Classifications

• • G—PHYSICS
  • G11—INFORMATION STORAGE
  • G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
  • G11B7/00—Recording or reproducing by optical means, e.g. recording using a thermal beam of optical radiation by modifying optical properties or the physical structure, reproducing using an optical beam at lower power by sensing optical properties; Record carriers therefor
  • G11B7/24—Record carriers characterised by shape, structure or physical properties, or by the selection of the material
  • G11B7/241—Record carriers characterised by shape, structure or physical properties, or by the selection of the material characterised by the selection of the material
  • G11B7/242—Record carriers characterised by shape, structure or physical properties, or by the selection of the material characterised by the selection of the material of recording layers
  • G11B7/244—Record carriers characterised by shape, structure or physical properties, or by the selection of the material characterised by the selection of the material of recording layers comprising organic materials only
  • G11B7/245—Record carriers characterised by shape, structure or physical properties, or by the selection of the material characterised by the selection of the material of recording layers comprising organic materials only containing a polymeric component
• • G—PHYSICS
  • G11—INFORMATION STORAGE
  • G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
  • G11B7/00—Recording or reproducing by optical means, e.g. recording using a thermal beam of optical radiation by modifying optical properties or the physical structure, reproducing using an optical beam at lower power by sensing optical properties; Record carriers therefor
  • G11B7/24—Record carriers characterised by shape, structure or physical properties, or by the selection of the material
  • G11B7/241—Record carriers characterised by shape, structure or physical properties, or by the selection of the material characterised by the selection of the material
  • G11B7/242—Record carriers characterised by shape, structure or physical properties, or by the selection of the material characterised by the selection of the material of recording layers
  • G11B7/244—Record carriers characterised by shape, structure or physical properties, or by the selection of the material characterised by the selection of the material of recording layers comprising organic materials only

Definitions

• This invention relates to methods for improving writing and reading in a 3- dimensional optical memory.
• a 3 -dimensional memory apparatus employing various substituted stilbenes as the active medium was disclosed in WO 03/070689.
• the active medium is irradiated for writing the data where the reading is done by measuring the fluorescence.
• fluorescent dyes tend to be highly insoluble materials as a consequence of their large conjugated systems and the fact that many fluorescent dyes are ionic compounds.
• formulations for disk production, and disks themselves can be unstable with respect to phase separation and crystallization of the dye material resulting in non-uniform disks.
• Some solubility enhancement can be achieved by the choice of appropriate counter-ions, the addition of solubilizing substituents to the molecular structure, and the appropriate engineering of the media matrix.
• a disk medium comprising very high concentrations of chromophores which will retain its necessary optical properties and lifetime are extremely difficult to obtain.
• 3D data storage solutions also suffer from problems relating to the formation of the data patterns within them.
• the nonlinear optical processes that are used to transform unwritten points in the media to written points require high laser powers that make commercialization difficult, and the fact that data is written by light limits its resolution and therefore the data density and indirectly the transfer rate (since there is a limit to the rate at which a disk may be spun).
• Another drawback of photochromic media is the destructive reading. This effect is essentially a small amount of "writing" that occurs every time a data point is read. Consequently, after a limited number of read cycles the data may become effectively erased.
• optical data media with high concentration of chromophore and in particular active chromophore are provided. Such media facilitate enhanced writing of data, as will be further explained below.
• Cooperativity between different active groups within the photochromic medium, whereby one light-activated group enhances the reactivity to light of another, adjacent group, may play a role and provide the basis for the enhanced writing effect.
• Preferred chromophores for flouoresenctic optical memory are such in which there is minimal overlap between the spectrum of absorption and the spectrum of fluorescence.
• Methods for enhancing the write ability in a photochromic medium are provided. Also provided are modified photochromic media useful for storing data.
• the method for enhancing light-induced conversion of an active photochromic group from one state to another in a polymer-based photochromic medium comprises providing conditions permitting cooperativity between chemical groups in said medium.
• a method for producing a polymer-based photochromic medium with an enhanced light-induced conversion of an active photochromic group from one state to another comprising polymerizing the medium under conditions yielding a high concentration of the active photochromic group in said medium.
• Also provided by the invention is a method for enhancing the light-induced conversion of an active photochromic group from one state to another in a polymer- based pliotochromic medium, comprising providing a medium having a high concentration of the active photochromic group.
• a particular embodiment is the enhancement of data inscription in a 3- dimentional optical memory.
• the overlap between the absorbance-emission spectra of the chromophore should be substantially less than 70% for multilayer optical storage, preferably less than 50%, at times less than 35% and most preferably less than 20%.
• polymer-based photochromic media prepared or provided as defined above. Further provided are three-dimensional storage media comprising said polymer-based photochromic media.
• polymer-based photochromic medium denotes a polymeric medium that comprises photochromic groups that can change their state from one state to another upon electromagnetic irradiation at an appropriate wavelength, e.g. an irradiation of ultraviolet (UV), visible or near infrared (IR) light.
• the change of state may be a switch between cis and trans configurations, between spiropyran and merocyanine forms, between the "open” and “closed” forms of diarylethenes and fulgides, between the two forms of phenoxynaphthacene quinines or a change in orientation e.g. in azobenzenes, that leads to a change in reactivity or the locality to irradiation.
• marks may be inscribed that differ in their optical interaction in unmarked regions.
• the dimensions of the marks are usually dictated by the ability to focus light and are typically wavelength or subwavelength dimensions so as are the localities of interest in photochromic media .
• a polymer-based photochromic medium may be prepared by polymerization of monomers with a photochromic moiety (photochromic monomers) or co-polymerization of photochromic monomers and non-photochromic monomers, as will be detailed further below.
• the polymer may be of different kinds.
• a preferred polymer is an acrylic polymer. - A -
• enhanced light-induced conversion denotes a light-induced change in the state of the photochromic group that either occurs at a faster rate, requires less light irradiation pulses or shorter pulses or requires a lower light flux than without enhancement.
• photochromic group at times referred to also as “active chromophore" denotes a group that can undergo an electromagnetic irradiation-induced change leading to a change of state of a locality of a photochromic medium from a non- mark to a written marked location or vise versa, in a manner as defined above.
• An active chromophore is a chromophore that is reactive to electromagnetic irradiation, typically but not exclusively in the UV, visible or near infrared light, such that it is capable of changing its state.
• An active chromophore has at least two states and switches from one state to another at a one wavelength different than the switch back or a switch to a further state.
• the concentration ratio between the two states may represent a data unit. Reading of the data may be performed through measurement of an optical response that is different for different states of the active chromophore, such optical response may be light scattering or fluorescence at given three-dimensional location of the medium.
• high concentration refers to a concentration greater than 5% (wt%), preferably greater than 10, 20%, 30%, 40% or even 50%, most preferably above 60%, at times above 70%, desirably above 80% and most desirably above 90%. These wt% concentration are calculated on the basis of the overall weight of the monomers with a photochromic moiety versus the entire weight of monomers in the medium.
• Cooperative effect refers to an effect by which a non-linear positive increase in writing sensitivity is caused by an increase of the concentration of the active chromophore.
• a photochromic medium having a concentration of the active chromophore substantially higher than concentration obtained in prior art media is provided.
• the active chromophore may be homogeneously distributed throughout the medium such that the concentration of the active chromophore in each volume unit is substantially the same.
• the modified photchromic medium may be a non homogeneous medium comprising localities of higher concentration of active chromophore separated by regions of lower concentration of active chromophore or regions that do not contain a photochromic substance.
• Such media may be prepared by the methods described below and referred to as “nano-particles" and "partial solvatation".
• the important requirement from said chemical heterogeneity is that the scale of the heterogeneity features will be significantly smaller than the wavelengths of the interrogating/ interacting light.
• the chemically heterogeneous medium is optically homogeneous and the light passing through the medium does not suffer aberrations by the chemical heterogeneity.
• a photochromic medium which may be part of a 3 -dimensional (3D) optical memory may be advantageous for the data reading and writing characteristics of the medium.
• the present invention is directed to an improved photochromic medium, particularly, but not exclusively, such intended for data storage, said medium comprising a supporting polymeric substance and an active chromophore bound to the supporting substance.
• the photochromic medium according to one embodiment being characterized in that the active chromophore is included in the polymeric substance at a weight percent concentration greater than 5% (wt%), preferably greater than 10, 20%, 30%, 40% or even 50%, most preferably above 60%, at times above 70%, desirably above 80% and most desirably above 90%.
• the photochromic medium according to another embodiment is characterized in that the polymeric substance comprises a non-active additive altering the microenvironment of the active chromophore.
• the polymeric material is typically an essentially transparent medium.
• the essential transparency means that a substantial portion of the electromagnetic irradiation will penetrate therethrough without being absorbed or scattered.
• the increase in chromophore concentration may be achieved by chemically modifying the chromophore structure at locations which do not alter its chromophoric characteristics by binding it to ploymerizable monomeric moieties.
• a modified monomer is produced and by polymerizing such modified monomers or by co- polymerizing these modified monomers with other monomers, a polymer is produced in which the pliotochromic groups are linked to the polymeric matrix, either pending or in the polymeric backbone.
• the polymer is a homopolymer comprised of polymerizable active chromophore monomer.
• concentration of the chromophoric group may be achieved through the use of different modified monomers, each with slightly different solubility characteristics in a monomelic solution.
• the total concentration of the basic chromophoric moiety in the pliotochromic medium is then the sum of the concentrations of the somewhat different compounds all having the identical basic chromophoric moiety.
• the polymer is a polymethyhnethacylate based polymer wherein the active chromophoric groups are stilbene derivatives of the following formula (I):
• Ax 1 and Ax 2 are phenyl groups optionally independently substituted with one or more groups selected from -C ⁇ aUcyls, -OC 1-6 alkyl, -SC ⁇ alkyl and, -C 1-6 OH, thiols and their salts, NR 1 R", R' and R" being independently hydrogen or C 1-6 alkyl;
• R 1 and R 2 are s ⁇ bstituents selected from nitriles selected from -(CH 2 ) n CN, n being O, 1 or 2, halides, carboxylic acids, their esters, or a nitro compound selected from - (CBb) n NO 2 , n being 0, 1 or 2.
• C 1-6 alkyls may be straight or branched alkyls, preferably a methyl, ethyl, propyl, isopropyl, butyl, sec-butyl or tert-butyl as well as pentyl groups; the nitrile is preferably a -CN group and the nitro compound is preferably an -NO 2 group.
• a polymerizable active chromophore monomer useful in accordance with the invention is preferably a compound of the following formula (II):
• M is a polymerizable monomeric moiety.
• M are acrylic monomers such as methylmethacrylate (MMA) and methylacrylate (MA) derivatives or a styrene-based monomer.
• photochromic-modified monomers are those of the following formula (III):
• X is methyl or hydrogen; n is an integer of 2 to 7; Y is hydrogen or a linear or branched alkyl moiety having 1 to 8 carbon atoms optionally substituted with halogens.
• polymerizable active chromophore monomers of the following formula (IV) and (V) (also referred to herein as “eMMA” and “eAA”, respectively):
• a preferred Styrene derivative is that in which Y is ethyl
• the photochromic group is preferably one with no or small absorbance-emission overlap.
• the active chromophore may be bonded in the non chromophoric supporting polymeric medium in a non homogeneous distribution, such that the medium comprises discrete portions of high concentration of active chromophore separated by non photochromic supporting medium.
• the active medium in the form of a monomelic active medium is incorporated together with a similar monomer into a different, non compatible polymer, thereby creating an interpenetrating network (IPN).
• IPN interpenetrating network
• the "eMMA” or “eAA” together with the appropriate monomer are incorporated into polystyrene to form an interpenetrating network of "eMMA” or “eAA” within the styrene network which serves as a structural frame for the polymeric chromophore "eMMA” or "eAA”.
• Fig. 1 illustrates the temperature dependence of the rate of "writing" in a disc made according to the present invention.
• Fig. 2 illustrates the fluorescence of 4 points in an ePMMA based chromophoric medium emitted during the "writing" process at a temperature of 90°C.
• Fig. 3 illustrates the fluorescence of 4 points in an ePMMA based chromophoric medium emitted during the "writing" process at a temperature of 30°C.
• Fig. 4 illustrates 2D scanning of the 4 spots of figure 2.
• Fig. 5 illustrates 2Dscanning of the 4 spots of figure 3.
• Fig. 6 illustrates a ID scan through the 3 lower spots of figure 2.
• Fig. 7 illustrates a ID scan through the 3 lower spots of figure 3.
• Fig. 8 illustrates the absorbance and emission spectra of the compound (I) of the present invention demonstrating the very small overlap between the absorbance and emission.
• Fig. 9 illustrates the huge overlap between absorbance and emission in two commercial fluorescent dyes, (A) Fluorescin and (B) Rhodamine.
• the present invention provides a modified photocliromic medium where the concentration of the active cnromophore may be increased, however not necessarily in a homogeneous manner.
• the high concentration of the active chromophore may be 100%, i.e. the entire medium is comprised of the active chromophore.
• Use of very high concentrations of active chromophores in a photochromic medium is beneficial to the photocliromic medium.
• the photochromic medium is part of a 3 -dimensional (3D) optical memory such a high concentration gives rise to a major improvement (further to linear improvements) in data writing characteristics.
• the increase in the chromophore concentration is more efficient than merely an increase in the power required to write data and/or data resolution.
• the underlining mechanism common to both ATE and other cooperative effects is a neighboring cooperative effect, where neighboring chromophores affect each other increasing the rate of writing (e.g. the rate in which the active chromophore isomerizes, under the appropriate electromagnetic irradiation, from one form to another).
• the microenvironment can be fine-tuned to supplement and enhance cooperativity by designing the matrix such that the chromophores are surrounded by appropriate chemical groups that have appropriate molecular freedom of movement, and are connected to the polymer chain affecting its mobility. This may be achieved by the addition of additives and co-monomers. Such a microenvironment gives the polymer chains appropriate mobility and an appropraite polarity. Such additives are described in co-pending, co-owned application entitled "Additives and co-monomers for the enhancement of polymer properties".
• the ATE is a light-induced local heating of a material that is undergoing some kind of photochemical process.
• the heating does not necessarily derive from the measured photochemical process, and is usually thought of as an undesirable phenomenon.
• the photochromic molecule when data is being written, the photochromic molecule undergoes photochemical excitation, absorbing energy from the incident light.
• the energy of the excited state of the molecule may be lost in at least one of the following three ways: (i) by fluorescence or direct energy transfer to another molecule; (ii) by chemical reaction e.g. photochromic isomerization, or (iii) by a nonradiative thermal loss of energy.
• these may be defined generally as effects that cause nonlinear improvements in processes as a function of concentration.
• a photocliromic media it can be described as an increased probability that an active chromophore will switch its state if it is spatially close to another chromophore that is undergoing switching of its own state. Most likely, a significant improvement in the probability of switching only occurs in a photochemically excited chromophore.
• the other cooperative effects can be envisaged as follows. When a photochromic molecule (chromophore) switches its state, it causes a local disturbance in the molecular-level structure of the medium. This disturbance results in stresses and free volume changes around nearby chromopliores, and these effects make it easier for these additional molecules to also switch.
• the cooperative effect also allows the improvement of data resolution in the same way the ATE does.
• the cooperative effect as described is clearly most important for chromophores that sweep out a large volume during isomerization, such as stilbenes undergoing a cis- trans isomerization, and the use of such photochromes is therefore a noted aspect of the invention.
• Another example of a cooperative effect occurs when energy transfer between chromophore molecules allows a single absorbance even to cause more than one chromophore to switch state, and a third example arises from the fact that high concentrations of nonlinear optical chromophores can exhibit large enhancements in their two-photon cross-sections.
• the strength of both the ATE and other cooperative effects depend on three important factors: (i) the amount of photochemical excitation and its concurency; (ii) the concentration of chromophores; (iii) the microenvironment of the chromophores.
• the importance of the amount of photochemical excitation may be utilized in order to achieve differentiation between the "reading” and "writing” of data in a 3D optical medium comprising the photchromophoric medium of the present invention.
• the "writing” is the process of irradiation resulting in the switching of a chromophore's state (e.g.
• cis-trans isomerization and "reading” is the process of irradiating the "written" data (marks and spaces) and ascertaining the various states of the chromophore. That is to say, a low incident laser power is used for reading data from the disk.
• This low power provides only a small amount of excitation (i.e. the concentration of chromophores in excited states is low, even if the concentration of all chromophores is high), so the ATE and other cooperative effects are both minimal and thus the isomerization of chromophores is not enhanced.
• the low power may be utilized in high peak power pulses that are temporally separated so that the processes such as heat accumulation do not become significant.
• the writing event is not necessarily limited to one pulse as long the energy transmitted to the medium is localized in space and time, thus a pulse train burst may be used, were preferably the burst duration is limited to less than a couple of hundreds of nanoseconds to minimize the spread of the writing event is space due to disk rotation and the heat diffusion period.
• One aspect of the invention is therefore the use of different incident laser powers for the reading and writing of data, in order to control the enhancement of chromophore switching.
• concentration of the chromopliore may also be utilized.
• the ATE and cooperative effects are both increased.
• a critical aspect of the invention is thus to synthesize disks that have very high concentrations of chromophores in order to achieve this enhancement. Normally, high concentrations of chromophores are not thought to be useful, since they cause problems in the excessive absorbance of signal light and for 1 -photon reading or recording erasing also of addressing beam, make the synthesis of media very difficult, and can cause unwanted photochemical reactions such as self-quenching.
• high concentration may yield a polymer that is too fragile, flexible or otherwise mechanically incompatible for disk manufacturing.
• the high concentration of the chromophore does not need to be uniform throughout the media, so long as the areas where the chromophore is present contain a high concentration.
• disks may contain a nanostructure where small localities containing high concentrations of chromophore reside within regions which contains no chromophore at all thus the nanostructure may provide the required mechanical properties.
• the disk may have a micro- or macro-structure (such as a layered or skeleton structure), where only the chromophore-bearing portions are used to actively store data (WO 03/070,689). Another option is use of a fortified macro-structure as described in US application No. 11/290,818.
• Such a multiphase system comprising small (sub-wavelength) localities containing high concentrations of chromophore reside within regions devoid of active chromophore may be constructed in the following manner.
• a cross-linked polymer hereinafter base polymer
• a monomer mixture comprised of chromophore-containing monomer and comonomer, e.g. eMMA, (disclosed in WO 03/070689).
• the monomer and the co-monomer are then polymerized and due to incompatibility with the base polymer, an interpenetrating network (IPN) would be formed ⁇ Structure and Properties of Polymeric Materials Ed. Clegg and Collyer, page 76.).
• IPN interpenetrating network
• the regions containing the photochromic polymer are physically and structurally reinforced by the complementing polymer.
• polymethylmethacrylate (PMMA) and polystyrene (PS) which are well known to produce an interpenetrating network may serve as one example.
• Other possible non compatible polymers are PBMA or PEMA together with the chromophoric active compound eMMA incorporated into a PS matrix with a similar effect, with the benefit of enhanced writability of the chromophoric medium eMMA.
• a cross linked disc of polystyrene may be swollen overnight with ethyl acrylate and cliromophores-monomer (eMMA, eAA or other derivatives of (I)) in at least 20%wt concentration, containing an initiator (e.g. azoisobutyronitrile (BPO). All the added monomers will be absorbed by the cross linked polystyrene, and on heating, give an IPN.
• the IPN can also be created in different order by first creating the acryl based matrix comprising the chromophore and swelling it with styrene and then polymerizing the styrene to create the IPN, or with other polymers that form noncompatible pairs e.g.
• polycarbonate and PMMA polybutyl(meth)acrylate (PBMA) or polyethyl(meth)acrylate (PEMA), or methacrylic acid ester (COPOLEX - being a mixture of benzylmethacrylate and 2,2 ⁇ bis[4-
• the particles are mixed in styrene or MMA and partially dissolved therein
• the styrene or MMA is polymerized, (e.g. using BPO as initiator) to create the supporing matrix.
• the level of the solvatation (that is controlled by the cross- linking) of the particle is such that there are no significant refractive index changes but on the other hand the cross-linking increases the level of the already existing incompatibility so as to create IPN on the micro environment level.
• Slightly crossed linked nano-particles composed of a copolymer of BMA and eMMA and a cross-linker are homogeneously mixed in partially polymerized MMA /PMMA/BPO mixture.
• the mixture is polymerized to such a degree that it does not dissolve the nano-particles.
• Polymerization of the supporting is completed e.g by accelerating the polymerization by raising the temperature.
• the mixture of functionalization that is produced at one or more places on the molecule may be optimized to provide the best possible differentiation between the molecules, while at the same time imparting other solubilizing function.
• this means that the functionalization will contain a mixture of groups that have a high degree of compatibility with the matrix, are highly flexible, and have markedly differing bulk.
• the method of synthesis of such a mixture is also addressed. It is possible to synthesize a number of discrete compounds and then mix them to create a mixture, but this approach is not economically efficient.
• a mixture of functionalization is introduced to one or more points in the chromophore structure, at a suitable point in the chromophore synthesis, by the use of a mixture of reactants in a single chemical reaction, or generally by the use of a plurality of products of complex chemical reactions whose products are functionally similar or functional non disturbing.
• a 671 nm solid-state laser (20 KHz 5 3x15 ns pulses) was used to write data, where the inscribed data was examined by measuring the fluorescence at the same point in the media using a 658 nm pulsed laser diode.
• a written point was defined as a point which produced 10% less fluorescence than a previously unprobed point, which is defined as modulation depth of 10%.
• Table 1 lists the laser power required to create written points in a storage medium comprising of different concentrations of eMMA.
• each fluorescence event results in the absorption of 6X10 "19 J
• a 10 wt% eMMA disk contains 1.4X10 molecules/gr
• the heat capacity of the media is ⁇ 1.5 degree per J/gr.
• a 10% (wt%) media is heated by ⁇ 30°C
• a 20% media is heated by twice the amount.
• eMMA has a 2-photon cross-section of around 250 GM, one may expect that each chromophore in the center of the diffraction limited focus point is excited several times in the above-mentioned experiments.
• at a 20 wt% concentration most chromophores are in intimate contact with at least one other chromophore, hence there is also no doubt that the chromophores begin to interact with each other at these very high concentrations.
• the improvement in data writing ability employing the ATE effect may also be demonstrated indirectly by showing that temperature is a critical aspect of data writing.
• Table 3 The results of writing data at different temperatures in a disk containing 10 wt% eMMA are presented in Table 3. In this case, pulse trains of different lengths and 3OW peak powers were used to write points in a media at different temperatures. The measurements were performed on a setup as described in US application No. 11/285,210. It was found that 16 pulses at 90°C were able to produce a more strongly written mark than 48 pulses at ambient temperature (RT).
• R is C 1-6 alkyl, preferably a methyl, ethyl, propyl or butyl group.
• the solubility-enhancement of eMMA may be accomplished by modifying only the ethoxy "tail" group, so that the polymerizable acrylate group may still be used for polymerization. In this case, since only one group is utilized, a much smaller total number of compounds are generated from the same mixture of alkyl halides.
• An example of a method for this synthesis is given in Scheme II.
• the phenol (III) is reacted with the mixture of alkyl bromides by a Williamson procedure, followed by the removal of the methyl ether protecting group from the mixture of products (IV) to yield a mixture of phenols (V).
• This mixture is reacted with an alkylating agent containing a polymerizable group to give the final mixture of solubility-enhanced eMMA analogs (VI).
• a third possibility for obtaining molecular structures differing in their chemical structures while maintaining the active chromophoric site would be to modify the length of the "spacer" group that connects the chromophore to the polymerizable group (3 carbon atoms in (VI).
• the invention further describes a method to address the above problems associated with many photochromic media.
• a blank disk is delivered to the user in a state that is optimized for the "write” process, such that sensitive, high-yielding, and fast data writing can be achieved.
• the disk (or parts thereof) is subjected to a process that in some way changes the structure of the disk from a "write-optimized” state to a "read-optimized” state.
• This state allows the written data to be read many times with optimized signal strength and little or no destruction of data.
• the switching between "read-optimized” and “write-optimized” states may be a one-way process (resulting in a "WORM” medium) or a reversible process (resulting in a potentially rewritable medium).
• the derivatives of the compound of formula (I) and in particular the compound (“eMMA") preferably used according to the present invention is photoisomerizable between trans and cis configurations.
• the trans configuration is characterized by (i) a much higher fluorescence than the cis; (ii) the trans configuration has a large 2-photon absorption cross-section; (iii) similar one-photon absorption.
• the compound (V) has a high Stokes shift of over 100 nm (Fig. 8), with very little overlap between its absorbance (maximum at 375nm) and emission (maximum at 485 nm) spectra. This means that it may be used at a concentration of over 20 wt% in a media in accordance with the present invention before absorbance of the emission becomes a serious problem. This is in contrast with previous 3 -dimensional data storage fluorescent memories, containing chromophores such as spiropyrans, diarylethenes, or phenoxynaphthacene quinones, or well-known laser dyes. These compounds all have large overlap, and thus only small concentrations of the active chromophore could be used, limiting these memories significantly in their signal strength, to the point that certain key technologies (e.g. 2-photon fluorescent readout) could not be used with them.
• chromophores such as spiropyrans, diarylethenes, or phenoxynaphthacene
• the principal point enabling efficient reading in a chiOinophoric medium of the present invention is not a high Stokes shift, but the reduction of the absorbance-emission overlap.
• the present invention concerns the overall overlap between the absorbance and emission spectra of the chromophore where the Stokes shift is only one of the possibilities of reducing the overlap. This overlap is defined partly by the Stokes shift and partly by the breadth of the peaks. A compound with a low Stokes shift might have excellent (small) overlap if its peaks are narrow enough, and vice versa. It should be noted that for a photochromic media, there must be low overlap between the "read" fluorescence and the absorbance spectra of both forms of the photochromic molecule.
• absorbance-emission overlap is operatively defined as the amount of emission (in %) that is absorbed during its passage through 1 cm of a 0.0 IM solution of the emitting chromophore, wherein this operative definition enables the comparison of different materials.
• This is comparable to the loss of signal that occurs as the "read" signal escapes from a high concentration fluorescent media.
• the loss is less than 10%.
• Rhodamine Green For most fluorophores it is considerably higher.
• the present invention pertains to molecules with an overlap of less than -70%, for multilayer and 3D homogeneous optical storage, preferably less than 50%, at times less than 35% and most preferably less than 20%.
• Disks containing chromophores linked to a poly(acrylate) chain can be made by copolymerizing MMA with a chromophore-containing monomer, e.g. "eMMA” or “eAA” (structures below).
• a solution of the chromophore-containing monomer and -0.2% AIBN (a radical initiator) in MMA is prepared at 60-65 C, and is put into a mold in the shape of a disk. The mold is lowered into a water bath which is held at 60 °C for 18 hours, after which the mold is cooled and opened to obtain the disk.
• a photochrome polymer comprising eMMA as the active chromophoric medium was mounted in a temperature-controlled read/write apparatus, and data spots were written at different temperatures.
• the irradiation time required to write spots as a function of temperature is plotted in Figure 1. As shown in the figure, a temperature change from 15°C to 9O 0 C increases the writing speed up to tenfold, perfectly demonstrating the heating concept.
• a photochromic polymer comprising ePMMA as the active chromophoric medium was mounted in a temperature-controlled read/write apparatus, and data spots were written at different temperatures.
• a difference in modulation between spots written at 9O 0 C and spots written at 3O 0 C, when both are read at 3O 0 C were found.
• Spots written at 9O 0 C showed about twice the modulation of spots written at 3O 0 C. Additional experiments were conducted to quantify the possible improvement factor in different polymer matrices at different temperatures.
• the sample was put on a copper holder that was heated using a power resistor.
• the holder was connected to a PI Nanocube in order to provide compensation for holder expansion during heating.
• a calibration cycle was done, in which the offsets resulting from expansion were measured. 4 spots were written at 9O 0 C. Each spot was written for 20 seconds. The same pattern was written at 3O 0 C.
• the reading of the written data (at 3O 0 C and at 9O 0 C) as reflected by the fluorescence was monitored and can be seen in the following Figures 2 (for the writing at 90 0 C) and 3 (for the writing at 30 0 C). As can be seen, the fluorescence at Fig. 2 is much more pronounced (0.98-0.84) than the fluorescence in Fig.
• MMA 500g is taken and to it, 2Og of Magnesium Sulfate (anhydrous) is added.
• a Magnetic stirring bar is put in the laboratory bottle, and the mixture is left to stir at room temperature for 30 minutes. After 30 minutes, the Mixture is filtered (using standard filtering apparatus, and kept in a sealed laboratory bottle.
• a stock solution of benzoylperoxide(BPO) and MMA, i.e. BPO/MMA (5%wt BPO in MMA) is prepared by taking 2.75g of BPO (75% initiator/25% Water) and adding it to 39g of MMA. 3 g of Magnesium Sulfate (anhydrous) and a magnetic stirring bar is added to the mixture, and the bottle is left to stir at room temperature for 30 minutes.
• the solution is then filtered and the subsequent solution stored in a sealed lab bottle.
• 2Og of PMMA (ATO VODlOO) is taken and placed in a drying dish.
• the material is placed in a vacuum oven at 90 0 C with Vacuum for 4-6 hours. Cooling should be done in the oven, with the vacuum on, and when cool, the PMMA granules should be put in a sealed lab bottle.
• the eMMA is generally hydrophobic, but it should be stored in a sealed bottle and prior to use can be kept in a dessicator (with either vacuum or silica gel- to dry it).
• 9Og of the dried MMA material is taken and to it 1Og of the dried PMMA added.
• a magnetic stirring rod is added and the mixture is left stirring at 70 C for 4-6 hours, (this allows the PMMA to dissolve in the MMA and gives a Polymer/Monomer stock solution.)
• the material is pumped through a 5 micron Sartorius in line filter (using Argon), into pre-heated flask such that there is a minimal difference in temperature.
• the flask is then closed and vacuum applied for 30 seconds to 1 minute in order to degas the mixture.
• the filtered and degassed mixture is poured into the pre-heated (by hot air) mold (80mm diameter/3mm thickness) using the open syringe and adaptor. Care should be taken when closing the gasket so as to make sure that no air bubbles enter.
• the molds are placed in a oven that has been pre-heated to 70 C.
• the polymerisation process takes about 24 hours to complete. After 24 hours at 70 C, the oven should be raised to 110 ° C for 90 minutes or so. This allows full cure of the Polymer.
• the disks should be left to cool in the molds and if necessary, the molds can be placed in a freezer in order to ease opening them up.
• the obtained disk is perfectly transparent and yellow with very low internal haze.
• DCP DiCumyl Peroxide
• the DCP is a solid that melts at 38 ° C, and will mix into the molten eMMA/DLP immediately.
• the mixture was pour mixture into a mold that has been pre-heated to 115-12O 0 C (the glass used in the mold should have been treated by 2% dimethyldichlorosilane in Hexane).
• the mold in then sealed and left overnight to polymerize. A crossedlinked disc is obtained (due to the multi-functional impurities in the eMMA).
• Example 6 50% eMMA: 50% trichlorobenzene.
• Example 7 100% eMMA.
• Example 8 75% eMMA; 25% dichlorobenzene.
• Example 9 75% eMMA; 25% trichlorobenzene.
• Example 10 45% eMMA: 45% tricholobenzene: 10% styrene.
• Example 11 45% eMMA: 45% tricholobenzene: 10% divinylbenzene.
• the product has good solubility in THF, hot chloroform ( ⁇ 5.5% w/w) and hot toluene (12.5% w/w).

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• 2006
  • 2006-01-12 US US11/813,846 patent/US20090283727A1/en not_active Abandoned
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  • 2006-01-12 EP EP06700671A patent/EP1842185A1/en not_active Withdrawn
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