Two-Photon Three-Dimensional Optical Storage Memory

Two-Photon Three-Dimensional Optical Storage Memory

(Parte 1 de 4)

Copyright 2009 by the American Chemical Society VOLUME 113, NUMBER 49, DECEMBER 10, 2009

Two-Photon Three-Dimensional Optical Storage Memory

A. S. Dvornikov, E. P. Walker, and P. M. Rentzepis* Department of Chemistry, UniVersity of California at IrVine, IrVine, California 92697

ReceiVed: June 16, 2009

We describe the design and construction of ultrahigh capacity three-dimensional, 3D, optical storage devices that operate by two-photon absorption. The molecular systems and their properties that are used as two photon media for writing and one photon for accessing the stored information within the volume of the device are presented in some detail and the nonlinear two-photon absorption mechanism is briefly visited. The optical system and its components, which facilitated writing and reading, are also described and the bit density, bit error rate, store and access speeds, cycle times, and stability of the materials under various experimental conditions are also topics addressed in this review. The first ever storage of terabyte data in a removable storage disk is described in detail.

1. Introduction

Computertechnologycontinuesto progressat suchan increasing ratethatit createsan evenlargerneedfor high-performancestorage devicesthatmuststore,retrieve,andprocesshugevolumesofdataat extremely high speeds. Improvementsin silicon technologyare bringingcomputerperformancetoapointwherethememorycapacity andinput-outputratesarebecomingthelimitingfactors;consequently, themajorcomponentthatwillmodulatethepracticallimitsof highspeedcomputingis thoughtto be the memorythat storesthe huge amountofdatatheindustryandgovernmentgenerate.Toaccomplish thesetasksneededfortheparallelexecutionof tasks,thenecessityof a reliablecompactandterabytecapacitymemoryisbecomingalmost mandatory.

To that effect, research to find means to store large amounts of information in small volumes, capable of large bandwidths and parallel access continues in many fronts and threedimensional (3D) storage has the promise to provide a solution to these needs. Basic research and applied efforts that may lead to 3D information storage include mainly phase holograms1,2 and two-photon processes.3-9 This paper will be confined only to 3D storage by means of two-photon absorption and in particular is a review of our studies and result in the conception, design and construction of two-photon 3D terabyte storage devices.

To ensurea quantumjumpin storagedensityandprocessing,the input-output speed must includeparallelprocessing,which most probablymeansthatan all-opticalstoragedevicemustbe advanced and utilized.In the case of two-photon3D storagethe densityof informationstoredisdependentuponthereciprocalofthewavelength λtothepowerofthedimensionusedtostoreinformation.Forexample, if the informationis stored in one dimension,then the densityis proportionally1/λ. Thisrelationshipalsosuggeststhattheinformation storagedensityismuchhigheratshort,UVwavelengthsthanvisible light.Forexample,thetheoreticalstoragedensityfora 2D devicethat operatesat 200 nm is 2.5 × 109 bits/cm2, whereasfor a similar3D storagememorythe densitymay be as highas 1.2 × 1014 bits/cm3 .

Inthefollowingsectionswepresentthebasictwo-photonabsorption mechanismandmethodforwritingandretrievinginformationstored withinthebulkof thedevice.In addition,a numberof thematerials and theirrelevantpropertiesthatare usedas two photon3D media are presentedand the system(s)employedfor storingTerabytesof datain a DVD-typedisksdescribed.

2. Two-Photon Mechanism

The theoretical bases for two-photon processes were estab- lished in the early 1930s.10,1 The probability for a two-photon

* Corresponding author. Tel.: (949) 824-5934. Fax: (949) 824-2761. E-mail:

10.1021/jp905655z C: $40.75 2009 American Chemical Society Published on Web 10/26/2009

transition to occur may be expressed as a function of three parameters: line profile, transition probability for all possible two-photon processes, and light intensity. These factors are related by

where γif is the spectral width; i, k, and f are the initial, intermediate, and final states, respectively; 1 and 2 refer to the two laser beams; Rik and Rkf are matrix elements; I1 and I2 are the intensitiesof the two laser beams; k1 and k2 are wave vectors; el and e2 are polarization vectors; ωki is the center frequency; ν is velocity; and Pif is the two-photon transition probability. These two-photon transitions may also allow for the popula- tion of molecular levels that are forbidden for one-photon processes such as g f g and u f u in contrast to the g f u and u f g transitionsthat are allowed for one-photon processes. The first factor describes the spectral profile of a two-photon transition and corresponds to a single-photon transition at a center frequency with a homogeneous width γif. The second factor describes the transition probability for the two-photon transition. This second factor is the sum of products of matrix elements RikRkf for transitions between the initial state i and the intermediate molecular level k or between k and the final state f. Often a virtual level is introduced to describe the two- photon transition. The frequencies of ω1 and ω2 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, sometimes advantageous to populate the final level

Ef by means of two different energy photons with ω1 +ω2)

(Ef - Ei)/h rather than by two equal photons. The third factor shows that the transition probability depends upon the product of the intensities I1 and I2. In the case where the photons are of the same wavelength, the transition probability depends upon

I2, it will therefore be advantageous to utilize short pulses that such as picosecond and femtosecond pulses.

Such a two-photon absorption process makes it possible to preferentially excite molecules inside a volume rather than the surface, which is the case for most storage devices. This is possible because the wavelength of each photon alone is longer, has less energy than the energy gap between the ground state and first allowed electronic level. However, if two photons collide simultaneously at any point within the volume of the storage media 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 photons collide at a point within the volume, absorption occurs only at the place of pulse overlap. At the point where the two beams interact, the absorption induces a molecular change in structure

AlexanderS. Dvornikovgraduatedfrom Moscow State University,Russia, in 1976 and received his Ph.D. in physical chemistry from the Institute of Chemical Physics Academy of Sciences, Moscow, in 1983. He was a researcher at the Institute of Chemical Physics from 1976, where he studied the mechanism and kinetics of photochemical reactions of organic dyes and photochromic materials. He joined the staff of the University of California, Irvine in 1989. In 1995, he also joined Call/Recall, Inc., where he was a Head of Materials Division and VP, involved in developing 3-D optical memory technology. He has published over 100 research papers in scientific journals and books and holds several patents.

Edwin P. Walker receivedhisPh.D.inOpticalSciencesfromtheUniversityof Arizonain1998.Hisworkincludesopticalsystemdesignandprototyping,optical testingandsystemalignment,opticaldatastorage,focusandtrackingservosystems, deformablemirrorsystemintegration,design/tolerancing/prototyping/integrating stateofthearthighperformanceopticalsystems.RecentlyheisworkingatBoeing as a SeniorOpticalScientist/Engineer.He alsoperformed7 yearsof pioneering researchanddevelopmentonvolumetricopticaldatastoragesystemsandrecording/ readoutsystemintegrationwith Call/Recall.He has over 20 publicationsin the opticsarea,1 patentand somepending,including1 bookchapterpublished.

Peter M. Rentzepis received the Ph.D. degree in physics and chemistry from the Universityof Cambridge,U.K. He joinedBell Laboratories,Murray Hill, NJ, as a Member of the Technical Staff and later as Head of the Physical and Inorganic Chemistry Department. He was appointed Presidential Chair and Professor of Chemistry and Electrical Engineering and Computer Science at UCI in 1985. He is a Founder of Call/Recall Inc. He pioneeredtwo-photon3D opticalstorageand has publishedover 400 papers, 5 books, and has over 80 patents. Dr. Rentzepis has received several awards in chemistry and physics and is a member of several scientific societies including the U.S. National Academy of Sciences and of other countries.

13634 J. Phys. Chem. A, Vol. 113, No. 49, 2009 Dvornikov et al.

and in effect creates a new molecule that is distinct from the unexcited molecules. The two molecular structures, the original and the one created by the two-photon absorption, become the write and read forms of a 3D optical storage memory, respectively.

3. Writing and Reading in a 3D Format

A 3D memory provides several desirable properties that may not be found in today’s electroopticinformationstorage devices:

2. Random and parallel access. 3. Fast writing and reading rates (nanosecond range). 4. Small size and low cost. 5. Minimal cross talk between adjacent bits. 6. High reading sensitivity. The operations that enable one to store, retrieve, and erase information within a 3D volume are: 1. Writing: information is recorded by two-photon absorption at any preselected place within the volume of the 3D medium. 2. Reading: information is retrieved from the memory in bit or parallel form. 3. Erasing: information recorded in any part of the memory may be erased and new informationstored using the appropriate wavelength.

Currently,computerinformationis storedand read in the form of binary code. The two stages of the binary code: zero, 0, and one, 1, in our case they may be thought to be 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 spiropyrans after the simultaneous absorption of two photons. The structure of a typical material used has two distinct forms: the write, closed form, and the read, 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, the media are excited by two-photon absorption of either a 1064 nm photon and a 532 nm photon equivalent to one 355 nm photon or two 532 nm photons, corresponding to a 266 nm photon.

Figure 1b displays the energy level diagram along with the molecular structures of the write and read forms of one of the molecules that we used to store data. By using laser beams with the wavelengths,shown in Figure 1, and by translatingthe beam along the axes of the memory device, which may be in the form of a cube or a disk, Figure 1a, we store the data in the form of spots within the volume of the memory device. The information can be stored in a page 2D disk format with many disks stored next to each other within a memory volume. Excitation and storage may be achieved also by using one tightly focused beam inside the volume of the 3D device, Figure 1a.

A complication may 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 such that the write form neither absorbs the fluorescence wavelengths of the read form nor it emits fluorescence.

The read cycle operates in a manner similar to that for the write cycle except that the read form absorbs at longer wavelengths than the write form; therefore, the reading laser source wavelengthis longer than the write. Following excitation of the written bits the fluorescence is detected by a photodiode array or charge-coupled device (CCD) and is processed as 1 in the binary code. The longer wavelengths of the reading light ensure that only the written molecules will absorb this radiation. Absorption of the two forms and the fluorescence spectrum of a written bit are shown in Figure 1d. Self-absorption of the fluorescence by adjacent written molecules does not affect the reading process because the largest segment of the fluorescence is emitted at longer wavelengths than the absorption band. Because 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 photodiode arrays 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.

Information can be stored not only at a bit at a time rate but also in a page format. Passing one beam of the written laser pulse through an SLM (Spatial Light Modulator) that contains the information to be stored in a 2D page format and image inside the cube where the plane of the other beam intersect it yields a two-photon process that results in the writing of a complete 2D page in the bulk of the cube or disk. In the case where the information of an entire page is to be accessed at once, rather than in a bit by bit mode, a single low intensity thin plane beam is used to illuminate the entire written 2D page to induce fluorescence by one photon process of the entire 2D page. The fluorescence is picked-up by a CCD and processed.

Figure 1. (a) Cube and disk storage devices. (b) Two-photon process schematic diagram. (c) Photochromic reaction, write and read forms. (d) Spectra of write and read forms.

Feature Article J. Phys. Chem. A, Vol. 113, No. 49, 2009 13635

4. Materials

A vast number of molecules may be used as materials for 3D devices, including photochromic materials, phosphors, photoisomers, and semiconductors. One of the most important properties that a two-photon medium must have is high twophoton cross-section. Excellent progress has been made in this area by several researchers,6,7,12-15 where very high two photon cross-section materials were designed and synthesized.

A few types of the molecules that have been used as 3 D storage media are described in the next section. 4.1. Spiropyrans. One of the first memory materials that we utilized for recording information in 3D format by two-photon absorption were photochromic spiropyrans (SP).3,16 Spiropyran molecules may exist in two isomeric forms: cyclic spiro form A and open merocyanine form B, Scheme 1. Exposure of the colorless form A to UV light results in the heterolitic cleavage of the C-O bond followed by isomerization to the colored merocyanine from B. Typical absorption spectra of forms A and B and the fluorescence spectrum of form B are shown in Figure 1d.

The spiro form A that corresponds to zero in the binary code is stable; however, the written merocyanine form is thermally unstable and reverts back to the original form after a few hours at room temperature. The most direct and simplest means for stabilizing this structure is to lower the temperature below the -30 °C activation energy; however, low temperatures are not desirable for practical applications. 4.2. ReversibleDimerizationof PolycyclicAromaticCompounds. Other photochromic materials studied in our laboratories that show promise for use as 3D memory devices are dimers of polycyclic molecules such as anthracenes.17

The photodimers are formed by excitation of the corresponding monomers, and the dimers revert back to monomers when exposed to UV light, Figure 2. The dissociation of the dimer, write form, results in the regeneration of a conjugated double bond molecule that exhibits a red-shifted absorption band. The monomer,writtenform, has its long wavelengthabsorptionband in the 300-400 nm region, while the dimer is blue-shifted and has practically no absorption at wavelengths longer than 300 nm. The monomer, written form emits with a fluorescence quantum yield of 0.3, while the dimers are practically void of any fluorescence. Because both dimer and monomer forms are stable and possess the high absorption cross-section and high quantumefficiencyfor both dissociationof dimersand monomer fluorescence, this photochromic system is potentially attractive as media for 3D memory and other optical devices. 4.3. Fulgides and Fulgimides. Photochromic fulgides, a class of organic compounds known to be capable of reversible light induced change in their structure18,19 are thermally stable in both forms and exhibit high photoreactionefficiencyand high fatigue resistance to repeated writing-reading-erasing cycles. Photochromic fulgides are promising candidates for many technological applications including use in recording media, particularly in erasable 3D optical storage devices. However, because photochromic fulgides do not fluoresce in either form we synthesized the fluorescing fulgides, shown in Scheme 2, which undergoes reversible photoisomerization that generates the cyclic structure C that fluoresces,20 Figure 3.

The absorption spectrum of the colored C form, Figure 3, is red-shifted by 130 nm compared to the open, E, form and in addition the C form exhibits a very low absorption in the region of 330-400 nm. Because of the significant Stokes shift of the spectra, we were able to convert the open form, almost quantitatively, into the cyclic, closed, colored form by light excitation at 350 nm. The colored form can be reversed to the open form by excitation at ∼500 nm. During the photoinduced process, only the formation of the E form was observed, which is the preferred configuration for the cyclization process. The coloration/bleachingcycles that correspondto the write and read forms can be repeated many times without noticeable decomposition of the material, Figure 4.

All isomeric forms of these 2-indolylfulgides show excellent long-term, room temperature, thermal stability. In contrast to other previously investigated fulgides, the colored, read form C, of the 2-indolylfulgidesemits a broad-bandfluorescencewith maximum intensity at about 610 nm, Figure 3. The fluorescence spectrum is Stokes shifted by approximately 100 nm from the absorption. No fluorescence from the E form was detected.


Figure 2. Reaction scheme and absorption spectra of (a) dimer, (b) monomer, and (c) fluorescence spectrum of anthracene monomer in PMMA.


Figure 3. Absorption and fluorescence spectra of isomeric forms of 2-indolylfulgide.

13636 J. Phys. Chem. A, Vol. 113, No. 49, 2009 Dvornikov et al.

Not only do fulgimides,21 Scheme 3, which are derivatives of fulgides,exhibitthe excellentphotochromicpropertiesof their precursor fulgides, but in contrast to fulgides, they are also chemically stable to acid or base-catalyzed hydrolysis. In addition, a very attractive aspect of these fulgimides is that the introductionof various substituentsat the nitrogen atom position of the imide ring can be used as a linking group to prepare photochromic copolymers, photochromic liquid crystals, photoregulated binding of proteins, and photochromic Langmuir- Blodgett films.2-25 4.4. Photochromic Copolymers. For molecules to be suitable as storage media they must be uniformly dispersed in a polymer matrix at relatively high concentration ∼10-1 M. A desirable means to achieve uniform dispersion and high concentration of the storage media is to form a photochromic cross-linked copolymer. We have synthesized such copolymers by copolymerization of the photochromic molecule and the corresponding monomer.26 To this effect, we polymerized methyl methacrylate monomer and the photochromic 2-indolylfulgimides containing groups capable to copolymerization and formedthe opticallyclear photochromiccross-linkedcopolymer, shown in Scheme 4.

These copolymerswere found to be photochromicand exhibit photoinduced reversible transformation from the write to read form, and the written bits emitted intense fluorescence. Similar to the case for the colored forms of the pure fulgimides, we also determined that the efficiency of the write/read cycle in the copolymer was essentially the same as for unpolymerized fulgimide molecule in solution. 4.5. Nondestructive Readout Molecular Memory. There is a commonproblemassociatedwith rewritableopticalmemory materials: destructive readout. In most if not all photochromic molecules proposed so far, the read and erase states have the same absorption band; therefore, some erasing, while reading, is unavoidable. This is expected because the vast majority of photochemical reactions and the onset of emission, in condense media, proceed from the same lowest electronic-excited state of the molecule.

(Parte 1 de 4)