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Langmuir 2006, 22, 94-100
Doped Colloidal Photonic Crystal Structure with Refractive Index Chirping to the [111] Crystallographic Axis Jeong-Ho Park, Won San Choi, Hye Young Koo, Jae-Chul Hong, and Dong-Yu Kim* Center for Frontier Materials, Department of Materials Science and Engineering, Gwangju Institute of Science and Technology, 1 Oryong-Dong, Buk-Gu, Gwangju 500-712, South Korea ReceiVed July 29, 2005. In Final Form: October 19, 2005 A three-dimensionally ordered array of close-packed colloidal spheres, a photonic crystal structure in which the refractive index of the medium interstitial lattice in a colloidal crystal spatially changes in the [111] crystallographic axis, is demonstrated. The colloidal photonic crystal structure with refractive index chirping was produced by infiltration of a monomer and organic dopants with a high refractive index into a silica opal, followed by interfacial gel polymerization. The resulting photonic crystal structure has a gradually varying stop band at each different (111) plane in the facecentered cubic (fcc) crystal structure at a normal incidence. This novel structure exhibited optical characteristics that have band-gap broadening by the superposition of stop bands at each plane of the crystal with different dielectric functions. Moreover, the refractive index perturbation in the [111] fcc opal also showed a defect state within a pseudo-photonic band gap. This new type of photonic crystal structure should be useful for the band-gap engineering of photonic-band-gap materials.
Introduction Three-dimensional photonic crystals (PCs) and photonic-bandgap (PBG) materials are a unique class of materials with a periodically modulated dielectric constant. These ordered dielectric structures with sizes comparable to the wavelength of electromagnetic (EM) waves have attracted considerable interest for possible applications because the propagation of EM waves can be manipulated by defining allowed and forbidden energy gaps in the photon dispersion spectrum. The absence of propagation of EM modes inside the structures gives rise to distinct optical phenomena.1,2 One of the representative bottom-up approaches used to prepare 3D photonic crystal structures is the spontaneous self-organization of spherical colloidal particles. This approach has been proposed as a possible route for the creation of PBG structures.3,4 Such artificial opal structures have been widely studied because of the ease and low-cost process for growing a 3D periodic structure even though they cannot exhibit a full PBG.5,6 Infiltration with materials with a sufficiently high refractive index might permit them to be converted into macroporous ordered inverse opal structures upon removal of the original colloidal crystal materials.7,8 By virtue of this infiltration technique, a 3D photonic crystal would have the potential to exhibit complete PBG frequencies over which an EM wave cannot propagate in any direction.9,10 Numerous previous attempts have been made to fabricate different structures in which a variety of materials can * To whom correspondence should be addressed. Address: Department of Materials Science and Engineering, Gwangju Institute of Science and Technology (GIST), 1 Oryong-dong, Buk-gu, Gwangju 500-712, South Korea. Phone: +82-62-970-2319. Fax: +82-62-970-2304. E-mail:
[email protected]. (1) Yablonovitch, E. Phys. ReV. Lett. 1987, 58, 2059. (2) John, S. Phys. ReV. Lett. 1987, 58, 2486. (3) Vlascov, Y. A.; Bo, X. Z.; Sturm, J. C.; Norris, D. J. Nature 2001, 414, 289. (4) Park, S. H.; Xia, Y. Langmuir 1999, 15, 266. (5) Tarhan, ˆI. ˆI.; Watson, G. H. Phy. ReV. Lett. 1996, 76, 315. (6) Bertone, F.; Jiang, P.; Hwang, K. S.; Mittleman, D. M.; Colvin, V. L. Phys. ReV. Lett. 1999, 83, 300. (7) Mı´guez, H.; Meseguer, F.; Lo´pez, C.; Lo´pez-Tejeira, F.; Sanchez-Dehesa, J. AdV. Mater. 2001, 13, 393. (8) Lopez, C. AdV. Mater. 2003, 15, 1679. (9) Busch, K.; John, S. Phys. ReV. E 1998, 58, 3896.
be infiltrated into the interconnected nanosize voids of opals to observe various phenomena in the optical regime, thus representing a promising approach for “band-gap engineering” by controlling parameters such as the refractive index,11,12 periodicity,13,14 or space-filling factor15 of a colloidal photonic crystal. In addition, one of the unique characteristics of a 3D colloidal crystal is the potential for manipulating the defect state. In the case of a synthetic opal, perturbation of the dielectric function in the crystal structure could generate an impurity mode within the donor and acceptor state. The formation of defects inside a 3D photonic crystal might allow for the propagation of photons at a specific frequency within the otherwise-forbidden band gap. Various PC structures with incorporated defect modes have been reported previously, in attempts to investigate the defect modes of 3D photonic crystals. Yablonovitch et al. first reported a defect mode within the optical stop bands by adding or removing dielectric materials from one of the unit cells in Yablonovite structures.16 Other approaches have involved adding differentdiameter spheres,17,18 inserting a dielectric planar layer19-21 or a different-sized colloidal layer symmetrically inside a colloidal opal,22,23 using asymmetrically heterogeneous colloidal structures (10) Blanco, A.; Chomski, E.; Grabtchak, S.; Ibisate, M.; John, S.; Leonard, S. W.; Lopez, C.; Meseguer, F.; Miguez, H.; Mondia, J. P.; Ozin, G. A.; Toade, O.; van Driel, H. M. Nature 2000, 405, 437. (11) Ozake, M.; Shimoda, Y.; Kasano, M.; Yoshino, K. AdV. Mater. 2002, 14, 514. (12) Kamenjicki, M.; Lednev, I. K.; Asher, S. A. J. Phys. Chem. B 2004, 108, 12637. (13) Iwayama, Y.; Yamanaka, J.; Takiguchi, Y.; Takasaka, M.; Ito, K.; Shinohara, Tadaomi; Sawada, T.; Yonese, M. Langmuir 2003, 19, 977. (14) Lee, Y. J.; Braun, P. V. AdV. Mater. 2003, 15, 563. (15) King, J. S.; Graugnard, E.; Summers, C. J. AdV. Mater. 2005, 17, 1010. (16) Yablonovitch, E.; Gmitter, T. J. Phys. ReV. Lett. 1991, 67, 3380. (17) Pradhan, R. D.; Tarhan, ˆI. ˆI.; Watson, G. H. Phys. ReV. B. 1996, 54, 13721. (18) Gates, B.; Xia, Y. Appl. Phy. Lett. 2001, 78, 3178. (19) Palacios-Lio´n, E.; Galisteo-Lo´pez, J. F.; Jua´rez, B. H.; Lo´pez, C. AdV. Mater. 2004, 16, 341. (20) Te´treault, N.; Mihi, A.; Mı´guez, H.; Rodrı´gez, I.; Ozin, G. A.; Meseguer, F.; Kitaev, V. AdV. Mater. 2004, 16, 346. (21) Fleischhaker, F.; Arsenault, A. C.; Kitaev, V.; Peiris, F. C.; von Freymann, G.; Manners, I.; Zentel, R.; Ozin, G. A. J. Am. Chem. Soc. 2005, 127, 9318. (22) Wostyn, K.; Zhao, Y.; de Schaetzen, G.; Hellemans, L.; Matsuda, N.; Clays, K.; Persoons, A. Langmuir 2003, 19, 4465. (23) Zhao, Y.; Wostyn, K.; de Schaetzen, G.; Clays, K.; Hellemans, L.; Persoons, A. Matsuda, N.; Schoonheydt, R. A. Appl. Phy. Lett. 2003, 82, 3764.
10.1021/la052071n CCC: $33.50 © 2006 American Chemical Society Published on Web 12/03/2005
Doped Colloidal PC with RefractiVe Index Chirping
of different sphere diameters,24 and burying line channel-like defects.25,26 Jiang et al. reported that a multilayer with three blocks of colloidal crystals with colloidal spheres of three different sizes showed newly generated mini-bands as well as partial superposition of the PBG spectra of the individual layers. It has also been reported that the effective bandwidth of this multilayer structure could be increased by overlapping many colloidal crystal layers with different-sized spheres, an approach that might be useful for optical filters.27 Recently, a PC structure with an airfilling-fraction gradient along the [111] axis using the plasmaetching process was reported for enhanced coupling to slow photon modes.28 In this article, we report on a novel photonic crystal structure in which the refractive index of the interstitial lattice of the colloidal crystal was gradually changed along the [111] axis of the face-centered cubic (fcc) opal crystal. This structure was prepared by infiltrating a mixture of monomer and high-refractiveindex dopants into the interstitial voids in the colloidal crystal and then applying interfacial gel polymerization.29 In the case of interfacial gel polymerization, the polymerization reaction is accelerated in the gel state formed at the interface between the polymer and monomer because of a “gel effect”.30 Whereas the monomer can easily diffuse into the gel phase during the gel polymerization, dopant molecules cannot, since high-refractiveindex dopants with bulky structures such as aromatic compounds have larger molecular volumes than the monomer. Thus, the growing gel layer excludes dopant molecules during the polymerization, and the dopants become more concentrated in the direction of polymerization growth to form a nearly quadratic refractive index profile.31 We have recently demonstrated that this interfacial gel polymerization technique can be applied to the infiltration of a colloidal crystal structure. The resulting colloidal crystal structure with a refractive index distribution was shown to be a tunable optical filter depending on the positional variation when the refractive index profile was engaged in the vertical direction of the fcc [111] axis.32 In terms of the direction of refractive index (RI) distribution in an fcc opal structure, an infiltrated colloidal opal structure with a graded refractive index distribution chirping the fcc [111] axis could have more interesting optical properties. Consequently, this 3D PC structure would have a gradually varying stop band at different (111) layers inside the opal, as shown in Figure 1. Therefore, the resulting PC structure would have a relatively wider pseudo-L band because of the gradually varying dielectric function along the [111] axis. Moreover, under certain conditions, the local perturbation in the refractive index profile of [111] could generate a defect state in the effective band gap. Experimental Section Monodisperse SiO2 spheres (0.49 ( 0.03 µm) were purchased from Duke Scientific (Palo Alto, CA). The ordered colloidal crystal was prepared on a clean glass substrate by sedimentation of a solution (24) Egen, M.; Voss, R.; Griesebock, B.; Zentel, R. Chem. Mater. 2003, 15, 3786. (25) Lee, W.; Pruzinsky, S. A.; Braun. P. V. AdV. Mater. 2002, 14, 271. (26) Vekris, E.; Kitaev, V.; von Freymann, G.; Perovi, D. D.; Aitchison, J. S.; Ozin, G. A. AdV. Mater. 2005, 17, 1269. (27) Jiang, P.; Ostojic, G. N.; Narat, R.; Mittleman, D. M.; Colvin, V. L. AdV. Mater. 2001, 13, 389. (28) von Freymann, G.; John, S.; Kitaev, V.; Ozin, G. A. AdV. Mater. 2005, 17, 1273. (29) Ishigure, T.; Sato, E.; Nihei, E.; Koike, Y. Jpn. J. Appl. Phys. 1998, 37, 3986. (30) Odian, G. Principles of Polymerization, 3rd ed.; John Wiley & Sons: New York, 1991; Chapter 3. (31) Zhang, Q.; Wang, P.; Zhai, Y. Macromolecules 1997, 30, 7872. (32) Park, J.-H.; Choi, W. S.; Koo, H. Y.; Kim, D.-Y. AdV. Mater. 2005, 17, 879.
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Figure 1. Schematics of the refractive-index-chirped PC structure. (a) An infiltrated PC structure with a narrow pseudo-band gap due to the homogeneous dielectric function. (b) An infiltrated opal structure with a superposition of stop gaps from the individual (111) layer due to the refractive-index-chirped to the [111] fcc opal crystal. of a silica colloidal dispersion composed of 4 vol % silica dispersed solution in a 1:1 mixture of ethanol and deionized water from a Milli-Q water system. The glass substrates were cleaned with piranha solution (H2SO4/H2O2 ) 3:1) for 10 min and then rinsed with deionized water and dried in a stream of filtered nitrogen gas. Colloidal crystals were obtained after evaporation at 45 °C for 24 h in and oven under a nitrogen flow and were then dried at 110 °C in a vacuum oven for 12 h to remove residual water. In the infiltration process, methyl methacrylate (MMA, Aldrich), n-butylmercaptan (n-BM, Aldrich) were used as the monomer and chain-transfer reagent, respectively. Here, MMA was extensively purified by a previously reported method.33 Diphenyl sulfide (DPS, nD ) 1.632, Aldrich) was used as a high-refractive-index dopant without further purification. Traces of Rhodamine B (RB, Aldrich) dye were added to the monomer solution as an indicator to qualitatively measure the change in refractive index during the polymerization reaction. As shown in Figure 2, the procedure used to prepare the refractiveindex-chirping photonic crystal is as follows. A prepolymer layer [poly(methyl methacrylate), PMMA] with a thickness of 1 mm was prepared by the bulk polymerization of purified MMA monomer in the bottom of a glass cell. A previously prepared SiO2 colloidal crystal was placed at the top of the glass cell using a spacer with a thickness of 1 mm as shown in Figure 2. Subsequently, a wellmixed neat solution of 20 or 30 vol % DPS, 0.1 wt % n-BM, and 10 ppm RB in MMA was immersed in the SiO2 colloidal crystal inside the glass cell. Finally, a gel polymerization reaction was carried out at 80 °C in a nitrogen environment for 48 h. For the preparation of a homogeneously doped sample, the same procedure was employed, but without the preparation of a prepolymer layer. For measurement of the optical transmission spectrum, light from a single-mode optical fiber from a white light source (AQ4303B, Ando) was focused on the sample using an objective lens. The light transmitted through the sample was focused on a multimode fiber coupled to an OSA (optical spectrum analyzer, Agilent 86140B). The resolution bandwidth and sensitivity of the OSA were set at 0.5 nm and -80 dBm, respectively. Using this arrangement, the spot size in the measurement could be determined by adjusting the focal plane of the sample position between the input and output optical fibers (
