme is cross-talk between planes. However, writing with three-dimensional resolution in thick media can be accomplished by using media which are non-linearly photoactive. Consider, for example, a focused Gaussian beam well below the saturating intensity, incident on a physically thick but optically thin absorbing sample. In the case where the optically active medium is linear, the same amount of energy is absorbed in each plane perpendicular to the axis of the incident beam, irrespective of the distance from the focal plane, because the net flux passing through each plane is approximately the same. Since the photoactivity is of a linear photoactive medium is proportional to absorption, planes above and below the particular plane being addressed are strongly contaminated. Where the photoactive media is quadratically dependent on intensity, however, net excitation per plane falls off with the inverse of the square of the distance from the plane being addressed. Therefore, information can be written in the plane being addressed without significantly contaminating adjacent planes, if the planes are sufficiently spaced. Several approaches to three-dimensional optical data storage have been investigated. These include: holographic recording on photorefractive media (Poch, Introduction to Photorefractive Nonlinear Optics. New York:John Wiley and Sons (1993) and Gunter et al., eds., Topics in Applied Physics, Vols. 61 & 62 Photorefractive Materials and Their Applications I and II, Berlin: Springer-Verlag, (1989 (Vol. 61) and 1990 (Vol 62))); hole burning (Moerner. ed.. Persistent Spectral Hole Burning: Science and Applications Berlin: Springer (1987)), and photon echo (Kim et al., Opt. Lett.. 14:423-424 (1989)).

U.S. Patent No. 5,289,407 to Stickler et al. employs confocal microscopy to write information in a three-dimensional two-photon active liquid acrylate ester blend photopolymer as submicron volume elements of altered index of refraction. Each element is either written (characterized by a changed index of refraction) or unwritten (characterized by an unchanged index of refraction). The pattern of inhomogeneities in the three-dimensional photopolymer are then detected by differential interference contrast or confocal microscopy. The writing speed is slow (on the order of 10 ms), although significant improvement is said to be possible. A more fundamental disadvantage to the method, however, is the need to use a light in the blue region of the visible spectrum to read the stored data. Most polymers have reduced transparency in the blue region, and, consequently, the use a blue read light limits the depth at which data can be read.

Two-photon based data storage in polymer systems have also been described in Parthenopoulos et al., "Three-dimensional Optical Storage Memory," Science. 245:843-845 (1989); Parthenopoulos et al., "Two-photon Volume Information Storage in Doped Polymer Systems," J. Appl. Phys., 68:5814-5818 (1990); Dvornikov et al., Accessing 3D memory Information by Means of Nonlinear Absorption," Opt. Comm., 1 19:341-346 (1995); U.S. Patent No. 5,268.862 to Rentzepis; and U.S. Patent No. 5,325,324 to Rentzepis et al. In these systems, two beams (532 nm and 1064 nm) were made to intersect in the bulk of the polymer sample containing a spirobenzopyran dispersed therein. At the point of intersection, the spirobenzopyran undergoes two-photon absorption and transformation to a form which fluoresces when excited by two 1064 nm photons. Each data point could assume one of two states (exposed or unexposed), and, in this manner, data was stored as an three-dimensional array of binary information. The lifetime of the transformed state of the sprirobenzopyran was on the order of minutes at room temperature and on the order of days in dry ice. These lifetimes, though suitable for some applications, do not meet the lifetime requirements of many data storage applications.

U.S. Patent No. 5,912,257 to Prasad et al. discloses a method for recording data in a three-dimensional matrix, including a plurality of dye molecules. A first volume element in the three-dimensional matrix is exposed to actinic radiation for a duration and at an intensity effective to alter detectably a fraction between 0.3 and 0.7 of the dye molecules contained therein. The detectably altered dye molecules are substantially uniformly dispersed in the first volume element. The method provides a three-dimensional matrix including a plurality of dye molecules having the formula:

wherein D is an electron donating group;

Q is an electron acceptor selected from the group consisting of electron acceptors having the formulae:

and

W is an electron accepting group, R³ is selected from the group consisting of substituted or unsubstituted alkyl or substituted or unsubstituted aryl moieties, n is an integer from 0 to 4,

A, B, and C are substituents of their rings and are each independently selected from the group consisting of alkyl, alkoxy, hydroxyalkyl, sulfoalkyl, carboxyalkyl, and hydrogen, and Y is a counterion

dispersed in the matrix. When Y is an anion, such as when A and B are hydrogen, D is -NR'R², Q is -(C₅H₄N)-R³. and R¹, R², and R³ are unsubstituted alkyl or hydroxyalkyl, Y is preferably tetra-substituted borate, such as tetramethylborate, tetrapropylborate, tetratolylborate, diphenyldimethylborate, diphenylditolylborate, or, more preferably, tetraphenylborate. The solubility of the styryl compound in the matrix may be modified by altering the nature of R¹, R², and R³ Generally, the greater the polarity of the groups and the greater the number of polar groups, the more soluble the dye in the matrix. In many applications suitable solubility is achieved where A and B are hydrogen, and where R¹, R², and R³ are selected from the group consisting of unsubstituted alkyl hydroxyalkyl, sulfoalkyl, and carboxyalkyl. Where n is 0, D is -NR^]R², and Q is -(C₅H₄N)-R³, preferred compounds are those where A and B are hydrogen, R² is unsubstituted alkyl, and R¹ and R³ are selected from the group consisting of hydroxyalkyl, sulfoalkyl, and carboxyalkyl.

The disclosed data storage methods and media have approximately

10¹² volume elements per square centimeter. Each of the volume elements can store a single bit, digital information of approximately 8 bits, or analog information.

Because of its ability to store analog data, such as grayscale value or color density values, these methods and media are particularly useful for storing or archiving a series of two-dimensional black and white or color images, such as frames of a movie. However, data stored in accordance with this method can be read only with the use of two-photon excitation lasers. These lasers are not easily portable and difficult to use in the field.

For these and other reasons, the need exists for data storage media having the capacity to store information in three dimensions which is secure, more easily and quickly accessible and means to read such information are easily portable.

SUMMARY OF THE INVENTION

The present invention relates to a method for reading a three- dimensional data storage device, including: a) providing a data storage medium including a three-dimensional matrix and a plurality of dye molecules dispersed therein, wherein the dye molecules are capable of a fluorescence change induced by multiple-photon excitation; b) inducing the fluorescence change of the dye by multiple-photon excitation under conditions effective to write an information code in a selected portion of the medium; c) inducing one-photon excitation in the fluorescence-changed dye; d) detecting a fluorescence emission in the one-photon excited dye portion; and e) correlating the fluorescence with the dye molecules contained in the selected portion that are detectably altered effective to retrieve the information code. The information code can be written on multiple levels within the three-dimensional data storage device.

Another advantage of the present invention is the ability to provide compact or hand held readers which is made possible by the use of diode lasers for single photon excitation.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic diagram illustrating a method according to the present invention for storing data.

Figure 2 is a schematic diagram illustrating a method according to the present invention for reading data.

Figure 3 A-D show 3-dimensional bar codes written using a two- photon process inside a dye doped PMMA block and read back using single photon excited read back system.

Figure 4 shows read back images using single photon confocal detection.

Figure 5 shows the original scanned photograph A and the image B recovered from the storage media.

Figure 6 shows a visualization of medical information storage such as an entire medical history (diagnostic tests, X-ray, MRI, fingerprints, dental records, etc.) on a bracelet.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a three-dimensional storage device and method of preparation thereof. The present high capacity data storage device has been developed using a two-photon approach with highly efficient synthesized molecules. This approach allows for 3-D storage of data in compact plastic media. As described further herein, this technology can be extended to a 3-D identification (bar code) system where information can be written in a more secured form. A three dimensional bar code writer/reader system has been developed. This involves writing multiple layers of bar codes in a dye-doped polymer matrix, using a highly localized two-photon process. This allows bar codes to be read using a scanning laser beam coupled with confocal detection of single photon fluorescence. This technique can also be used to write multiple layers of photographs or images (X-rays, fingerprints, immunization record, etc.) for security identification purpose. We have also demonstrated the ability of this technique to write multiple layers of photographs or images for security identification purpose. This can be used to make security identification labels with secret identification codes well concealed deep inside the label. Also, the method of the present invention is suitable for producing medical bracelets and military dogtags. The storage matrix can be in any form, for example, in the form of a polymer block or a thin polymer sheet which can be used as adhesive security label. The fact that the images/barcodes are written inside the storage matrix provides the advantages of a scratch proof and tamper proof security/identification tags.

Thus, the present invention enables the making of secure and flexible medical bracelets and dogtags. When used as medical bracelets and dogtags, highly efficient two-photon chromophores can be produced for this specific application. A wide variety of dye materials can be infiltrated into many different kinds of polymeric materials. A system was developed for writing information into the storage matrix using two-photon technology. This writer is comparatively expensive and bulky but can be used in a centralized facility where individual dogtags, medical bracelets, and the like, are encoded. The reader can be a durable, portable system carrying an inexpensive solid state laser, coupled, for example, with a palm PC. Additionally, software for reading by a wide variety of readers can be easily developed or adapted by a person skilled in the art. Accordingly, the recognition and use of diode lasers for single photon excitation in accordance with the present invention has made possible the ability to provide compact or hand held readers previously unavailable. In this method a tightly focused pulsed IR beam changes the optical properties of the polymer medium doped with two-photon absorbing dyes at the focused spot. The optical change can be change in absorption wavelength or change in emission wavelength, or photo-bleaching of the dye or even physical change like shrinkage in the medium due to polymerization. To demonstrate this technique we used Poly-methyl methacrylate (PMMA) doped with dye AF240 (7-benzothiazol-2- yl-9,9-diethylfluoren-2-yl)diphenylamine) developed at Airforce Research Laboratory at Dayton, as the barcode storage medium. A tightly focused pulsed IR beam (A TkSaphire laser operating at 800 nm with pulse width 80fs and a repetition rate of 90MHz as light source and a high NA objective for focusing) was used to write barcodes in to the dye doped polymer matrix mounted on a computer controlled scanning stage. Software and hardware was developed in accordance with known practices to convert the computer generated images or barcodes to be written in to the matrix. The alpha numeric characters were converted into barcode images in bitmap format using commercially available barcode generating software. Software was used to read this bitmap image format, translate this image into XY stage motion, and control the optical shutter and focusing motor (Z stage) when writing the information into the storage medium. In the case of the dye referred to as AF240, the written spot's linear absorption and fluorescence properties are red shifted compared to unwritten region. Here the written spot shows an absorption near 500 nm and emission around 570 nm making it possible to use a readback system constituting of a 488 nm line of argon laser as the excitation source and confocal detection of the emission at 570 nm. Similar techniques were used to write barcodes into different polymer/dye composites utilizing change in emission of the dye or two-photon induced polymerization or refractive index changes.

Using this technique we were able to write multiple layers of barcodes/images in a single polymer block at a vertical separation of 10 microns, and up to a depth of couple of hundred microns. This technology will enable individuals to be able to carry their entire medical history (diagnostic tests, X-ray, MRI, fingerprints, dental records, etc.) on a card, wristband, or dogtag and security identification labels with secret identification codes well concealed deep inside the label. Figure 6 shows another embodiment, where medical information can be encoded into a few millimeter area cross section of a plastic sheet having less than a millimeter thickness. Using highly efficient two-photon absorbers, this technology could be particularly useful for military personnel, where entire medical files and 201 files could be stored in dogtags. These devices could be easily read in field situations using hand held laser readers. The data can be stored digitally, below the surface of the device to protect the integrity of the information from surface damage. This information would also be secure due the 3-D nature of the storage media and the wavelength specificity of the reader, making the information accessible only to authorized personnel.

To make PMMA blocks, the dyes were dissolved in the monomer (MMA) and then polymerized to provide the dye doped polymer block. Many other kinds of polymer sheet materials (e.g., polyester, polyethylene) are also suitable for use in the present invention. Preformed plastic sheets, and bulk materials can be infiltrated with dyes by a simple process of controlled diffusion. This approach has been successfully applied to various polymers like acrylic, polycarbonate, terephthalate, polyethylene, polystyrene and polyvinylacetate systems without causing any change in the physical properties of the polymer. As the change in fluorescence is a property of the dye, any kind of organic or inorganic matrix to incorporate dye can be used. Thus, the choice of storage matrix is very flexible.

Suitable polymers are disclosed in U.S. Patent No. 5,912,257 to Prasad, et al., which is hereby incorporated by reference in its entirety.

Suitable dyes useful in the present invention include any dye having good two-photon absorption and showing a change in its fluorescence properties after two-photon absorption, which then can be excited by one-photon absorption and gives an emission at a wavelength other than the one-photon absorption wavelength. From a series of dyes (AF240 is a member of this series) developed at Airforce Research Laboratory at Dayton, most of the dyes had this property at varying levels. Similarly, some dyes developed in our laboratory (e.g., APSS 4-[N- (2-hydroxyethyl)-N-methyl) amino phenyl]-4-(6-hydroxyhexyl sulfonyl)stilbene also show similar properties to a lesser extent. Suitable dyes are disclosed in U.S. Patent No. 5,912,257 to Prasad, et al., which is hereby incoφorated by reference, in its entirety.

For example, a tightly focused pulsed IR beam (A Ti:Saphire laser operating at 800 nm with pulse width 80fs and a repetition rate of 90MHz as light source and a high NA objective for focusing) was used to write barcodes in to the dye doped polymer medium mounted on a computer controlled scanning stage. The writing process utilizes two-photon excitation induced fluorescence change of the dye for storing information, such as barcodes. In two-photon excitation, simultaneous absorption of two photons at a higher wavelength ( lower energy) occurs which is equivalent to the absorption of a single photon at a lower wavelength (higher energy). For e.g, in case of Dye AF240, simultaneous absorption of two photon occurs at a wavelength of 800 nm, which is equivalent to the absorption of single photon at 400 nm and fluoresces at 490 nm. After the two photon excitation, the dye changes its absorption and emission properties to give an emission at around 580 nm when excited at around 488 nm. Any material which has similar shift in absorption and emission properties can be used as the dye material. Further this can be extended to use three photon or any other multi-photon process, as the resolution increases with higher order multi-photon processes. The only draw back in these cases is that the resolution of single photon confocal read back cannot be increased along with the resolution of higher order multi-photon writing. Even sequential two-photon absorption induced optical changes in the media may be used for the same purpose. Sequential two-photon absorption, in which the absorption of two photons occur through a non-virtual intermediate energy level doesn't require high peak power pulsed lasers. This in turn introduces the possibility of using this process with a CW laser for writing information.

Barcodes generated using a commercially available software, as shown in Figure 3, was used as an input into software which controls the XY stage, focus motor and the laser shutter to write the barcodes into the storage media. Commercially available software converts the alpha numeric characters into a barcode image in bitmap format which in turn is read by software. This software reads this image and translates this information as the motion of the XY stage and controls the focusing motor (Z stage) and optical shutter to write the data into the storage medium.

A commercial confocal microscope is shown in Figure 2 as the readback system with 488 nm line of a Kr:Argon laser as the excitation. This read back system can be custom made for this purpose without using a bulky commercial microscope. Instead of Kr: Argon laser, a commonly available diode laser giving out 980 nm light can be frequency doubled to get 490 nm excitation for the reader. Recent developments in the diode laser technology have made available 490 nm diode lasers, which can be used for this reader. Reading is done with a galvanometer mirror scanners, which scans the written layer with the excitation light, and detecting the fluorescence signal through a confocal aperture. Different layers of written layers can be selected by focusing the excitation light at different layers, using a focus motor (stepper motor). Furthermore, the reader can be customized for a specific wavelength, which will be invisible without knowing the correct readout excitation wavelength.

The present invention provides improvements over currently available technologies which include the following. Information is not stored on the surface of the media but rather within the media, which protects the integrity of the information. The present invention enables the use of any polymer for data storage. Multiple layers of barcodes can be written one below another. In contrast, current technologies can store information only in two-dimensions. The present invention enables customization of the excitation and emission wavelengths of the storage media by properly selecting the dyes, which makes the information more secure.

Figures 3A-D show 3D bar codes written using a two-photon process inside a dye doped PMMA block and read back using single photon excited read back system. Here Figures 3A and 3B show the images of the two layers of bar codes stored inside the polymer block. These images are stored in a 180 μm X 180 μm square area. Figure 3C shows the vertical cross section of the two layers showing positions of these layers inside the storage media. The first layer is at a depth of 30 μm from the surface and the second layer is written at separation of

40 μm from the first layer. Figure 3D shows the three dimensional image recreation of the storage medium with its written bar codes, using optical sectioning of the layers by confocal microscopy.

More specifically, suitable dyes and dye compositions for use in the present invention can include the following:

C 24 H 22 N 2S 398.56

AF 1 83 C29H20N2S - 428.55

Many other compounds which have similar structures also can be used to varying degrees of efficiency. The matrix material can be any material which is capable of dispersing the styryl compound. For example, the matrix can be a liquid in which the compound is either suspended, so as to form stable solid dispersion, such as a colloid, or dissolved. Suitable dispersing liquids include alcohol solvents, such as methanol, ethanol, isopropanol, and n-butanol; ketone solvents, such as acetone, methyl ethyl ketone, and cyclopentanone; ester-containing solvents, such as ethyl acetate and isopropyl acetate; ether solvents, such as tetrahydrofuran, diglyme, and dioxane; chlorinated hydrocarbons, such as methylene chloride, chloroform, and carbon tetrachloride; acetonitrile; pyridine; dimethylformamide; and dimethylsulfoxide. Alternatively, the compounds of the present invention can be incorporated into various polymeric matrix materials to produce compositions which are useful in two-photon pumped cavity lasing, in infrared beam detection, and in optical limiting. That is to say, the styryl compounds, set forth above, may be incorporated into such materials as acrylic and methacrylic polymers, styrene polymers, vinyl halide polymers, cyanoethylated cellulosic materials, aminoplastic resins, polyester resins, cellulose acetate polymers such as cellulose acetate butyrate, etc., nitro cellulose, cellulose propionate, and cured epoxy-type polymers. Examples of polymeric matrix materials which may be used with the styryl compounds of the present invention include: polymers, i.e., homopolymers and copolymers, of polyol(allyl carbonate) monomers, polymers, i.e., homopolymers and copolymers, of polyfunctional acrylate monomers, polyacrylates, poly(alkylacrylates), such as poly(methyl methacrylate), cellulose acetate, cellulose triacetate, cellulose acetate propionate, cellulose acetate butyrate, poly(vinyl acetate), poly(vinyl alcohol), poly(vinyl chloride), poly(vinylidene chloride), polyurethanes, polycarbonates, poly(ethylene terephthalate), polystyrene, copoly(styrene-methyl methacrylate) copoly(styrene-acrylonitrile), poly(vinyl butyral) and polymers, i.e., homopolymers and copolymers, of diallylidene pentaerythritol, particularly copolymers with polyol (allyl carbonate) monomers. e.g., diethylene glycol bis(allyl carbonate), and acrylate monomers.

Blends of the aforesaid transparent polymers are also suitable as matrix materials. Preferably, the matrix material is an optically clear polymerized organic material prepared from a polycarbonate resin, such as the carbonate-linked resin derived from bisphenol A and phosgene, which is sold under the trademark LEXAN; a poly(methyl methacrylate), such as the material sold under the trademark PLEXIGLAS; poly (2 -hydroxy ethyl methacrylate); and polymerizates of urethanes and epoxy materials. The resultant compositions of matter may be formed into such articles as discs, plates, films, rods, and the like, by any known molding, casting, spray drying etc. technique. The various esters of acrylic acid and methacrylic acid which may be used to form the polymers comprising the major constituent of the compositions of the present invention are those having the formula:

O

CH₂ ^C C O R⁵

R⁴

wherein R⁴ is hydrogen or a methyl radical and R⁵ is an alkyl radical having from 1 to 6 carbon atoms, inclusive. Compounds, which are represented by the above formula and, consequently, which may be used in the present invention include methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate. n-butyl acrylate, isobutyl acrylate, t-butyl acrylate, amyl acrylate, hexyl acrylate, methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, t-butyl methacrylate, amyl methacrylate, hexyl methacrylate, 2-hydroxyethyl methacrylate, and the like.

The acrylic and methacrylic acid esters may be polymerized alone or in combination with other ethylenically unsaturated monomers in amounts such that the final polymer has a preponderance of the acrylic or methacrylic acid ester therein, i.e., at least 51%, by weight, based on the total weight of the monomers. Comonomers useful for this purpose are set forth hereinbelow.

The styrene monomers, which may also be employed to produce the compositions of the present invention, are those having the formula R⁶

wherein R⁶ is hydrogen or a lower alkyl radical having 1 to 4 carbon atoms, inclusive, and R⁷ is hydrogen, a lower alkyl radical having 1 to 4 carbon atoms, inclusive, or a halogen radical. Suitable monomers represented by the above formula include styrene, methyl styrene, ethyl styrene, propyl styrene, butyl styrene, chloro styrene, bromo styrene, fluoro styrene, iodo styrene, α-butyl styrene, α-methyl methylstyrene, α-methyl ethylstyrene, α-butyl ethylstyrene, α-ethyl chlorostyrene, α-propyl iodostyrene, and the like.

These styrene monomers may also be polymerized alone or in combination with other ethylenically unsaturated monomers.

The vinyl halide monomers which may be used to produce the compositions of the present invention are well known in the art and generally vinyl chloride is the most practical for reasons of availability and cost. However, vinyl fluoride has become more important in recent years and its use is also contemplated herein. These vinyl halide polymers may be used as pure homopolymers. however, inasmuch as commercially available polymeric vinyl halide resins generally are produced containing minor amounts, i.e., up to about 2.0% of copolymeric material, resins of this sort are also applicable herein. Commercially available poly (vinyl chloride) also, for example, may contain about 1.0% or less of other constituents such as vinyl acetate, in copolymeric form. These polymers are also useful herein. These vinyl halides may additionally be employed with varying amounts of comonomers, generally in amounts as indicated above in regard to the esters of acrylic and methacrylic acids.

Examples of applicable comonomeric compounds which may be copolymerized with the acrylates, styrenes and vinyl halides set forth above in amounts less than about 50%, by weight, based on the total weight of the monomers, include the unsaturated alcohol esters, more particularly the allyl, methallyl, crotyl, 1 -chloroallyl, 2-chloroallyl, cinnamyl, vinyl, methvinyl, 1-phenylallyl, butenyl, etc., esters of saturated and unsaturated aliphatic and aromatic monobasic and polybasic acids such, for instance, as acetic, propionic, butyric, valeric, caproic, crotonic, oxalic, malonic, succinic, glutaric, adipic, pimelic, suberic, azelaic, sebacic, citraconic, mesaconic, itaconic, acetylene dicarboxylic aconitic, benzoic, phenylacetic, phthalic, terephthalic, benzoylphthalic, etc., acids; the saturated monohydric alcohol esters, e.g., the methyl, ethyl, propyl, isopropyl, butyl, sec. -butyl, amyl, etc.; esters of ethylenically unsaturated aliphatic monobasic and polybasic acids, illustrative examples of which appear above, vinyl cyclic compounds (including monovinyl aromatic hydrocarbons), e.g., styrene, o-, m-, and p-chlorostyrenes, -bromostyrenes, -fluorostyrenes, -methylstyrenes, -ethylstyrenes, -cyanostyrenes, the various polysubstituted styrenes such, for example, as the various ditri-, and tetra-chlorostyrenes, -bromostyrenes, -fluorostyrenes, -methylstyrenes, -ethylstyrenes, -cyanostyrenes, etc., vinyl naphthalene, vinyl-cyclohexane, vinyl furane, vinyl pyridine. vinyl dibenzofuran, divinyl benzene, trivinyl benzene, allyl benzene, diallyl benzene, N-vinyl carbazole, the various allyl cyanostyrenes, the various alpha-substituted styrenes and alpha-substituted ring-substituted styrenes, e.g., alpha-methyl styrene. alpha-methyl-para-methyl styrene, etc.; unsaturated ethers, e.g., ethyl vinyl ether, diallyl ether, ethyl methallyl ether, etc.; unsaturated amides, for instance, N-allyl -caprolactam, acrylamide, and N-substituted acrylamides, e.g., N-methyl acrylamide, N-allyl acrylamide, N-methyl acrylamide, N-phenyl acrylamide, etc.; unsaturated ketones, e.g., methyl vinyl ketone, methyl allyl ketone, etc.; methylene malonic esters, e.g., methylene methyl malonate, etc.; ethylene; unsaturated polyhydric alcohol (e.g., butenediol, etc.) esters of saturated and unsaturated. aliphatic and aromatic, monobasic and polybasic acids.

Other examples of monomers that can be copolymerized are the vinyl halides, more particularly, vinyl fluoride, vinyl chloride, vinyl bromide, and vinyl iodide, and the various vinylidene compounds, including the vinylidene halides, e.g. vinylidene chloride, vinylidene bromide, vinylidene fluoride, and vinylidene iodide. other comonomers being added if needed in order to improve the compatibility and copolymerization characteristics of the mixed monomers.

More specific examples of allyl compounds, that can be copolymerized are allyl alcohol, methallyl alcohol, diallyl carbonate, allyl lactate, allyl alphahydroxyisobutyrate, allyl trichlorosilane, diallyl methylgluconate, diallyl tartronate, diallyl tartrate, diallyl mesaconate, the diallyl ester of muconic acid, diallyl chorophthalate, diallyl dichlorosilane, the diallyl ester of endomethylene tetrahydrophthalic anhydride, triallyl tricarballylate, triallyl cyanurate, triallyl citrate, triallyl phosphate, tetrallyl silane, tetrallyl silicate, hexallyl disiloxane, etc. Other examples of allyl compounds that may be employed are given, for example, in U.S. Patent No. 2,510,503, which is hereby incorporated by reference.

Among the monomers which are suitable for use in carrying out the present invention are, for example, compounds such as acrylonitrile, and other compounds, e.g., the various substituted acrylonitriles (e.g., methacrylonitrile, ethacrylonitrile, phenylacrylonitrile, etc.), the various N-substituted acrylamides and alkacrylamides, for instance, N-dialkyl acrylamides and methacrylamides, e.g., N-dialkyl acrylamides and methacrylamides, e.g., N-dimethyl, -diethyl, -dipropyl, -dibutyl, etc., acrylamides and methacrylamides and the like.

The cyanoethylated cellulosic materials employed in the formation of the compositions of the present invention may be prepared from the cellulose of wood pulp or wood fiber after removal of the lignin and the like therefrom. Additionally, α-cellulose flock, regenerated cellulose fibers such as viscose, cotton linters, and natural cellulose materials such as cotton, jute, ramie, and linen may be used in such forms as fibers, yarns, fabrics, raw stock, batting and the like. Additionally, the cellulosic material may be non-fibrous, e.g., in the form of felted or webbed materials. The fibrous forms of the cellulose may be employed in woven or knitted condition. It is also within the scope of the present invention to employ methyl cellulose, ethyl cellulose, and the like as the starting material.

The cyanoethylation of the cellulosic materials may be carried out by reacting the natural or regenerated cellulosic material with acrylonitrile in various ways. The physical properties of the resultant products will vary with the nature of the cellulosic material, its molecular weight, the method of treatment and the like. However, said properties are affected most noticeably by the extent to which the cellulosic material has been cyanoethylated.

The cyanoethylation of the cellulosic material is usually defined in one of two ways, i.e., either by its nitrogen content, expressed in weight percent of nitrogen, or by a decimal fraction representing the number of cyanoethyl groups introduced per anhydroglucose unit. This decimal fraction is usually referred to as the "degree of substitution." Complete cyanoethylation of cellulose generally corresponds to a nitrogen content of about 13.1% or slightly above, and a degree of substitution of about 3. A nitrogen content of at least 10% and a corresponding degree of substitution of about 2.3 is generally present in the most commonly available materials.

At low degrees of substitution, that is, a degree of substitution up to about 2, cyanoethylation does not greatly alter the solubility or the physical appearance of the cellulose, i.e., the fibrous characteristics thereof are generally retained. However, as the degree of substitution increased progressively above 2, the fibrous characteristics of the cellulose gradually diminish and resemblances of the product to a thermoplastic resin, become increasingly apparent. Additionally, the product develops a solubility in certain organic solvents which the cellulosic material did not have.

As mentioned above, substantially any cellulosic material can be utilized in the production of the compositions of the present invention. Cellulose, and some chemically related compounds, are structurally polymers of anhydroglucose, and different polymers are generally classified in terms of the number of anhydroglucose units in a molecule. Chemically, an anhydroglucose unit is a trihydric alcohol, one hydroxyl group being a primary hydroxyl and the other two being secondary. Celluloses are predominately 1 to 4 unit polymers, the number of polymerized units usually being referred to as the degree of polymerization. As with any other polymer, each cellulosic polymer is a mixture of polymers of different molecular weight and it is the average degree of polymerization which determines the classification of the ultimate product. The celluloses used in the present invention generally have a degree of polymerization of at least about 2000, although those celluloses having degrees of polymerization below 2000 are also useful herein. The viscose rayons for example, have a degree of polymerization of from about 250 to 350. Natural cotton has a degree of polymerization of about 850 to 1000 and many wood pulp derivatives have a degree of polymerization in excess of 1000. All these celluloses however, may be used in the practice of the present invention.

The cyanoethylation procedures used to form the starting composition of the present invention do not form part of the instant invention and any known procedure for achieving this result may be employed. One such method is shown, for example, in U.S. Patent No. 2,332,049. which is hereby incoφorated by reference. Additional procedures are shown in U.S. Patent Nos. 2,375,847, 2,840,446, 2,786,736, 2,860,946, and 2,812,999, which are all hereby incoφorated by reference. In general, the procedure for preparing the cyanoethylated celluloses involves reacting a cellulosic material with acrylonitrile in the presence of an alkali and precipitating and washing the resultant cyanoethylated produce. Generally, the amount of acrylonitrile which is used is 10-20 times the amount of cellulosic material being treated. The particular alkali employed is not critical and such materials as potassium hydroxide and sodium hydroxide may be used. A good general procedure is to employ about 2.5 to about 7.0 weight percent of alkali, based on the weight of the cellulosic material.

The aminoplast resins employed in the practice of the present invention are synthetic resins prepared by the condensation reaction of an amino (including imino) or amido (including imido) compound with an aldehyde. Resinous condensates of this type, as well as methods for their preparation, have been shown innumerable times in the prior art, and adequate disclosures of them may be found in, for example, U.S. Patent Nos. 2,197,357, 2,310,004, 2,328,592 and 2,260,239, which are hereby incoφorated by reference. Melamine is a suitable aminotriazine reactant for preparing the heat-curable or potentially heat-curable partially polymerized aminotriazine-aldehyde resinous reaction products which are used in the practice of the present invention, but other aminotriazines, e.g., mono- di-, and tri-substituted melamines, such as the mono-, di- and trimethylmelamines, and the like, guanamines, such as formoguanamine, acetoguanamine, benzoguanamine, and the like, as well as mixtures of aminotriazines, may be utilized as reactants. Similarly, formaldehyde, typically in aqueous solution, is a common aldehyde reactant, but other aldehydes, e.g., acetaldehyde propionaldehyde, butyraldehyde, benzaldehyde, and the like, or compounds engendering aldehydes, e.g., paraformaldehyde, hexamethylenetetramine, and the like, may also be employed. The properties desired in the finished product and economic considerations are among the elements which will determine the choice of the particular aminotriazine and aldehyde employed.

The mole ratio of aldehyde to aminotriazine in such resinous reaction products is not critical, and may be within the order of from about 1.5:1 to about 4:1, respectively, depending on the nature of the starting materials and the characteristics desired in the final product, but it is preferred that the mol ratio be within the order of from about 2: 1 to about 3:1, respectively.

Conventional reaction conditions are observed in preparing the aminotriazine-aldehyde resins, i.e., the aldehyde and the aminotriazine may be heat-reacted at temperatures ranging from about 40° C. to reflux temperature, i.e. about 100° C, for periods of time ranging from about 30 to 120 minutes, at a pH ranging from about 7.0 to 10, preferably in an aqueous medium. Any substance yielding acidic or alkaline aqueous solutions may be used to regulate the pH, for example, alkaline materials such as alkali metal or alkaline earth metal oxides, e.g., sodium, potassium or calcium hydroxide or sodium or potassium carbonate; mono-, di-, or tri-alkylamines, e.g., triethylamine or triethanolamine; alkylene polyamines or polyakylene polyamines, e.g., 3,3'-iminobispropylamine, and the like.

Other amido or imido compounds having at least two aldehyde-reactable hydrogen atoms attached to amidogen nitrogen atoms may also be used in preparing the aminoplast resins used in the present invention. For example, urea and those of its derivatives which have been commonly used in the preparation of aminoplast resinous compositions, such as for example the alkylureas, e.g., mono- and dimethylurea, halourea and the like may be used.

The properties of the thermosetting aminoplast resins can be further modified, if desired, by incoφorating various other substances into the aminotriazine-aldehyde resin. Included among such substances are plasticizers such as the α-alkyl-D-glucosides, e.g., α-methyl-D-glucoside, disclosed in U.S. Patent No. 2,773,848 to Lindenfelser, which is hereby incoφorated by reference, methylol derivatives corresponding to the general formula:

R⁸ X N CH₂OH

wherein R⁸ represents an alkyl, aryl, or aralkyl group, R⁹ represents a hydrogen atom or an alkyl, alkylol, aryl or acyl group, and X represents,

O

e.g., N-methylol p-toluenesulfonamide (which may be formed in situ by the addition of p-toluenesulfonamide to an amidogen-formaldehyde reaction mixture) and the like, or combinations of these glucosides and methylol derivatives, e.g., a mixture of α-methyl-D-glucoside and p-toluenesulfonamide, as disclosed in U.S. Patent No. 2,773,788 to Magrane et al, which is hereby incoφorated by reference.

The aminoplast resinous molding materials may be prepared by first impregnating a fibrous filler, such as chopped α-cellulose, with an aminoplast resin, in syrup form, drying the impregnated material to a low volatile content, usually in the order of about 10% or less, converting the dried material to a fine, fluffy powder while blending it with various commonly employed additives, such as curing catalysts, pigments, mold lubricants, and the like, and finally densifying and granulating the powdered molding composition, thus converting it to a form especially suited for commercial molding techniques and to which the styryl compounds may be added.

The polyester resins employed in the practice of the present invention may be either thermoplastic or thermosetting. They are all relatively well known in the art and are prepared by reacting polycarboxylic acids, or their anhydrides, with polyhydric alcohols. The thermosetting polyesters are prepared using a procedure wherein at least one of the reactive components contains α,β-ethylenic unsaturation. By following this procedure, resinous, essentially linear esterification or condensation products containing a plurality of ethylenically unsaturated linkages distributed along the backbones of their polymer chains are produced.

The use of α,β-ethylenically unsaturated polycarboxylic acids provides a convenient method of introducing ethylenic unsaturation into the polyester resins. It is preferred to employ α,β-ethylenically unsaturated dicarboxylic acids, such as maleic, fumaric, citraconic, γ,γ-dimethylcitraconic, mesaconic, itaconic, α-methylitaconic, γ-methylitaconic, teraconic, and the like, as well as mixtures thereof, but minor amounts of α,β-ethylenically unsaturated polycarboxylic acids containing three or more carboxyl groups, such as aconitic acid and the like, together with the particular α,β-ethylenically unsaturated dicarboxylic acid or acids chosen, may also be used. Whenever available, the anhydrides of any of the aforementioned α,β-ethylenically unsaturated polycarboxylic acids may be substituted for said acids in whole or in part.

Any of the large class of polyhydric alcohols ordinarily used in preparing reactive polyester resins may be employed in the practice of the present invention. While dihydric alcohols, and especially saturated aliphatic diols, are commonly-used co-reactants in the preparation of the polyester resins, it is not mandatory that all of the polyol used be of this type, in that small amounts, e.g., usually up to about 10% of the total equivalents of hydroxyl groups present in the esterification mixture, of polyols having more than two hydroxyl groups may also be employed. Among the dihydric alcohols which may be employed are saturated aliphatic diols such as ethylene glycol, propylene glycol, butylene glycol. diethylene glycol, dipropylene glycol, triethylene glycol, tetraethylene glycol, butanediol-1,2, butanediol-1,3. butanediol-1,4, pentanediol-1,2, pentanediol-1,3, pentanediol-1,4, pentanediol-1,5, hexanediol-1,2, hexanediol-1,3, hexanediol-1,4, hexanediol-1,5, hexanediol-1,6, neopentyl glycol and the like, as well as mixtures thereof. Among the polyols having more than two hydroxyl groups which may be employed in minor amounts, together with the above-mentioned diols, are saturated aliphatic polyols such as glycerol, trimethylol ethane, trimethylol propane, pentaerythritol, dipentaerythritol, arabitol, xylitol, dulcitol, adonitol, sorbitol, mannitol, and the like, as well as mixtures thereof. In forming the thermoplastic polyester resins useful herein, the above alcohols are reacted with non-polymerizable polycarboxylic acids, i.e., acids which are saturated or which contain only benzenoid unsaturation, such as oxalic, malonic, succinic, glutaric, adipic, pimelic, suberic, azelaic, sebacic, malic, tartaric, tricarballylic, citric, phthalic, isophthalic, terephthalic. cyclohexanedicarboxylic, endomethylenetrahydrophthalic, and the like, as well as mixtures thereof.

These saturated acids may be used alone to form thermoplastic resins or in combination with the above-mentioned unsaturated acids in the formation of thermosetting resins in order to impart many beneficial properties thereto. For example, nonpolymerizable polycarboxylic acids having only two carboxyl groups, and no other reactive substituents, may be employed to impart a desirable degree of flexibility which may not be achieved by the use of the α,β-ethylenically unsaturated polycarboxylic acids alone. Where such nonpolymerizable polycarboxylic acids are employed, the amount thereof should constitute at least about 20% but not more than about 80% of the total equivalents of carboxyl groups present in the esterification mixture. Preferably, such nonpolymerizable polycarboxylic acids may be employed in amounts ranging from about 25% to about 75% of the total equivalents of carboxyl groups present in the esterification mixture. Halogenated unsaturated polycarboxylic acids may also be employed in the preparation of the thermosetting polyester resins of the present invention for puφoses of imparting various desirable properties thereto as mentioned above in regard to the saturated acids. Examples of halogenated acids which may be used include monochloro- and monobromomaleic, monochloro- and monobromofumaric. monochloro- and monobromomalonic, dichloro- and dibromomalonic, monochloro- and monobromosuccinic, α,β-dichloro- and dibromosuccinic, hexachloroendomethylene-tetrahydrophthalic, and the like, as well as mixtures thereof. Whenever available, the anhydrides of any of these halogenated acids may also be substituted therefore in whole or in part.

Among the halogenated polyols that may be employed are 2,2'- chloromethylpropanediol-1,3, adducts of hexachlorocyclopentadiene with unsaturated polyols, such as butenediols, pentenediols, and the like, and adducts of hexachlorocyclopentadiene with polyols having three or more hydroxyl groups, one of which is etherified with an unsaturated alcohol reactive with hexachlorocyclopentadiene. Among the latter are compounds such as 3-[l, 4.5,6,7.7- hexachlorobicyclo - (2.2.1) - 5 - hepten-2-yloxyl]-l,2-propanediol. which is the adduct of hexachlorocyclopentadiene with vinyl glycerol ether, 3-[l,4,5,6,7,7- hexachlorobicyclo-(2.2.1)-5-hepten-2-yl]-methoxy - 1,2 - propanediol, which is the adduct of hexachlorocyclopentadiene with allyl glycerol ether, adducts of hexachlorocyclopentadiene with vinyl and allyl ethers of pentaerythritol, and the like. Mixtures of these halogenated polyols may also be employed, if desired.

The esterification mixtures, from which both the thermoplastic and the thermosetting polyester resins employed in the practice of the present invention are prepared, are generally formulated so as to contain at least a stoichiometric balance between carbonyl and hydroxyl groups. Thus, where a diol and a dicarboxylic acid are employed, they are usually reacted on at least a mol to mol basis. In common commercial practice, a small excess of polyol, usually in the range of from about 5% to about 15% excess, is employed. This is done primarily for economic reasons, i.e., to insure a rapid rate of esterification.

Both types of polyester resins used in the practice of the present invention are formed in the manner customarily observed in the art. Thus, the particular polycarboxylic acid or acids and polyol or polyols employed are reacted at elevated temperatures and atmospheric pressure. Since resinifying reactants of this type are prone to develop undesirable color when in contact with air at elevated temperatures, it is generally considered good practice to conduct the esterification reaction in an inert atmosphere, such as can be obtained by bubbling an inert gas, e.g., carbon dioxide, nitrogen, and the like, through the esterification mixture. The reaction temperature is not critical, thus the reaction will preferably be carried out at a temperature which usually will be just below the boiling point of the most volatile component of the reaction mixture, generally the polyol.

The esterification mixture should be sufficiently reacted so as to ultimately produce a polyester resin having an acid number not appreciably more than about 75. It is preferred to employ polyester resins having acid numbers ranging from about 30 to about 50.

Further details pertaining to the preparation of polyester resins of the types employed in the practice of the present invention are disclosed in U.S. Patent No. 2,255,313 to Ellis and in U.S. Patent Nos. 2,443,735 to 2,443,741, inclusive, to Kropa, and these patents are hereby incoφorated into the present application by reference.

The thermosetting polyester resins of the present invention, in combination with the styryl compounds, may be cross-linked by the addition of a suitable cross-linking agent.

The polyester resins are cross-linked by admixing them with a monomer compound containing the polymerizable CH₂=C< group to give a composition that may be cured to a stable thermoset condition. One may use about 10 parts by weight of the monomeric material to about 90 parts by weight of the polyester resin up to about 60 parts of the monomeric material to about 40 parts of the polyester resin. The preferred embodiment, however, is to use from about 25 parts of the monomeric material to about 35 parts of the monomeric material with about 75 parts to about 65 parts, respectively, of the polyester resin.

The monomeric material containing the polymerizable CH₂=< group which may be used in the practice of the present invention, has a boiling point of at least 60°C. Among the polymerizable monomeric materials that will find use in our invention are those such as styrene, sidechain alkyl and halo substituted styrenes such as alpha methylstyrene, alpha chlorostyrene, alpha ethylstyrene and the like or alkyl and halo ring-substituted styrenes such as ortho. meta and paraalkyl styrenes such as o-methylstyrene, p-ethylstyrene, meta-propylstyrene, 2,4 - dimethylstyrene, 2,5 - diethylstyrene, bromostvrene, chlorostyrene, dichlorostyrene. and the like. Still further, one can make use of the allyl compounds such as diallyl phthalate, tetrachlorodiallyl phthalate, allyl alcohol, methallyl alcohol, allyl acetate, allyl methacrylate, diallyl carbonate, allyl lactate, allyl alphahydroxyisobutyrate, allyl trichlorosilane, allyl acrylate. diallyl malonate, diallyl oxalate, allyl gluconate, allyl methylgluconate, diallyl adipate, diallyl sebacate, diallyl citraconate. the diallyl ester of muconic acid, diallyl itaconate. diallyl chlorophthalate, diallyl dichlorosilane, the diallyl ester of endomethylene tetrahydrophthalic anhydride, the diallyl ester of tetrachloroendomethylenetetrahydrophthalic anhydride, triallyl citrate, triallyl phosphate trimethallyl phosphate, tetrallyl silane, tetrallyl silicate, hexallyl disiloxane and the like. These monomeric materials may be used either singly or in combination with one another. When the thermosetting polyester resin is combined with the cross- linking monomeric material, it is desirable to incoφorate therein a polymerization inhibitor in order to prevent premature gelation of the resinous composition, particularly if it is expected that said composition will be subjected to prolonged periods of storage or if it is expected that it will be subjected to temperatures significantly higher than room temperature. With the polymerization inhibitor, the resinous composition will remain stable at room temperature for months without noticeable deterioration. Amongst the polymerization inhibitors may be used are any of those which are conventially known and used in the art such as hydroquinone, benzaldehyde, ascorbic acid, isoascorbic acid, resorcinol, tannin, symmetrical di - (beta-naphthyl) - p - phenylene diamine, phenolic resins, sulfur compounds and the like. The concentration of the inhibitor is preferably and as a general rule less than 1% by weight is usually sufficient. However, with the preferred inhibitors, e.g., polyhydric phenols and aromatic amines, one may make use of such small amounts of 0.01% to 0.1%, by weight. The thermosetting polyester resins can readily be solidified without benefit of catalyst by the application of heat or by the application of heat and pressure. However, in such an operation without benefit of a catalytic agent the time element makes it desirable to incoφorate into the composition conventional polymerization catalysts such as the organic superoxides, the alcoholic and acidic peroxides. Among the preferred catalysts are: the acidic peroxides, e.g., benzoyl peroxide, phthalic peroxide, succinic peroxide and benzoyl acetic peroxide; fatty oil acid peroxides, e.g., coconut oil acid peroxides, lauric peroxide, stearic peroxide and oleic peroxide; alcohol peroxides, e.g., tertiary-butyl hydroperoxide, usually called tertiarylbutyl peroxide and teφene oxides, e.g., ascaridole. Still other polymerization catalysts might be used in some instances, e.g., soluble cobalt salts (particularly the linoleate and naphthenate), p-toluene sulfonic acid, aluminum chloride, stannic chloride and boron trifluoride and azobisisobutyronitrile.

The above polymer matrix materials are usually transparent, but may be translucent or, in some applications, opaque to visible light. Preferably, the polymer does not linearly absorb or only weakly absorbs infrared radiation in the region from 750 to 1200 nm. The polymer matrix material is selected based on the application to which the composition is to be put. For instance, as detailed below, where the application requires a film, such as for infrared detection, the polymer is preferably a film-forming polymer, such as the polyurethane coating material EPOXYLITE #9653-2 (Epoxylite Corp., Irvine CA). On the other hand, where a three-dimensional material is preferred, such as for use in optical limiting or in two- photon pumped cavity lasing, casting polymers, such as poly(HEMA) or EPO- TEX301 (Epoxy Technology, Inc., Billerica, MA) are preferred.

Another class of suitable matrix materials are sol-gel glasses, preferably those having bulk glass densities of from about 0.5 to about 1 g/cm³ and refractive indices of from 1.4 to 1.5. A styryl compound of the present invention and a polymerizable monomer, preferably poly(hydroxyethyl methacrylate), are impregnated into the bulk glass. The monomer is then polymerized by heating, by irradiation, or by the passage of time at room temperature. Optionally, the monomer may contain polymerization initiators, such as 2,2'-azobisisobutyronitrile ("AIBN"), preferably in a initiator to monomer mole ratio of from 0.25 to 2%. The styryl compound and monomer may be introduced simultaneously or sequentially. Simultaneous impregnation is preferred but requires that the styryl compound be soluble in the monomer. Furthermore, the monomer must have a surface tension which permits penetration of the monomer into the sol gel bulk glass. In the latter regard, for many glasses, alkyl methacrylate is preferred to hydroxyalkyl methacrylates. However, many compounds of the present invention are only marginally soluble in alkyl methacrylates. In this situation, an alternative impregnation method is preferred. First, the styryl compound, dissolved in a suitable solvent, such as a ketone solvent, is contacted, by immersing, spraying, dripping, brushing, and the like, with the bulk sol gel glass. The solvent is removed. and the dye-doped glass is then contacted with a monomer solution, optionally containing a polymerization initiator, for 1/2 to 72 hours, at from room temperature to about 80°C, to impregnate the glass with the monomer. Polymerization of the monomer, such as by heating, by irradiating, or by passage of time at near-room temperatures from 25 °C to 50°C, preferably in a sealed container in the absence of oxygen, completes formation of the sol gel composition.

Alternatively, the procedures used to impregnate the sol gel with the styryl compound and polymer can be used to introduce the compound and polymer into a Vycor glass having pore size from about 2θA to about 1 OOA, preferably from about 35 to about 5θA. Vycor glasses suitable for use in the compositions of the present invention are commercially available, for example, from Corning Glass Inc.. Corning, New York.

As indicated above, the compositions of the present invention can be in the form of fiber. Alternatively, the compositions can be formed into a free standing film, preferably having a thickness of from about 0.001 to about 1 mm. The composition can also be coated as a film on a substrate, such as paper, a polymer film, a metal sheet, or glass. Preferably, the composition forms a film from about 0.01 to about 0.05 mm thick on the substrate.

The composition can also be in the form of a three dimensional article, preferably having two parallel faces, such as a rod. The faces can be polished by conventional methods, such as by manual grinding using a diamond grinding wheel, by abrading the surface using abrasives, such as silicon carbide paper, preferably with increasing grit ranging from 60 to 2000 and preferably using a lubricant, such as water, or by polishing on cloths with 10 to 0.1 μm grade diamond paste, preferably using an automated grinding and polishing machine, such as the METASERV™ 2000 (Buehler VK Ltd., Coventry, England), or by combinations thereof. Polishing is best effected by sequentially performing the above steps.

Using the aforementioned methods, compositions containing from about 0.001 to about 0.1 M of styryl compound are achieved. In general, it is preferred that the styryl compound be as concentrated as possible without forming aggregates. Aggregate formation is minimized and compound concentration is maximized when the styryl compound's concentration in the matrix material is from about 0.0015 to about 0.01 M.

The styryl compounds and compositions of the present invention have strong two-photon absoφtion with a cross section that is significantly greater than commercial dyes, such as Rhodamine, DCM, and DMP. The compounds also exhibit intense emission having a wavelength from about 300 to about 680 nm when excited by infrared laser radiation. These properties make the styryl compounds and compositions useful active materials in a variety of applications, such as infrared beam detection, two-photon induced optical power limiting, and two-photon pumped lasing.

The above described method can be used to convert infrared radiation having wavelengths corresponding to the two-photon absoφtion spectra of the compounds of the present invention, or, more specifically, from about 700 to about 1300 nm, to radiation having wavelengths from about 350 to about 680 nm. The method is particularly useful for converting infrared radiation produced by a Nd- YAG laser although other infrared sources, such as, for example, Ti-sapphire, ruby, Alexandrite, semiconductor diode, Nd-YLF, and Nd-glass lasers, can be converted. The emitted radiation can be coherent (laser) radiation or it can be incoherent (nonlaser) radiation, such as when the compounds or compositions of the present invention absorb two-photons and fluoresce. With regard to the production of fluorescent radiation, the compounds and compositions of the present invention, can be used as fluorophores in two-photon based microscopy and two-photon based imaging as described in Tsien, "Fluoroscence Imaging Creates a Window on the Cell," Chem. Eng. News, pp. 34-44 (July 18, 1994) and Denk et al.. "Two-Photon Laser Scanning Microscopy," Science. 2:73-76 (1990), which are hereby incoφorated by reference.

A laser is also provided by the present invention. The laser includes a source capable of producing infrared radiation, and a styryl compound of the present invention. The compound is positioned at a location where infrared radiation from the source exposes the compound. The compound converts the infrared radiation to visible radiation. Construction details of the laser, including appropriate pump sources and cavity optics, are the same as those used in conventional (solution) dye lasers, such as those described in Hecht, which is hereby incoφorated by reference. The infrared laser source is preferably a Q-switched pulsed Nd-YAG laser having a pulse width of from 1 ns to 100 ns, a spectral width of less than 10 cnr^!, an angular divergence of from 0.5 mrad to about 2.5 mrad, and a repetition rate of from 0.1 Hz to about 1 kHz. To achieve cavity lasing, two parallel plane reflective surfaces, such as dielectric-coated mirrors, can be employed. The pump beam can be coupled into the cavity by any of the conventional methods, such as by focused normal incidence. The two-photon pumped lasing mechanism, by which the above laser is believed to operate, has several advantages. Most dyes dissociate easily when pumped by ultraviolet or visible light but are more resistant to infrared pumping. Therefore, the life of the laser dye is extended by the two-photon pumping mechanism. Additionally, in two-photon pumped lasing, absoφtion of the pump beam by the medium is very small. As a result, the bulk of the medium can be used rather than only the surface layer, as is the case in conventional one-photon lasing. Consequently, the gain length can be made very large, for example, by using waveguides or optical fibers doped with the compounds of the present invention. As a result, highly efficient lasing can be achieved. The present invention relates to a method for recording data in a three-dimensional matrix which contains a plurality of dye molecules. Preferably, the dye molecules are uniformly distributed in the matrix material. That is, the concentration of dye molecules in any arbitrarily selected volume element within the matrix is the same as the concentration of the dye molecules within the matrix taken as a whole. The dye molecules can be of any suitable concentration, but, preferably, the concentration of the dye molecules is as high as possible, limited, of course, by the ability of the matrix material to disperse uniformly the dye molecules therein without aggregation or other phenomenon which adversely impacts uniform distribution. Preferably, the concentration of the dye molecules in the matrix is from about 0.001 M to about 0.4 M and, more preferably, from about 0.01 M to about 0.05 M.

The matrix is preferably made of a material which substantially uniformly disperses the dye molecules. The material from which the matrix is made is also preferably transparent, more preferably substantially transparent, most preferably having a transmittance of greater than 75%. to the actinic radiation used to store the data and to the actinic radiation that will be used to read the data.

Suitable matrix materials for dispersing the dye molecules include polymers, such as those described above in connection with compositions comprising a matrix material and a styryl dye. Preferred polymers are poly(methyl methacrylate) and poly(2-hydroxyethyl methacrylate). The above polymer matrix materials are usually transparent, but may be translucent or, in some applications, opaque to visible light. Preferably, the polymer does not linearly absorb or only weakly absorbs infrared radiation in the region from 750 to 1200 nm. The polymer matrix material is selected based on the absorbance spectrum (more accurately the two-photon absorbance spectrum), the solubility of the dye in the matrix material, cost, diffusion rate of the dye molecule through the matrix material, and the like.

Another class of suitable matrix materials are sol-gel glasses, preferably those having bulk glass densities of from about 0.5 to about 1 g/cm³ and refractive indices of from 1.4 to 1.5. A styryl compound of the present invention and a polymerizable monomer, preferably poly(hydroxyethyl methacrylate), are impregnated into the bulk glass. The monomer is then polymerized by heating, by irradiation, or by the passage of time at room temperature. Optionally, the monomer may contain polymerization initiators, such as 2,2'-azobisisobutyronitrile ("AIBN"), preferably in a initiator to monomer mole ratio of from 0.25 to 2%. The styryl compound and monomer may be introduced simultaneously or sequentially. Simultaneous impregnation is preferred but requires that the styryl compound be soluble in the monomer. Furthermore, the monomer must have a surface tension which permits penetration of the monomer into the sol gel bulk glass. In the latter regard, for many glasses, alkyl methacrylate is preferred to hydroxyalkyl methacrylates. However, many compounds of the present invention are only marginally soluble in alkyl methacrylates. In this situation, an alternative impregnation method is preferred. First, the styryl compound, dissolved in a suitable solvent, such as a ketone solvent, is contacted, by immersing, spraying, dripping, brushing, and the like, with the bulk sol gel glass. The solvent is removed, and the dye-doped glass is then contacted with a monomer solution, optionally containing a polymerization initiator, for 1/2 to 72 hours, at from room temperature to about 80°C, to impregnate the glass with the monomer. Polymerization of the monomer, such as by heating, by irradiating, or by passage of time at near-room temperatures from 25°C to 50°C, preferably in a sealed container in the absence of oxygen, completes formation of the sol gel composition. Alternatively, the procedures used to impregnate the sol gel with the styryl compound and polymer can be used to introduce the compound and polymer into a Vycor glass having pore size from about 2θA to about 100 A, preferably from about 35 to about 50 A. Vycor glasses suitable for use in the compositions of the present invention are commercially available, for example, from Corning Glass Inc., Corning, New York.

Alternatively the storage medium can be any polmeric sheet or bulk media into which the dyes can be post-infiltrated using plastic infiltration technology. This technology allows for preformed plastic sheets and bulk materials to be infiltrated with dopants like dyes by a simple process of controlled diffusion. This approach has been successfully applied to various polymers like acrylic, polycarbonate, terephthalate, polyethylene, polystyrene and polyvinylacetate systems without causing any change in the physical properties of the polymer. This dye infiltrated polymer sheets or bulk plastics also can be used as a storage media.

The three-dimensional matrix having dye molecules dispersed therein preferably has two parallel faces. The faces can be polished by conventional methods, such as by manual grinding using a diamond grinding wheel, by abrading the surface using abrasives, such as silicon carbide paper, preferably with increasing grit ranging from 60 to 2000 and preferably using a lubricant, such as water, or by polishing on cloths with 10 to 0.1 μm grade diamond paste, preferably using an automated grinding and polishing machine, such as the METASERV™ 2000 (Buehler VK Ltd., Coventry, England), or by combinations thereof. Polishing is best effected by sequentially performing the above steps. Alternatively, the faces can be trimmed on an ultramicrotone with a glass knife.

Dye molecules suitable for the practice of the present invention include any dye which can be detectably altered by actinic radiation. Preferred dyes are those which are detectably alterable by two-photon processes. The two-photon processes can be, for example, a change in absoφtion and emission properties of the dye molecules, which in turn can be detected by another light source at the new absoφtion wavelength which will make the dye to emit at the new emission wavelength. For example, the dye molecule can be a colored photochromic dye, such as a spirobenzopyran or a spirooxazine (including a spirobenzoxazine, and a spironaphthoxazine), which, upon exposure to actinic radiation, is converted via a two-photon upconversion process to a form which lacks this color. Other suitable photochromic dyes are those disclosed in Brown, ed., Photochromism, vol. 3 in Weissberger's Techniques of Organic Chemistry, New York:Wiley Interscience (1971), which is hereby incoφorated by reference.

Dyes suitable for use in the methods of the present invention include styryl dyes, such as (4-[N-(2-hydroxyethyl)-N-methyl)aminophenyl]-4'-(6'- hydroxyhexylsulfonyl)stilbene) ("APSS"). Preferably, the dye is AF240 (7- benzothiazol-2-yl-9,9-diethylfluoren-2-yl)diphenylamine) as there is a significant change in absoφtion and emission before and after two-photon excitation. Further they have very strong two-photon absoφtion, making it an ideal material for usage in the storage media.

The three-dimensional matrix material includes a first volume element, which, according to the method of the present invention, is exposed to actinic radiation. The size of the volume element is not critical to the practice of the present invention, but small volume elements, such as those having a volume of from about 0.001 μm³ to about 10 μm³, preferably from about 0.01 μm³ to about 1 μm³, are preferred. Most preferably, the volume element is sized so as to be the smallest volume which can be uniquely addressed by the actinic radiation used. Where a focused laser beam having a Gaussian cross section is employed, uniquely addressing means that the volume outside of the volume element is exposed to an intensity no more than 10% of the intensity to which the volume element is exposed. The shape of the volume element is likewise not critical. Typically, a hexahedral shape or ellipsoidal shape is employed having dimensions on the order of tenths of microns, such as 0.5 x 0.5 x 0.8 microns.

In most cases the matrix will contain more than one volume element. These volume elements can be discrete (i.e. non-overlapping with one another), or they can be overlapping or they can be continuously overlapping. Preferably, each of the elements is sufficiently separated from other volume elements so that the exposing actinic radiation, when directed at one of the volume elements, does not expose other (particularly adjacent) volume elements to an intensity and for a duration effective to detectably alter the dye molecules contained in the other (particularly adjacent) volume elements. Most preferably, the volume elements are separated by distances sufficient so that each volume element can be uniquely addressed by the actinic radiation.

As used herein, actinic radiation includes electromagnetic radiation, such as ultraviolet, visible, near infrared, infrared radiation, or combinations thereof. The actinic radiation can be monochromatic or polychromatic and, preferably, has a non-zero intensity at a wavelength at which the dye absorbs, preferably from about 660 to about 1300 nm. It can be coherent, incoherent, polarized, laser, pulsed laser, focused laser, or diffuse radiation. The actinic radiation is preferably high intensity radiation in the range from 660 to 1300 nm. Preferably, it is laser radiation in the form of a laser beam.

A variety of laser sources emitting in the range from 660 to 1300 nm are available. Suitable sources will be apparent to the skilled practitioner and are summarized, for example, in Hecht, which is hereby incoφorated by reference. One particularly useful laser source is a mode-locked Ti-sapphire laser, preferably operated at 790 to 800 nm and having pulse durations as short as possible (typically on the order of tens of femtoseconds). Another laser source well suited for irradiating the dye in the practice of the present invention is a Q-switched pulsed Nd-YAG laser having an output of 1060 nm. Spectral widths of less than 10 cm^-1 are preferred. The angular divergence of the laser can be from about 0.5 mrad to about 2.5 mrad, depending on the distance of the laser from and the size of the volume element being irradiated. Repetition rates of from 0.1 Hz to about 500 MHz are suitable. Because of the high intensities generally needed to effect two-photon processes, it is preferred that the laser be a pulsed laser having a pulse duration as short as possible, preferably ranging between several tens of femtoseconds and several nanoseconds, and having pulse peak powers of several hundreds of megawatts.

As indicated above, optimization of the process of the present invention requires that the data be written in a third dimension, which requires that the actinic radiation selectively access volume elements in planes below those in the surface of the matrix and that these planes be as closely spaced as possible. This can be achieved by manipulating the actinic radiation prior to the radiation entering the matrix material. One such manipulation involves focusing laser radiation provided in the form of a laser beam. Methods for focusing laser beams are well known to those in the art and are described in Hecht, which is hereby incoφorated by reference. One focusing technique uses an confocal microscope, such as those described in U.S. Patent No. 5,034,613 to Denk et al. ("Denk"), which is hereby incoφorated by reference. By adjusting the microscope optics, the vertical location of the focal point in the matrix can be selected, and, in this manner, a volume element in the interior of the matrix can be selectively exposed.

Alternatively, the actinic radiation can be laser radiation provided in the form of two or more laser beams made to intersect at the volume element to be exposed. The two or more laser beams can intersect at right angles to each other (in the case where number of beams is two or three), or two or more of the two or more laser beams can intersect at an oblique angle. The two or more laser beams can be provided by a single laser, the beam from which is split by one or more beam splitters into a plurality of beams, each of which is then directed by conventional optics to intersect at the volume element to be exposed. Optics and methods suitable for producing two beams in this manner are described, for example, in Hecht. which is hereby incoφorated by reference. Alternatively, the two or more laser beams can be provided by two or more lasers.

To store a multiplicity of data points, the data storage method of the peasant invention can be carried out for another volume element (e.g., a second volume element, a third volume element, and so on). This involves moving the laser beam relative to the matrix to another volume element and exposing the another volume element (e.g. the second volume element, the third volume element, and so on) to laser radiation for a duration and at intensity effective to alter detectably a fraction of the dye molecules contained in the another element.

In the case where the laser beam is a focused laser beam, such as with an confocal microscope, this can be effected by shifting the laser beam relative to the matrix in an X-Y plane within the matrix and shifting the focal point of the laser beam relative to the matrix material along a Z axis. Alternatively, this can be done by moving the matrix material in the X-Y plane through the focal point. As used in this context, the Z axis is coincident with the laser beam and the X-Y plane is orthogonal to the laser beam. Shifting the focal point relative to the matrix material can be achieved by moving the matrix material in a Z direction relative to the focusing optics or by adjusting the focusing optics so that the focal point moves relative to the matrix material or both. Although the method used to move the matrix relative to the laser beam is not critical to the practice of the present invention, to store data with temporal and spatial efficiency, it is desirable that the movement be accurate and that it be carried out quickly. As indicated above, this can be done by mechanically moving the matrix material in one or more of the three dimensions and scanning the beam in the remaining of the three dimensions. The method employed, depends, in part, on whether the data is to be stored serially (that is, whether temporally adjacent data storage operations are conducted in spatially adjacent volume elements) or randomly and on whether the data to be stored is spatially digital or analog. A variety of methods for scanning the matrix are known to those skilled in the art, and any of these are suitable for practicing the present invention. For example, the matrix can be scanned using a stepper motor or a continuous motor connected to a mechanism for translating the rotational motion of the motor to linear motion. Such a translating mechanism can be, for example, a rack and pinion mechanism or to a screw mechanism. Matrix scanning can also be effected with a plurality of magnetic coils driven by a voltage source, preferably, a computer controlled voltage source. The matrix can also be scanned by applying a voltage to change the dimensions of a piezoelectric material which is in contact with the matrix or with a stage supporting the matrix. Scanning the matrix in two of the three dimensions can also be achieved using a rotating disk format, such as those employed in compact disk ("CD") systems and other conventional commercial data storage products.

Alternatively, the laser beam can be scanned optically by using scanning mirrors in the optical path of the laser beam. Further details regarding beam scanning are available, for example, in Denk, which is hereby incoφorated by reference.

Irrespective of whether the X-Y scanning is effected optically or by stage scanning (i.e., by moving the matrix), in cases where the laser beam is focused, such as with a confocal microscope, the focal point can be scanned (in the Z direction). In the case where a confocal microscope is used, the focal position of the focal point relative to the matrix can be adjusted by rotating the focus control knob, such as with a stepper or continuous motor. Alternatively, the position of the focal point relative to the matrix can be controlled by moving the Natarex in the Z direction, for example, with a stepper motor connected to a means for translating rotational motion to linear motion or with electromagnetic coils, as described above for controlling X-Y position.

Scanning can be effected in an analog manner or in a digital manner. In analog scanning the volume elements overlap in one of the three dimensions. This is generally effected by moving the matrix continuously in the one analog dimension, such as with a continuous motor or with a electromagnetic coil having a ramp voltage applied thereto. Alternatively, one of the scanning mirrors can be continuously moved to effect a continuous movement of the beam in one of the X or Y directions. Yet another alternative is to adjust the position of the focal point in a confocal microscopy set-up by rotating the focusing knob in a continuous fashion so as to provide analog data storage in the Z direction. The data stored in each of the volume elements can be binary, digital, or analog. Independently of this, the data stored in each of the volume elements can be stored as binary, digital, or analog data. Interconversion of binary, digital, and analog data, such as by electrical or electronic manipulations, is well known in the art. Whether the data is stored in binary, digital, or analog form depends on the number of possible states which each of the volume elements can assume when it is exposed.

As indicated above, data is stored in each of the elements by exposing the volume element to actinic radiation for a duration and at an intensity effective to detectably alter a fraction of the dye molecules contained therein. When a single intensity/duration combination is used to expose each of the exposed volume elements, the fraction of dye molecules detectably altered in each of the exposed volume elements is the same. Preferably, this fraction is greater than 0.6, more preferably, greater than 0.7, and, most preferably, greater than 0.8. Conversely, the unwritten state is characterized by a fraction of detectably altered dye molecules, preferably, less than 0.4, more preferably, less than 0.3, and, most preferably, less than 0.2. When each of the exposed volume elements is exposed to one of a finite number, N, greater than one of intensity /duration combinations, the fraction of dye molecules detectably altered in each of the exposed volume elements will have one of N+l potential values. (Here, one is added to N to account for the unexposed volume element, in which the fraction of detectably altered molecules is not be detectably different than zero.)

For example, when the number of intensity /duration combinations employed is 3, the data storage medium can include, in addition to a first volume element, a second volume element and a third volume element, each of which contain a fraction of detectably altered dye molecules. The fraction of the dye molecules detectably altered in the second volume element is detectably different than the fraction of the dye molecules detectably altered in the first volume element, and the fraction of the dye molecules detectably altered in the third volume element is detectably different than the fractions of the dye molecules detectably altered in the first and second volume elements. Thus, each volume element in this data storage medium can be used, for example, to store hexadecimal data in hexadecimal form without converting the hexadecimal data to binary form. Alternatively, each volume element can be used to store 4 bits of binary data.

As yet another illustration of the data storage media of the present invention wherein data is stored in digital form, consider a data storage medium containing, in addition to the first volume element, 254 additional volume elements where the fraction of the dye molecules detectably altered in each of the 254 additional volume elements is detectably different than the fraction of the dye molecules detectably altered in each of the other 254 additional volume elements and in the first volume element. Each volume element of this data storage medium is thus able to store, for example, 8 bits of binary data or, alternatively, ASCII text without converting the ASCII text to binary form.

In principle, the value of N can depend on the number of dye molecules per volume element, the ability to selectively focus the actinic radiation on the volume element being exposed, the effect of perturbing the stored data by each reading cycle, the anticipated number or reading cycles, the diffusion rate of the dye molecules through the matrix, the time for which the data needs to be stored, and the tolerance for error. Data stored using intensity /duration combinations in excess of N is considered to be stored as analog data.

As indicated above, the fraction of dye molecules detectably altered in each volume element is depends on two factors: (1) the intensity of the exposing actinic radiation and (2) the duration of exposure. Generally, it is preferred to hold one of these factors constant and to adjust the other so that the fraction of dye molecules detectably altered correlates with the data to be stored. Intensity can be adjusted by, for example, passing the actinic radiation through an attenuator, such as a rotatable dichroic mirror. By changing the angle of the dichroic mirror with respect to the path of the actinic radiation, the intensity of the actinic radiation transmitted through the dichroic mirror can be modulated. The duration for which the actinic radiation exposes the volume element can be adjusted, for example, by placing a shutter in the path of the actinic radiation and controlling the length of time for which the shutter is open. In cases where the dye molecules are two-photon active and exposure is modulated temporally (such as with a shutter), the fraction of molecules detectably altered correlates linearly with exposure duration. Intensity modulation, on the other hand, gives rise to a quadratic dependence of fraction of molecules detectably altered on intensity. The methods and data storage media of the present invention are particularly well-suited for the storage of two-dimensional images, such as pictures, photographs, charts and graphs, and the like. The two-dimensional image comprises a two-dimensional array of pixels. These pixels may be discrete (i.e. non- overlapping) in both directions, or, alternatively, they may be overlapping or continuous in one of the two dimensions and discrete in the other dimension.

Each pixel has a value associated with it. For example, in the case of the black-and-white image, the value associated with each pixel can be its gray level, determined, for example, by a densitometer. In the case where the image is a color image, the value associated with each pixel can be, for example, the density of one of the colors making up the color image. Typically, color images can be broken down into three primary colors and a gray level, and these can be determined using a densitometer with an appropriate color filter.

The two-dimensional array of pixels is mapped to a two-dimensional array of volume elements in the three-dimensional matrix. This is done by exposing a volume element in the two-dimensional array of volume elements to actinic radiation for a duration and at an intensity effective to alter a fraction of the dye molecules contained in the volume element which correlates to the value associated with the corresponding pixel. The value can be binary, as in the case of a line drawing or a half-tone picture. Alternatively, it can be digital, as in the case where the image is stored as a stepped gray scale, or it can be analog, as in the case where the image is stored as a continuous gray scale.

As indicated above, the image can be divided into discrete pixels, and these discrete pixels can be mapped to the data storage medium of the present invention as discrete volume elements. In this embodiment, the value of a pixel is sampled, such as with a densitometer, and the value is converted to an analog, digital, or binary signal. A volume element is selected to receive the data from the image, such as by moving the matrix or by moving the laser beam or beams delivering the actinic radiation. The volume element is then exposed for a duration and at an intensity controlled by the signal. For example, the signal can be fed to a motor which controls the angle of a dichroic mirror through which the beam delivering the actinic radiation passes, and the angle of the mirror (and, thus, the intensity of the beam passing there through) can be adjusted to correlate to the signal. Alternatively, the signal can be used to control (through, for example, a solenoid) the time for which a shutter through which the actinic radiation passes is opened. After completion of the data recording operation for the first pixel, the value of the next, preferably adjacent, discrete pixel is sampled, and a second signal is generated. The matrix is moved relative to the actinic radiation optics to access a second volume element which is spatially disjoint from the first volume element, and the second volume element is exposed under the control of the signal from the second pixel. This process is repeated until every pixel making up the two- dimensional image is stored in the two-dimensional array of volume elements. In the case where the pixels are overlapping in one of the dimensions, these overlapping pixels are mapped to the data storage medium of the present invention as overlapping volume elements. In this embodiment, the value of a pixel is sampled, converted to a signal, and used to control the exposure of a first volume element. The device used to sample pixel value is then shifted to a new pixel, which, in part, overlaps the pixel just sampled. The matrix is shifted relative to the actinic radiation optics to provide access to a second volume element which overlaps the first volume element to the extent that the second pixel overlaps the first pixel, and the second volume element is exposed under the control of the signal from the second pixel. By repeating the operation, the entire two-dimensional image is mapped to the two-dimensional array.

In the case where the pixels are continuous in one of the two dimensions of the image, the image can be scanned along the continuous dimension with a device for measuring the value being recorded, such as gray level or color density, to produce a continuous signal. The matrix is moved continuously (relative to the actinic radiation optics) at a rate corresponding to the rate of scanning of the image. For example, where the continuous dimension of the image being recorded is 20 mm, and the corresponding dimension of the matrix's two-dimensional array is 2 mm, the rate of scanning the matrix can be one-tenth the image scan rate. While moving the matrix continuously relative to the actinic radiation optics, the intensity the actinic radiation can be modulated by continuously adjusting the angle of a dichroic mirror in response to the continuous signal generated by the image scanning device. When scanning and exposing in the continuous dimension is complete, the image scanning device is shifted in the second dimension, the position of the matrix relative to the optics is shifted in the second dimension of the matrix's two- dimensional array, and the scanning and exposing operations are repeated. In this manner, the entire image can be raster scanned and recorded in the data storage medium of the present invention.

The method of the present invention can be used to map a second value of the same image to a second two-dimensional array of volume elements in the same matrix. The second value can be, for example, the color density of a second color. By repeating this operation, for a third color and for the gray level of the two-dimensional image, a full color image can be stored in the data storage medium of the present invention.

The method of the present invention can also be used to record a plurality of images in adjacent two-dimensional arrays of volume elements. The plurality of images can be, for example, the frames of a movie or other time-evolved scene or scenes. By reading the X-Y plane quickly, so that each two-dimensional image is reconstructed before the human brain can "see" it, and by scanning the Z dimension at a rate which corresponds to the rate at which the images were made (such as, in the case of a movie, at the frames per second rate), the movie or time- evolved scene or scenes can be replayed.

The data stored in, for example, the first volume element of the data storage medium of the present invention can be read by detecting the fraction of dye molecules contained in the first volume element that are detectably altered. During data writing using the two-photon process, the dye molecules in the written region are detectably altered. During readback, it is exposed to an actinic radiation which is selected such that only the molecules altered by the two photon writing, absorb these radiation and emit at another wavelength. Typically, the actinic radiation used for reading is not the same as the actinic radiation used for storing. For example, where the actinic radiation used to store data at the first element is electromagnetic radiation of a particular wavelength, which is suitable to induce changes in the dye material using the two-photon process, the actinic radiation used to read the data is an electromagnetic different wavelength which can excite the altered dye molecules to give out an emission at a third wavelength. A preferred embodiment of the data storage method of the present invention is depicted in Figure 1. Matrix 13 is placed on an X-Y stage 9 either mounted on a microscope generally indicated at 20 and which may be a confocal microscope, or kept independently. Matrix 13 is exposed through high numerical aperture ("N.A.") objective lens 6, such as a Nikon planapo 60X (1.4 N.A.), with 80 fs pulses of, for example, 800 nm wavelength light from a TLSapphire laser diagrammatically illustrated at 1. Laser beam 3 is supplied to the focusing objective via beam steering optics consisting of dichoric mirror 2 and other ND filters, optical shutters etc. shown as 4 and finally optionally, beam scanning optics, diagrammatically illustrated at 5. Matrix 13, carried on stage 9, is translated in the X direction by a stepper motor 10 and in the Y direction (peφendicular to the plane of the paper) by a second stepper motor(not shown). By controlling the stepper motor 10 and the stepper motor not shown, stage 9 is translated in the X-Y plane and a two- dimensional array of volume elements in matrix 13 are defined. Stage 9 is moved in the Z direction by stepper motor focus controller 8. All the stepper motors (10, 8 and the third one which is not shown) and the beam scanning are controlled by an IBM compatible computer 12 through control electronics shown as 11. By moving stage 9 in the Z direction, laser beam 3 is made to focus at different X-Y planes. In this manner different twodimensional planes of volume elements are defined, so that three-dimensional stacks of data can be written into matrix 13. The intensity of laser beam 3 is modulated by rotatable galvanometric mirrors inside the scanning system shown as 5. Appropriate software was developed in accordance with conventional techniques to control all the required components of the writer.

The reading process can be carried out by exposing the entire matrix to actinic radiation and selectively detecting fluorescence only from the first volume element; by selectively exposing the first volume element and detecting fluorescence from the entire matrix; or by selectively exposing the first volume element and selectively detecting fluorescence only from the first volume element.

Selectively exposing the first volume element in the reading process can be effected by the same methods and devices discussed above in relation to selectively exposing particular volume elements for data storage puφoses. In particular, a confocal microscope is preferably used to selectively expose the first volume element to the actinic radiation used to read the data stored in the first element. The confocal microscope can also be used to detect selectively fluorescence from the first volume element. Using a confocal microscope to detect the emitted fluorescence is particularly preferred because the adjustable confocal pin hole provided in the collection optics of the confocal microscope minimizes background fluorescence collected from dye molecules above and below the plane of focus. Thus, even though dye molecules above and below the first volume element may be inadvertently exposed to actinic radiation of sufficient intensity and for sufficient duration to cause them to undergo two-photon fluorescence, the confocal microscope limits fluorescence detection to the dye molecules in the first element.

Preferably, the actinic radiation is of an intensity and duration insufficient to detectably alter the dye molecules contained in the first element, so that the data stored therein remains unchanged after the reading process. By reading the stored data with actinic radiation of an intensity and duration insufficient to detectably alter the dye molecules, the data storage media of the present invention can be used as "write once, read many" ("WORM") data storage media.

Reading a plurality of data points stored in different volume elements of the data storage medium of the present invention requires that the actinic radiation used to read the data or the detection optics or both be moved to a second volume element. This can be done by moving the matrix, such as by using stepper or continuous motors or electromagnets, for example, as described above with respect to data storage. Alternatively, where the actinic radiation laser radiation is in the form of one or more laser beam(s), the volume element being read can be selected by adjusting the position of the laser beam(s) or, where a focused laser beam is employed, by adjusting the laser beam's focal point, for example, as described above with respect to data storage.

Figure 2 illustrates a typical configuration for reading data from the data storage medium in accordance with the present invention. The optical memory formed by the foregoing process may be read by successively imaging each of the data-containing planes of matrix 13 by directing a continuous wave (CW) laser beam 16 from laser, for e.g, 488nm line from a Kr:Ar laser, 14 into matrix 13 carried by stage 9, using the optics of confocal microscope 20. Laser beam 16 (indicated by solid arrowed lines) is reflected by dichroic mirror 15 and is focused by objective mirror 6 onto focal plane 21 in matrix 13. The position of stage 9 in the X-Y plane is controlled by two stepper motors, one (10 ) controlling the position in the X direction, and the second one (not shown) controlling the position in the Y direction. ( Alternatively, laser beam 16 is translated in the X-Y plane by optional beam scanning optics 5.) By adjusting stepper motor 8 which controls the position in Z direction, the vertical location (along the Z axis) of focal point 21 in matrix 13 can be selected. Thus, by adjusting vertical position and the position of stage 9 (or the optional beam scanning optics 5), laser beam 16 can be made to focus at a selected volume element within matrix 13. At focal point 21, altered dye molecules in the selected volume element are excited and fiuoresce. Fluorescence produced by the dye molecules in the selected volume element of matrix 13, indicated by dotted arrows 17 in Figure 2, travels back through microscope 20, retracing the optical path of laser beam 16. Fluorescence 17 passes through objective lens 6, through optional beam scanning optics 5, and to dichoric mirror 15. Because the fluorescence 17 emitted by the dye molecules in matrix 13 is of different wavelength than laser beam 16, fluorescence 17 passes through dichroic mirror 15 and is directed by stationary mirror 22, preferably through confocal aperture 18, to a suitable detector, such as photomultiplier 19. The output of photomultiplier 19 can be acquired into the computer 12 and can be viewed using the reader software. Laser beam 16 can scan each layer of matrix 13 in the X-Y plane to produce a corresponding image, and, by successively moving the matrix 13 along the Z direction using the stepper motor control 8, and there by changing the focus of the laser beam on various planes, each layer of the matrix 13 can be read.

The present invention is further illustrated by the following examples

EXAMPLES

Example 1 - Barcode Storage

A PMMA (poly-methyl methacrylate) polymer block doped with the new two-photon chromophore (2% by weight) AF240 (7-benzothiazol-2-yl-9,9- diethylfluoren-2-yl)diphenylamine ), obtained from the polymer branch of the US Airforce Research Laboratory, was used as the data storage medium. Dye doped Poly(methyl methacrylate) ("PMMA") blocks were made by dissolving AF240 in the methyl methacrylate monomer ("MMA") and then polymerizing the monomer to yield the dye-doped polymer block storage medium. This dye was chosen since it exhibits good two-photon absoφtion and shows a change in fluorescence properties after two-photon absoφtion. A tightly focused pulsed IR beam, a Ti:Saphire laser (TSUNAMI from SPECTRA-PHYSICS pumped by a diode pumped solid state laser, MILLENNIA also from SPECTRA-PHYSICS) operating at 800 nm with pulse width 80fs and a repetition rate of 82MHz as light source and a high NA objective for focusing was used to write barcodes in to the dye doped polymer medium mounted on a computer controlled scanning stage. Using this technique we wrote multiple layers of barcodes/images in a single polymer block at a vertical separation of 10 microns, and up to a depth of couple of hundred microns.

The written spot's linear absoφtion and fluorescence properties were red shifted compared to the unwritten region of the storage medium. The written spot shows a reasonably broad absoφtion around 500 nm and an emission around 570 nm making it possible to use a read back system constituting a 488 nm line of a Kr: Argon laser as the excitation source and confocal detection of the emission at 570 nm. Reading is done with a galvanometer mirror scanner, which scans the written layer with the excitation light, and detecting the fluorescence signal through a confocal aperture. Different layers of the written layers can be selected by focusing the excitation light at the desired layer, using a focus motor (stepper motor).

Example 2: Multilayer Storage and Retrieval

The storage media used (AF240 doped PMMA block) was prepared similar to the one described in the Example 1. A series of images separated by 6 μm in the z direction (starting at a depth of 50μm from surface ), were written in the volume of the polymer block by the two-photon process, by tightly focused Ti:Sapphire laser beam, as described earlier. Fig. 4 shows the read back images using single photon confocal detection. In this case the excitation source used was 514nm line from an Argon laser and the emission at ~570nm from the dye molecules altered by the two-photon writing was detected using a confocal microscope. Although the image contrast is very good in the single photon read back, the overlap of different written layers (6 μm apart) is visible, indicating that higher separation between the layers (around lOμm.) is necessary for the total elimination of cross-talk between written layers.

Example 3 : Image Storage and Retrieval

Again the material and methods used for the storage are similar to Example 1. Here a scanned photograph was written into the dye doped PMMA matrix. The information was retrieved using single photon confocal detection with 488nm line from a Kr: Argon laser as the excitation source. Figure 5 shows the original scanned photograph A and the image recovered from the storage media B.

Although the invention has been described in detail for the puφose of illustration, it is understood that such detail is solely for that puφose, and variations can be made therein by those skilled in the art without departing from the spirit and scope of the invention which is defined by the following claims.

Claims

WHAT IS CLAIMED:

1. A method for reading a three-dimensional data storage device, comprising: a) providing a data storage medium comprising a three-dimensional matrix and a plurality of dye molecules dispersed therein, wherein said dye molecules are capable of a fluorescence change induced by multiple-photon excitation; b) inducing said fluorescence change of said dye by multiple-photon excitation under conditions effective to write an information code in a selected portion of said medium; c) inducing one-photon excitation in said fluorescence-changed dye; d) detecting a fluorescence emission in said one-photon excited dye portion; and e) correlating the fluorescence with the dye molecules contained in the selected portion that are detectably altered effective to retrieve said information code.

2. The method of claim 1, wherein said selected portion is on multiple levels within said device.

3. The method of claim 1, wherein said dye molecules are substantially uniformly dispersed in said three-dimensional matrix.

4. The method according to claim 1, wherein the detecting is carried out using a confocal microscope.

5. The method according to claim 1, wherein the matrix comprises polymethyl methacrylate, poly-2-hydroxyethyl methacrylate, polyester, polyethylene, or mixtures thereof.

6. The method according to claim 1 , wherein the multi-photon absorbing dye comprises 7-benzothiazol-2-yl-9,9-diethylfluoren-2-yl)diphenylamine; 4-[N-(2- hydroxyethyl)-N-methyl) amino phenyl] -4 '-(6-hydroxy hexyl sulfonly)stilbene);

AF240 C24H22N2S 398.56

AF183 C 29 H 20 2 S - 428.55

or mixtures thereof.

7. The method according to claim 1 , wherein said providing comprises dissolving the multi-photon absorbing dye in a polymerizable monomer and polymerizing the monomer to form the polymer medium.

8. The method according to claim 1, wherein said providing comprises infiltrating the multi-photon absorbing dye into a polymer sheet.

9. The method according to claim 1, wherein said inducing said fluorescence change is repeated to write multiple layers of information codes in the polymer medium.

10. The method according to claim 1. wherein said inducing said fluorescence change is carried out so that the information code is written inside the polymer medium.

11. The method according to claim 1 , wherein said multi-photon excitation comprises two-photon excitation.

12. The method according to claim 1, wherein said multi -photon excitation comprises three-photon excitation.

13. The method according to claim 1 , wherein the information code is in the form of a bar code.

14. The method according to claim 1 , wherein the storage device is in the form of an identification tag.

15. The method according to claim 1 , wherein the storage device is a medical bracelet.

16. The method according to claim 1, wherein the storage device is a military dogtag.

17. The product of the method of claim 1.