JP2001522119A - Multilayer photochromic optical data disk - Google Patents

Abstract

(57) Abstract The 2.5-dimensional optical storage medium 101 is formed from multilayer photochromic molecules embedded in a polymer matrix separated by a transparent separation layer 107. The medium 101 moves to a position in three dimensions and is written with two-photon / single-beam information. The information written in this way is read out in bit units or page units. In the latter case, the area is illuminated and its fluorescence is detected by a CCD camera.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[Reference of related application]

This application is related to US Provisional Application No. 60/063, filed October 31, 1997.
No. 797, the disclosure of which is incorporated herein by reference in its entirety.

[0002]

FIELD OF THE INVENTION

The present invention provides three-dimensional (3D) optical memory, WORM (write-once) drive, WER (
The present invention relates to methods and systems for use in optical data storage systems, such as write, erase, and read) drives, and more specifically to methods for writing information to multi-layer discs or other media using a single laser. It is about the system.

[0003]

[Description of Related Technology]

One of the known techniques for optical data storage is to use 2D to 3D photochromic media.
There is a two-photon writing / fluorescence reading of the beam. This technology is mainly described in Peter M.
Developed by Rentzepis and his co-workers, see for example US Pat.
Nos. 268,862 and 5,325,324, and in his and others cited below. The information is recorded in two crossed beams: a fundamental beam of picosecond YAG: Nd laser radiation and a second harmonic. The inherent pulse duration is about 30 picoseconds and its pulse energy is a few millijoules. This method can be recorded on a page-by-page basis, but requires significant changes to existing computer architectures.

[0004] Another known technique includes single-photon writing and retardation reading in multilayer photochromic matrices, photopolymers and photorefractive crystals. This technology is mainly S
. Developed by Kawata and others, and is described, for example, in the citations of those listed below. This technique has disadvantages of poor spatial resolution and high energy cost.

[0005] The following references show existing technology.

[0006] Hunter, F, Kiamilev, S .; Esener, D.A. Part
henopoulos, and P.C. Rentzepis, Appl. Opt. ,
Vol. 29 (14) pp. 2058-2066, 1990 S. Dvornikov, J .; Malkin and P.M. M. Rentze
pi, J .; Phys. Chem. , Vol. 98 (27) pp. 6746-
672, 1994 S. Dvornikov, P .; M. Rentzepis, Opt. Com
m. , Vol. 136 pp. 1-6, 1997, Peter M. Rentzepis: U.S. Pat. No. 5,268,862, 1
993; Peter M. Rentzepis, Sadik Esener:
U.S. Patent No. 5,325,324, 1994. Toriumi, J .; M. Herrmann, S .; Kawata, Opt
. Lett. , Vol. 22 (8), p. 555-557, 1997 Kawata, R .; Juskaitis, T .; Tanaka, T .; Wi
lson, S.M. Kawata, Appl. Opt, Vol. 35 (14), pp
. 2466-2470, 1996. Weaver, L. et al. A. Houston, K .; Iwata, T .; L. Gust
afon, J .; Phys. Chem. , Vol. 96 pp. 8956,199
2 years C. Xu, W.C. W. Webb, J. et al. Opt. Soc. Am. B. , Vol. 1
3 (3), pp. 481-491, 1996 S. He, L .; Y. Ning Cheng, J .; D. Bhawalkar
, P. N. Prasad, JOSA B, Vol. 14 (5), p. 1079
-1087, 1997 E. FIG. Spence, P.M. N. Kean, W.C. Sibett, Opt. Le
tt, Vol. 16 (1), pp 42-44, 1991 J. P. Dymott, A .; I. Ferguson, Opt. Lett.
Vol. 19 (23), p. 1988-1990, 1994 Tamura, C.I. R. Doerr, L .; E. FIG. Nelson, H .; A. H
aus, E .; P. Ippen, Opt. Lett, Vol. 19 (1), p.
46-48, 1994; J. Delfyett, L .; Florez, N.M. Stoffel, T .; G
Mitter, N.M. Andrewakis, G .; Alphanse, W.C. Ge
islik, Opt. Lett, Vol. 17 (9), p. 670-672,
1992 I. N. Kovalenko, A .; A. Filippova, {Establishment Theory and Mathematical Statistics}, Moscow, Higher Education, 1982, p. 227 N.R. I. Korotev, S.M. A. See Magnitskii, V .; V. Shu
bin and N.I. T. Sokolyuk, Jpn. Appln. Phys. , V
36, pp. 423-425, 1997

Summary of the Invention

In view of the foregoing, it is apparent that there remains a need in the art for a method and system for optical data storage that provides high information density at a reasonable energy cost within existing computer systems.

Accordingly, it is a first object of the present invention to provide a method and system for optical information storage using a single laser (single wavelength writing radiation) for writing.

Another object of the present invention is to provide a method and system that provides a simple scheme for recording information.

Another object of the present invention is to provide a method and system for obtaining high spatial resolution using femtosecond (shortest) laser pulses.

[0010] Another object of the present invention is to provide an existing computer architecture, especially a CD-ROM.
And a method and system using the DVD-ROM standard.

To achieve these and other objectives, the present invention provides a method for generating 2.5-dimensional images from a photochromic medium using two-photon writing and fluorescence reading with a single beam.
A method and system for optical data storage will be provided. Reading can be performed either sequentially (bit-wise) or in parallel (page-wise). The organic photochromic molecule used for the medium has a colorless type A and an excited or colored type B.
The molecules are embedded in a polymer matrix, have excellent optical properties, and exhibit high spatial resolution limited only by molecular size. Information is recorded on the early form A of the photochromic molecule by two-photon absorption. On the other hand, the information
Can be read out and reproduced from the excited photochromic molecule.

The present invention provides a variant of 3D optical data storage in which the medium has a multilayer photochromic structure, such as a multilayer fluorescent disk. The media has a photosensitive layer separated by an inert (non-photoactive) polymer layer. This structure is single beam, two-photon, bit-wise writable, and its high resolution is derived from the small thickness of the photoactive layer and the non-linearity of the two-photon method. The multilayer structure provides a 2.5 dimensional optical memory compatible with existing CD and DVD technologies.

The use of a single beam instead of a two beam for recording allows the use of extremely short, ie, femtosecond-order, light pulses. Mode-locked femtosecond CW lasers capable of generating femtosecond lasers with repetition rates from 10 MHz to 10 GHz are suitable for use in the present invention. Reading can be performed by reading out the fluorescence generated by either single or two-photon absorption. Single-photon absorption allows for page-by-page reading of information.

[0014]

[Detailed description of preferred embodiments]

A preferred embodiment will be described in detail with reference to the drawings. In all the figures, the same elements are denoted by the same reference numerals.

[0015] The storage medium comprises a transparent multilayer polymer photochromic matrix, which is disposed on a flat substrate surface, for example, by spin coating. A cross section of the 2.5D matrix is shown in FIG. As shown in the figure, the 2.5D matrix 101 is composed of alternating photosensitive layers 105 and separating layers 10.
7 is provided. The photosensitive layer 105 can be about 1 μm thick, and the separation layer 107 can be about 30 μm thick. The photosensitive layer 105 contains a polymer in which photochromic molecules are embedded. For example, naphthacenepyridone may be used to increase the sensitivity of a photochromic substance. Of course, any other structure may be used as long as the performance according to the present invention is maintained. The separation layer 107 is
It can be made of any transparent polymer or other material. Here, "transparent" means transparent to a wavelength used for reading and writing, which will be described in detail below.

The information recording on the medium 101 is based on a photo-coloring reaction by two-photon absorption of a laser beam narrowed down in a photochromic medium. Next, the photochromic effect used for information recording will be described.

Considering the standard relaxation time of a molecule in the excited state of a complex organic molecule, the relaxation time is in the range of picoseconds to nanoseconds, and the time interval between successive laser pulses is determined by the single excited state of the photochromic molecule. Is enough to completely eliminate. That is, such excitation is necessary to cause a colored photoreaction. Thus, light absorption and chemical reaction are temporally separated, so that light absorption and chemical reaction can be described separately. In the simplest case, the excitation of the photochemical reaction can be explained using a two-level model.

Other processes such as reverse photochemical reaction, decomposition, energy transfer, and light absorption by the polymer host and the reaction product are ignored. Assuming that the photoreaction quantum efficiency is constant, the type A concentration does not change during the light pulse propagation, and then some of the excited molecules change to type B according to the quantum efficiency ψ. On the other hand, the remaining molecules return to the initial state at the moment when the next light pulse is incident.

Assuming that all light pulses are the same and the light density of the medium is not large,
The number of molecules converted from type A to type B after propagation of k pulses is obtained as follows.

(Equation 1)

Here, ∫Ф ² dt can be calculated when the pulse energy and the pulse time profile are known.

(Equation 2)

Where τ is the pulse duration, I (t / τ) is the pulse intensity profile, g
Is a type coefficient. For a Gaussian profile, g = 0.664, I (t / τ
) = Sech ² (t / τ) g = 0.533. Therefore, equation (1) can be rewritten as:

(Equation 3)

Here, n _B (k) is the local concentration of the colored type B of the photochromic substance after the same light pulse action, and n ₀ is the overall photochromic substance concentration (n ₀ = n _A + n _B ).
, N _A is the initial concentration of type A of the photochromic material, Ф is the local photon flux density during single pulse propagation, and δ is the two-photon cross section. These equations take into account quantity changes. Thus, these equations hold for any pulse energy.

By comparing equation (3) with the equation for a photochemical reaction excited by a single photon, the effective cross section can be defined, and equation (3) can be rewritten in the same format as the equation.

(Equation 4)

Here, σ _eff = σ for single-photon excitation, and σ _eff = δgE / τ for two-photon excitation.

After a single pulse propagation, the concentration of type B is defined by:

(Equation 5)

In the case of 2σ _eff E≪1, the quantity change can be ignored, and the equation (4) becomes simple.

(Equation 6)

In the case of a single pulse excitation light reaction,

(Equation 7)

[0028]

If the number of pulses is large, equation (6) can be transformed into an exponential form.

(Equation 8)

If the irradiation light amount kE is replaced by _Δt , the kinetics of the photochemical reaction in the steady irradiation can be explained by using equation (8).

As an example, the properties of a 3D optical memory device can be predicted. The average power of the femtosecond laser radiation is 15 mW (according to these CD standards), the pulse repetition rate is 1 GHz (see the disclosure of SD Yakubovich et al.), The pulse duration is 1
00fs, the two-photon cross section of the photochromic molecule is 10 ⁻⁴⁷ cm ⁴ s (exactly a value for Rh-6G dye, used to increase sensitivity, and dye AF-5
It is assumed that the quantum efficiency and the type coefficient of the photoreaction are close to 1 and the writing beam diameter is 0.8 μm. Using these parameters:

(Equation 9)

According to equation (6), after propagation of a single pulse, only about 1% of the molecules are converted to type B. If the fluorescence quantum efficiency of the colored form of the photochromic material is quite large, this percentage of the stored form B is sufficient to reliably read the information. As a result, in a 3D memory device having such parameters, information can be written at a speed of 1 Gbit / s. The energy required to write one bit of information is equal to 15 pJ (see below).

Since the energy varies inversely with the square root of the pulse duration τ,
The shortest possible pulse should be used. In addition, shortening the pulse duration can increase the threshold for photodisruption of the medium. For τ of about 100 fs, the threshold for polymers such as PMMA reaches a value of about 10 TW / cm ² .

Obviously, short pulses can be used to record information by photochromic two-photon excitation of photochromic molecules. However, using a pulse of τ <50fs,
Difficulties arise in any medium due to refractive index scattering. Therefore, in the present invention, a laser that generates a pulse in the range of 50 to 100 fs is considered to be optimal. That is, Ti: sapphire laser, Cr: LiSAF laser, Er-doped fiber laser, semiconductor laser system or CPM dye laser.
In the present invention, a prototype of a 3D optical data storage device was developed using a Ti: sapphire laser.

When the pulse energy exceeds the 0.5σ _eff value, saturation occurs due to resonance, and the number of upper and lower levels becomes substantially equal, and even if the pulse energy is further increased, there is no light reaction product. It only causes a significant increase. This increase limit of the first pulse is, from equation (4), equal to (n _B / n ₀ ) _smax = 0.5 °. Therefore, when it is necessary to accumulate a large amount of coloring patterns and the quantum efficiency of coloring is not sufficiently high, it is impossible to record information bits with a single pulse. By increasing the number of write pulses, the unrestricted portion of the molecule can be converted to a colored form, as in equations (3, 6). The concentration dependence of the associated type B on the effective density of the pulsed radiation after the propagation of one or several laser pulses and the number of writing pulses at different effective radiation densities of the pulses are shown in FIGS. B). Due to the resonant saturation of two-photon absorption (FIG. 2A), the relevant type B concentration limit is:
Pulse _{k: (n B / n 0} ) max = defined by 1- ^(1-0.5ψ) Number of ^k.
Therefore, an increase in the accumulation of colored mold is achieved only by an increase in k (FIG. 2 (B)
)).

It is important to consider the reverse photochemical reaction, the dynamic inequality of photochromic molecules embedded in the polymer main material and the resonance energy transfer between excited and non-excited molecules. This requires correction of the true dynamic action of the writing action shown in FIGS. 2A and 2B and represented by equations (3), (4) and (5). The correction of the deviation of the relational expression is made possible by taking into account the relation of the quantum efficiency to the concentration of the relevant type B. (The dependence of the quantum efficiency of the photochemical reaction on the relative concentration of type B, which was reliably obtained experimentally, will be described later.) In the development of a practical device for rewritable 2.5D optical data storage, one bit is used. Efforts have been made to minimize energy consumption per information write. It is natural that the basic photochromic material parameters, digital information density per page, and information writing speed, and information reading sensitivity are specified. The above described simple model of the process performed during the two-photon writing of the digital information into the photochromic data store allows the evaluation of the appropriate writing conditions.

Next, a 1-bit recording standard will be introduced. A bit is considered to be recorded if the smallest detectable type B concentration (n _B / n ₀ ) _min in the volume maintained at the diffraction-limited laser beam focal spot is accumulated.

The minimization of the energy consumption per bit is achieved by a simple minimization of the total energy W _s = kES, where k is the number of pulses characterized by the density of the energies E and S, Pit area). The total energy is nonlinearities in the 2-photon absorption, the photochromic material portion _{(n B} / n _{0) min} type A
Required to convert from to type B. Obviously, W _s value is not important for the two-photon write pulse parameters. Equations (3) and (4), which explain the kinetics of type B accumulation in non-linear two-photon excitation of photochemical reactions with short laser pulse trains, are:
It is necessary to introduce a dimensionless parameter of "cast energy consumption" per information bit writing. This parameter is defined by the pulse photon flux density, the number of pulses k required for writing one bit of information, the two-photon light absorption cross section δ, the pulse duration τ, and the pulse type coefficient g, and has the following relationship.

(Equation 10)

As described above, minimization of the parameter W defines an optimization method of two-photon writing.

The solution of equation (4) for the value of W yields a relationship between W and (n _B / n ₀ ) _min .

[Equation 11]

Under the two-photon excitation condition, the quantum efficiency of the direct photochemical action A → B resulting from the color-type accumulation is expressed as a parameter in the last relational expression.

Clearly, equation (9) is a non-monotonic function W (k) that depends on the parameter ψ,
Shows the quantum efficiency of direct photochemical reactions. However, various (n_B/ N₀)_minAnd ψ
There is no possibility to find an analytical expression for the minimum value of W for. According to equation (9) in image format
Then, when the calculation of the appropriate energy consumption per 1-bit writing of information is executed, FIG.
It becomes a curve as shown in FIG. This curve shows the minimum value of W for a specific number of pulses.
Becomes (n_B/ N₀)_minDefine the combination of and ψ. Thus, the coordinates (n_B/
n₀)_minThe area inside and ψ is divided into several small parts,
The optimal number of resources is defined. Separately, equation (6) (W / k) = (σ_effE)^1/2Can be explained according to It is clear that the optimum W / k equation changes greatly (see FIG. 2 (
A)), Equation (4) σ for all pulses in the optimal recording_effObey E
It is naturally expected that the information format is significantly different from 0.5. For accurate calculations,
The W / k of the optimal recording mode has a low level of quantum efficiency ψ and a low value of the type B molecule (n_B/ N ₀ )_minEqual to 0.8 for the required relative accumulation of. Also, W / k is represented by ψ and (
n_B/ N₀)_minHas a bias that increases slightly as it approaches threshold 1.
. Therefore, FIG. 3 shows that not only the required number of pulses, but also ψ and (n_B/ N ₀ )_minAlso determines the cast energy consumption per bit of information written to
Give the possibility to justify.

If the information recording speed is obtained from equation (9) or given from the known value of the casting energy consumption per bit of information predicted by FIG. 3, the required laser power P and pulse The repetition rate f is easily derived.

(Equation 12)

Here, d is the waist diameter of the focused laser beam, and λ is the wavelength of the emitted light in vacuum.

The most reliable way to minimize the required laser power is to use a photochromic material that features a large value two-photon absorption cross section. For example, 1
When using a laser of 10 pJ per 00 fs, δ = 10 ⁻⁴⁷ cm ⁴ s is required to obtain an effective cross section σ _eff close to a single photon absorption cross section in the case of a diffraction-limited light beam diameter. In this way, saturation of two-photon light absorption occurs. New photochromic materials use sensitizers to improve performance. Clearly, materials with high values of photoisomerization quantum efficiency are suitable.

An important aspect of write optimization is to determine the optimal number of laser pulses having a given duration and time profile to divide the write energy and minimize the energy of the entire pulse. That is, we define the cast energy consumption W per information bit write, the optimal number of pulses to write one information bit k,
Recording radiation E _opt ,

(Equation 13)

The optimal energy of a single pulse of was found. However, if the laser used to write the information produces only pulses with energy significantly less than the calculated E _{opt, the} number of pulses required to write one information bit will be high, and the cast energy per information bit write will be large. Consumption is very different from the optimal value. In this case, according to equation (6), the consumption of cast energy per writing of information bits increases in proportion to the square root of the required number of pulses.

[Equation 14]

The recording-reading-erasing process is characterized by the quantum efficiency of the corresponding photochemical reaction. Quantum efficiency values can change during the photochemical reaction process. In the case of a polymer matrix, different changes in quantum efficiency occur due to: 1) reverse photochemical reaction, 2) molecular dynamic inhomogeneity due to heterogeneity due to interaction with the polymer main material, 3). Energy transfer between high concentration photoisomers of photochromic molecules,
4) The difference in photoreaction rate along the film depth due to light absorption in a sample with high light density.

Importantly, realizing a WER optical data storage device requires erasing information in the same manner as writing (two photons that generate a photochemical erasure reaction).

The information reading of the 2.5D optical memory device can be performed by reading the fluorescence excited by single or two-photon absorption. In single photon readout, "page unit" (parallel)
Reading can be used. In both cases, the readout process is performed simultaneously with the information erasure, by colored erasure of the photochromic molecules. Reading in page units is performed by a CCD camera. If the read beam irradiates the sample at Brewster's angle, interference due to reflected light can be avoided.

For steady-state irradiation, the solution of the equation defines the number of colored molecules of the photochromic material.
This molecule is erased in the unit volume of the effective layer of the data storage medium after illuminating the sample with readout radiation having a constant photon flux density の 間 に during a time t. Also, the total number of fluorescent photons emitted from the volume during the time period is:

(Equation 15)

Here, n _B (0) is the initial concentration of the coloring type, ψ _A and _{Ｂ B} are the fluorescence and erasing quantum efficiency, respectively, and σ _B is the light absorption cross section of the coloring type molecule at the readout wavelength.

The total number of photons emitted from 1 cm ³ is given by:

(Equation 16)

In single-photon excitation of fluorescence, partial erasure of information is performed not only on the layer to be read but also on all layers of the medium. From equation (16), the ratio of the fluorescent photons emitted after undergoing the backlight reaction B → A to the molecules to be “erased” is equal to the quantum efficiency ratio (fluorescence to photoquenching). For this reason, this read method inherently has a limit on the number of read cycles.

All readout systems have an inherent detection limit determined by the minimum number of photons emitted from a unit volume during the readout time required to detect the presence of colored information pits. Let ΔF _min be the minimum number of photons that can be read by the system to obtain a discriminable signal (detection limit). The maximum number of read cycles, N _max , can be predicted for the initial concentration of type B as the integer part of the following equation:

[Equation 17]

Here, NA is the aperture diameter of the object to be read out expressed in numerical values.

Obviously, to obtain the maximum number of read cycles, the illumination density during all read cycles should be varied to provide a constant number of fluorescent photons equal to the detection limit in all cycles. However, in practice, the simplest method is to keep the irradiation density constant during the entire cycle of reading information. In this case, the maximum number of read cycles in a single layer is defined by:

(Equation 18)

At a large N _max , the number of reads at the optimum read density E ^opt is approximately given by the following equation.

[Equation 19]

Further, the optimum read irradiation density is E ^opt = eF ₀ / N _max .

To analyze the effect of the fluorescence of other layers during a page-by-page read, assume that the matrix has M layers. Assume that the integrated absorption of readout radiation in all layers with recorded information is small. The information density in a single layer has the property of the parameter η = S _rec / S. Here, S _rec is an area colored during information recording, and S is an entire page area. Image constant k, the difference between the inside and outside of the signal intensity of the colored pit ratio outside of the signal intensity of the colored pit is defined by _{k = (I p -I 0)} / I 0.

It is assumed that the defined depth of the object is considerably smaller than the distance between the information layers. In this case,
Fluorescence imaging is shown in FIG. Symbols J-1, J, and J + 1 represent images of three continuous layers 105 of the multilayer matrix 101 formed by the CCD camera 401 having the target panel 403. An image is formed by the minute object 405. It is assumed that the distance between the information layers is equal. The light beams forming the pit image are arranged in a cone defined by an angled object aperture α. If the object size is large, α will be substantially the same for all image layers. In FIG. 4, an image formed on the layer J on the surface of the CCD camera 401 arranged on the target panel 403 appears. It is noted again that all layers are irradiated at the same time.

Next, the image contrast for the pits arranged on the optical axis in the J layer is calculated. According to FIG. 4, in addition to the radiation emitted exactly from the pits considered, the radiation enters the point of the pit image, from the image area S of the adjacent layer and from the area corresponding to the radiation from the other layers of the matrix. The emitted radiation is collected by the target portion.
The image area S is defined by a circle where the cone α intersects the pit image point under consideration and the corresponding vertex of the image plane of the adjacent layer. The corresponding beam is indicated by the solid line 407 in FIG. All pits located close to the readout layer illuminate one area, similar to area S on the surface of the CCD camera target (in this case, the beam is represented by dashed line 409 in FIG. 4). .

It is assumed that a pit image is formed by fluorescence emission of power P. The brightness of the pit image _is an I p = _P / _S p. Here, _Sp is an area of a single pit image. The radiance from this pit on the surface of the adjacent layer image is _Sp / S times lower.
Here, the radiance from all the adjacent layer pits is I _pn = ηI _p . Similarly, the radiation from the back surface of the adjacent layer (from the region not containing the colored pits) is I _0n = I−ηI ₀ . The radiation levels from all matrix layers due to geometric similarity are equal to the considered radiation levels of the neighboring layers. Here, the total alias emission _Ita from all layers on the image plane of the layer J is expressed by the following equation.

(Equation 20)

Accordingly, the contrast of the pit image obtained during reading of information from the matrix having the M layer is expressed by the following equation.

(Equation 21)

Here, when M = 1, k _M = k, and for k → ∞, equation (21) changes to the following equation.

(Equation 22)

Next, the probability of a multilayer matrix read error is predicted. The required noise level depends on the pattern recognition method. Suppose that a two-level encoding of information is used, and that the information recording is done in a way that recognizes the pit placement on the surface of any layer. In this case, the signal from each pit can be averaged over the total number of pixels (n) on the CCD matrix corresponding to one pit of information. The sample size in this case is n
be equivalent to.

The signal and sound intensities are generally distributed values, and the average values are as follows,

(Equation 23)

The distribution variance of this value is the same and equal to σ.

The definition of the sound level and its variance is made for a large area and thus for a large number of CCD pixels. This is why the statistical certainty of the definition of these values outweighs the statistical certainty of the signal level definition. This is because the latter is defined by the information of the single pit area. It is also assumed that the accuracy of the signal level calculation or the calculation of the minimum value is high.

In a state where noise is present, information included in the pit volume discrimination criterion can be extracted. The contrast of the information read is known, and is equal to k _M. If the noise value is not taken into account, the pit signal exceeds the sound level by ΔI.

(Equation 24)

Here, the signal I averaged by the sample size is shown as an actual example. The symbols are as follows.

(Equation 25)

[0072]

(Equation 26)

Since σ and σ are known, the read error occurrence probability is equal to the probability that the read signal obtained from the colored pit becomes I _1/2 or less. This probability is defined by the following equation.

[Equation 27]

Here,

[Equation 28]

Here, the above equation is a standard normal distribution function.

During the information recording, when the uncolored pits have the same value as defined by the equation (24), the read error occurrence probability becomes the following equation.

(Equation 29)

The assumption of the similarity between the signal and the sound scattering described above also corresponds to the case where the readout system noise is greater than the data storage medium noise. In principle, the reverse situation also occurs when the random non-uniform media properties are large. The relative variance of signal and sound inherent in a single-layer matrix is equal to each other, and the signal variance is clearly greater than the sound variance of a multi-layer matrix. The minimum error occurrence rate in this case is based on the signal definition standard (23
) Cannot be calculated. Therefore, this criterion is changed. However, the case considered is realistic because low-level signal reading requires increasing the number of read cycles and because the uniformity of the data storage medium is the least important, technical parameter. is there.

If the information reading consists of a device based on a sequential method, for example a mechanism similar to a CD or DVD drive, the intensity of the reading laser radiation is precisely focused in the selected effective layer, The influence of the fluorescence of the adjacent layer on the read signal contrast is reduced. The use of two-photon excitation of the fluorescence of the photochromic molecules in the colored mold with a femtosecond continuous train of laser pulses can further reduce the noise signal from the nearby effective layer.

When erasing all information recorded in the medium, information erasing can be performed by two-photon excitation light erasing to confirm the spatial position of information in a selected layer of the data storage medium, or by single-photon excitation light. Performed by either erasure. The modification of the first erasure is the same as that of the information recording, and the feature of the second modification is already completed as a process accompanying the information reading.

The prototype of the manufactured 2.5D optical memory device is the table top setup 501 shown in FIG. 5, and includes, as basic components, a recording laser, information erasing and reading, an optical modulator, and image writing / reading. Module, a precision 3D positioning stage with a photochromic matrix holder, and a control computer (not shown in the schematic). A schematic diagram of the setup is shown in FIG.

Recording is made with radiation of a cw femtosecond Ti: sapphire laser (λ = 800-860 nm) 503 focused on a selected photosensitive layer of the multilayer polymer matrix 505. This laser has an average power of 150 mW and a repetition rate of 100 M
Generate a 100 fs pulse in Hz. Argon-ion 5.6-W "Innov
a-316} laser ("coherent") 507 is used for pumping. The recording beam is focused using a microscope objective lens (x40, NA = 0.65) 509 supported by the universal head 511. The universal head is also attached to Princeton Instruments CCD camera 513. The matrix 505 is held, and is moved three-dimensionally by the step driver 515. The setup 501 also includes an electro-optical gate 517, an acousto-optical gate 519, and optionally a mirror 521.

FIG. 6 schematically shows a universal head in a recording mode. The recording radiation light after passing through the electro-optical gate 517 becomes a parallel light beam by the lens system 601 and becomes one beam. This beam covers most of the aperture of the microscope objective lens 509,
It has adequate beam divergence to minimize objective lens aberrations. Inside the universal (recording and reading) head 511, the Ti: sapphire laser radiation is reflected by the dichroic mirror 603 and directed toward the microscope objective lens 509.

The pulse energy of the recording radiation changes from 10 to 500 pJ per pulse. On the other hand, the maximum peak power density inside the photochromic layer reaches 100 GW / cm ² . The exposure time for recording a single dot is in the range of 1 to 10000 μs corresponding to a pulse train of 100 to 1,000,000 pulses. To read the information page, the image of the selected layer is focused on the matrix of the CCD camera 513 with the same microscope objective 509 used in the recording. Fluorescence is wavelength 5
Excited with 14 nm argon-ion laser radiation. FIG. 7 schematically shows the universal head in the reading mode. The emitted light is applied to the sample at the Brewster's angle from the opposite side through the transparent quartz plate 701 of the driver 515. The beam is 1
It has a diameter sufficient to uniformly illuminate a 20 × 160 μm page surface.

The universal head 511 is stationary, and a layer to be selected is selected by moving the sample to 3D. Movement is by three step motors, driver 515
To drive the precision platen. Spatial movement accuracy is 0.22μm for all axes
It is.

The writing radiation has a pulse train duration controlled by an electro-optical gate. On the other hand, the read time is controlled by the acousto-optical gate.

[0086] Sample movement, gate switching and CCD camera operation are computer controlled.

The storage medium sample is a transparent multilayer polymer photochromic matrix, and is disposed on the surface of a circular (diameter 20 mm, thickness 5 mm) quartz plate by a spin coating method. The thickness of the photosensitive layer is about 1 μm, and the thickness of the separation layer is about 30 μm. In the present invention, PMMA or a copolymer thereof is used together with polystyrene and acrylomitrel as a main material of the photosensitive layer. The separation layer was made of polyethylene terephthalate. This specification reports the results obtained using naphthacenepyridone (NP) as the photochromic material. Most results are in NP No. 10 obtained. The main photochemical properties, such as colorless and colored extinction coefficients, the fluorescence and photoreaction quantum efficiency of this substance in toluene solution are given in the cited papers.

NP No. FIG. 8 shows a typical absorption spectrum of a polymer film having a thickness of 1 μm and containing 10.

The NP-containing polymer film is transparent in the near infrared region. Therefore, 2.
Two-photon writing to all layers of the 5D light matrix body is possible. The absorption bands of type A and type B in the visible region are clearly separated (450 nm and 530 nm). For this reason, both types are selectively excited.

For both NP Nos. A representative normalized fluorescence spectrum of type 10 is shown in FIG.

The two types of NP Nos. Although the 10 fluorescence spectra overlap, information can be read accurately and efficiently due to the large difference between the positions of the absorption bands of type A and type B and their quantum efficiency.

One of the many characteristics of 2.5D memory devices is the spatial resolution in all recording and reading cycles. Move horizontally at 1 μm intervals (less than single dot diameter) 3
FIG. 10 shows an image of an information pit written by dots. The lateral dimensions of the pits are about 1.7 μm and the vertical dimensions are 2 μm larger (as expected), ie about 3.7 μm. Therefore, the lateral dimension of the pit is 0.2 μm larger than the waist diameter of the writing beam. Due to the non-linearity of the two-photon writing process, the size of the pit to be written should be smaller than the waist diameter of the writing beam. Visible enlargement of the pit size does not impose any significant restrictions on the storage medium, but is believed to be related to the write process saturation effect. Saturation levels are greatly affected by photodisruption, backlight reactions, light absorption by other levels of molecules, photoreaction quantum efficiency by heterogeneity of the molecules embedded in the polymer main material, and the like.

Most of these effects not only decrease the resolution but also greatly affect the speed and energy change of information recording and erasing. In addition, these effects affect optical memory devices and other important properties, such as the number of record-erase cycles achieved.

The above theoretical predictions have been made for ideal recording and reading conditions. A more detailed analysis should be done to explain the real process. The effects of the backlight reaction, reduced quantum efficiency during the photoreaction, oxygen effects, and differences between the single and two-photon absorption spectra should be considered. All of these factors significantly reduce the parameters of recording, reading and erasing information.

As described above, the described setup can be used as a photochromic material tester as well as a pilot prototype of a 2.5D optical memory device based on two-photon and fluorescence recording of information. In addition to two-photon excitation of photochromic molecules, non-fixed single-photon excitation with femtosecond laser pulses is also effective and can be studied using this setup.

In the designed setup, information can be written, read and erased in principle. That is, two-photon single-beam “bit-by-bit” writing and fluorescent “page-by-page” single-photon reading 2.5D femtosecond optical data storage are possible. The 2.5D data storage prototype ran stably and was able to perform information writing in 1, 2, 3 and 5-layer polymer photochromic naphthacenepyridone matrices.

The NP derivative was chosen for several reasons. Photochromic materials used for 3D optical data storage must meet some stringent requirements. to date,
No suitable photochromic material has been found. NP has high quantum efficiency of photocoloring and type B fluorescence and low extinction constant. On the other hand, these materials have high photodisruption quantum efficiency and low two-photon absorption cross section. The light erasure reaction limits the type B accumulation and the number of read cycles during information recording. On the other hand, NP-based data storage media provide an efficient 2.5D optical memory device.

FIG. 11 shows the results of writing and reading a graphic image having a high plot resolution in a five-layer optical matrix. The photosensitive PMMA layer is about 1 μm thick, contains 0.1 M of NP, and alternates with a 30 μm layer of polyethylene terephthalate. The recording was performed at a wavelength of 860 using a 50 mW Ti: sapphire laser radiation.
nm and an exposure time of 1 ms for each dot. For information reading, the page is illuminated with about 100 mW / cm ² radiation. The image accumulation time of the CCD camera was 1 s. The obtained spatial resolution is 1.7 μm and the image contrast is about 2. In the above set-up, the read speed was limited by the low read beam intensity (100 mW).

The preferred embodiment according to the present invention has the following configuration. Photochromic materials and appropriate femtosecond lasers are selected to achieve two-photon writing of information. The multilayer data storage medium can be made according to FIG. 1, and the 2.5D optical data storage device can be basically the same as the prototype outlined in FIG. Those skilled in the art who have disclosed the present invention can easily develop software to encode information, control information recording, control a reading process performed using a CCD camera, and decode information. It is.

A multilayer data storage medium according to a preferred embodiment is formed as a matrix comprising a thin (about 1 μm thick) layer of a polymer having photochromic molecules embedded therein. Using the right laser, two-photon recording of information and fluorescence readout are achieved. The setup includes a computer controlled 3 comprising a data storage medium holder, a write and read laser beam, a micro objective lens for focusing the write beam and transferring the page image during reading of information on the CCD camera.
D Includes precision carriage.

Although the preferred embodiment according to the present invention has been described above in detail, it is easy for those skilled in the art who have disclosed the present invention to realize the following other embodiments within the scope of the present invention. You will understand. A molecule having a large two-photon absorption cross section (eg, AF-50) can be used as a sensitizer for a photo-coloring reaction by energy transfer that excites photochromic molecules during the recording process by increasing the sensitivity of the storage medium. To achieve a WORM (write-once) type of 2.5D optical memory, irreversible coloring reactants can also be used in certain photochromic media (eg, lactone-type coloring of organic dyes). Semi-reversible WORM drives are based on the coloration of spiropyran bound to polymer molecules, which are irreversibly colored by a photochemical reaction but return to a colorless form upon heating. Therefore, the present invention should be construed as limited only by the appended claims.

[Brief description of the drawings]

FIG. 1 is a sectional view showing an optical information storage medium according to a preferred embodiment of the present invention.

FIG. 2 is a characteristic diagram showing the relationship between the pulse energy density value and the effective cross section after the effect of some pulses, and the relationship between the number of pulses and the accumulation of the coloring type.

FIG. 3 is a diagram showing the relationship between the quantum efficiency of photo-coloring and the required number of laser pulses for writing 1-bit information with respect to the accumulation of a required type B;

FIG. 4 is a schematic diagram showing image information from a multilayer fluorescent matrix in reading.

FIG. 5 is a schematic diagram showing the structure of a prototype of the present invention.

FIG. 6 is a schematic diagram showing the universal head in a write mode.

FIG. 7 is a schematic diagram showing the universal head in a read mode.

FIG. 8: Naphthacenepyridin on type B accumulation while illuminating the medium using blue light.
No. The figure which shows the absorption spectrum of the polymer thin film containing 10.

FIG. 9. Naphthacenepyridone No. in type A and type B The figure which shows 10 standardized fluorescence spectra.

FIG. 10 is a view showing an image of a pit (hole) drawn at three points horizontally arranged at a distance of about 1 μm (pit dimensions are 1.7 μm long and 3.7 μm wide). ).

FIG. 11 shows five fluorescent images obtained from five layers of a 2.5D optical medium (the images are recorded directly on each other, the thickness of the photosensitive layer is 1 μm and the distance between the layers is 30 μm) Meters apart).

──────────────────────────────────────────────────続 き Continuation of front page (81) Designated country EP (AT, BE, CH, CY, DE, DK, ES, FI, FR, GB, GR, IE, IT, LU, MC, NL, PT, SE ), OA (BF, BJ, CF, CG, CI, CM, GA, GN, GW, ML, MR, NE, SN, TD, TG), AP (GH, GM, KE, LS, MW, SD, SZ, UG, ZW), EA (AM, AZ, BY, KG, KZ, MD, RU, TJ, TM), AL, AM, AT, AU, AZ, BA, BB, BG, BR, BY, CA, CH, CN, CU, CZ, DE, DK, EE, ES, FI, GB, GE, GH, GM, HR, HU, ID, IL, IS, JP, KE, KG, KP , KR, KZ, LC, LK, LR, LS, LT, LU, LV, MD, MG, MK, MN, MW, MX, NO, NZ, PL, PT, RO, RU, SD, SE, SG, SI, SK, SL, TJ, TM, TR, TT, UA, UG, US, UZ, VN, YU, ZW (72) Inventor Shuvin, Vladimir Voy Russia, 117192 Moscow, Michlinsky PI, 54-3-143 (72) Inventor Malakoff, Dmitry A. Moscow, Russia (72) Inventor Levich, Eugene Buoy United States of America, New York, 10036 New York, 330 West 45 Street, Apartment 9 El ( 72) Inventor Markin, Jacob N, Israel, 77461 Ashdod, Hanabie Habakok 13/2 F-term (reference) 4F100 AK01A AK01C AK01E AT00A AT00C AT00 E BA05 BA06 BA07 GB41 JL14B JL14D JN01B JN01D JN17 JN17A JN17C JN17E YY00A YY00C YY00E 5D029 JA04 JB13 JB35 LB07 LC08 5D090 AA01 BB12 BB18 BB19 CC01 CC04 CC14 DD01 FF08 FF08 FF01

Claims (26)

[Claims] 1. A light, comprising: a plurality of photosensitive layers each including a base material in which photochromic molecules are embedded; and a plurality of separation layers for separating the photosensitive layers from each other. Data storage medium. 2. The optical data storage medium according to claim 1, wherein said base material comprises a polymer. 3. The optical data storage medium according to claim 1, wherein the separation layer is transparent at a light wavelength at which the photochromic molecules are photochromic. 4. The optical data storage medium according to claim 1, wherein said photochromic molecule comprises a naphthacenepyridone molecule. 5. The optical data storage medium according to claim 1, wherein the photochromic molecule is selected from the group consisting of spiropyran, fulgide, thiondigoid color, and phenoxynaphthacenequinone. 6. The optical data storage medium according to claim 1, wherein said photosensitive layer is 0.1-1 micrometer thick and said separation layer is 1-50 micrometer thick. 7. The optical data storage medium according to claim 6, wherein the number of the photosensitive layers is at least 20. 8. A three-dimensional matrix of a photochromic material having optical characteristics that are changed by two-photon absorption of coherent radiation light, the matrix is arranged in three dimensions, and a part of the matrix is arranged at a predetermined position. A method of irradiating a predetermined position with the coherent light according to the information stored in the matrix to change a light characteristic of the portion of the matrix. . 9. The method of claim 8, wherein the light characteristic is fluorescence. 10. The method according to claim 9, wherein the fluorescence is anisotropic. 11. The apparatus according to claim 9, further comprising a step of reproducing the information by detecting the fluorescence generated by absorbing the readout radiation light by the photoreaction of the portion. Optical data storage method. 12. The method according to claim 11, wherein the absorption is two-photon absorption.
3. The method of optical data storage according to 1. 13. The method of claim 1, wherein the absorption is a single photon absorption.
2. The method for optical data storage according to item 1. 14. The method of optical data storage according to claim 11, wherein said readout radiation is polarized. 15. The method for optical data storage according to claim 14, wherein said reproducing step comprises detecting whether or not anisotropy of said fluorescence generated by two-photon absorption is present. 16. The method of optical data storage according to claim 14, wherein said reproducing step comprises a positive / negative detection of anisotropy of said fluorescence generated by two-photon absorption. 17. The method of optical data storage according to claim 14, wherein said readout radiation has a polarization modulated for single photon absorption. 18. The method according to claim 11, wherein said reproducing step comprises a page-by-page readout for simultaneously irradiating a plurality of portions of said medium and simultaneously detecting fluorescence from said plurality of portions. Optical data storage method. 19. The method according to claim 18, wherein the fluorescence from the plurality of portions is detected by a CCD camera. 20. The method of optical data storage according to claim 18, wherein said readout radiation is polarized. 21. The method of claim 8, wherein the photochromic material is oriented in a field direction that orients a magnetic dipole moment of a molecule in the photochromic material. 22. A driving means for supporting and moving the medium three-dimensionally and arranging a part of the medium at a predetermined position according to information to be recorded, and emitting coherent light for changing optical characteristics of the medium. An apparatus for recording information in a photochromic medium, comprising: laser means; and modulation means for modulating the coherent radiation according to the information. 23. The laser means, wherein the laser means is a femtosecond laser having a pulse duration of substantially 50-100 fs, wherein the repetition rate is substantially 100 MHz.
24. The device of claim 23, wherein the device has a wavelength outside the range of absorption and fluorescence wavelengths of the photochromic medium. 24. The apparatus according to claim 23, wherein said driving means comprises means for moving said medium to record information in a multilayer manner. 25. The laser of claim 2, wherein the laser means comprises means for focusing the coherent radiation to form an ellipse in the portion of the medium.
An apparatus according to claim 3. 26. The apparatus according to claim 23, wherein said modulating means comprises means for adjusting the duration and intensity of said coherent radiation to control the width of a pit recording said information. The described device.