46 OPTICS LETTERS / Vol. 35, No. 1 / January 1, 2010

Two-photon-induced polarization-multiplexedand multilevel storage

in photoisomeric copolymer film

Yanlei Hu,1 Zhoushun Zhang,2 Yuhang Chen,1 Qijin Zhang,2 and Wenhao Huang1,*1Department of Precision Machinery and Precision Instrument, University of Science and Technology of China,

Hefei, Anhui 230026, China2CAS Key Laboratory of Soft Matter Chemistry, Department of Polymer Science and Engineering,

University of Science and Technology of China, Key Laboratory of Optoelectronic Science and Technology,Anhui Province, Hefei, Anhui 230026, China*Corresponding author: whuang@ustc.edu.cn

Received August 4, 2009; revised November 17, 2009; accepted November 22, 2009;posted December 4, 2009 (Doc. ID 115269); published December 24, 2009

We present a two-photon-induced polarization-multiplexed and multilevel data storage method with a bis-azobenzene copolymer film. A polarization-adjustable femtosecond pulsed laser is used as writing beam toinduce anisotropy, and the recorded information is retrieved by a CCD sensor from the film with correspond-ing polarized illumination. It is found that the optical axis of bisazobenzene molecules can be reorientedunder two-photon excitation by the polarized femtosecond laser via a photoisomerization process.Polarization-multiplexed and multilevel storage is demonstrated by using this method. The capability tocombine both advantages of these distinct techniques makes it a novel approach to obtain higher opticaldata density. © 2009 Optical Society of America

OCIS codes: 210.0210, 210.4680, 210.4770, 190.4180.

To overcome the storage density limitation deter-mined by the diffraction limit of light, many newtechniques, such as holographic storage [1,2], near-field storage [3–5], and two-photon (2P) 3D storage[6,7], have been intensively researched in the pastdecades. Because of its high spacial resolution, 2P ex-citation data storage by focusing intense laser pulsesinside the medium has shown great potential toachieve high volume density. A variety of materialshave been used successfully in 2P excitation datastorage, including photochromic [6], photobleaching[7], and photorefractive materials [8].

Multiplexed and multilevel storage are other ap-proaches that can further increase optical data den-sity in the domain of 2D storage. Compared with thecurrent storage technologies, which are focusing onshortening the laser wavelength and increasing theNA of the objective to obtain smaller spot size, bothmultiplexed and multilevel storage have the capabil-ity to multiple the storage density without changingthe optical parameters and mechanical systems. Mul-tiplexed storage (e.g., wavelength-multiplexed orpolarization-multiplexed storage) provides an ap-proach to increase the information density by storingmultiple and individually addressable data at thesame recording surface. It has been demonstratedthat nine different data pages can be recorded at thesame position of the medium surface by utilizing bothpolarization and wavelength multiplexed opticalmemory with a plasmonic gold-nanorods-based re-cording material [9]. However, the data are not eras-able because of the irreversible photothermal reshap-ing of the gold nanorods. A rewritable polarization-encoded multilayer data storage method was appliedwith a polystyrene film doped with azo dye2,5-dimethyl-4-(p-nitro-phenylazo)anisole to multi-

0146-9592/10/010046-3/$15.00 ©

plex six letters in three layers, but an amount of un-desired cross talk went with the readout bits [10]. Aswith multiplexed storage, multilevel storage, usingsignal amplitude modulation or signal waveformmodulation, offers considerable improvement to stor-age capacity and data transfer speed at a low cost[11,12]. The ability to support good compatibilitywith other developing technologies while increasingstorage capacity by log2�N� times (N level) over con-ventional binary memory has been the key in its de-velopment.

Although so many storage approaches have beenintensively exploited and each has its own merits, re-ports on techniques integrating two or more ap-proaches are still scarce. In this Letter, for the firsttime to our best knowledge, we present the evidenceof the capability to combine both advantages of mul-tiplexed and multilevel methods by two-photon-induced polarization storage in photoisomeric copoly-mer film.

Bisazobenzene copolymer is chosen as the mediumfor two-photon-induced polarization storage. Investi-gations into polarization-multiplexed storage revealthat azo material is a promising candidate because ofits geometrical isomerization and photostabilityamong a variety of materials [10,13]. Polymers con-taining bisazobenzene chromophores, which have twoalternated azo bonds between three benzene rings,are considered to have a better storage effect that isdue to their larger nonlinear optical properties, en-hanced birefringence, and thermal stability in com-parison with similar derivatives containingazobenzene chromophores [14]. Recently we showedthat anisotropy can be induced in poly[(methyl methacrylate)-co-4-{(2-methacryloyloxyethyl)oxy}- 4�-(4-

nitrophenylazo)azobenzene)](poly(MMA-co-M2BAN))

2010 Optical Society of America

January 1, 2010 / Vol. 35, No. 1 / OPTICS LETTERS 47

film under 2P excitation, and recorded data wereread out by a confocal laser scanning microscope inreflection mode [15]. We prepared the poly(MMA-co-M2BAN) film samples with thickness of 5 �m by adrop-coated method [15]. The polymer sample has amaximum absorbance of 3.1 �a.u.� at 370 nm. Thehigh absorbance makes it possible for 2P excitationby femtosecond laser pulses at wavelength of 800 nm.

The optical configuration for recording and readingdata is illustrated in Fig. 1. A linearly polarizedTi:sapphire femtosecond pulsed laser beam withwavelength of 800 nm, pulse duration of 80 fs, andrepetition rate of 80 MHz was applied as the record-ing light source for 2P excitation. A Glan–Taylorprism was used to create pure linear polarizationstate of the recording beam. The laser was tightly fo-cused inside the sample by a high-NA objective(40�, NA=0.65). A piezo-driven nanopositioningstage and a shutter controlled synchronously by acomputer were utilized to determine each bit locationand exposure time. Unlike the optical setup reportedin [15], the recorded data were read out by a trans-mission type microscope. Light emitted from a halo-gen lamp was used as a reading beam after beingconverted into parallel rays and polarized. The read-ing beam passed though the sample and thenreached the CCD sensor. The detected signal was fi-nally transmitted to the computer to form a magni-fied image.

Polarization-multiplexed storage in poly(MMA-co-M2BAN) film was accomplished as follows. First, the0° (horizontally) polarized femtosecond laser was in-troduced into the sample for writing a bit sequence.The exposure time of each bit was 35 ms, and the av-erage continuous power of the incident laser was16 mW (corresponding to an average peak power of2.5�103 W). Then the Glan–Taylor prism was ro-tated by 45° to write another bit sequence with thesame parameters. The interval of adjacent bits was4 �m. It is found that when the polarization of thereading beam is the same as the recording beam,bright spots are monitored, and the spots fade outwhen the polarization of the reading beam is devi-ated with an angle of 45°. As shown in Figs. 2(a) and2(b), different bit sequences present themselveswhen read by different polarized light. When the po-larization of the reading beam is oriented at 22.5°,i.e., the middle of the two recording directions, all the

Fig. 1. Optical configuration for recording and reading

data.

bits are monitored by the sensor [see Fig. 2(c)]. Mean-while, the readout bit intensity is relatively lower, aswe expected.

Figure 2(d) shows the relationship between the in-tensity of the recorded bits and the reading beam po-larization. It is found that the angular dependency isconsistent with the curve of cos�2��, where � is theangle between the polarization directions of the re-cording beam and the reading beam. The maximumand minimum of readout intensities appear when theangles between polarization directions of the record-ing beam and the reading beam are 0° and 90°, re-spectively. Considering that the average intensity ofthe background is about 80, dark dots appear whenthe polarization direction of the reading beam is per-pendicular to that of the recording beam. It is note-worthy that when recorded below 12 mW with the ex-posure time of 35 ms, the dark dots cannot bedistinguished from the untreated area when read bya perpendicularly polarized beam, whereas brightdots can still be clearly identified by the parallelbeam. In other words, with a given exposure time,the phenomenon of mutation between bright dots anddark dots can be observed only on the condition thatthe bits are recorded above the threshold power. Asdiscussed above, the evidence of multilevel storage ispresented by the readout patterns with obviously dif-ferent gray levels when recorded by different polar-ized beams (see Fig. 3). The pattern was recorded bymoving the tightly focused laser in the film with thespeed of 3.6 �m/s and an average continuous power

Fig. 2. Polarization-multiplexed storage in poly(MMA-co-M2BAN) film and the readout bit intensity as a function ofthe polarization direction of the reading beam. (a), (b), and(c) are three kinds of bit sequence in the same region readout by beams of different polarizations. (a) and (b) are re-corded by 0°, 45° polarized laser beam and read out by 0°,45° polarized illumination, respectively. (c) is read out by22.5° polarized illumination. (d) The readout bit intensityas a function of the polarization direction of the readingbeam. The circles are the experimental bit intensity, andthe solid curve is the theoretical curve of cos�2��, where � isthe angle between the recording polarization and the read-ing polarization.

of 10 mW. A 0° polarized beam was first used to write

48 OPTICS LETTERS / Vol. 35, No. 1 / January 1, 2010

the two outer Olympic rings, and then a 90° polarizedbeam was used to write the other three rings in themiddle. The polarization directions of the readingbeam were [Fig. 3(a)] 0°, and [Fig. 3(b)] 90°. As shownin Fig. 3, bright rings and dark rings appear in read-out images. This indicates that information with dif-ferent gray levels can be stored conveniently by re-cording beams with different polarization states. Themultilevel information recording is realized by usingthe gray value to differentiate the levels, namely, sig-nal amplitude modulation. Moreover, two-photon-induced polarization multiplexed and multilevel stor-age in poly(MMA-co-M2BAN) film is rewritable,associated with the reversible trans–cis–trans photoi-somerization process of the bisazochromophores.

In conclusion, two-photon-induced polarizationmultiplexed storage in poly(MMA-co-M2BAN) filmwith negligible cross talk and multilevel storage havebeen presented. It is demonstrated that polarizationmultiplexed and multilevel storage can be integratedtogether. Our results provide the possibility formultilayer, polarization-multiplexed, and multileveloptical data storage, which can significantly increasethe data density.

Fig. 3. Multilevel storage in poly(MMA-co-M2BAN) film.The bright rings shown in (a) are recorded by 0° polarizedbeam, and the dark rings are recorded by the 90° polarizedbeam. The polarization direction of the reading beam is (a)0° and (b) 90°. The scale bar is 10 �m.

The authors acknowledge financial support fromNational Natural Science Foundation of China(NSFC), 50875250, 50703038, 50773075, and50533040.

References

1. P. J. van Heerden, Appl. Opt. 2, 387 (1963).2. G. Berger, M. Dietz, and C. Denz, Opt. Lett. 33, 1252

(2008).3. T. Matsumoto, Y. Anzai, T. Shintani, K. Nakamura,

and T. Nishida, Opt. Lett. 31, 259 (2006).4. E. Betzig, J. K. Trautman, R. Wolfe, E. M. Gyorgy, P. L.

Finn, M. H. Kryder, and C.-H. Chang, Appl. Phys. Lett.61, 142 (1992).

5. B. D. Terris, H. J. Mamin, D. Rugar, W. R.Studenmund, and G. S. Kino, Appl. Phys. Lett. 65, 388(1994).

6. D. A. Parthenopoulos and P. M. Rentzepis, Science 245,843 (1989).

7. D. Day and M. Gu, Appl. Opt. 37, 6299 (1998).8. Y. Kawata, H. Ishitobi, and S. Kawata, Opt. Lett. 23,

756 (1998).9. P. Zijlstra, J. W. M. Chon, and M. Gu, Nature 459, 410

(2009).10. X. Li, J. W. M. Chon, S. Wu, R. A. Evans, and M. Gu,

Opt. Lett. 32, 277 (2007).11. Y. Tang, J. Pei, Y. Ni, L. Pan, H. Hu, and B. Zhang,

Opt. Express 16, 6156 (2008).12. K. Kiyono, M. Horie, T. Ohno, T. Uematsu, T.

Hashizume, M. P. O’Neill, K. Balasubramanian, R.Narayan, D. Warland, and T. Zhou, Jpn. J. Appl. Phys.40, 1855 (2001).

13. M. Maeda, H. Ishitobi, Z. Sekkat, and S. Kawata, Appl.Phys. Lett. 85, 351 (2004).

14. Z. Zheng, L. Wang, Z. Su, J. Xu, J. Yang, and Q. Zhang,J. Photochem. Photobiol., A 185, 338 (2007).

15. Z. Zhang, Y. Hu, Y. Luo, Q. Zhang, W. Huang, and G.Zou, Opt. Commun. 282, 3282 (2009).