Mesoscopically Periodic Photonic Crystal Materials for Linear and Nonlinear Optics and Chemical Sensing
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ding the CCA lattice into a hydrogel polyacrylamide network (Figure 3). These resulting solid photonic crystals can be prepared such that the acrylamide and the colloidal particles occupy a small percent of the sample volume, which consists mostly of water. The medium surrounding the spheres can be modified because other solvents can be diffused into the polymerized array to replace the water. Although the hydrogel-linked CCA undergoes swelling and shrinkage as the solvent medium is changed, the array ordering is maintained. Single-crystal films of this array can be prepared in which the {111) planes of the fee lattice are well-oriented and parallel to the surface. Sunkara et al. at Oklahoma have more recently developed an alternative polymerization strategy for forming solidified CCA materials.10 Optically Nonlinear Photonic Crystals We11'12 as well as others13 proposed that optically nonlinear photonic crystals could be fabricated using optically non-
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- impurities Introduction Over the past decade, we have been working to develop intelligent photoniccrystal materials with unique properties, which will be useful in a number of technological areas. These photonic-crystal materials utilize mesoscopically periodic arrays of spherical particles as their active optical elements and are easily fabricated chemically by the use of crystallinecolloidal-array (CCA) self-assembly techniques. Crystalline colloidal arrays are mesoscopically periodic fluid materials, which efficiently diffract light meeting the Bragg condition.1"4 These photonic-crystal materials consist of arrays of colloidal particles that self-assemble in solution into either face-centered-cubic (fee) or bodycentered-cubic (bec) crystalline arrays1'5 (Figure 1), with lattice constants in the mesoscale size range (50-500 nm). Just as atomic crystals diffract x-rays that meet the Bragg condition, CCAs diffract ultraviolet, visible, and near-infrared light, depending on the lattice spacing;2"4 the diffraction phenomena resemble those of opals, which are close-packed arrays of monodisperse silica spheres.6 The CCA however can be prepared as 44
macroscopically ordered arrays of nonclose-packed spheres. This self-assembly is the result of electrostatic repulsions between colloidal particles, each of which has numerous charged surface functional groups. We have concentrated on the development of CCAs that diffract light in the visible spectral region and generally utilize colloidal particles of ~100-nm diameter.7 These particles have thousands of surface charges, which result from the ionization of surface sulfonate groups. The nearest-neighbor distances are often >200 nm.
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Photonic-Crystal Diffraction Devices These bec or fee cubic arrays are wellordered and the arrays strongly diffract light in the visible spectral region (Figure 2). All light meeting the Bragg condition is diffracted, whereas adjacent spectral regions freely transmit. We earlier demonstrated the use of these CCAs as narrow-band optical diffraction filters.10'3 We more recently
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