Patent Application
20080055686
These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:
Some aspects of the present invention and general scientific principles used herein can be more clearly understood by referring to U.S. Patent Application 2005/0136333 (Ser. No. 10/742,461), which was published on Jun. 23, 2005; co-pending application having Ser. No. 10/954,779, filed on Sep. 30, 2004; and co-pending application having Ser. No. 11/260,806, filed on Oct. 27, 2005; all of which are incorporated herein by reference in their entirety. It should be noted that with respect to the interpretation and meaning of terms in the present application, in the event of a conflict between this application and any document incorporated herein by reference, the conflict is to be resolved in favor of the definition or interpretation provided by the present application.
As used herein, the term “aromatic radical” refers to an array of atoms having a valence of at least one comprising at least one aromatic group. The array of atoms having a valence of at least one comprising at least one aromatic group may include heteroatoms such as nitrogen, sulfur, selenium, silicon and oxygen, or may be composed exclusively of carbon and hydrogen. As used herein, the term “aromatic radical” includes but is not limited to phenyl, pyridyl, furanyl, thienyl, naphthyl, phenylene, and biphenyl radicals. As noted, the aromatic radical contains at least one aromatic group. The aromatic group is invariably a cyclic structure having 4n+2 “delocalized” electrons where “n” is an integer equal to 1 or greater, as illustrated by phenyl groups (n=1), thienyl groups (n=1), furanyl groups (n=1), naphthyl groups (n=2), azulenyl groups (n=2), anthraceneyl groups (n=3) and the like. The aromatic radical may also include nonaromatic components. For example, a benzyl group is an aromatic radical which comprises a phenyl ring (the aromatic group) and a methylene group (the nonaromatic component). Similarly a tetrahydronaphthyl radical is an aromatic radical comprising an aromatic group (C6H3) fused to a nonaromatic component —(CH2)4—. For convenience, the term “aromatic radical” is defined herein to encompass a wide range of functional groups such as alkyl groups, alkenyl groups, alkynyl groups, haloalkyl groups, haloaromatic groups, conjugated dienyl groups, alcohol groups, ether groups, aldehyde groups, ketone groups, carboxylic acid groups, acyl groups (for example carboxylic acid derivatives such as esters and amides), amine groups, nitro groups, and the like. For example, the 4-methylphenyl radical is a C7 aromatic radical comprising a methyl group, the methyl group being a functional group which is an alkyl group. Similarly, the 2-nitrophenyl group is a C6 aromatic radical comprising a nitro group, the nitro group being a functional group. Aromatic radicals include halogenated aromatic radicals such as 4-trifluoromethylphenyl, hexafluoroisopropylidenebis(4-phen-1-yloxy) (i. e., —OPhC(CF3)2PhO—), 4-chloromethylphen-1-yl, 3-trifluorovinyl-2-thienyl, 3-trichloromethylphen-1-yl (i.e., 3-CCl3Ph-), 4-(3-bromoprop-1-yl)phen-1-yl (i.e., 4-BrCH2CH2CH2Ph-), and the like. Further examples of aromatic radicals include 4-allyloxyphen-1-oxy, 4-aminophen-1-yl (i.e., 4-H2NPh-), 3-aminocarbonylphen-1-yl (i.e., NH2COPh-), 4-benzoylphen-1-yl, dicyanomethylidenebis(4-phen-1-yloxy) (i.e., —OPhC(CN)2PhO—), 3-methylphen-1-yl, methylenebis(4-phen-1-yloxy) (i.e., —OPhCH2PhO—), 2-ethylphen-1-yl, phenylethenyl, 3-formyl-2-thienyl, 2-hexyl-5-furanyl, hexamethylene-1,6-bis(4-phen-1-yloxy) (i.e., —OPh(CH2)6PhO—), 4-hydroxymethylphen-1-yl (i.e., 4-HOCH2Ph-), 4-mercaptomethylphen-1-yl (i.e., 4-HSCH2Ph-), 4-methylthiophen-1-yl (i.e., 4-CH3 SPh-), 3-methoxyphen-1-yl, 2-methoxycarbonylphen-1-yloxy (e.g., methyl salicyl), 2-nitromethylphen-1-yl (i.e., 2-NO2CH2Ph), 3-trimethylsilylphen-1-yl, 4-t-butyldimethylsilylphenl-1-yl, 4-vinylphen-1-yl, vinylidenebis(phenyl), and the like. The term “a C3-C10 aromatic radical” includes aromatic radicals containing at least three but no more than 10 carbon atoms. The aromatic radical 1-imidazolyl (C3H2N2—) represents a C3 aromatic radical. The benzyl radical (C7H7—) represents a C7 aromatic radical.
As used herein the term “cycloaliphatic radical” refers to a radical having a valence of at least one, and comprising an array of atoms which is cyclic but which is not aromatic. As defined herein a “cycloaliphatic radical” does not contain an aromatic group. A “cycloaliphatic radical” may comprise one or more noncyclic components. For example, a cyclohexylmethyl group (C6H11CH2—) is a cycloaliphatic radical which comprises a cyclohexyl ring (the array of atoms which is cyclic but which is not aromatic) and a methylene group (the noncyclic component). The cycloaliphatic radical may include heteroatoms such as nitrogen, sulfur, selenium, silicon and oxygen, or may be composed exclusively of carbon and hydrogen. For convenience, the term “cycloaliphatic radical” is defined herein to encompass a wide range of functional groups such as alkyl groups, alkenyl groups, alkynyl groups, haloalkyl groups, conjugated dienyl groups, alcohol groups, ether groups, aldehyde groups, ketone groups, carboxylic acid groups, acyl groups (for example carboxylic acid derivatives such as esters and amides), amine groups, nitro groups, and the like. For example, the 4-methylcyclopent-1-yl radical is a C6 cycloaliphatic radical comprising a methyl group, the methyl group being a functional group which is an alkyl group. Similarly, the 2-nitrocyclobut-1-yl radical is a C4 cycloaliphatic radical comprising a nitro group, the nitro group being a functional group. A cycloaliphatic radical may comprise one or more halogen atoms which may be the same or different. Halogen atoms include, for example; fluorine, chlorine, bromine, and iodine. Cycloaliphatic radicals comprising one or more halogen atoms include 2-trifluoromethylcyclohex-1-yl, 4-bromodifluoromethylcyclooct-1-yl, 2-chlorodifluoromethylcyclohex-1-yl, hexafluoroisopropylidene-2,2-bis (cyclohex-4-yl) (i.e., —C6H10C(CF3)2C6H10—), 2-chloromethylcyclohex-1-yl, 3-difluoromethylenecyclohex-1-yl, 4-trichloromethylcyclohex-1-yloxy, 4-bromodichloromethylcyclohex-1-ylthio, 2-bromoethylcyclopent-1-yl, 2-bromopropylcyclohex-1-yloxy (e.g., CH3CHBrCH2C6H10O—), and the like. Further examples of cycloaliphatic radicals include 4-allyloxycyclohex-1-yl, 4-aminocyclohex-1-yl (i.e., H2NC6H10—), 4-aminocarbonylcyclopent-1-yl (i.e., NH2COC5H8—), 4-acetyloxycyclohex-1-yl, 2,2-dicyanoisopropylidenebis(cyclohex-4-yloxy) (i.e., —OC6H10C(CN)2C6H10O—), 3-methylcyclohex-1-yl, methylenebis(cyclohex-4-yloxy) (i.e., —OC6H10CH2C6H10O—), 1-ethylcyclobut-1-yl, cyclopropylethenyl, 3-formyl-2-terahydrofuranyl, 2-hexyl-5-tetrahydrofuranyl, hexamethylene-1,6-bis(cyclohex-4-yloxy) (i.e., —OC6H10(CH2)6C6H10O—), 4-hydroxymethylcyclohex-1-yl (i.e., 4-HOCH2C6H10—), 4-mercaptomethylcyclohex-1-yl (i.e., 4-HSCH2C6H10—), 4-methylthiocyclohex-1-yl (i.e., 4-CH3SC6H10—), 4-methoxycyclohex-1-yl, 2-methoxycarbonylcyclohex-1-yloxy (2-CH3OCOC6H10O—), 4-nitromethylcyclohex-1-yl (i.e., NO2CH2C6H10—), 3-trimethylsilylcyclohex-1-yl, 2-t-butyldimethylsilylcyclopent-1-yl, 4-trimethoxysilylethylcyclohex-1-yl (e.g., (CH3O)3SiCH2CH2C6H10—), 4-vinylcyclohexen-1-yl, vinylidenebis(cyclohexyl), and the like. The term “a C3-C10 cycloaliphatic radical” includes cycloaliphatic radicals containing at least three but no more than 10 carbon atoms. The cycloaliphatic radical 2-tetrahydrofuranyl (C4H7O—) represents a C4 cycloaliphatic radical. The cyclohexylmethyl radical (C6H11CH2—) represents a C7 cycloaliphatic radical.
As used herein the term “aliphatic radical” refers to an organic radical having a valence of at least one consisting of a linear or branched array of atoms which is not cyclic. Aliphatic radicals are defined to comprise at least one carbon atom. The array of atoms comprising the aliphatic radical may include heteroatoms such as nitrogen, sulfur, silicon, selenium and oxygen or may be composed exclusively of carbon and hydrogen. For convenience, the term “aliphatic radical” is defined herein to encompass, as part of the “linear or branched array of atoms which is not cyclic” a wide range of functional groups such as alkyl groups, alkenyl groups, alkynyl groups, haloalkyl groups, conjugated dienyl groups, alcohol groups, ether groups, aldehyde groups, ketone groups, carboxylic acid groups, acyl groups (for example carboxylic acid derivatives such as esters and amides), amine groups, nitro groups, and the like. For example, the 4-methylpent-1-yl radical is a C6 aliphatic radical comprising a methyl group, the methyl group being a functional group which is an alkyl group. Similarly, the 4-nitrobut-1-yl group is a C4 aliphatic radical comprising a nitro group, the nitro group being a functional group. An aliphatic radical may be a haloalkyl group which comprises one or more halogen atoms which may be the same or different. Halogen atoms include, for example; fluorine, chlorine, bromine, and iodine. Aliphatic radicals comprising one or more halogen atoms include the alkyl halides trifluoromethyl, bromodifluoromethyl, chlorodifluoromethyl, hexafluoroisopropylidene, chloromethyl, difluorovinylidene, trichloromethyl, bromodichloromethyl, bromoethyl, 2-bromotrimethylene (e.g., —CH2CHBrCH2—), and the like. Further examples of aliphatic radicals include allyl, aminocarbonyl (i.e., —CONH2), carbonyl, 2,2-dicyanoisopropylidene (i.e., —CH2C(CN)2CH2—), methyl (i.e., —CH3), methylene (i.e., —CH2—), ethyl, ethylene, formyl (i.e., —CHO), hexyl, hexamethylene, hydroxymethyl (i.e., —CH2OH), mercaptomethyl (i.e., —CH2SH), methylthio (i.e., —SCH3), methylthiomethyl (i.e., —CH2SCH3), methoxy, methoxycarbonyl (i.e., CH3OCO—), nitromethyl (i.e., —CH2NO2), thiocarbonyl, trimethylsilyl ( i.e., (CH3)3Si—), t-butyldimethylsilyl, 3-trimethyoxysilypropyl (i.e., (CH3O)3SiCH2CH2CH2—), vinyl, vinylidene, and the like. By way of further example, a C1-C10 aliphatic radical contains at least one but no more than 10 carbon atoms. A methyl group (i.e., CH3—) is an example of a C1 aliphatic radical. A decyl group (i.e., CH3(CH2)9—) is an example of a C10 aliphatic radical.
As defined herein, the term “optically transparent” as applied to an optically transparent substrate or an optically transparent plastic material means that the substrate or plastic material has an absorbance of less than 1. That is, at least 10 percent of incident light is transmitted through the material at at least one wavelength in a range between about 300 and about 800 nanometers. For example, when configured as a film having a thickness suitable for use in holographic data storage said film exhibits an absorbance of less than 1 at at least one wavelength in a range between about 300 and about 800 nanometers.
As defined herein, the term “volume element” means a three dimensional portion of a total volume.
As defined herein, the term “optically readable datum” can be understood as a datum that is stored as a hologram patterned within one or more volume elements of an optically transparent substrate.
As defined herein, the term “photochromic conversion” refers to the property of a molecule that it can be converted from a stable state A to a stable state B by a wavelength λA. This conversion is accompanied by a change in the visible absorption spectrum and the refractive index of the material.
As defined herein, the term “reversible photochromic conversion” refers to the property of a molecule that it can be converted from a stable state A to a stable state B by a wavelength λA and subsequently from the stable state B to the stable state A by a wavelength λB.
As noted, holographic data storage relies upon the introduction of localized variations in the refractive index of the optically transparent substrate comprising the photochemically active dye as a means of storing holograms. The refractive index within an individual volume element of the optically transparent substrate may be constant throughout the volume element, as in the case of a volume element that has not been exposed to electromagnetic radiation, or in the case of a volume element in which the photochemically active dye has been reacted to the same degree throughout the volume element. It is believed that most volume elements that have been exposed to electromagnetic radiation during the holographic data writing process will contain a complex holographic pattern, and as such, the refractive index within the volume element will vary across the volume element. In instances in which the refractive index within the volume element varies across the volume element, it is convenient to regard the volume element as having an “average refractive index” which may be compared to the refractive index of the corresponding volume element prior to irradiation. Thus, in one embodiment an optically readable datum comprises at least one volume element having a refractive index that is different from a (the) corresponding volume element of the optically transparent substrate prior to irradiation. Data storage is achieved by locally changing the refractive index of the data storage medium in a graded fashion (for example, continuous sinusoidal variations), and then using the induced changes as diffractive optical elements.
As used herein, the term “dynamic range” is a measure of the data storage capacity of the holographic storage medium. It is related to the number of detectable holograms which can be recorded in the medium and can be equivalently considered as the total refractive index change of storage medium material.
The capacity to store data as holograms is also directly proportional to the ratio of the change in refractive index per unit dye density (Δn/N0) at the wavelength used for reading the data to the absorption cross section (σ) at a given wavelength used for writing the data as a hologram. The refractive index change per unit dye density is given by the ratio of the difference in refractive index of the volume element before irradiation minus the refractive index of the same volume element after irradiation to the density of the dye molecules. The refractive index change per unit dye density has a unit of cm3. Thus in an embodiment, the optically readable datum comprises at least one volume element wherein the ratio of the change in the refractive index per unit dye density of the at least one volume element to an absorption cross section of the at least one photochemically active dye is at least about 10−5 expressed in units of centimeter.
Sensitivity (S) is a measure of the diffraction efficiency of a hologram recorded using a certain amount of light fluence (F). The light fluence (F) is given by the product of light intensity (I) and recording time (t). Mathematically, sensitivity is given by equation (3),
wherein I is the intensity of the recording beam, “t” is the recording time, L is the thickness of the recording (or data storage) medium (example, disc), and n is the diffraction efficiency. Diffraction efficiency is given by equation (4),
wherein λ is the wavelength of light in the recording medium, θ is the recording angle in the media, and Δn is the refractive index contrast of the grating, which is produced by the recording process, wherein the dye molecule undergoes a photochromic conversion.
The absorption cross section is a measurement of an atom or molecule's ability to absorb light at a specified wavelength, and is measured in square cm/molecule. It is generally denoted by σ(λ) and is governed by the Beer-Lambert Law for optically thin samples as shown in equation (5),
wherein N0 is the concentration in molecules per cubic centimeter, and L is the sample thickness in centimeters.
Quantum efficiency (QE) is a measure of the probability of a photochemical transition for each absorbed photon of a given wavelength. Thus, it gives a measure of the efficiency with which incident light is used to achieve a given photochromic conversion. QE is given by equation (6),
wherein “h” is the Planck's constant, “c” is the velocity of light, σ(λ) is the absorption cross section at the wavelength λ, and F0 is the photochromic conversion fluence. The parameter F0 is given by the product of light intensity (I) and a time constant (τ) that characterizes the photochromic conversion process.
Typically, the photochemically active dyes undergo a light induced chemical reaction when exposed to light with a wavelength within the absorption range to form at least one photo-product. This reaction can be a photo-decomposition reaction, such as oxidation, reduction, or bond breaking to form smaller constituents, or a molecular rearrangement, such as a sigmatropic rearrangement, or addition reactions including pericyclic cycloadditions. Thus in an embodiment, data storage in the form of holograms is achieved wherein the photo-product is patterned (for example, in a graded fashion) within the modified optically transparent substrate to provide the at least one optically readable datum.
Photochemically active dyes that are particularly suited for the current invention are dyes that can undergo a reversible photochromic conversion. Thus, in one embodiment, data storage in the form of holograms is achieved wherein the photo-product is patterned (for example, in a graded fashion) within the modified optically transparent substrate by irradiation with a wavelength λ1 to provide the at least one optically readable datum, while irradiation with a wavelength λ2 is provided to further control the photochromic conversion amplitude in the volume element. The irradiation with a wavelength λ2 can either occur simultaneously or sequentially to the irradiation with the wavelength λ1.
Typically, the reversible conversion does not have the same time constant in both directions. Furthermore, the quantum efficiencies for the two reaction pathways and the absorption cross sections of the reaction products are not necessarily the same. Examples of suitable reversible photochromic dyes comprise vicinal diarylethenes, fulgides and fulgimides, spiropyrans, spirooxazines, naphtopyrans and combinations thereof.
Examples of suitable diarylethenes that can be used as photochemically active dyes include but are not limited to diarylperfluorocyclopentenes, diarylmaleic anhydrides, diarylmaleimides, or a combination comprising at least one of the foregoing diarylethenes. The vicinal diarylethenes can be prepared using methods known in the art. The diarylethenes are present as open-ring or closed-ring isomers. In general, the open ring isomers of diarylethenes have absorption bands at shorter wavelengths. Upon irradiation with ultraviolet light, new absorption bands appear at longer wavelengths, which are ascribed to the closed-ring isomers. The absorption spectra of the open and closed-ring isomers may depend on the substituents of the thiophene rings, naphthalene rings or the phenyl rings. The absorption structures of the open and closed-ring isomers may depend upon the upper cycloalkene structures. For example, the open-ring isomers of maleic anhydride or maleimide derivatives show spectral shifts to longer wavelengths in comparison with the perfluorocyclopentene derivatives.
An exemplary class of vicinal diarylethene compounds can be represented by generic structure (I),
wherein “e” is 0 or 1; R1 is a bond, an oxygen atom, a substituted nitrogen atom, a sulfur atom, a selenium atom, a divalent C1-C20 aliphatic radical, a halogenated divalent C1-C20 aliphatic radical, a divalent C3-C20 cycloaliphatic radical, a halogenated divalent C1-C20 cycloaliphatic radical, or a divalent C2-C30 aromatic radical; Ar1 and Ar2 are each independently a C2-C40 aromatic radical, or a C2-C40 heteroaromatic radical; and Z1 and Z2 are independently a bond, a hydrogen atom, a monovalent C1-C20 aliphatic radical, divalent C1-C20 aliphatic radical, a monovalent C3-C20 cycloaliphatic radical, a divalent C3-C20 cycloaliphatic radical, a monovalent C2-C30 aromatic radical, or a divalent C2-C30 aromatic radical. It should be noted that each of the aromatic radicals Ar1 and Ar2 are identical or different as are the groups Z1 and Z2. It will be understood by those skilled in the art that Ar1 may differ in structure from Ar2 and that Z1 may differ in structure from Z2, and that such species are encompassed within generic structure I and are included within the scope of the instant invention.
In another embodiment, e is 0, and Z1 and Z2 C1-C5 alkyl, C1-C5 perfluoroalkyl, or CN. In still another embodiment, e is 1, and Z1 and Z2 are independently CH2, CF2, or C═O. In yet another embodiment, Ar1 and Ar2 are each independently an aromatic radical selected from the group consisting of phenyl, anthracenyl, phenanthrenyl, pyridinyl, pyridazinyl, 1H-phenalenyl and naphthyl, optionally substituted by one or more substituents, wherein the substituents are each independently C1-C3 alkyl, C1-C3 perfluoroalkyl, C1-C3 alkoxy, or fluorine. In yet another embodiment at least one of Ar1 and Ar2 comprises one or more aromatic moieties selected from the group consisting of structures (II), (III), and (IV),
wherein R3, R4, R5, and R6 are hydrogen, a halogen atom, a nitro group, a cyano group, a C1-C10 aliphatic radical, a C3-C10 cycloaliphatic radical, or a C2-C10 aromatic radical; R7 is independently at each occurrence a halogen atom, a nitro group, a cyano group, a C1-C10 aliphatic radical, a C3-C10 cycloaliphatic radical, or a C2-C10 aromatic radical; “b” is an integer from and including 0 to and including 4; X and Y are selected from the group consisting of sulfur, selenium, oxygen, NH, and nitrogen substituted by a C1-C10 aliphatic radical, a C3-C10 cycloaliphatic radical, or a C2-C10 aromatic radical; and Q is CH or N. In one embodiment, at least one of R3, R4, R5, and R6 is selected from the group consisting of hydrogen, fluorine, chlorine, bromine, C1-C3 alkyl, C1-C3 perfluoroalkyl, cyano, phenyl, pyridyl, isoxazolyl, —CHC(CN)2.
The vicinal diarylethenes can be reacted in the presence of actinic radiation (i.e. radiation that can produce a photochemical reaction), such as light. In one embodiment, an exemplary vicinal diarylethene can undergo a reversible cyclization reaction in the presence of light (hν) according to the following equation (V):
where X, Z R1 and e have the meanings indicated above. The cyclization reactions can be used to produce holograms. The holograms can be produced by using radiation to effect the cyclization reaction or the reverse ring-opening reaction. The cyclization reaction is a photochromic reaction, whereby a form change results in change in refractive index. Typically, hν′ is lower in energy (longer wavelength) than hν. Typically, if the cyclization reaction is initiated at an ultra violet wavelength, then the reverse ring opening reaction typically occurs at a visible or infrared wavelength.
As described above, cyclization reactions can be used to produce holograms. The holograms can be produced by using radiation to effect the cyclization reaction or the reverse ring-opening reaction. Thus, in an embodiment, a photo-product derived from a vicinal diarylethene can be used as a photochemically active dye. Such photo-products derived from the vicinal diarylethene can be represented by a formula (VI),
wherein “e”, R1, Z1, and Z2 are as described for the vicinal diarylethene having formula (I), A and B are fused rings, and R8 and R9 are each independently a hydrogen atom, an aliphatic radical, a cycloaliphatic radical, or an aromatic radical. One or both fused rings A and B may comprise carbocyclic rings which do not have heteroatoms. In another embodiment, the fused rings A and B may comprise one or more heteroatoms selected from the group consisting of oxygen, nitrogen, and sulfur.
A single wavelength bit wise holographic recording system 10 is illustrated in
As shown schematically in
The photochromic conversion process of the dyes can be analyzed using a rate equation model. The total density of the dye molecules is N0, the density of dyes in the ring-open form is N(t), and a beam, for example blue beam, has the intensity Ib, For a single wavelength photochromic conversion process the photochromic conversion dynamics of the dyes for a ring-open to ring close cyclization reaction, can be described using the following rate equation:
where σb is absorption cross-section of the dyes at the blue wavelength when the dyes are in the ring-open form, ηb is quantum efficiency of the dye transition from the ring-open to the ring-close form when a photon at blue wavelength is absorbed, λb is the blue wavelength, h is the Planck constant, c is the speed of light.
Reversible diarylethene dyes can be bleached in a reversible fashion as indicated above. This result is illustrated in
For a dual wavelength photochromic conversion process where a blue beam effects the cyclization reaction and a green beam effects the reverse ring opening reaction, the rate equation is given by
where σg is absorption cross-section at the green wavelength when the dyes are in the ring-close form; Ig, the green beam intensity, λg the green wavelength, ηg is quantum efficiency for transition from the ring-close to the ring-open form when a photon at green wavelength is absorbed.
Solution to this rate equation is:
where Nt→∞ is density of the dyes in the ring-open at the steady state, τ is photochromic conversion time constant for the dye transition from the ring-open to the ring-close form for dual wavelength photochromic conversion with both blue and green illumination, τb is the photochromic conversion time constant for the dye transition from the ring-open to the ring-close form if there is only a blue beam illumination with intensity Ib, and τg is the photochromic conversion time constant for the dye transition from the ring-close to the ring-open form if there is only a green beam illumination with intensity Ig.
Photochromic conversion fluence F (mJ/cm2) and normalized photochromic conversion amplitude A are two parameters that can be used to describe a photochromic conversion process. The photochromic conversion fluence F is a product of the beam intensity I and time constant τ of the photochromic conversion process. Normalized photochromic conversion amplitude A is a ratio of a change of the transmitted power to the initial transmitted power at the beginning of the photochromic conversion. Both parameters are determined by the internal properties of the dyes (such as absorption cross-section, quantum efficiency) and light beam intensities, and are independent of dye concentrations and material uniformity. Both parameters can be measured experimentally.
The single wavelength photochromic conversion fluence for example, blue and green illumination, Fb, Fg respectively are given by
Physically, Fb is the photochromic conversion fluence, when there is only a blue beam illumination, and Fg is the photochromic conversion fluence when there is only a green beam illumination. The dual wavelength (blue and green) illumination photochromic conversion fluence is given by
For a dual wavelength, blue beam and green beam, illumination, the cyclization photochromic conversion fluence F is given by equation 11. This equation shows that the cyclization photochromic conversion fluence is directly impacted by the intensity ratio Ig/Ib. When Ig goes to zero, i.e., no green beam illumination, F equals Fb, as expected. As the intensity ratio increases, photochromic conversion fluence F decreases.
The normalized cyclization photochromic conversion amplitude A is given by
As the intensity ratio increases, the normalized photochromic conversion amplitude decreases. The normalized photochromic conversion amplitude A and the photochromic conversion fluence F have the same dependence on the intensity ratio Ig/Ib. F and A are mutually related by: F=FbA.
In a single-bit system, each bit is an interference grating produced by two counter-propagating beams, typically with Gaussian intensity profiles that overlap at their focuses. Ideally, the bit has a size of the beam waist in transverse dimensions and a size of a couple of Rayleigh range (ZR) in longitudinal dimension. Data bits are arranged layer by layer. The distance between two adjacent layers could be as small as twice that of the bit depth. Data capacity increases linearly with number of layers.
Assume a recording beam to be a focused Gaussian beam at blue wavelength, and fluence used for recording a single bit to be F0, with N recording layers in a disk. At a fixed location in layer L(i), a total fluence at this location during recording (all data bits) at a different layer L(j) is ˜F0/2, assuming half of the bits are 1 and half are 0. This is roughly independent of the distance D between these layers, as beam exposure time at that location scales as D2, while beam intensity scales as 1/D2 (due to a longer distance between the two layers), and these two factors cancel each other.
In a single-wavelength technique, in an N layer system, total fluence experienced by the layer L(i) during recording all the other (N−1) layers is (N−1)F0/2. This is a background fluence that consumes material dynamic range but does not contain any data information. Compared with the fluence F0 for recording a bit, this background fluence is ˜N/2 times higher. This means that the dynamic range that is usable for data is reduced to ˜(2/N) of the total dynamic range of the material, i.e., the majority of the material dynamic range is wasted as a result of undesirable background dye photochromic conversion during recording at adjacent layers. The usable dynamic range scales down linearly with the number of layers N. The higher the number of data layers, the greater the loss of the dynamic range. This presents a very serious problem for high capacity storage.
Now considering the case when dual wavelengths, for example, blue and green are used, with the blue beam forming the ring-closed product and the green beam reversing this process to regenerate the initial ring-open form. The green beam is therefore added to control the cyclization photochromic conversion process. Assuming the green beam is a plane wave throughout the sample, i.e., intensity Ig is uniform, for a fixed location at the layer L(i), total fluence of the blue light experienced by that location during recording at a single different layer is still F0/2. However, in the dual wavelength case, background photochromic conversion amplitude is not determined by the fluence of the blue beam. Rather, it depends on the intensity ratio of the green beam to the blue beam at that location. The higher the intensity ratio, the lower the normalized photochromic conversion amplitude. Furthermore, the total background photochromic conversion amplitude is not a summation of background photochromic conversion from all other layers. Rather, it is limited by the highest background photochromic conversion amplitude from a single layer due to the balance of the backward and forward photochromic conversion dynamics induced by the two wavelengths. The highest background photochromic conversion for L(i) comes from recording at its adjacent layer L(i−1) or L(i+1), as the intensity ratio Ig/Ib is the smallest in this situation. Assuming the distance between two adjacent layers is 4ZR, blue beam intensity Ib at L(i), adjacent to the recording layer, is ˜(¼)2Ib,0, Ib,0 is blue beam intensity at the recording layer. Based on the two-wavelength photochromic conversion analysis presented above, the normalized photochromic conversion amplitude at L(i) is 1/(1+16α), where α=(Fb/Fg)(Ig/Ib,0), where Ib,0 is the intensity at the focal point (peak intensity). The normalized photochromic conversion amplitude at the recording layer is 1/(1+α). In one embodiment, the focal point is found with the recording volume element. The usable dynamic range is 1/(1+α)-1/(1+16α). By adjusting Ig, α can be adjusted. The usable dynamic range could be ˜60% if α is ˜0.25. Compared with the single wavelength photochromic conversion scenario, in the case of dual wavelength photochromic conversion, the usable dynamic range does not demonstrate a decaying scaling relationship with the number of layers. This is a very significant advantage for multi-layer storage.
In accordance with one embodiment of the present invention is a method for bit-wise recording of holographic data. The method disclosed herein enables higher data capacity by preventing the loss of dynamic range due to background illumination during recording. The method includes irradiating an optically transparent substrate simultaneously with a holographic interference pattern and a photochromic conversion control illumination.
In one embodiment, the holographic interference pattern and the photochromic conversion control illumination irradiate the optically transparent substrate simultaneously. In another embodiment, the holographic interference pattern and the photochromic conversion control illumination irradiate the optically transparent substrate sequentially. In a non-limiting example, the optically transparent substrate may be illuminated by the holographic interference pattern followed by the photochromic conversion control illumination or vice versa. In a further embodiment, the time periods of irradiation of the holographic interference pattern and the photochromic conversion control illumination, overlap. In a still further embodiment the time period of illumination of the holographic interference pattern is a subset of the time period of the irradiation of the photochromic conversion control illumination. In a non-limiting example, the photochromic conversion control illumination starts before the onset of the illumination by the holographic interference pattern and is on for a period of time after the illumination by the holographic interference pattern has ceased. In a further example, the irradiation by holographic interference pattern and the photochromic conversion control illumination start simultaneously and end at different times.
In a further embodiment, the control beam illuminates a volume of the optically transparent substrate overlapping at least in part a volume illuminated by the holographic interference pattern. In a still further embodiment, the photochromic conversion control illumination is a beam at an angle to the recording beams. In one embodiment, the angle is in a range of about 0 degrees to plus or minus 180 degrees. In a further embodiment, the angle is in a range of about plus or minus 0 degrees to plus or minus 90 degrees.
In some embodiments, the first wavelength is selected to be in a range from about 350 nanometers to about 450 nanometers. In further embodiments, the first wavelength is selected to be in a range from about 375 nanometers to about 425 nanometers. Similarly, in some embodiments, the second wavelength is selected to be in a range from about 450 nanometers to about 900 nanometers. In further embodiments, the second wavelength is selected to be in a range from about 500 nanometers to about 700 nanometers.
The ratio of the intensity of holographic interference pattern can be altered to obtain a desired photochromic conversion amplitude in a selected holographic medium. In one embodiment, I2/I1 is in a range from about 0.02 to about 4. In accordance with another embodiment of the present invention is a dual wavelength system for holographic data recording. The system includes a holographic interference pattern generating source, wherein the interference pattern generating source emits at a first wavelength λ1 and an intensity I1. The system further includes a photochromic conversion control source, wherein the photochromic conversion control source emits at a second wavelength λ2 not equal to λ1 and an intensity I2.
Referring to
In accordance with another embodiment of the present invention, is a pre-recorded holographic storage medium. The pre-recorded holographic data storage medium is prepared by a method including the steps of providing an optically transparent substrate including a photochemically active dye and irradiating the optically transparent substrate with a holographic interference pattern and a photochromic conversion control illumination producing an optically readable datum corresponding to a volume element. In one embodiment the holographic data storage medium includes greater than 4 recorded layers in the thickness of the holographic data storage medium. In a further embodiment, the holographic data storage medium includes greater than 10 recorded layers in the thickness of the holographic data storage medium. In a still further embodiment, the holographic data storage medium includes greater than 20 recorded layers in the thickness of the holographic data storage medium. In one embodiment, the holographic data storage medium includes greater than 40 recorded layers in the thickness of the holographic data storage medium. In some embodiments of the present invention, the data storage medium has an areal density of individual data bits greater than 0.01 bits of data per square micron.
Optically transparent plastic materials may be advantageously employed in the preparation of the optically transparent substrate. Optically transparent plastic materials used in producing holographic data storage media (such as the optically transparent substrate) can comprise any plastic material having sufficient optical quality, e.g., low scatter, low birefringence, and negligible losses at the wavelengths of interest, to render the data in the holographic storage material readable.
Organic polymeric materials, such as for example, oligomers, polymers, dendrimers, ionomers, copolymers such as for example, block copolymers, random copolymers, graft copolymers, star block copolymers; and the like, or a combination comprising at least one of the foregoing polymers can be used. Thermoplastic polymers or thermosetting polymers can be used. Examples of suitable thermoplastic polymers include polyacrylates, polymethacrylates, polyamides, polyesters, polyolefins, polycarbonates, polystyrenes, polyesters, polyamideimides, polyaromaticates, polyaromaticsulfones, polyethersulfones, polyphenylene sulfides, polysulfones, polyimides, polyetherimides, polyetherketones, polyether etherketones, polyether ketone ketones, polysiloxanes, polyurethanes, polyaromaticene ethers, polyethers, polyether amides, polyether esters, or the like, or a combination comprising at least one of the foregoing thermoplastic polymers. Some more possible examples of suitable thermoplastic polymers include, but are not limited to, amorphous and semi-crystalline thermoplastic polymers and polymer blends, such as: polyvinyl chloride, linear and cyclic polyolefins, chlorinated polyethylene, polypropylene, and the like; hydrogenated polysulfones, ABS resins, hydrogenated polystyrenes, syndiotactic and atactic polystyrenes, polycyclohexyl ethylene, styrene-acrylonitrile copolymer, styrene-maleic anhydride copolymer, and the like; polybutadiene, polymethylmethacrylate (PMMA), methyl methacrylate-polyimide copolymers; polyacrylonitrile, polyacetals, polyphenylene ethers, including, but not limited to, those derived from 2,6-dimethylphenol and copolymers with 2,3,6-trimethylphenol, and the like; ethylene-vinyl acetate copolymers, polyvinyl acetate, ethylene-tetrafluoroethylene copolymer, aromatic polyesters, polyvinyl fluoride, polyvinylidene fluoride, and polyvinylidene chloride.
In some embodiments, the thermoplastic polymer, used in the methods disclosed herein as a substrate, is made of a polycarbonate. The polycarbonate may be an aromatic polycarbonate, an aliphatic polycarbonate, or a polycarbonate comprising both aromatic and aliphatic structural units.
Examples of useful thermosetting polymers include those selected from the group consisting of an epoxy, a phenolic, a polysiloxane, a polyester, a polyurethane, a polyamide, a polyacrylate, a polymethacrylate, or a combination comprising at least one of the foregoing thermosetting polymers.
The photochemically active dye may be admixed with other additives to form a photo-active material. Examples of such additives include heat stabilizers; antioxidants; light stabilizers; plasticizers; antistatic agents; mold releasing agents; additional resins; binders, blowing agents; and the like, as well as combinations of the foregoing additives. The photo-active materials are used for manufacturing holographic data storage media.
Cycloaliphatic and aromatic polyesters can be used as binders for preparing the photo-active material. These are suitable for use with thermoplastic polymers, such as polycarbonates, to form the optically transparent substrate. These polyesters are optically transparent, and have improved weatherability, low water absorption and good melt compatibility with the polycarbonate matrix. Cycloaliphatic polyesters are generally prepared by reaction of a diol with a dibasic acid or an acid derivative, often in the presence of a suitable catalyst.
Generally, the photochemically active dyes and polymers used for forming the optically transparent substrate, and the holographic data storage medium should be capable of withstanding the processing conditions used to prepare the holographic data storage medium, for example during a step in which the photochemically active nitrone and any additional additives which may be present are compounded with a polymer powder and subsequently molded into data storage discs.
In an embodiment, the photochemically active dye is present in an amount from about 0.1 to about 10 weight percent, based on the total weight of the optically transparent substrate, and has a UV-visible absorbance in a range between about 0.1 and about 1 at a wavelength in a range between about 300 nm and about 800 nm. Such dyes are used in combination with other materials, such as, for example, binders to form photo-active materials, which in turn are used for manufacturing holographic data storage media.
In one embodiment, a film of an optically transparent substrate including an optically transparent plastic material and at least one photochemically active dye is formed. Generally, the film is prepared by molding techniques using a molding composition that is obtained by mixing the dye with an optically transparent plastic material. Mixing can be conducted in machines such as a single or multiple screw extruder, a Buss kneader, a Henschel, a helicone, an Eirich mixer, a Ross mixer, a Banbury, a roll mill, molding machines such as injection molding machines, vacuum forming machines, blow molding machine, or then like, or a combination comprising at least one of the foregoing machines. Alternatively, the dye, and the optically transparent plastic material may be dissolved in a solution and films of the optically transparent substrate can be formed from the solution.
In one embodiment a data storage composition including a photochemically active dye and a thermoplastic polymer is injection molded to form an article that can be used for producing holographic data storage media. The injection-molded article can have any geometry. Examples of suitable geometries include circular discs, square shaped plates, polygonal shapes, or the like. The thickness of the articles can vary, from being at least 100 micrometers in an embodiment, and at least 250 micrometers in another embodiment. A thickness of at least 250 micrometers is useful in producing holographic data storage disks that are comparable to the thickness of current digital storage discs.
Without further elaboration, it is believed that one skilled in the art can, using the description herein, utilize the present invention to its fullest extent. The following examples are included to provide additional guidance to those skilled in the art in practicing the claimed invention. The examples provided are merely representative of the work that contributes to the teaching of the present application. Accordingly, these examples are not intended to limit the invention, as defined in the appended claims, in any manner.
Several samples of diarylethene (structural formula VII) doped thermoplastic disks were prepared. The dye (VII) was prepared according to standard procedures known in the art. The dye was blended with polycarbonate optical quality powder and the blend injection molded to form small disks. The disks were about 5 cm in diameter and 1 mm in thickness. The diarylethene concentration was about 0.26 wt %.
The power of the blue beam incident on the sample was fixed at 11.70 mW (milliwatts), while the power of the green beam was varied from 0.99 mW to 135 mW. Spot size of the blue beam on the sample was 4.1 mm in diameter. Spot size of the green beam was 6 mm in diameter and it projected an elliptical spot on the disk. Monitoring of the photochromic conversion process lasted for a few hundred seconds, until a steady state was reached.
Table 1 shows averages of photochromic conversion fluence and normalized photochromic conversion amplitude at three different Ig/Ib levels. As the intensity ratio Ig/Ib increases from 0.028 to 3.8 (i.e., by a factor of about 100), both the photochromic conversion fluence and normalized photochromic conversion amplitude decrease by roughly a factor of 3. At an intensity ratio of 0.028, the photochromic conversion fluence is 2385 mJ/cm2, which is very close to that in a single blue beam illumination case (˜2434 mJ/cm2).
While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.
