Research activities on materials for wavelength-multiplexedoptical recording in Japan

Uichi Itoh and Toshiro Tani

We describe recent research activities on materials for future wavelength-multiplexed optical recordingsystems in Japan. Organic materials are the major objectives in this field: photochromic materials and theyoung field of possible photochemical hole burning materials. As for the latter, we introduce our results onorganic dye molecules in polymer systems and discuss matrix effects on hole spectrum and the possibility ofmatrix designing. For the former, recent progress in new materials for erasable and/or multiplexed memoryand the relationship of the matrix, performed in universities, governmental laboratories, or industries, isreviewed briefly.

1. Basic Research on Hole Burning Materials in ETL

Persistent spectral hole burning is essentially a se-quential two-photon process (burning process by thefirst photon and the observation process by the secondone). Various kinds of nonphotonic phenomenon mayproceed between the two photonic processes and evenduring the first burning process. This makes the phe-nomena fairly complicated to understand. For exam-ple, the hole widths observed in systems of organic dyemolecules doped in glass matrices are usually 2 or 3orders of magnitude larger than those (twice of 1/T 2 )estimated from the simple phenomenological lineshape theory.1 It is considered so far that this ismainly due to the nonequilibrium nature of the disor-dered matrices.

For the hole burning phenomena to appear, the exis-tence of the solid matrix is essential as well as thephotoreactive molecule itself. Hole broadening phe-nomena can be a distinctive tool for obtaining informa-tion on the excited state of the photoactive moleculeand on the interactions with its surroundings. In cer-tain situations, the latter can be the leading factor ofthe broadening, which should not be interpreted by thesimple T2 relaxation properties of the excited state ofthe dopant molecules, which determines the so-called

When this work was done both authors were with ElectrotechnicalLaboratory, Umezomo 1-1-4, Sakura-mura, Niihari-gun, Ibaraki305, Japan; U. Itoh is now with Kodak Japan, Tokyo 105, Japan.

Received 23 April 1987.0003-6935/88/040739-04$02.00/0.C 1988 Optical Society of America.

homogeneous linewidth. We utilized hole burningspectroscopy as a microscopic probe of amorphousstates and have been investigating various kinds ofglass system by this method. 2 3

One way to investigate the nature of the hole width isto try to form and detect an intrinsic hole width byoptimizing experimental conditions. Here the wordintrinsic means that the hole width can be understoodtotally by the dynamical aspects of the dopant mole-cules. T2 is relaxation time as stated above, and thisT2 should be the same as obtained by other dynamicalobservations such as photon echo and fluorescence linenarrowing. Recent findings indicate that burning in-tensity and burning temperature are significant fordeducing intrinsic hole width. 4 5 So far as we know,there has been no experimental evidence for the obser-vation of the intrinsic linewidth. However, lowerburning intensity and lower burning temperatureseem to make the hole width close to its lifetime-limited value even in an amorphous matrix. In anycase, these may support the usefulness of this methodto reveal the dynamical aspect of the molecules in thesolid matrix.

The other way is to find some correlation betweenhole broadening properties, which should be matrixmediated, and any other physical parameters of thematrix. This offers some possibility of forming a con-crete image of the interactions and the irreversiblenature of amorphous systems, which we describe here.We deal with the hole width problem from the point ofview of annealing processes, photoinduced and ther-mally induced ones. Our final purpose is to get acomprehensive understanding on the microscopic na-ture of amorphous solids by photochemical hole burn-ing (PHB). It should be emphasized here that thisfundamental knowledge will be helpful in obtaining

15 February 1988 / Vol. 27, No. 4 / APPLIED OPTICS 739

some guiding principles for designing the materials forthe wavelength-multiplexed future optical data stor-age systems. 6 7

In Table I, various glasses are listed which were usedas matrices in our study. We have been investigatingthe matrix effects on the hole width systematically.From these experimental results we obtained someconjecture on the structure of glasses. This may offerone possible model on the nature of the two-level sys-tems (TLS's) in the glass matrices and also give aninterpretation on the hole broadening mechanism.

We used quinizarin [1,4-dihydroxy-9,10-anthraqui-none (DAQ)] as a PHB active molecule. DAQ is one ofthe most extensively investigated PHB molecules. Itsground state is stable with intramolecular hydrogenbonds. In the matrices observed, DAQ does not seemto make any chemical bonds to its surroundings.Hence it is electrically neutral and possesses an electricdipole moment. Therefore, in the first approxima-tion, DAQ can only interact with surroundings by elec-tric dipoles in the matrix. A draft of the randomsystem which contains DAQ is shown in Fig. 1. Thematrix is composed of poly(ethylene) glass and electricdipoles mainly localized on the oxycarbonyl groups.The glassy part is represented by screen dots and can-not be detected by DAQ directly, while dipoles areexpressed by arrows which interact with DAQ throughdipole-dipole interaction. The range of this interac-tion is larger than those of other possible electron-

Table I. Glasses used for Observing Matrix Effects

Inorganic glasses

(3-D network)a-SiO 2 (silica glass)

Organic glasses

(Small molecules)EtOH:MeOHEPA,EPa2MTHF etc.

(Polymers)Poly(olefin)Poly(alkyl methacrylate) etc.

a E, ether; P, isopentane; A, ethanol.

I H H

H H

H CH3

H C=0OH

/ ::.<<'E:?,l .................'000i

:,, , s. M *0 : : ?;;I :00 2b i 0 e ; ;:: O ~ / : i .0007i0;5t 2X000 0 0 c2X?5><StS;t 0 \ti > :0S

0 ;;fifL SN w of Citg Sj :d; aa: jfii2:Egi/

Fig. 1. Schematic models of amorphous polymer matrix and hereindoped quinizarin molecules and hole spectra for each case. Screendots correspond to poly(ethylene) glass and arrows to electric di-

poles mainly localized around oxycarbonyl groups.

phonon couplings in this system, such as intermolecu-lar hydrogen bond.

Interestingly this simple story can most easily berealized by using polymer matrices. Figure 2 showsstructures of the polymer molecules which we used asmodel systems of Fig. 1. These polymers have meth-acrylic units in common on the straight backbonechain, and electric dipoles are mostly localized aroundthese units. The bulk density of these electric dipolescan be intentionally changed in two ways. Group I is amain chain modification which consists of copolymerof ethylene and methacrylic acid (EMA) and theirhomopolymers (PE, PMAA).6 Group II is a side chainmodification which consists of poly(alkyl methacry-late) molecules.7 In the latter case, the local density ofelectric dipoles along the chain is not changed frommolecule to molecule, but the bulk density in the glasswhich is made from each polymer molecule is de-creased by increasing the length of the alkyl chain atthe end of the side group.

One result of these model matrices is shown in Fig. 3.The saturated hole width in each polymer at -4.7 K,obtained with a burning intensity of 0.1-0.8 mW/cm2

for 20-40-min burning, is plotted as a function of thebulk density of electric dipoles in the figure. Numbersindicate the number of carbon atoms of the alkyl chain

H H H CH3

H H H C=OOH

PE : Poly(ethylene) PMAA : Poly(methacryl ic acid) EMA : Poly(ethyfene-co-methacryl ic acid)

II H CH3+ C-~ C)n

H COT o6R

R=CH3 PMMA : Poly(methyl methacrylate)

C2H5 PEMA Poly(ethyl methacrylate)

C4H, PBMA : Poly(n-butyl methacrylate)

C6H13 PHMA : Poly(n-hexyl methacrylate)

Fig. 2. Structures of polymer molecules.

740 APPLIED OPTICS / Vol. 27, No. 4 / 15 February 1988

E

I0U~0Ill

INDUSTRY

2~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

o o-

- 0, PMAA

209 p't

4 ,

,0"

12 -18 "'

EMA " ° °0,0

( PE I I I I I

0 0.2 0.4 0.1Density of Electric Dipoles (u.)

Fig. 3. Saturated hole width as a function of estimated density ofelectric dipoles in polymer matrices. Numbers indicate the num-

bers of carbon atoms of alkyl groups in side-chain modification.

Shikonin/EtOH:MeOH 4.8K

0.56mWcm2, 55min

OH 0 O

nOH 0

559 555 551Wavelength (nm)

Fig. 4. Multiple hole burning of shikonin in EtOH:MeOH glass.

in each polymer. The strong correlation between thehole width and density of electric dipoles in the matrixis seen in the figure. This indicates that the saturatedhole width may result from a photoinduced irrevers-ible hole broadening which may occur during or justafter burning (see Ref. 8). Furthermore, the TLS'swhich are responsible for this irreversible broadeningshould be strongly correlated to the dipole densities inthe polymer glasses. A simple model shown in Fig. 1seems to be basically correct to describe our randomsystems. Based on this model, we can also under-stand, even if qualitatively, the annealing properties ofthe hole width observed in this series of polymer glassmatrices.

So far, we would like to restrict our discussion of holebroadening mechanism to the quinizarin/polymer sys-tems described above. However, a qualitatively simi-lar result is reported in case of metal-free tetraphenyl-porphyrin/polyalkyl methacrylate polymers.9 Thecriteria of saturation broadening are the most impor-tant problems in the PHB spectroscopy (see Ref. 10).

MITI.

UNIV.

PHOTO REACrIVEMATERIAL

85-93 /

-60 B Yen

HGH EFFICIENT APHOTO CHEMICAL PROCESSES 02 B Yc

86 - 89 . e

CHEMISTRY MATERIAL SCI. DEVICE

Fig. 5. Project and budget of optical mass storage of Japan in 1987.Abscissa indicates orientation from basic science to application,

while the ordinate indicates location of activities.

We expect that these findings will lead to the possi-bility of controlling the hole width by designing thestructure of a polymer matrix, which will be extendedfurther to some development of structural modifica-tion of various kinds of glass matrix.

In addition to the matrix effect, we are also interest-ed in the mechanism of hole burning, in particular, theelectric structures of photoactive molecules. In caseof DAQ and its related molecules, we have examined sofar more than twelve kinds of molecule in which thenumber and the position of OH- or NH 2-groups arechanged in the anthraquinone skeleton. The exis-tence of two functional groups which can form intra-molecular hydrogen bonds and possess electron-do-nating power at both 1 and 4 positions seems to benecessary for the appearance of the persistent hole inDAQ series. Intramolecular charge transfer seems tobe essential for the formation of the 0-0 band wellseparated from other higher absorption bands. Fur-thermore, we found that the photoproduct of quini-zarin can be related to the light-induced changes inboth functional groups which are stabilized by inter-molecular hydrogen bonding with the polar matrix.11-13

In Fig. 4, multihole burning spectra of shikonin, adelivative of naphthazarin, in EtOH:MeOH glass isshown as an example of the DAQ family stated above.Shikonin is also noteworthy as a prototype for molecu-lar design to assemble the matrix effects into a pho-toactive molecule.

II. Research Activities in Japan

We introduce an overview of research activities inJapan which include manpower activities, project/budget, and national projects like photoreactive mate-rials in the future technologies project of MITI. In thenational project, the photochromic materials and thenewborn field of possible PHB materials are con-tained. A review of recent activities in photochromicmaterials is given with some typical results obtained inindustries, universities, and national laboratories.References 14-16 give details of the items described.

15 February 1988 / Vol. 27, No. 4 / APPLIED OPTICS 741

-1 I I I I I I I I I I

6

Since the new research and development project onphotoreactive materials was started in Nov. 1985, re-search activities related to materials for wavelength-multiplexed optical recording are also increasing inJapan. In Fig. 5, 1987 projects and budgets of opticalmass memory in Japan are shown schematically.

References

1. J. Friedrich, H. Wolfrum, and D. Haarer, "Photochemical Holes:A Spectral Probe of the Amorphous State in the Optical Do-main," J. Chem. Phys. 77, 2309 (1982); J. Friedrich and D.Haarer, "Transient Features of Optical Bleaching as Studied byPhotochemical Hole Burning and Fluorescence LineNarrowing," J. Chem. Phys. 76, 61 (1982).

2. T. Tani, H. Namikawa, K. Arai, and A. Makishima, "Photo-chemical Hole-Burning Study of 1,4-DihydroxyanthraquinoneDoped in Amorphous Silica Prepared by Alcoholate Method," J.Appl. Phys. 58, 3559 (1985).

3. T. Tani, U. Itoh, H. Anzai, T. Moriya, and A. Itani, "Photochem-ical Hole Burning of 1,4-Dihydroxyanthraquinone and Its Rela-tives in Organic Glass: Matrix Effects," in Twelfth Interna-tional Conference on Photochemistry, 4-9 Aug. 1985, 3P89.

4. H. P. H. Thijssen and S. Volker, "Pitfalls in the Determinationof Optical Homogeneous Linewidths in Amorphous Systems byHole-Burning. Influence of the Structure of the Host," Chem.Phys. Lett. 120, 496 (1985).

5. K. K. Rebane, "Zero-Phonon Lines, Photoburning of SpectralHoles, Information Storage, Photon Echo and Solid Systems,"at Second International Conference, Unconventional Photoac-tive Solids, Cleveland, 9-12 Sept. 1985.

6. T. Tani, A. Itani, Y. Iino, and M. Sakuda, "Matrix Effects onHolewidth as Observed by Photochemical Hole Burning of 1,4-Dihydroxyanthraquinone in Poly(ethylene), Poly(methacrylic

Acid) and Their Copolymer Systems," to appear in J. Chem.Phys. (Jan. 15, 1988).

7. T. Tani, Y. Iino, M. Sakuda, and A. Itani, under preparation.8. K. K. Rebane and A. A. Gorokhovskii, "Hole-Burning Study of

Zero-Phonon Linewidths in Organic Glasses," J. Luminesc. 36,237 (1987).

9. K. Arishima, T. Nishi, and H. Hiratsuka, "Optimization ofPolymer Structure for Photochemical Hole Burning Memory inPorphyrin Derivative/Polymer Systems," in Second SPSJ In-ternational Polymer Conference, Tokyo, 18-21 Aug. 1986,1C17.

10. L. Kador, G. Schulte, and D. Haarer, "Relation Between Hole-Burning Parameters and Molecular Parameters: Free-BasePhthalocyanine in Polymer Host," J. Phys. Chem. 90, 1264(1986).

11. F. Graf, H.-K. Hong, A. Nazzal, and D. Haarer, "Zero-PhononPhotochemistry of Hydrogen Bonded Hydroxy Quinizarin asStudied by Photochemical Hole Burning," Chem. Phys. Lett.59, 217 (1978).

12. F. Dtissler, F. Graf, and D. Haarer, "Light Induced ProtonTransfer in Dihydroxyanthraquinone as Studied by Photo-chemical Hole Burning," J. Chem. Phys. 72, 4996 (1980).

13. Y. ino, T. Tani, M. Sakuda, H. Nakahara, and K. Fukuda, "HoleBurning of 1,4-Disubstituted Anthraquinone Molecules,"Chem. Phys. Lett. 140, 76 (1987).

14. S. Arakawa, H. Kondo, S. Tamura, N. Asai, and J. Seto, "OpticalRecording Media Incorporating Novel Organic PhotochromicMaterials," in Technical Digest, Conference on Lasers andElectro-Optics (Optical Society of America, Washington, DC,1986), paper F13.

15. E. Ando, J. Miyazaki, and K. Morimoto, "J-Aggregation of Pho-tochromic Spiropyran in Langmuir-Blodgett Films," Thin SolidFilms 133, 21 (1985).

16. T. Tamaki, T. Kokubu, and K. Ichimura, "Regio- and Stereose-lective Photodimerization of Anthracene Derivatives Includedby Cyclodextrins," Tetrahedron 43, 1485 (1987).

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742 APPLIED OPTICS / Vol. 27, No. 4 / 15 February 1988