6746
J. Phys. Chem. 1994,98, 61466752
Spectroscopy and Kinetics of Photochromic Materials for 3D Optical Memory Devices A. S. Dvornikov, J. Makin,+and P. M. Rentzepis’ Department of Chemistry, Unlversity of California. Imine, California 92717-2025 Received: February 17, 1994; In Final Form April 20, 1994.
The spectra and kinetics of photochromic molecules have beem studied by means of two-photon excitation. All intermediate states and species have been identified and their formation and decay rates, from the picosecond to the millisecond range, measured, and the reaction mechanism has been deduced. The methods and optical system for the utilization of these molecules in 3D computer memory storage devices are presented.
Introduction The need for materials suitable for electronic devices has generated one of the most intense areas of research. To a large extent, materials are the limiting factors for the employment of many new devices including displays and computer memories. As advances in silicon technology continue to increase the data density per unit area, the 1/0 speed is also drastically increased.’ Even with these improvements it is rather a matter of time when more dense memory devices with higher 1/0 speed will be mandatory to run in conjunction with parallel processors. There are several means being investigated for achieving this goal or at least satisfying most of the present day and near-future highdensity,high-speed requirements. These include high-resolution silicon memories, 2D optical disks, and 3D optical devices. The optimum results will obviously be achieved by 3D volume devices because of the vast information storage capability within a small volume and the ability to input and retrieve megabytes of informationwithin a very short period of time. There are several means proposed which may lead to practical 3D storage memory devices. These include phase holograms? persistent hole buming,3 and two-photon processes.c7 Each of these has the potential for immensestorage capacity,extremely fast 1/0speeds,and parallel processing compatibility. The main reason that none of these techniques have found practical application in devices yet is the lack of materials with the necessary if not optimum physical and chemical properties. Several excellent papers have been published on hole burning3 and phase holograms,* where each method, its potential uses, advantages, and problems to solve are discussed. We shall not deal any further with the first two techniques, and instead we shall focus our attention here on the materials which have several of the properties needed in order for them to be useful in two photon 3D memory devices. In fact, we shall restrict this paper mostly to one- and two-photon spectroscopy of the intermediates and kinetics of some photochromic molecules which have the potential of being used in memory devices. Then, on the basis of the experimental data, we will propose a mechanism for the two-photon-induced photochromic process. Because 3D volume storage is achieved by the nonlinear by the nonelinear excitation of the photochromic molecules, we will present the salient features of two-photon absorption processes. By means of such two-photon process, as will be shown later, one is allowed to excite molecules at preselected places within the volume and thus induce into the molecules the changes necessary for the writing and reading of information in 3D space. Because the excitation is achieved by two-photon processes, the initial selection rules for the transition follow the two-photon selection rules which are different than the ones operative in one-photon
t Present address: Weitzmann Institute of Science Rehovot, Israel. * To whom correspondence should be addressed.
*Abstract published in Advance ACS Absfracis, June 15, 1994.
0022-36S4/94/2098-6146%04.50/0
processes. However, the reaction kinetics and mechanism subsequent to the excitation process are similar in the one- and two-photon processes. Owing to the large size of the molecules used, the density of states at the level of excitation is very large; therefore,within the excitationbandwidth there is a large number of allowed states for both one- and two-photon excitations. In this paper, first we shall present some aspects of two-photon processes and then the photochemistry and kinetics of the molecular species used. Subsequently,we shall describe in some detail the mechanism for writing and reading of information within 3-dimensional optical memory devices.
Principles of Two-Photon Processes Intense laser beams may induce multiphotonabsorptionwhich causes the population of excited states.'^^ The rate of absorption depends upon the intensity of the exciting laser source, I, to the power of the process:
R = cNIN where N is the order of the process, Le., N = 2 for a two-photon process, and GN is the multiphoton cross section. The energy of the final populated state Ef is
E, - Ei = Nhw and the transition probability can be expressed as the product of terms
Pif = r i f R I N where y a is the spectral profile. This expression also suggests that Doppler broadening may be highly suppressed by the absorption of two photons from two counterpropagating beams.
where i, v, and f correspond to initial, virtual, and final states, respectively. When kl = kl, the two beams propagate in the same direction, and the bandwidth achieves its maximum value. However, for kl = -k2, counterpropagating beams, there is no Doppler effect, and the spectrum presents its pure homogeneous line width Tif. Rif is a function of the matrix elements between the ground, i, and final state, f. These states are coupled via the virtual level v, RiV,Rvk. It has been shown that the only virtual levels which 0 1994 American Chemical Society
Photochromic Materials for 3D Optical Memory Devices may make a contribution to theoscillator strength of the transition are the ones which are in near resonance with a real level. The third term is of theutmost importance for this study because it suggests that the two-photon transition probability depends upon the product of the two incident intensities. Assuming that the two photons o1and w2 are of the same frequency, then the transition probability will depend upon the square of the intensity. It is, therefore, of considerable advantage to use ultrashort laser pulses because of their high intensities. Note that the two photons absorbed need not be of the same wavelength. All that is required is that the two photons interact simultaneously with a molecule, and therefore the process may be quite adequately described by a two-level Hamiltonian equation. The parity rule demands that a two-photontransition occurs between levels with the same parity, g g, u u,in contrast to the one-photon transitions where the parity rule is g u, u g. Thus, states are populated by twophoton transitions which could not be reached via a one-photon transition from the ground state. A virtual two-photon process has the additional practical property of allowing for the preferential excitation of molecules situated within a 3D volume, in preference to molecules on the surface. For a two-photon process where both photons have the same frequency, the signalto-noise ratio is 3: 1;however, by utilizing photons with carefully selected frequencies, it is possible to greatly increase this ratio. The reason for the preferential absorption within the voluem is due to the fact that the wavelength of each photon alone is longer and has less energy than the energy gap between the ground state and first allowed electronic level. However, if two beams are used and the energy sum of the two laser photons is equal to or larger than the energy gap of the transition, then absorption will take place albeit with a much lower crosssection than one-photon transition. It is also important to note that there is no real level, only a virtual state connecting the ground and excited states, at the wavelength of either beam; therefore, neither photon may be absorbed alone. When two such photons interact in a molecule within the volume, absorption occurs only at the place of pulse overlap. This is in sharp contrast to the sequential two-photon process where each photon is in resonance with allowed real state. In practice, as would be described in some detail further on, the virtual transition allos for accessing any preselected place within the 3D volume while sequential two-photon excitation writes on the surface first. The following section of this paper will deal with the properties of the materials used for 3D memory and their photochemistry of conversion from the write to read forms.
- - - -
Photochemistry of Spiropyrans There are many molecules which exhibit photochromic properties; however, very few have the necessary characteristics required to be considered for utilization in optical devices. These include the separation of the absorption spectra of the two photochromic states, which helps avoid cross-talk. Also, both photochromic states should be stable at room temperature and neither be reactive in the dark or revert from one form to another without light irradiation. In addition, the fatigue developed as a result of cycling from read to write or read lo6times or more should be minimal. A class of photochromic molecules which satisfy many of these requirements is spiropyrans. Spiropyrans (SP) exhibit two distinct photochromic forms, separated by about 150 nm, which suggests that they might be useful as optical material. Spiropyran molecules are composed of two *-electron moieties which are set orthogonallyto each other. The absorption of this molecule is composed of two individual absorption spectra rather than that of a completely conjugated molecule. Because of the absence of conjugation the absorption occurs in the UV region, and we will refer to it as the colorless form, A; see Scheme I. Excitation of this moleculeby two photons initiates a population in an electronically excited state with subsequent bond cleavage which allows rotation of the moieties resulting in the formation
The Journal of Physical Chemistry, Vol. 98, No. 27, 1994 6741
SCHEME 1 hv
I
Me
B
A
of a new, nearly planar structure. This form is referred to as the colored, merocyanineform, B (Scheme 1). Thea-electron system extends, in this planar form, throughout the molecule and is responsible for the red shift found in the absorption spectrum of this form. This large absorption Stokes shift between the original and merocyanineform minimizescross-talk. Many papers dealing with the photocolorationmechanism of photochromic spiropyrans, chormenes, and spiroxazineshave appeared in the literature.lSz7 However, the relationship between the structure of spiropyrans and their photochromic properties is still not understood completely. The steps of the transformation of spiropyran form A, to merocyanine form B, was found to depend upon (a) quantum yield of the photochemical reaction A B, (b) rate constant of the thermal reaction B A, and (c) absorption, emission, and fluorescence properties of forms A and B. The rate constants and the absorption spectra of both forms of most spiropyran molecules are either known or can be calculated.10 Spiropyrans are found to have the desired properties for 3D devices; however, they also have a large disadvantage for use as 3D memory which is the Occurrence of the back thermal reactions B A. This back-reactionessentially erases the written information after a short period of time. In addition, both forms frequently undergo other isomerization reactions that are irreversible and ultimately induce the conversion of the original material to photoinactive species. In this paper we shall discuss new classes of photochromic spiropyrans which promise to be more suitable than benzospiropyrans for use in 3D devices because of their favorable spectroscopic properties and slow if not absent B A conversion.
-
-
-
-
Isobenzofuran Spiropyrans In a previous paper” we have shown that picosecond photoexcitation of a solution of SP I1 in tolune or heptane with 355-nm laser pulses generated a short-lived intermediate which had its maximum absorption at 460 nm. This intermediate decays with a lifetime of 60 ps and forms a long-lived intermediate with ,A, at 550 nm (Figure 1). This intermediate continues to grow up to about 35 ns and then decays with a microsecond rate.
Because of its long lifetime, the state with a maximum absorption intensity at 550 nm is assigned to the relatively stable isomer of the merocyanine form B. This isomer has also been observed by means of nanosecond and microsecond absorption spectroscopy. Thespectra recorded in the picosecond, nanosecond, and microsecond ranges are identical; therefore, we conclude that the same isomer is present in this long time range. The rate of formation of B is found to be the same as the decay of the short-lived intermediate with A,, at 460 nm (Figure 1). This data suggests that the 460-nm intermediate may be the precursor of isomer B. Isomer B, in turn, may be formed from eitherthe& andT1 stateofAoranother isomerofthemerocyanine form. By means of nanosecond spectroscopy,it is found that the triplet state of the closed form (Figure 1) has a lifetime of about
Dvornikov et al.
6748 The Journal of Physical Chemistry, Vol. 98, No. 27, 1994 0.1
I
0 1 350
I
4.b
5.40
6.40
7
Wavelength(nm)
Figure 1. Transient absorption spectra of SP I1 in toluene: (1) 10 ps, (2) 60 ps, (3) 850 ps, and (4) 10 ns after excitation.
I
~~
450
&
550 ‘ 6
0
I
0
I 650
71
Wavelength (nm) Figure 3. Transient absorption spectra of SP Ill in toluene: (1) 10 ps, (2) 300 ps, and (3) 20 p after excitation.
SCHEME 2
Ill R=N$; IV R=Br V R5H
w
decrease in the ratio of the short wavelength maximum intensity to the long wavelength one. We believe that this spectral and kinetic behavior may be explained by the appearance of a ciscisoid isomer X with an absorption maximum at 380 nm. This isomer is subsequently completely transformed into the merocyanine isomer B after 100 ps with absorption maximum at 405 and 570 nm. The 380-nm band is formed with the rate of -(2-3) X 1Olo s-l, which suggests a singlet state.
-
XI
400
500
600
700
Wavelength (nm)
Figure 2. Transient absorptionspectra of 6’-methoxyindolinobenmpu~ pyran in toluene.
250 ns. Therefore, the T I state of form A cannot be the direct precursor of isomer B. The kinetics of the 460-nm band growth in the optical density shows that the formation of an intermediate, which we designate as X,is completed in less than 6 ps. Taking into account the high rate of the formation of the X state and in addition the reported values for the lifetimes of the SIand TI states of the closed form of SP, we conclude that the formation of the X intermediate may originate either from higher electronically excited states S, and T, (n > 1) or from a nonthermalized Franck-Condon SIsinglet state. It is known that the formation of the cis-cisoid isomer of nitrosubstituted indoline spiropyrans is the primary product of the C-0 bond cleavage, and this isomer is formed in the triplet state (3X).Quenching of this state leads to the formation of the closed f0rm.1~ However, in recent papers on unsubstituted indoline spiropyran, Ernsting16J4has reported the immediate appearance of a similar X transient with absorption A, at 400 nm and having the geometry of the closed form. Our picosecond transient absorption experiments of 6’methoxyindolinobenzpiropyran in toluene show (Figure 2) that immediately after excitation with a 25-ps, 355-nm laser pulse an intermediate is formed with two distinct absorption bands in the region of 500450 and 350-450 nm. The absorption intensity of the long wavelength, 500650-nm band increases during the first 100 ps, and after that the spectrum remains stable for about 15 ns. The short wavelength maximum 350-450 nm also rises from 0 to 50 ps, and after 100ps its intensity achieves a constant value. A time-dependent red shift in the wavelength maximum from 380 nm at 25 ps to 405 nm at 100 ps was observed, as well as a
Dithiolan Spiropyrans When solutionsof dithiolanespiropyranswith structures, shown in Scheme 2, are irradiated by two photons, a new colored species is formed in less than 1 ms (Figure 3). No participation of the triplet state of the closed form was observed, and the introduction of triplet donors such as fluorene or oxygen were not found to have any influence in the quantum yield of the photocoloration process. Picosecond excitation of spiropyran I11 solution (Figure 3) in toluene induces the formation of a short-lived intermediate with A, = 490 nm and a decay lifetime of 150 ps. The decay of this product, whichis assigned to thecis-cis isomer of the merocyanine form, is responsible for the formation of a relatively stable isomer of the open form, B, with ,A, = 540 nm. The spectrum of this isomer remains unchanged for several microseconds. Our experiments show that for all spiropyrans of thedithiolane and benzodithiolane series studied formation of the stable isomer of the colored form is complete within 25 ps. The kinetics of thermal decoloration of this isomer, B, were found to be first order (Table 1) in all of the dithiolane and benzodithiolane spiropyrans series studied, for which no interconverting isomers were observed, with the exception of spiropyran 111. Our data suggest that there is only one stable isomer of the merocyanine form in spiropyrans of the dithiolane and benzodithiolane series. The relative thermodynamic stability of the isomers of the merocyanine form of the spiropyrans and the rate constants of their interconversionsare determined to a considerable extent by the heterocyclic segment of the spiropyran molecule. For dithiolane spiropyrans annelation of the benzene ring in the heterocyclic part of this molecule does not change the
Photochromic Materials for 3D Optical Memory Devices
The Journal of Physical Chemistry, Vol. 98, No. 27, 1994 6749
SCHEME 3
h
IX R,=OCH,, %=H; X R,=H, %=OCH, XI R,=N02,%=H
W
400 500 Wavelength (nm)
300
600
700
Figure4. Absorptionspectra of SPIIIclosed and open forms: (1) toluene, 298 K, before irradiation; (2) toluene, 353 K, before irradiation; (3) 3-methylpentane,77 K, after U V irradiation.
TABLE 1: Kinetics and Spectroscopic Properties of SP
111-VI1 111
IV V VI VI1
17 000
540 510 480 500 545
16 000 20 000 32 900 25 000
16.7 1.7 0.8 0.02 2.3
0.14 0.59 0.54 0.24 0.38
0.15 0.05 0.025
absorption spectrum of form A. This is supported by the fact that in the dithiolane series the long-wavelength absorption band of the closed form is completely determined by the chromene fragment. Light absorption excites the chromene moiety, which cannot be deactivated by means of energy transfer to the heterocyclic portion of the molecule. Therefore, the decrease in the quantum yield, @E, observed for A B upon annelation of the heterocyclic part may be attributed to the increase in the radiationless deactivation of the photochemically active excited states of the chromene. Part of the deactivation is achieved by radiationless energy transfer to the heterocyclic moiety which is lower in benzodithiolane compounds than in dithiolane compounds. In addition, the heterocyclic part of the molecule should not have any significant influence on the spectroscopic and photochemical properties of the closed form. Therefore, the spectra of the lower electronically excited states of form A and the luminescence properties are determined by the chromene portion of the molecule. In contrast to all other classes of spiropyrans, it is possible for the spiropyrans of the dithiolane series to transform almost all of the closed form A into open merocyanine form B.28 This is evidenced by the almost complete disappearance of the 345-nm absorption band of the closed form (Figure 4). Even in cases where the absorption of thecolored form is at 345 nm, the residual optical density does not exceed -10%. Knowing the initial concentration of the spiropyrans under investigation, we can easily determine the extinction coefficient of the merocyanine form B at 77 K where no B A reaction occurs. The values €8 obtained were used to determine thequantum yields of the photocoloration of spiropyrans of the dithiolane series (Table 1). Because some error may be introduced into the determination of the €6 coefficients by the irreversible photodecomposition of form A under UV light irradiation, we determined the quantum yields of the irreversible decomposition @dec: of the closed form of such spiropyrans. A solution of the spiropyran in toluene was irradiated at the absorption band maximum of the closed form. The irreversible decrease in the optical density of the absorption band of the closed form ADh(A) was used to calculate ads.
-
-
where ADx(A) is the irreversible decrease in the optical density
m
XII
at the wavelength X(A), CA(A)is the extinction coefficient at the wavelength X(A), Vis the volume of solution being irradiated (dm3), N is the Avogadro number, I is the intensity of the irradiation (photonsfs), and t is the irradiation time (s). The values of obtained by this method and listed in Table 1 attest to the fact that the introduction of a nitro group into the chromene fragment of the molecule results in a drastic decrease in its photostability, which is in agreement with the data obtained for other classes of ~piropyrans.2~ Therefore, the value of cg used in Table 1 represents only a mean value of the experimentally obtained values of €8 for several structurally similar spiropyrans. Photochromic Properties of Spiropyrans in Polymer Films Spiropyrans can be dispersed in many polymers including PMMA and PVA. The solid solutions are easy to make in any form and are stable under light irradiation. We have measured the photochemical behavior of various photochromic systems composed of spiropyrans dispersed in polymer matrice~.~OJl In a polymer matrix unimolecular processes may deviate from firstorder kinetics.32-34 Because of the existence of kinetically inequivalent sites in the polymer,34which in spiropyrans should be associated with different degrees of steric hindrance to the rotation of the merocyanine form, that is necessary for it to adopt the configuration in which cyclization to the chromene ring of the spiropyran molecule can take place. The steric interactions are determined by the size of the heterocyclic fragment of the spiropyran or of the chromene fragment substituents. It is important therefore to clarify the effect and nature of the substituent in the chromene fragment and the heterocycle as a whole on the kinetics of the dark reaction of the photochromic spiropyrans. UV irradiation of spiorpyrans in solid PMMA, at the same concentration as in solution, generates the corresponding colored form with absorption spectra similar to those in liquid solution, but the quantum yield of the photocoloration is 3-4 times lower. The decay kinetics of the colored form of the spiropyrans dispersed in the polymers, with structures, shown in Scheme 3, wefe found to deviate considerably from first order (Figures 5 and 6 ) . In terms of the rate for the reaction in the polymer matrix, we can clearly divide all spiropyrans into two groups: (1) those with the nitro group in the chromene fragment which exhibit longlived color forms-lifetimes of the order of 1-2 h-and (2) those with any substituents other than nitro which show a reverse reaction-with a lifetime on the order of a second. The reverse reaction of the merocyanine form in the dark to revert to the closed form, may be represented as a two-step process35
Bcis-cis
-
A
where (1) is the cis-trans isomerization of the merocyanine form to the cis isomer, which has the more favorable configuration for cyclization (with the closest approach of 0-to the spiro carbon), and (2) is the cyclization reaction itself, Le., the nucleophilic addition of 0-. In liquid solutions the decay rate of spiropyrans without a nitro group in the chromene fragment is controlled
Dvornikov et al.
6750 The Journal of Physical Chemistry, Vol. 98, No. 27, 1994
TABLE 2: Kinetics and Spectroscopic Properties of SP I in Different Polymer Hosts and Solvents
x
.d Y
-d
0.5
solvent/ wlvmer
fluorescence Quantum yield
MMA HEMA PMMA PHEMA
< I x 10-3 3.0 x 10-3 1.6 X 1k2 5.0 X 10-*
thermal decay constant, s-1
7.2 x 8.6 X 4.0X 3.0 x
10-3 10-4 10-4 10-5
lifetime, s
14 1200 2500 33000
m
.Y Y
0“ 0.0 50
0
100
150
200
Time (min) Figure 5. Thermal decay kinetics of merocyanine forms of (1) S P XI, (2) S P XIII, and (3) S P XI1 in P M M A at 20 OC.
solvatochromism in all types of nitro-substituted spiropyrans and increase in the order of the CpClo bond. Increase in the bond order reduces the rate of isomerization, which becomes the ratedetermining step in the nitro-subustituted spiropyrans, even in liquid solutions. In the polymer matrix the inhibition of isomerization in the nitro-substituted spiropyrans as a result of the increase in the rigidity of the medium also causes an anomalously large increase in the lifetime of the meracyanine form. In our experiments on writing and reading information within a 3D volume, the SPmolecule is usually dispersed homogeneously in a polymer matrix. Although the original structure of SP is stable, the written form, merocyanine, whose red fluorescence is the means for reading the information, is unstable and in PMMA, at room temperature, reverts to the original form with a rate of 7.2 X 10-2 s for SP I. Because the fluorescence efficiency is a crucial parameter for the “read” process, we measured its quantum yield as a function polarity and hardness of the host medium. The fluorescence of Rhodamine 6G with its known quantum yield of @R 1,036was used as a standard for the measurement of the fluorescence of dispersed SP molecules efficiency in two polymer matrices, poly(methy1 methacrylate) (PMMA) and poly(2hydroxyethyl methacrylate) (PHEMA), and the corresponding monomeric solutions MMAISP and HEMAJSP. Because both Rhodamine 6G and our samples emit a t the same 500-600-nm wavelength region, comparison of the data becomes more reliable. The fluorescence quantum yield, asp, was calculated using the expression
-
0.00
0
5
10
15
20
25
Time (sec) Figure 6. Thermal decay kinetics of merocyanine forms of S P VIII-X in PMMA at 20 OC.
mainly by the rate of the cyclization stage, since the order of the bond about which the merocyanine chain rotates is close to the single bond and is the same in all such compounds.24 The rate constants for the various compounds are therefore controlled by the degree of delocalization of the charges 6+ and 6- and consequently by the Hammett constants of the substituents. The rigid polymer matrix primarily affects the first stage of the reaction, since this encounters the greatest resistance from the medium during the rotation of the merocyanine fragment in the solid polymer. Therefore, it becomes the rate-determining step for rotation in the polymer matrix. The rate of rotation (isomerization) of the merocyanine chain is controlled primarily by the order of the bond about which the molecule rotates. The bond order C9-Cl0 in all the spiropyrans merocyanine forms with one heterocycle and without the nitro group are approximately the same. Therefore, in the polymer matrix the rate of decay of spiropyrans without the nitro group is approximately the same despite the differences in the Hammett constants of the substituents. The rate constants for the compounds which have the same chromene fragment but different heterocycles when dissolved, in MMA, were found to differ by almost a factor of 100. This differences is found to be practically eliminated in the solid polymer solution. The slight difference in the kinetics observed for the reverse,reactionof these compounds in the polymer is due to the different orders of C9-Cl0 bond resulting from the change in heterocycle basicity. Introduction of the nitro group into the chromene fragment entails redistribution of the electron density in the merocyanine, which is apparent at thevery least from theappearance of negative
where the optical densities of Rhodamine 6G and S P samples are designated as ODR and ODs, respectively, and the fluorescence intensities of Rhodamine 6G and SP as ZR and is, respectively. The fluorescence quantum yields, thermal decay constants, and lifetime of S P I in HEMA, PHEMA, MMA, and PMMA are listed in Table 2. The data show that SP I has a rather low quantum yield, 3 X 1k3-5 X 10-3, compared to the quantum yield for the fluorescence of Rhodamine 6G. One of the reasons for the low quantum yield of the SP written form is isomerization which occurs at a greater rate than the fluorescence decay lifetime and thus decreases the fluorescence intensity. We have shown previously that polar solvent stabilizethe written polar structure and attenuate the rateof isomerization. Similarly hard polymer matrices limit the ability for molecular group movement; consequently, the isomerization rate becomes lower, resulting in a more stable merocyanine form with an increase in the fluorescence quantum yield. The data in Table 2 show that the fluorescence quantum yield and lifetime of SP increases by an order of magnitude from HEMA and PHEMA. A similar increase in the fluorescence quantum yield and lifetime is found for MMA and PMMA. However, the forward reaction rate remains as fast as in PHEMA as was in HEMA and PMMA. The higher fluorescence quantum yield and the longer lifetime for the written form of PS were observed in PHEMA polymer matrix, which is both a hard and polar matrix. W e must note that the fluorescence quantum yield of Spin PMMA and PHEMA is low compared to that of Rhodamine 6G, yet the SP emits with enough intensity to be detected and used for information or reading purposes.
Photochromic Materials for 3D Optical Memory Devices
The Journal of Physical Chemistry, Vol. 98, No. 27, 1994 6751
,.----
L 4 0 Polychrmw”
W
Figure 7. Experimental system for writing and reading of information
in 3D.
5w
Mx)
7w
800
Wavelength (nm)
Figure 8. Two-photon-induced fluorescence spectrum and energy dependenceoffluorescenceintensityafSPI merocyanineform in PMMA.
There are several other photochromic materials which can be used forsuchdevices;theseincludeamongothersdimerJmonomer transformation and diarylethenes.?* Writing and Reading of Information in 3D In our experiments on writing and reading of information in 3D format, we utilized different spiropyran molecules, dispersed homogeneously in polymer matrices such as poly(methy1)methacrylate) or polystyrene. To write information in a 3D storage memory device which contains SP molecules as the recording medium requires excitation in the ultraviolet region of thespectrumbecauseSPabsorbsat-266and355 nm. Excitation to this state is provided by absorption of two photons, either one 1064-nm photon and a 532-nm photon which is equivalent to a 355-nm transition or two 532-nm photons, corresponding to a 266-nm photon energy. It is important to note, however, that two-photon absorption induced by each beam separately may also occur when the individual beams are sufficiently intense to result in background noise. The problem can be eliminated by utilizing two beams of different wavelength such that one beam cannot induce any two-photon background noise, Le., its wavelength is longer than necessary to induce a two-photon transition. Such for example is the case with a 1064-nm beam interacting with a spiropyran molecule which absorbs at 355 and 266 nm which is 3 and 4 times this energy but not at twice. However, when the 1064-nm beam is combined with a 532-nm beam, excitation of the spiropyran molecule to the first excited state at 355 nm takes place. Figure 7 displays the experimental system utilized for the twophoton process towriteand read. Thewritten bitsappear colored because the writing process involves the generation of the open form which absorbs in the 550-nm region. For most of the discussionpresentedthetwobeamspropagateorthogonaltoeach other; however, similar results may be achieved by two counterpropagating beams or in fact any other pulse colliding arrangement. Each configuration has advantages over the others; however, the physics is essentially the same. Figure 7 shows a scheme for the orthogonal pulse interaction used to write and read in 3D space. The information can be stored not only at a bit at a time rate hut also with equal ease in a page format and in fact with severalpages superimposedwithin the memory volume simultaneously. The procedureusedto the“read” the informationwritten within the volume of the memory is similar to the ‘write” cycle except that the “reading” form absorbs at longer wavelengths than the “write” form; hence, one or both laser interacting pulses must be at longer wavelengths than the ones used for writing. When the written formofthemoleculeisexcitedby two-photonabsorption, the molecule fluoresceswith a lifetime of 5 ns. The fluorescence spectrum is located at longer wavelength than the absorption of
Fgure9. Photographicreproductionofaone-page.6.3 Mbit, information
stored inside a 3D memory device. both the write and read forms. This fluorescence is detected by a photodiode or charge coupled device (CCD) and is processed at 1 in the binary code; in this scheme a nonwritten spot would correspond to zero. The proper selection of materials which provide widely separated spectra are extremelyimportant because they assure that only the molecules that have been written will absorb thereading radiationand consequentlyinducefluorescence to be emitted only from the part of the written memory that is being excited. The fluorescence emitted by the written form of SPI afterexcitation with two 1064-nmphotonsisshownin Figure 8. That the process indeed proceeds by two-photon absorption is verified by the square dependence of fluorescenceintensity vs laser energy (see Figure 8, insert). Because the-reading” is based on fluorescence, a zero background process, this method has the advantage of a high “reading” sensitivity. Extremely low-level fluorescence measurements are possible by the use of photomultipliers or charge coupled devices which are capable of singlephoton detection and very low background noise when operated at low temperatures. The -5-11s fluorescencelifetime in essense
6752 The Journal of Physical Chemistry, Vol. 98, No. 27, 1994 becomes the limiting speed of the reading process. A complication may arise from the fluorescence of the written form which if absorbed by an adjacent SP moleculewill induce false information to be read or cross-talk. In addition, if the original, “write” form emits, after excitation, this fluorescence will be “read” by the detector as written information. Therefore, in order to avoid reading while writing, the close form of the molecules chosen must not fluoresce in the “write” form. No fluorescence from the closed form of the SPI molecules used in our experiments has evern been observed. We have been able to make these molecules stable in both forms for several months a t 3 OC. Other molecules with similar photochromic properites have been made which are stable at room temperat~re.’~Utilizing these photochromic materials, which are of the SP famil~,~O we have been able to write, store for a long time, and read several pages of 6.3 Mbit each simultaneously inside the volume of a 1-cm cube. A photograph of a single6.3 Mbit page, whereall bits werestored simultaneously, in a cube is shown in Figure 9. The information was stored by passing a 532-nm picosecond laser pulse through a lithographic mask. The mask pattern then was imaged by means of a set of cylindrical lenses inside the cube where the 532-nm pulses was intersected by a 1064-nm picosecond pulse, thus indicating a two-photon process resulting in excitation of the dispersed photochromic molecules. The excited-state molecules relaxed into the written form affecting the storage of the mask format inside the cube. The image seeing in the picture of Figure 9 is due to the fluorescence of the written information induced by two intersecting 1060-nm picosecond pulses. A more detailed information concerning the writing and reading system maybe found in refs 38 and 39. Summary
We have presented time-resolved experimental data on the two-photon spectroscopy of some photochromic molecules. The transient species and states involved in the photochemical reaction have been identified, and their rates of formation and decay have been measured from the picosecond to the millisecond range. Based on the data, a mechanism of the reaction has been deduced. In addition, we show that these materials may be suitable for use in memory devices and describe the method and system used for writing, storing, and reading information in 3D memory devices. Acknowledgment. This work was supported in part by the United States Air Force, Rome Laboratory, and ARPA under Contract F 30602-93-C-023 1. References and Notes (1) Optical ComputingHardware; Lee, S. H., Johns, J., Eds.; Academic Press: New York, 1993. (2) Hesselink, L.; Bashaw, M. C. Opt. Quantum Electron. 1993, 25, S611. (3) Persistent Hole Burning: Science and Applications; Moerner, W. E., Ed.; Springer: Berlin, 1987.
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