© Copyright 2003 American Chemical Society
FEBRUARY 18, 2003 VOLUME 19, NUMBER 4
Letters Optically Tunable Gelled Photonic Crystal Covering Almost the Entire Visible Light Wavelength Region Yumie Iwayama,† Junpei Yamanaka,*,† Yoshihiro Takiguchi,‡ Masaomi Takasaka,‡ Kensaku Ito,§ Tadaomi Shinohara,| Tsutomu Sawada,⊥ and Masakastu Yonese† Faculty of Pharmaceutical Sciences, Nagoya City University, 3-1 Tanabe, Mizuho, Nagoya-city, Aichi 467-8603, Japan, Hamamatsu Photonics K.K. 5000 Hirakuchi, Hamakita-city, Shizuoka 434-8601, Japan, Faculty of Engineering, Toyama University, 3190 Gofuku, Toyama-city, Toyama 930-8555, Japan, Faculty of Science, Kyoto Sangyo University, Motoyama, Kamigamo, Kyoto-city, Kyoto 603-8555, Japan, and National Institute for Materials Science, 1-1 Namiki, Tsukuba-city, Ibaraki 305-0044, Japan Received August 20, 2002. In Final Form: December 4, 2002 By fixing charged colloidal crystals in a poly(acrylamide) hydrogel matrix, we fabricated photonic crystals whose diffraction peak wavelengths were tunable by applying mechanical stress. The reflection spectrum for a single crystal grain was measured by applying microspectroscopy under compression. The photonic band gap wavelength shifted linearly and reversibly over almost the entire visible light wavelength region (460-810 nm).
Introduction When wavelength-sized optically transparent dielectric materials are arranged periodically, they work as “photonic crystals”.1,2 There exists a photonic band structure in the crystal, and within the wavelength range of the photonic band gap, light waves cannot propagate through the medium.3,4 Recent efforts to fabricate such photonic crystals were made using the semiconductor lithography technique5-7 or the self-assembling nature of colloidal * To whom correspondence should be addressed. E-mail:
[email protected]. † Nagoya City University. ‡ Hamamatsu Photonics. § Toyama University. | Kyoto Sangyo University. ⊥ National Institute for Materials Science. (1) Yablonovitch, E. Phys. Rev. Lett. 1987, 58, 2059. (2) Joannopoulos, J. D.; Meade, R. D.; Winn, J. N. Photonic Crystals. Modeling the flow of light; Princeton University Press: Princeton, NJ, 1995. (3) Chow, E.; et al. Nature 2000, 407, 983. (4) Noda, S.; Chutinan, A.; Imada, M. Nature 2000, 407, 608.
particles.8-11 The products thus fabricated were rigid and durable enough to be expected as devices for ultrafast fiber optical communications.12 A millimeter-sized periodic structure can act as a photonic crystal in millimeter wavelengths; a submicrometer-sized structure, in a visible light region; and a few nanometer-sized ones, in the X-ray region. We used colloidal crystals13-22 of charged silica and polystyrene (5) Noda, S.; Tomoda, K.; Yamamoto, N.; Chutinan, A. Science 2000, 289, 604. (6) Campbell, M.; Sharp, D. N.; Harriosn, M. T.; Denning, R. G.; Turberfield, A. J. Nature 2000, 404, 53. (7) Toader, O.; John, S. Science 2001, 292, 1133. (8) Van Blaaderen, A.; Ruel, R.; Wiltzius, P. Nature 1997, 385, 321. (9) Pradhan, R. D.; Bloodgood, J. A.; Watson, G. H. Phys. Rev. B 1997, 55, 9503. (10) Vlasov, Y. A.; Bo, X.-Z.; Sturm, J. C.; Norris, D. J. Nature 2001, 414, 289. (11) Lu, Y.; Yin, Y.; Gates, B.; Xia, Y. Langmuir 2001, 17, 6344. (12) Per, H.; Kapon, E.; Moser, M. Nature 2000, 407, 880. (13) Russel, W. B.; Saville, D. A.; Schowalter, W. R. Colloidal Dispersions; Cambridge University Press: New York, 1989. (14) Ordering and Phase Transitions in Charged Colloids; Arora, A. K., Tata, B. V. R., Eds.; VCH: New York, 1996.
10.1021/la0207365 CCC: $25.00 © 2003 American Chemical Society Published on Web 01/18/2003
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Figure 1. Illustration of the Bragg peak (λm) shift for the immobilized charged colloidal crystal by mechanical compression. The lattice spacing of the crystal d ()λm/2) is reduced by decreasing the sample thickness from t0 to t.
latex particles with a diameter of about 100 nm for the visible light region. Due to the strong electrostatic interparticle interaction, they form a periodic structure showing body-centered cubic (bcc) symmetry with a lattice spacing of submicrometers. The charged colloidal crystals have peculiar features, which are not seen in closely packed particle structures (synthetic opal).8,10,11 Their lattice spacing is easily varied by changing the particle concentration Cp. Since the electrostatic interaction is isotropic and long-ranged, large three-dimensional crystal grains (about 1 cm) are easily formed spontaneously. Furthermore, because of the free spaces inside their structures, the photonic band gap energy and the dispersion relation of the refractive index of the charged colloidal crystal can be tuned by mechanical stress, as illustrated in Figure 1. To bring the crystal to practical use, the structure has to be fixed in a flexible matrix. Thus, we immobilized the colloidal particles by a polymer hydrogel, in a similar manner as the gelation of colloidal crystals reported by Kamenetzky et al.18 and Asher et al.19-22 Asher et al. also showed that the Bragg peak of the gelled crystal can be shifted by about 10% from the original value upon stretching.22 However, we used gelled colloidal crystals prepared at much lower particle and gel concentrations and were able to optically tune them in a quite wide range of wavelengths, covering almost the entire visible light region, by applying linear compression. Experimental Section Colloidal silica SI-80P (diameter ) 110 nm, charge number ) 510/particle) was purchased from Catalyst & Chemicals Industries Co., Kanagawa, Japan. Polystyrene latex (94 nm, 780/ particle) was synthesized by an emulsion-free polymerization method by Dr. Hiroshi Yoshida, Hashimoto Polymer Phasing Project, ERATO, Japan. They were thoroughly purified by dialysis and ion-exchange method as reported elsewhere.17 Under the low Cp conditions we adopted here, the charged colloidal crystals were destroyed even when the salt concentration was about 10 µM. To avoid contamination of ionic impurities from a glass wall, polystyrene, Teflon, or quartz apparatus were used for sample preparations, instead of glassware. They were (15) Yoshida, H.; Ito, K.; Ise, N. J. Chem. Soc., Faraday Trans. 1991, 87, 871. (16) Yamanaka, J.; Yoshida, H.; Koga, T.; Ise, N.; Hashimoto, T. Phys. Rev. Lett. 1998, 80, 5806. (17) Yoshida, H.; Yamanaka, J.; Koga, T.; Koga, T.; Ise, N.; Hashimoto, T. Langmuir 1999, 15, 2684. (18) Kamenetzky, E. A.; Mangliocco, L. G.; Panzer, H. P. Science 1994, 263, 207. (19) Weissman, J. M.; Sunkara, H. B.; Tse, A. S.; Asher, S. A. Science 1996, 274, 959. (20) Holtz, J. H.; Asher, S. A. Nature 1997, 389, 829. (21) Pan, G.; Kesavamoorthy, R.; Asher, S. A. Phys. Rev. Lett. 1997, 78, 3860. (22) Asher, S. A.; Holtz, J.; Liu, L.; Wu, Z. J. Am. Chem. Soc. 1994, 116, 4997.
Figure 2. A schematic drawing of the compression cell for microscopy and microspectroscopy. A, micrometer head; B, guide screw; C, polyacrylate plates (the upper one is equipped with a circular window); D, stage; E, quartz plates (thickness ) 1 mm); F, gel sample. carefully cleaned with purified water, until the electrical conductivity of the waste became less than 0.6 µS/cm2. Water used was purified by Milli-Q Simpli-Lab system (Millipore, Bedfold, MA), just before usage. Then these colloidal dispersions were mixed with aqueous solutions of acrylamide monomer (AAm), N,N′-methylene bis acrylamide (cross-linker, BIS), and 2,2′-azobis[2-methyl-N-[2hydroxyethyl]-propinonamide] (ABAP), which is a polymerization initiator curable by ultraviolet (UV) light. Mixed-bed ionexchange resin beads (AG501-X8 (D), Bio-Rad Laboratories, Hercules, CA) were put into the reaction solution to reduce ionic impurities to 1-2 µM. After the resin beads were removed, the solution was bubbled by Ar gas for 10 min to minimize contaminations by dissolved O2 and CO2.23 The solution was then kept stationary for 5-20 min in a disk-type reaction cell to form the crystal. Then UV light was illuminated by using a 400 W Hg lamp to cure the monomers to polymerize the gel. The gel thickness was about 2 mm. We chose Cp ) 0.8-3.3 vol %, at which the diffraction wavelength was 810-620 nm. The resulting gel had 5-10 vol % of polymer network. Namely, nearly 90% or more of the gelled crystal consisted of water. The microspectroscopy was performed by using an optical microscope (type Optiphoto, Nikon Co., Tokyo, Japan) and multichannel spectrophotometer (type MCPD-3000, Otsuka Electronics Co., Osaka, Japan). An incident light beam (light source, halogen lamp) was introduced to the sample through an objective lens, and a right-angle reflection from a circular area with a diameter of about 10 µm was collected. To observe the gelled crystal under the mechanical compression, we used a specially designed cell, shown in Figure 2, which was equipped with three micrometer-heads and two parallel quartz plates. A type PC-2400 spectrophotometer (Shimadzu Co., Kyoto, Japan) was used for macroscopic spectroscopy.
Results and Discussion Figure 3a shows an overview of the gelled photonic crystal of SI-80P silica ([AAm] ) 1.33 M, [BIS] ) 5 mM, [ABAP] ) 0.4 mg/ml; Cp ) 3.3 vol %), having a volume(23) O2 inhibits the radical polymerization reaction. CO2 gives carbonic acid when dissolved in water, which acts as an ionic impurity.
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Figure 4. Reflection spectrum from the single grain shown in Figure 3 under mechanical compression. The main peak was attributed to the first-order Bragg reflection at right angles to the (110) planes.
Figure 3. (a) Colloidal photonic crystal incorporated in a diskshaped poly(acrylamide) gel matrix, showing a Bragg diffraction of visible light. Sample: SI-80P silica, Cp ) 3.0 vol %. Bar, 5 mm. The crystal consisted of randomly oriented crystal grains a few hundred micrometers in diameter. (b-e) Reflection microscope images of single grains in the gelled photonic crystal under compression. Bars, 50 µm. The ratio of the thickness of the gel in the presence of compression (t) to that in its absence (t0, 2.15 mm): t/t0 ) 1.0 (b), 0.85 (c), 0.80 (d), and 0.75 (e).
filling polycrystalline structure. The gel thickness was 2.30 mm. The crystallite (crystal grain) is a few hundred micrometers in diameter. Due to the Bragg diffraction of visible light, it shows an iridescent color. Before the gelation, the crystal structure was destroyed by adding about 10 µM NaCl. Once it was gelled, however, the crystal structure was maintained even in 0.01 M NaCl, which suggests that the particles are strongly fixed by the gel matrix. The lattice spacing of the gelled crystal was tuned by setting it between two parallel quartz plates and compressing it to a given thickness measured in micrometers. Figure 3b-e are reflection microscope images of a gelled single crystal grain of the silica crystal ([AAm] ) 0.67 M, [BIS] ) 5 mM, [ABAP] ) 0.4 mg/ml; Cp ) 3.0 vol %), at different compression ratios, t/t0. Here, t and t0 indicate the thickness of the gel in the presence and absence of compression, respectively. As the grain was compressed, its color changed from reddish to bluish, corresponding to a decrease of the lattice spacing. Figure 4 shows the reflection spectrum of a single grain obtained by microspectroscopy. The continuous blue shift of the Bragg peak was observed under the compression. Beforehand, we confirmed with an ultra-small-angle X-ray scattering technique that the particles tended to be
Figure 5. λm against the compression ratio t/t0 for three photonic crystals. Solid lines were drawn assuming that λm is exactly proportional to t/t0. The broken line shows the theoretical values calculated by the plane-wave expansion method for sample c, the silica crystal shown in Figures 3 and 4. For samples a and b, latex particles were used. Cp ) 0.8 and 1.2 vol %, respectively.
arranged with a bcc symmetry and to make (110) planes parallel to the plate surface.16,17 Hence, the main peak was attributed to the first-order Bragg reflection at right angles to the (110) planes. Figure 5 shows the shift of the peak wavelength λm by the compression for three crystals designed to have different λm values. Latex particles were used for samples a and b ([AAm] ) 0.67 M, [BIS] ) 5 mM, [ABAP] ) 0.4 mg/mL in both cases; Cp ) 0.8 and 1.2 vol %, respectively), while sample c is a silica crystal (Cp ) 3.0 vol %, Figures 3 and 4). Closed and open symbols are obtained by microand macro- (beam size, 1 × 12 mm) reflection spectroscopy, respectively. The data points and bars are averaged values for 5-10 measurements, and standard deviations (3% on average), respectively. The lattice spacing of sample a determined from λm at t/t0 ) 1 by the Bragg relation was 300 nm, which is 3 times larger than the particle diameter. This large free space enabled a large deformation of the crystal without particle collision, resulting in the peak
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shift covering almost all wavelengths of visible light. When the stress was removed, the peak wavelength returned to the initial value immediately. The degree of peak shift was calculated for sample c based on the plane-wave expansion method24 (broken line). Here it was assumed that a basic lattice was compressed along a direction perpendicular to the (110) plane under a constant lattice volume. The theory predicted a roughly linear relation between λm and t/t0 over the visible wavelength region. Solid lines in Figure 5 were theoretically drawn by assuming that the λm is exactly proportional to t/t0, which shows good agreement with the observed values. This implies that the crystal grain of a submillimeter size changed in shape uniaxially along the same direction and with the same ratio as the macroscopic deformation of the gel. Although so far we focused on single grains near the gel surface, the linearity between λm and t/t0 still holds for the spectrum optically averaged over the grains inside the gel matrix. This suggests that the compression was homogeneous throughout the gel. It was demonstrated that a single grain in the gelled photonic crystal is applicable for tunable optical filters and switches. However, it is not ready for use because the size of grains is small and the directions are irregular. We fabricated photonic crystals containing equally orientated grains by introducing the charged colloidal dispersion between two parallel glass plates separated at about 100 µm.25 The area of the thin photonic crystal was larger than 1 cm2, and the whole surface reflected incident light of a specific wavelength at a certain angle meeting the Bragg condition. It worked as a centimeter-sized single crystal. We measured the transmission spectrum of the thin photonic crystal with an optical fiber spectrophotometer. A deep dip was observed at the wavelength of the Bragg diffraction, and the full width at half-depth of
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the dip was 7 nm, which is slender enough as compared with that from close-packed colloidal photonic crystals in the literature. Gelation of the thin photonic crystal is in progress. Pictures of the thin photonic crystal, the optical fiber spectroscopy system, and the spectrum are presented in the Supporting Information. Conclusion Using the specific nature of the charged colloidal crystals immobilized by the gel matrix, we could linearly tune the band gap over the wavelength range of almost the entire visible light range. These techniques can be used for particles with a high refractive index, which produce a larger band gap energy. Even when the band gap is shallow, one can use its dispersion relation in the refractive index,26 which is useful for modification of pulse lasers.27 This photonic crystal in a gel will become a valuable part of tunable optical devices. Acknowledgment. We express sincere gratitude to Professor N. Ise, Kyoto University, for his helpful discussions. Sincere gratitude is due to Drs. Yoshitsugu Hirokawa, Takuya Okamoto, Hiroshi Yoshida, and Professor Takeji Hashimoto, Hashimoto Polymer Phasing Project, ERATO, JST, for their helpful discussions and cooperation. Part of this research was done as the “Pilot Applied Research Project for the Industrial Use of Space” of the National Space Development Agency of Japan (NASDA) and Japan Space Utilization Promotion Center (JSUP). Supporting Information Available: Pictures of a photonic crystal in the narrow space and its transmission spectrum. This material is available free of charge via the Internet at http://pubs.acs.org. LA0207365
(24) Ho, K. M.; Chan, C. T.; SouKoulis, C. M. Phys. Rev. Lett. 1990, 65, 3152. (25) Sawada, T.; Suzuki,Y.; Toyotama, A.; Iyi, N. Jpn. J. Appl. Phys. 2001, 40, L1226.
(26) Kosaka, H.; et al. Appl. Phys. Lett. 1999, 74, 1370. (27) Imhof, A.; Vos, W. L.; Sprik, R.; Lagendijk, A. Phys. Rev. Lett. 1999, 83, 2942.
