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Bragg-Type Polarization Gratings Formed in Thick Polymer Films Containing Azobenzene and Tolane Moieties† Makoto Ishiguro, Daisuke Sato, Atsushi Shishido, and Tomiki Ikeda* Chemical Resources Laboratory, Tokyo Institute of Technology, R1-11, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan ReceiVed June 2, 2006. In Final Form: July 20, 2006 Holographic gratings were formed in thick polymer films containing azobenzene and diphenylacetylene (tolane) moieties in the Bragg regime. Amorphous polymers containing various contents of the azobenzene moiety with photosensitivity and the tolane moiety with large birefringence in the side chain were synthesized, and optically transparent thick polymer films were prepared. The films were irradiated with a linearly polarized beam from an Ar+ laser (488 nm), and the transmittance of a He-Ne laser beam (633 nm) through a pair of crossed polarizers, with the film between them, was measured to estimate a photoinduced birefringence (∆n). The value of ∆n increased with an increase in the tolane moiety content in the polymer films. When two linearly polarized beams at 488 nm were interfered in the film, a diffraction beam was observed, and the maximum diffraction efficiency (η) increased with the tolane moiety content. In the film containing 70 mol% of the tolane moiety, the highest η of 99% was achieved, and angular selectivity due to Bragg diffraction was clearly observed. We consider the cooperative molecular motion of the tolane moieties to be induced by the photoinduced change in alignment of the azobenzene moieties even if the polymers show no liquid-crystalline phase. When two orthogonal circularly polarized beams were allowed to interfere in the film, a Bragg-type polarization grating was formed. It was found that the value of η reached 90% within 920 ms.
Introduction Holography is a technique to record and reconstruct complete optical information about objects using amplitude and the phase of light.1,2 Therefore, holography has been of great interest for holographic data storage systems with high storage capacity, short access time, and a high data-transfer rate. In holographic materials, many aspects such as the high diffraction efficiency, fast response, high resolution, high sensitively, temporal stability, and rewritability must be considered. Even though a number of holographic materials have been proposed, azobenzene polymers seem to be one of the most suitable rewritable holographic materials because of their high sensitivity and reversibility.3-14 † Part of the Stimuli-Responsive Materials: Polymers, Colloids, and Multicomponent Systems special issue. * To whom correspondence should be addressed. E-mail: tikeda@ res.titech.ac.jp. URL: http://www.res.titech.ac.jp/∼polymer/. Tel: +81-45924-5240. Fax: +81-45-924-5275.
(1) Gabor, D. Nature 1948, 161, 777-778. (2) Kogelnik, H. Bell Syst. Technol. J. 1969, 48, 290-297. (3) (a) Todorov, T.; Tomova, N.; Nikolova, L. Opt. Commun. 1983, 47, 123126. (b) Todorov, T.; Nikolova, L.; Tomova, N. Appl. Opt. 1984, 23, 4309-4312. (c) Todorov, T.; Nikolova, L.; Tomova, N. Appl. Opt. 1984, 23, 4588-4591. (d) Nikolova, L.; Todorov, T. Opt. Acta 1984, 31, 589-588. (e) Todorov, T.; Nikolova, L.; Stoyanova, K.: Tomova, N. Appl. Opt. 1985, 24, 785-788. (f) Nedelchev, L.; Nikolova, L.; Matharu, A.; Ramanujam, P. S. Appl. Phys. B: Laser Opt. 2002, 75, 671-676. (g) Nikolova, L.; Todorov, T.; Dragostinova, V.; Petrova, T.; Tomova, N. Opt. Lett. 2002, 27, 92-94. (4) (a) Hvilsted, S.; Andruzzi, F.; Kulinna, C.; Siesler, H. W.; Ramanujam, P. S. Macromolecules 1995, 28, 2172-2183. (b) Ramanujam, P. S.; Holme, N. C. R.; Hvilsted, S. Appl. Phys. Lett. 1996, 68, 1329-1331. (c) Rasmussen, P. H.; Ramanujam, P. S.; Hvilsted, S.; Berg, R. H. J. Am. Chem. Soc. 1999, 121, 47384743. (d) Berg, R. H.; Hvilsted, S.; Ramanujam, P. S. Nature (London) 1996, 383, 505-508. (5) (a) Eich, M.; Wendorff, J. H. Makromol. Chem., Rapid Commun. 1987, 8, 59-63. (b) Eich, M.; Wendorff, J. H. Makromol. Chem., Rapid Commun. 1987, 8, 467-471. (c) Eich, M.; Wendorff, J. H. J. Opt. Soc. Am. B 1990, 7, 1428-1436. (6) (a) Rochon, P.; Batalla, E.: Natansohn, A. Appl. Phys. Lett. 1995, 66, 136-138. (b) Barrett, C. J.; Natansohn, A. L.; Rochon, P. L. J. Phys. 1996, 100, 8836-8842. (c) Xie, S.; Natansohn, A.; Rochon, P. Chem. Mater. 1993, 5, 403411. (7) (a) Bieringer, T. In Holographic Data Storage; Coufal, H. J., Psaltis, D., Sincerbox, G. T., Eds.; Springer Series in Optical Sciences; Springer: New York, 2000. (b) Bieringer, T. Macromol. Chem. Phys. 1995, 196, 1375-1390.
Azobenzene, which undergoes reversible changes in conformation between trans and cis isomers, is one of the most popular photochromic materials. Moreover, upon exposure to linearly polarized light, trans-azobenzene molecules are aligned perpendicular to an electric field of light through the repetition of trans-cis-trans isomerization cycles. Once trans-azobenzene molecules have fallen perpendicular to the direction of polarization of the light, they become inactive with respect to incident light (Weigert effect). In azobenzene polymers, a large photoinduced change in the refractive index can be brought about through the change in alignment of the chromophores. The diffraction efficiency (η) is the most basic factor used to evaluate the capability of holographic materials, and it depends on the diffraction types. There are two types of holographic recording: Raman-Nath and Bragg regimes. In the RamanNath regime, which is formed in thin films, multiple diffraction beams are observed, and the theoretical maximum η is 33.9%. (8) (a) Ikeda T.; Tsutsumi, O. Science 1995, 268, 1873-1875. (b) Ikeda, T. J. Mater. Chem. 2003, 13, 2837-2057. (9) (a) Yamamoto, T.; Hasegawa, M.; Kanazawa, A.; Shiono, T.; Ikeda T. J. Phys. Chem. B 1999, 103, 9873-9878. (b) Yamamoto, T.; Hasegawa, M.; Kanazawa, A.; Shiono, T.; Ikeda, T. J. Mater. Chem. 2000, 10, 337-342. (10) (a) Wu, Y.; Demachi, Y.; Tsutsumi, O.; Kanazawa, A.; Shiono, T.; Ikeda, T. Macromolecules 1998, 31, 349-354. (b) Wu, Y.; Demachi, Y.; Tsutsumi, O.; Kanazawa, A.; Shiono, T.; Ikeda, T. Macromolecules 1998, 31, 1104-1108. (c) Wu, Y.; Demachi, Y.; Tsutsumi, O.; Kanazawa, A.; Shiono, T.; Ikeda, T. Macromolecules 1998, 31, 4457-4463. (d) Wu, Y.; Zhang, Q.; Ikeda, T. AdV. Mater. 1999, 11. 300-302. (11) (a) Hasegawa, M.; Yamamoto, T.; Kanazawa, A.; Shiono, T.; Ikeda, T. AdV. Mater. 1999, 11, 675-677. (b) Hasegawa, M.; Yamamoto, T.; Kanazawa, A.; Shiono, T.; Ikeda, T. Chem. Mater. 1999, 11, 2764-2769. (12) (a) Yoneyama, S.; Yamamoto, T.; Hasegawa, M.; Tsutsumi, O.; Kanazawa, A.; Shiono, T.; Ikeda, T. J. Mater. Chem. 2001, 11, 3008-3013. (b) Yoneyama, S.; Yamamoto, T.; Tsutsumi, O.; Kanazawa, A.; Shiono, T.; Ikeda, T. Macromolecules 2002, 35, 8751-8758. (13) (a) Okano, K.; Shishido, A.; Tsutsumi, O.; Shiono, T.; Ikeda, T. J. Mater. Chem. 2005, 15, 3395-3401. (b) Okano, K.; Shishido, A.; Ikeda, T. AdV. Mater. 2006, 18, 523-527. (c) Okano, K.; Shishido, A.; Ikeda, T. Macromolecules 2006, 39, 145-152. (14) (a) Yu, H.; Okano, K.; Shishido, A.; Ikeda, T.; Kamata, K.; Komura, M.; Iyoda, T. AdV. Mater. 2006, 17, 2184-2188. (b) Yu, H.; Shishido, A.; Ikeda, T.; Iyoda, T. Macromol. Rapid Commun. 2005, 26, 1594-1598.
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Scheme 1. Synthesis Route for the Tolane Monomer Used in This Study
Scheme 2. Synthesis Route for the Azobenzene Monomer Used in This Study
On the contrary, Bragg diffraction can be obtained in thick films.15-18 The gratings formed in the Bragg regime exhibit only a single diffraction beam with a high angular selectivity, and η could reach 100%. Therefore, to achieve a large value of η as well as angular selectivity, it is necessary to induce Bragg diffraction using thick films. Because the change in alignment of mesogenic moieties can be cooperatively induced by that of azobenzene moieties, liquidcrystalline (LC) polymers containing an azobenzene moiety are known as high-performance holographic materials and have large optical anisotropy, reversibility, and high stability below the glass-transition temperature (Tg). We previously reported the formation of holographic gratings in the Raman-Nath regime.8-14 However, it is difficult to prepare optically transparent thick LC polymer films because of the light scatting attributed to an LC phase. In this article, we introduced a transparent unit with mesogenic and azobenzene moieties to prepare transparent thick polymer films and achieve a high η near 100% using the Bragg regime. The key point is that we can take advantage of the efficient change in the refractive index by the local cooperative molecular motion of the mesogenic moiety even if the LC phase is macroscopically eliminated by the incorporation of the transparent unit. In the Bragg regime, η is defined by eq 12
× 100 (λπd∆n′ cos θ)
η(%) ) sin2
(1)
where d is the film thickness, ∆n′ is the spatial modulation of the refractive index, λ is the wavelength of a reading beam, and θ is the incident angle of the reading beam. According to eq 1, (15) Minabe, J.; Maruyama, T.; Yasuda, S.; Kawano, K.; Hayashi, K.; Ogasawara, Y. Jpn. J. Appl. Phys. 2004, 43, 4964-4967. (16) Ha¨ckel, M.; Kador, L.; Kropp, D.; Frenz, C.; Schmidt, H. AdV. Funct. Mater. 2005, 15, 1722-1727. (17) Sutherland, R. L.; Natarajan, L. V.; Tondiglia, V. P.; Bunning, T. J. Chem. Mater. 1993, 5, 1533-1538. (18) (a) Schilling, M. L.; Colvin, V. L.; Dhar, L.; Harris, A. L.; Schilling, F. C.; Katz, H. E.; Wysocki, T.; Hale, A.; Blyler, L. L.; Boyd, C. Chem. Mater. 1999, 11, 247-254. (b) Trentler, T. J.; Boyd, J. E.; Colvin, V. L. Chem. Mater. 2000, 12, 1431-1438.
η is related to d and ∆n′. Therefore, if polymer films with large birefringence values are used, then high η and a fast response could be achieved. Diphenylacetylene (tolane) groups are one of the popular mesogens that show high birefringence.12,19 Therefore, we prepared a series of polymers containing the tolane moiety (mesogenic unit) and the azobenzene moiety (photoresponsive unit) and investigated the photoinduced change in birefringence and the formation of holographic gratings in the thick polymer films. Furthermore, we explored the formation of the polarization gratings by allowing the interference of two orthogonal circularly polarized beams in thick polymer films for the first time in the Bragg regime and achieved the short response time of 920 ms. Experimental Section Synthesis of Monomers and Polymers. The tolane monomer and the azobenzene monomer used in this study were prepared by the synthetic route shown in Schemes 1 and 2, respectively.12 Figure 1 shows the structures of the copolymers with the azobenzene moiety and the tolane moiety and their abbreviations used in this study. Copolymerization was performed in dry DMF with 1 mol % 2,2′-azobis(isobutyronitrile) (AIBN) as an initiator. A solution of the monomers and AIBN in dry DMF was degassed by three freezepump-thaw cycles and then sealed off. The monomers were allowed to copolymerize under vacuum at 60 °C for 48 h. The cooled solution was poured into methanol with stirring to precipitate the polymer. The polymer obtained was purified by precipitation from THF into a large excess of methanol three times and dried under vacuum. The molecular weights and thermodynamic properties of the polymers are shown in Figure 1. The molecular weight was measured by gel permeation chromatography (GPC; Japan Spectroscopy Co., model DG-980-50; column, Shodex GPC, models K802, K803, K804, and K805; eluent, chloroform) calibrated with standard polystyrenes. The thermodynamic properties were determined with a differential scanning calorimeter (DSC, Seiko I&E SSC-5200 and DSC220C) at a heating rate of 10 °C/min. At least three scans were performed in each sample to check the reproducibility. The mole fractions of (19) Takatsu, H.; Takeuchi, K.; Tanaka, Y.; Sasaki, M. Mol. Cryst. Liq. Cryst. 1986, 141, 279-287.
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Figure 3. Change in transmittance (a) and birefringence (∆n) (b) in the AE5TL20M75 film as a function of time. The film was irradiated with a linearly polarized beam. The value of ∆n was estimated by eq 3. Film thickness, 35 µm.
Figure 1. Chemical structure, abbreviations, molecular weights, and thermodynamic properties of the copolymers used in this study. Mn, number-average molecular weight; Mw, weight-average molecular weight; G, glassy; I, isotropic.
mW/cm2. The incident angle of the pumping beam was fixed at 7°. The intensity of the probe beam at 633 nm from a He-Ne laser (NEC Co., GLS5370, 1 mW) transmitted through a pair of crossed polarizers, with the sample film between them, was measured with a photodiode. The photoinduced birefringence was estimated from the change in transmittance. All experiments were carried out at room temperature. Figure 2b shows the optical setup for the formation of gratings. Two linearly s-polarized or orthogonal circularly polarized beams at 488 nm from the Ar+ laser were employed as writing beams. Two beams obtained with a beam splitter at equal intensity were interfered on the film at an incident angle of 7°. The total intensity of the writing beams was adjusted at 200 mW/cm2 unless otherwise noted. The formation of gratings was evaluated by monitoring the firstorder diffraction beam at 633 nm from the He-Ne laser with a photodiode in real time. The reading beam was incident to the film at an angle of R. All experiments were carried out at room temperature. The diffraction efficiency was defined as the ratio of the intensity of the first-order diffraction beam (I1) to that of the transmitted beam (I0) through the sample film as expressed by eq 2: η(%) )
I1 × 100 I0
(2)
Results and Discussion
Figure 2. Optical setup for the evaluation of the photoinduced birefringence (∆n) (a) and the formation of gratings (b). A, analyzer; BS, beam splitter; GP, Glan-Thomson prisms; L, lens; M, mirror; P, polarizer; PD, photodiode; QW, quarter-wave plate. the azobenzene moiety and the tolane moiety in the copolymers were determined by absorption spectroscopy with a UV-vis spectrometer (Jasco, V-550). Preparation of Films. Thick polymer films were prepared by hot pressing the copolymers with a pair of glass plates, with silica spacers with thicknesses of 10-100 µm between them, on a hot stage at 180 °C for 30 min and then annealing at 150 °C for 2 h. The film thickness was confirmed by UV-vis spectroscopy. Optical Setup. The optical setup for the evaluation of the photoinduced birefringence in the films is shown in Figure 2a. The polymer film was irradiated with a 488 nm linearly s-polarized beam from an Ar+ laser (Spectra Physics, Inc., BeamLok2065-7S) at 200
Photoinduced Alignment of the Polymers Containing the Tolane Moiety. In azobenzene-containing polymer films, optical anisotropy is generally induced by irradiation with a linearly polarized beam. Therefore, the photoinduced birefringence was investigated by exposing the azobenzene polymer films containing a tolane moiety to a linearly polarized beam at 488 nm. Figure 3a shows the photoinduced change in transmittance in the AE5TL20M75 film with a thickness of 35 µm. Before photoirradiation, no transmittance was observed, which means that the film is optically isotropic. However, upon photoirradiation with a pump beam for 40 min, the transmittance increased up to 100% and then decreased to reach a steady value. The photoinduced birefringence can be determined from the observed transmittance by eq 3
T(%) ) sin2
× 100 (πd∆n λ )
(3)
where T is the transmittance, d is the film thickness, ∆n is the birefringence, and λ is the wavelength of the probe beam (633 nm). Figure 3b shows the value of ∆n. Before irradiation, ∆n was 0, which means that the film is optically isotropic, whereas the value of ∆n gradually increased and rose to about 1.9 × 10-2 upon photoirradiation. This can be explained by the change in alignment due to the trans-cis-trans isomerization cycles of the azobenzene moieties. To confirm the
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Figure 4. Polarized absorption spectra of the AE5TL20M75 film before (a) and after irradiation (b). A| and A⊥ denote absorption parallel and perpendicular to the polarization direction of the pump beam, respectively. Film thickness, 35 µm.
Figure 5. Photoinduced birefringence (∆n) as a function of the tolane moiety content and film thickness. Film thicknesses: 9, 35 µm; 2, 60 µm; b, 100 µm. Exposure time, 40 min.
alignment of the azobenzene moieties, we measured polarized absorption spectra. Figure 4 shows the polarized absorption spectra of the AE5TL20M75 film with a thickness of 35 µm before (a) and after (b) irradiation of the pump beam. Here, A| and A⊥ denote the absorption parallel and perpendicular to the polarization direction of the writing beam, respectively. Before exposure to the writing beam, A| and A⊥ exhibited the same profile, indicating that the alignment of azobenzene moieties is random in the initial state. However, after irradiation, A⊥ became larger than A|. This result clearly suggests that the in-plane alignment of the azobenzene moieties was brought about perpendicularly to the polarization direction of the writing beam. Next, we investigated the effect of the tolane moiety content on ∆n. Figure 5 shows the maximum value of ∆n induced in the films containing various amounts of the tolane moiety. Photoirradiation was performed at 200 mW/cm2 for 40 min. In the 100-µm-thick films, the effect of the tolane content on ∆n was small. However, in thinner films, the value of ∆n increased with an increase in tolane content and rose to 2.4 × 10-2 in the 35µm-thick AE5TL70M25 film. It is clear that the alignment of the tolane moieties is cooperatively induced by the azobenzene
Figure 6. Diffraction pattern on the screen (a) and dynamics of the diffraction efficiency (b, c) in the AE5TL40M55 film. In b, the film was irradiated with two linearly s-polarized beams. In c, the film was alternately irradiated with two linearly s-polarized beams (2) and a single circularly polarized beam (1). Film thickness, 100 µm.
moieties because the increase in the tolane content enhances ∆n. It should be noted that even if the polymers show no LC phase an enhancement of the photoinduced birefringence was observed. Therefore, we might be able to utilize the optical transparency of amorphous polymers and the large change in the refractive index of LC polymers simultaneously.18 Formation of Holographic Gratings. Figure 6 shows the results for the formation of holographic gratings in the AE5TL40M55 film with a thickness of 100 µm. Upon exposure to the s-polarized writing beams, only a single diffraction beam was observed, which is attributed to the formation of gratings in the Bragg regime (Figure 6a). When we measured the intensity of the transmitted beam and the diffraction beam at the same time (Figure 6b), it was found that the value of η increased upon exposure to the writing beams, whereas the intensity of the transmitted beam decreased in contrast. It is considered that the intensity of the transmitted beam is transferred to the diffraction beam with no optical loss, thanks to the optical transparency of the thick polymer film prepared. The value of η reached 96% after 13 min, and η > 90% was kept after the writing beams were
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Figure 7. Effect of tolane moiety content on the diffraction efficiency and the response time. Film thickness, 100 µm.
Figure 8. Schematic illustration of a plausible mechanism of the cooperative molecular motions of the azobenzene and tolane moieties. The tolane moiety content increases from a to c, maintaining the same azobenzene moiety content.
turned off. We revealed that the formed grating could be erased by the exposure of the film to a single writing beam with circular polarization (Figure 6c). In addition, the grating was erased by thermal treatment above Tg. In this case, the recording-erasure cycles could be stably repeated more than 200 times. Recorded holograms showed a high diffraction efficiency of >70% even after 6 months when kept in the dark. Effect of the Tolane Moiety Content on the Formation of Gratings. We investigated the photoinduced birefringence of the polymer films and found that the tolane moiety contributed to the enhancement of the maximum ∆n as shown in Figure 5. Therefore, it is expected that the tolane moiety should play an important role in the formation of gratings. We explored the effect of the tolane moiety content on the maximum η and the response time, which is defined as the time required to reach the maximum η (Figure 7). We observed that the value of η monotonically increased with the tolane moiety content and finally reached 99% in the AE5TL70M25 film, which is nearly the theoretical maximum value in the Bragg regime (100%). However, the response time increased upon increasing the tolane moiety content. When the tolane moiety content was
