Molecular and Biomolecular Electronics - American Chemical Society

7 Two-Photon Three-Dimensional Optical Storage Memory

Downloaded by UNIV OF MISSOURI COLUMBIA on August 10, 2013 | http://pubs.acs.org Publication Date: May 5, 1994 | doi: 10.1021/ba-1994-0240.ch007

A . S. D v o r n i k o v a n d P . M. Rentzepis* D e p a r t m e n t o f C h e m i s t r y , U n i v e r s i t y o f C a l i f o r n i a , I r v i n e , I r v i n e , CA 92664

We present a means for constructing three-dimensional optical memory devices that operate by two-photon interaction. Photochromic molecules and semiconductors have been studied as materials for writing and reading information within the volume of a solid matrix. The mechanism for writing and reading the information, bit density, cycle times, and stability of the materials under various experimental conditions are the topics addressed.

C O M P U T E R T E C H N O L O G Y has progressed to such an extent that it has created an even larger need for high-performance devices that must store, retrieve, and process huge volumes of data at extremely high speeds (J). Improvements in silicon technology are bringing computer usage to a critical point where the memory capacity and input-output speed are the limiting factors. Therefore, the major component that will modulate the practical limits of high-speed computing is thought to be the memory. Owing to the huge data storage requirements, the need for the parallel execution of tasks and necessity of a compact, very high capacity, low-cost memory is becoming almost mandatory. The necessity, therefore, for the search to find means to store large amounts of information in small volumes cannot be overstated, nor can the requirement for these memories to be capable of large bandwidths and parallel access be overemphasized. Three-dimensional (3D) storage may provide a desirable solution to these needs. Research efforts that may lead to 3D information storage include persistent hole burning (2), phase holograms (3), and two-photon processes, especially using organic materials (4-6), semiconductors, and biomoleeules such as bacteriorhodopsin (7-JO). In this chapter we will restrict discussion to the last

* Corresponding author.

0065-2393/94/0240-0161$08.00/0 © 1994 American Chemical Society

In Molecular and Biomolecular Electronics; Birge, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1994.


162

MOLECULAR AND BIOMOLECULAR ELECTRONICS

topic, namely 3D storage by means of two-photon absorption and par ticularly in the utilization of organic photochromic materials (II). To make a quantum increase in the information density available and the input-output of information be suitable for parallel processing, an all-optical storage device must be advanced and utilized and a means for storing more information per unit volume must be found (12, 13). In the case of optical systems, the density of information stored is de pendent upon the reciprocal of the wavelength λ to the power of the dimension used to store information. For example, if the information is stored in one dimension, then the density is proportionally l/λ. This relationship also suggests that the information storage density is much higher at U V wavelengths and 3D devices than when visible light is used to store information in a two- (2D) or 3D medium. In a 2D memory, the theoretical storage density for a device that operates at 200 nm is 2.5 Χ 10 bits/cm , whereas for a similar 3D storage memory the density may be as high as 1.2 Χ 10 bits/cm . 9

2

14

3

In the following sections we will discuss the basis for a two-photon process, the means for writing and reading information within a volume storage device; describe the materials and methods used; and present a status report on their relevant molecular and spectroscopic properties.

Two-Photon Mechanism The theoretical bases for two-photon processes were established in the early 1930s (14). The probability for a two-photon transition to occur may be expressed as a function of three parameters: line profile, tran sition probability for all possible two-photon processes, and light inten sity. These factors are related by p. ~

2i£

f

y - r \w

- w

if

X

y k

l

-__w

2

- _vΓ (^ /K ΪΓ l

Rife * ei * Rfcf e (w

ki

-

Wi -

2

Ki·ϋ)

+ ) _L K 1Γ)1 M2 + j _ / ,(y„.Λ2 2

2

A

if/2

2

Β& · e · Rfc/* ei

+

2

(w

ki

-w -K -v) 2

2

•hh

(1)

2

where 7*/is the spectral width; i, k, a n d / a r e the initial, intermediate, and final states, respectively; 1 and 2 refer to the two laser beams; R** and Rfk are matrix elements; Ι and I are the intensities of the two laser beams; Κι and K are wave vectors; βχ and e are polarization vectors; iVtf is the center frequency; and P / is the two-photon transition probability. According to this postulate, two-photon transitions may also allow for the population of molecular levels that are forbidden for one-photon processes such as g g and u -> u in contrast to the g u and u g transitions that are allowed for one-photon processes. The first factor τ

2

2

2

f


7.

DVORNIKOV & RENTZEPIS

Two-Photon 3D Optical Storage Memory

163

describes the spectral profile of a two-photon transition. The first factor corresponds to a single-photon transition at a center frequency = ωγ + ω + ν(Κι 4- Κ ) with a homogeneous width 7^. The second factor describes the transition probability for the two-photon transition. This second factor is the sum of products of matrix elements R^R^/for tran sitions between the initial state i and the intermediate molecular levels k or between k and the final state / . Often a virtual level is introduced to describe the two-photon transition. The frequencies of ω! and ω can be selected in such a way that the virtual level is close to a real molecular state. This greatly enhances the transition probability, and it is, therefore, generally advantageous to populate the final level E / by means of two different energy photons with ω + ω = (Ef— E )/h rather than by two equal photons. The third factor shows that the transition probability depends upon the product of the intensities ίχ and I . In the case where the photons are of the same wavelength, then the transition probability depends upon I . It will therefore be advantageous to utilize lasers emit ting high-intensity light such as picosecond and subpicosecond pulses. Such a two-photon absorption process makes it possible to prefer entially excite molecules inside a volume in preference to the surface. This is possible because the wavelength of each beam 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. It is also important to note that there is no real level at the wavelength of either beam; therefore, neither photon may be absorbed alone. When two such pho tons collide within the volume, absorption occurs only at the place of pulse overlap. Theoretically the ratio of the signal to the background is 3:1; however, there are several means for increasing this ratio such as using a weak beam that can induce by itself a two-photon process and an intense laser pulse that cannot populate any allowed excited state by means of two photons. Another means for reducing the background intensity has been discussed by Birge (15). This two photon virtual pro cess is in sharp contrast to the sequential two-photon process where the first step involves the absorption of a single photon by a real spectro scopic level and therefore is not capable of volume storage. The principal difference in the two cases as far as their suitability for 3D volume mem ory is that the virtual case avails itself to writing and reading in any place within the 3D volume, whereas the sequential excitation is re stricted in writing and reading first at the surface. At the point where the two beams interact, the absorption will induce a molecular change that will distinguish this micro volume from the unexcited space. The two molecular structures, that is, the original and the one created by 2

2


2

χ

2

{

2

2



164

the two-photon absorption, are used as the write and read forms of a 3D optical storage memory, respectively.

Writing and Reading in a 3D Memory A 3D memory provides several desirable properties that may not be found in today's electrooptic devices. The major advantages of a 3D storage device are as follows: (1) immense information storage capacity, —10 bits/cm ; (2) random and parallel access; (3) nanosecond or faster writing and reading speed; (4) small size and low cost; (5) absence of mechanical or moving parts; (6) minimal cross talk between adjacent bits; and (7) high reading sensitivity. The operations that enable one to store, retrieve, and erase information within a 3D volume are as follows:


13

3

1. Writing: information is recorded at any place within the 3D medium. 2. Reading: information is retrieved from the memory. 3. Erasing: information recorded in any part of the memory is removed. Currently, information for a digital computer is stored in the form of binary code. The two stages of the binary code, zero, 0 and one, 1, may be thought of as the photochemical changes that lead to two distinct structures of the particular molecular species used as the storage medium. An example is provided by the changes in molecular structure occurring in photochromic materials such as spirobenzopyrans (SP) after the simultaneous absorption of two photons. The structure of a typical SP molecule is shown in Figure 1. SP has two distinct forms: the spiropyran or closed form and the merocyanine or open form. These two distinct forms provide the two states necessary for storage information in a binary format. Specifically the original closed form designates zero, whereas the open form designates one. To write information in a 3D device that contains SP requires excitation in the U V region of the spectrum because SP absorbs at —260 and 355 nm (see Figure 2). Excitation to this state is provided by the two-photon absorption of either a 1064-nm photon and a 532-nm photon equivalent to a 355-nm photon, or two 532-nm photons, corresponding to a 266-nm photon. The background to signal ratio for two collinear beams propagating at opposite directions is 1:3. Figure 1 displays the energy level diagram along with the molecular structures of the write and read forms of SP. By using laser beams with wavelengths shown in Figure 1 and by translating the beams along the axes of the memory device, which in this case is in the form of a cube,



7.



165

Figure 1. Top, Schematic representation of writing and reading in 3D by two-photon processes. Middle, Energy level diagram and shape ofSP in the write and read forms. Bottom, Structure of the SP in the write (I) and read (II) form.

the required spatial pattern is achieved in the form of colored spots within the volume of the memory. The information can be stored in a page format with several pages superimposed within a memory volume. A complication can arise from the presence of fluorescence from the excited, closed zero form which, if absorbed by adjacent molecules, would subsequently transform them to the read form and thus introduce cross talk between adjacent bits. To avoid such effects, molecules are chosen that do not fluoresce in the write form. No fluorescence from the write form of the SP molecules used in our experiments has been observed even at liquid nitrogen temperatures (5).



166


The read cycle operates similarly to the write cycle except that the read form absorbs at longer wavelengths than the write form; therefore, one or both laser beam wavelengths must be longer than the ones used for writing. The result of absorption by the written molecule is fluorescence. This fluorescence is detected by a photodiode or charge-coupled device (CCD) and is processed as 1 in the binary code. The longer wavelengths assure that only the written molecules will absorb this radiation, which induces fluorescence to be emitted only from the written volume of the memory. The fluorescence spectrum of a written bit is shown in Figure 2. Self-absorption of the fluorescence by adjacent written SP molecules does not affect the reading process because the largest segment of the fluorescence is emitted at longer wavelengths than the ab-

Pulse Energy, mJ

Figure 2. Top, Absorption spectra of write form (a); read form (b), and fluorescence of read form (c) of ISP in polymethyl methacrylate. Bottom, Laser power vs. fluorescence intensity (proof of two-photon process).



7.



167

sorption band. The small amount of fluorescence that may be absorbed by adjacent written bits will yield signals that are either too weak in intensity to be detected or that can be easily eliminated by means of electronic discriminators. Because the reading is based on fluorescence, a zero background process, this method has the advantage of a high reading sensitivity. Light detection by means of photomultipliers or CCDs makes possible single-photon detection measurements. The fluorescence was found to decay with — 5-ns lifetime, which in essence is the speed of the reading process. Erasing the information may be achieved either by increasing the temperature of the memory device to —50 °C or by irradiation with infrared light. By increasing the temperature, the written molecules achieve more energy than barrier separating write and read, causing the written molecules to revert to the original form.

Materials A vast number of molecules may be used as materials for 3D devices, including photochromic materials, phosphors, photoisomers, and semiconductors. One rather promising class of molecules that has been used in research on 3D storage are the spiropyrans. Spiropyrans are molecules of the general structure shown in Figure 1 and Table I. The photochromism, namely their change in molecular structure after light absorption, of spiropyrans is attributed to the photoinduced cleavage of the C - O bond of the pyran ring formed by isomerization to another form referred to as merocyanine dye (16). We designate this initial orthogonal structure as the write form, 0, in the binary code. Light at 350 nm or less induces absorption that initiates the photochemical opening of the pyran ring (see Figure 1) forming an isomer that, in turn, rearranges to a planar form as a result of thermal isomerization. This planar form designated as the read form, 1, in the binary code, is colored in appearance and absorbs intensely in the 550-nm region as shown in Figure 2. The intermediate state, between the orthogonal and planar species, has been identified by means of time resolved spectroscopy (17, 18). The final, colored species of SP is formed with a quantum yield that varies from — 10 to 98% (16). This variation depends in part on the structure of the original molecule and the host, that is, polymer or solvent. Generally, 6 - N 0 , 8-OCH -substituted compounds such as ISP and 2SP are more sensitive than other types of spiropyrans and have coloration quantum yields of about 50 to 70% in nonpolar solvents. The coloration quantum yield usually decreases with increasing of solvent polarity (19). The energy levels of the photochromic components depend upon the substituents and the nature of the matrix. Therefore, the choice of the molecule has a profound effect upon the operation, speed, and performance of 2

3


168

MOLECULAR AND BIOMOLECULAR ELECTRONICS Table I.

Spectra and Decay Kinetics of Read Form of Spiropyrans in Polystyrene and Poly (methyl methacrylate)

Absorption Fluorescence of Merocyanine ofMerocyanine Form (nm) Form (nm)


SP 1 2 3 4 5

PMMA

PS 609 630

580 615 580 585 565

645 690

1

2

PMMA

PS

PMMA

PS

kj; k (s' )

4 Χ ΙΟ" ; 2 Χ 10" Ι X 10"" ; 6 X 10~

625 670 620 605 610

3

4

2

4

1 Χ 10" ; 3 Χ 10" 5 Χ 10" ; 2 X 10~ 3 Χ 10" ; 2 Χ 10" 3

4

3

4

2

3

NOTE: ki and k , thermal decay constants; SP, spirobenzopyrans; PS, polystyrene; P M M A , poly (methyl methacrylate). 2

5

the overall memory device. The polymer matrix usually decreases the rate of molecular transformation and its quantum yield compared to liquids because of increase in viscosity. In addition solid devices are frequently preferable to liquids. The mechanism that drives the write and read process in these pho tochromic molecules may be presented by the following equation, where A represents the original closed form, 0, in binary code; A designates the electronic excited state of Α; ί is an intermediate species of veryshort lifetime; hvi, hv , hv , and hv are photon energies; v\ Φ v Φ v and usually ν > v > v . Β is the colored, open form binary code 1; B* 2

γ

2

3

4

3


2

3

7.



169

designates the excited state of Β that is formed after absorption of two photons; Δ is heat; and hv designates fluorescence. 3


A A'

I

I

Β

„ Β

2 hu, Β"

read

B + hu

B" Β

write

3

hu /A 4

erase

Other materials that are highly photochromic and that may be used for 3D devices are fulgides. Fulgides also undergo photoinduced ring opening. The quantum yields for the read and write forms have been measured (20> 21), and the lifetime of the process including the inter mediate states and kinetics are shown in Figure 3. Because of the lack of fluorescence, these materials are not considered suitable for appli cations where the reading process relies on the detection of fluorescence emitted by the written bits. For the case of nonfluorescing materials, reading can be realized by detection of the two-photon absorption of the written molecules. Another distinct set of materials that have some very interesting inherent properties for optical memories are the infrared phosphors (22,23). These materials emit in the visible region of the spectrum when irradiated with near-IR light. The Quantex Q-phosphors that we studied are semiconductor materials doped with rare earth electron traps. The writing process is achieved by the promotion of an electron from the valence to the conduction band by means of UV irradiation. The electron is trapped and remains there indefinitely because the potential energy well of the trap is much deeper than 1000 T. Reading of the information is achieved by irradiating the phosphor with near-IR light ~ 1 eV, which is of sufficient energy to excite the electron over the barrier. Subse quently the electron decays back into the valence band followed by visible light emission. Presently, two disadvantages are associated with the commercially available phosphors. The information may be read only once and the phosphor is not soluble in any polymer that we know. Therefore, a 3D block composed of these materials suffers from excessive light scattering. These materials however have the advantages of being stable at room temperature and because a very large number of electrons



170


may be excited into the conduction band, the storage of very large numbers of bits of information may be possible. Presently, we favor the use of photochromic materials such as spiropyrans over other materials for 3D memory devices because they possess most of the characteristics needed for writing and reading within a 3D device, such as high absorption cross-section, high photocolorarion efficiency, intense fluorescence of the read form, and the possibility to repeat the write-read-erase cycle more than 10 times. 6

Stability Several critical characteristics must be examined before materials may be considered suitable for 3D memory devices. These properties may include among others the stability at room temperature, fatigue as a function of the number of cycles performed, laser power requirements



7.



171

for writing and reading, cross talk, absorption cross-section, and fluo rescence quantum yield. Studies are currently performed in all of these areas. The closed form of SP materials that correspond to 0 in the binary or write form are stable. However, the open, read form is rather unstable and was found to revert to the original write form after a few hours at room temperature. The information in the SP read form was found to disappear via a multiexponential thermal decay. Such a nonexponential behavior is expected because of the many types of isomers and the many nonequivalent sites in the polymer matrix. However, the thermal decay kinetics may be satisfactorily approximated as a sum of two exponentials. The decay constants calculated under this approximation are shown in Table I. Replacement of one polymer by another with higher polarity may cause the decrease in the rate of dark decay of the merocyanine forms. Apparently, this effect can be attributed to relative stabilization of the polar structure of the merocyanine form by the polar environment of the matrix. The effect of the thermal decay processes on the stored information is obviously very detrimental to any device because it causes the information stored to be deleted. To alleviate this, the material is kept at low temperatures, that is, —40 °C or lower, where the decay of the read form is negligible. Because the open form is essentially a dianion with a positive charge located on the nitrogen and a negative on the oxygen (Figure 1), an ionic species such as an acid could bind these groups and anchor the molecule permanently onto the open, written structure. This was achieved by the use of H C l and a variety of organic and inorganic acids. Reaction of SP with H C l shifts the equilibrium from the closed form to the open form and renders the open form stable indefinitely at room temperature. It was possible to drive it back to the closed form by irradiation with U V light. The use of polar polymers such as poly(hydroxyethyl methacrylate) (PHEMA) for anchoring the two ends of written SP molecules has also improved the stability and lifetime of the read form. For example, while in the original molecule, the in formation decayed to l/e of its value within 70 min; at 20 °C the bridged material was found to have a lifetime of 1.6 Χ 10 s or 3 X 10 min at 3 °C (see Figure 4). As a consequence of this chemical binding, the activation energy of the bridged species has increased to 40 kcal as shown in the Arrhenius plot of Figure 4. 6

4

We have measured the decay kinetics from 77 to 273 Κ with an accuracy of ± 2 K, keeping the sample in a cryostat, thus allowing us to measure the photostability of the read form as a function of temperature. The sample was kept at a particular temperature, irradiated with 532nm photons, and then the fluorescence intensity of the read form was recorded as a function of the number of laser reading pulses. The decay of fluorescence intensity as a function of laser irradiation, at 0.5 mj, at various temperatures is shown in Figure 5 (top). These experiments




172

0.0032

0.0034

1/T

0.0036

Figure 4. a, Decay of read form of ISP in polymethyl methacrylate; top trace: at 77 K; bottom trace: at room temperature, h, Activation energy plot of bridged read form of ISP in poly (hydroxyethyl methacrylate). k is thermal decay rate constant and Τ is temperature in kelvins.

showed that the lifetime of the read form of SP, dissolved in a poly (methyl methacrylate) (PMMA) solid block, increase with decrease in temper ature (Table II).

Fatigue Fatigue, which for a photochromic molecule is defined as the gradual loss of information after repeated write and read cycles, is a very im portant property of the device. Fatigue places a limit on the maximum number of write and read cycles that can be performed with a particular single 3D memory unit. Fatigue, therefore, may be measured by the decrease in the fluorescence intensity of the written form as a function of the cycles performed. The effect of temperature and laser power on fatigue may also be important and has been studied extensively. In the studies presented here, we used a 1-cm P M M A cube in which 10~ M 2



7.



173

1 SP was homogeneously dispersed. A set of ~30-μιη spots were written inside this P M M A - S P cube. The material was maintained at ~ 7 7 Κ throughout the experiment. The reason for this low temperature was to ascertain that any decay measured was due to the reading process rather than to temperature. In these experiments, the written spot was illu minated with a laser light having the preselected energies shown in Figure 5 (bottom), and the change in the fluorescence intensity of the written spot was measured as a function of reading cycles. The data show that the energy of the beam plays a dominant role on the fatigue and the number of cycles that may be performed before the stored

0.0 Η

0.05 m j / c m "Έ—"

S—5

É"

2

i

OJ5mJ/cm

2

•04 .

I

-0.8

Reading cycles χ 10' Figure 5. Top, Fatigue of ISP in poly (methyl methacrylate) as a function of temperature; shown asfluorescenceintensity versus number of cycles at various temperatures at 0.5 mj/cm . Bottom, Fatigue as a function of laser energy for ISP. 2



1.3 Χ 10" (76) 4.9 Χ 10" (200) 5.8 Χ ΙΟ" (17)

PMMA

2

3

2

5.2 Χ 10" (190) 6.1 Χ 10~ (160) 3.5 Χ 10" (28)

o°c

3

2

3

4

2.7 Χ 10~ (370) 7.2 Χ 10~ (1400) 2.3 Χ 10" (430)

3

3

-20 °C

1

1.2 X 10~ (850)

-30 °C 3

2.4 Χ 10" (4100) 3.3 X 10~ (3000) 1.4 X 10~ (715)

-40 °C

3

4

4

2.7 X 10~ (3700) 2.9 X 10~ (3450) 4

4

Polymers

-60 °C

Photostability o f I S P i n V a r i o u s

1.2 X 10~ (8400)

-70 °C 4

2.5 Χ 10" (40000)

-80 °C

5

8.7

5.0

6.4

ΔΕ (kcal/mol)

NOTE: Values are thermal decay constant expressed in s with τ value in parentheses, expressed in s. 1 SP, 1 spirobenzopyran; ΔΕ, change in energy; P M M A , poly (methyl methacrylate); PS, polystyrene; PVA, poly (vinyl alcohol).

PVA

PS

20 °C

Polymer

T a b l e II.


Ω

ο g

I

Ο

w

ζα

>

ι

ο

-α

7.



175

information fades. The fatigue expressed as decrease in fluorescence intensity shows that one-half of the information is lost after 10 cycles when the energy of the beam is L 2 5 mj/cm . The same intensity decrease occurred after ~ 1 0 cycles when the power of the reading laser was 0.5 mj/cm . Practically no change was observed when a 0.05-mJ/cm energy laser beam was used to read the stored information. These results suggest that to utilize the SP material for the construction and utilization of a memory device, the energy of the laser should be ~ 5 0 μ] if the information is to read more than 10 times. 5

2

6

2

2


6

Effect of Polymer on Stability The stability of the written form was also measured as a function polarity and hardness of the host medium. The measurements were based on the decrease in the fluorescence intensity of the read form as a function of the polarity and hardness of the polymer matrix while keeping all other parameters, such as temperature, wavelength, and intensity of the laser beam constant throughout the experiment. The fluorescence quantum yields of SP solutions in methyl methacrylate (MMA), in 2hydroxyethyl methacrylate (HEMA), and in the corresponding polymers are listed in Table III. We have shown previously that polar solvents stabilize the written polar structure and attenuate the rate of isomerization. Similarly hard polymer matrices limit the ability for molecular group movement, thus making isomerization slower and consequently increase the fluorescence quantum yield. The data in Table III show that the fluorescence quantum yield of SP increases from H E M A to P H E M A from 3.0 X 10~ to 5.0 X 10~ . A similar increase on the fluorescence quantum yields is found for M M A and PMMA. The higher fluorescence quantum yield for the read form of SP was observed in P H E M A polymer matrices, which are both hard and polar. These data show that the choice of polymer has a strong influence in the stability of the read form. It follows, therefore, 3

2

Table III. Kinetic and Spectroscopic Properties of ISP JSP desolved in

MMA HEMA PMMA PHEMA

Fluorescence Quantum Yield

Molecular and Biomolecular Electronics - American Chemical Society

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