From Opals to Optics: Colloidal Photonic Crystals
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From Opals to
Optics: Colloidal Photonic Crystals Vicki L. Colvin
Introduction Over a decade ago, theorists predicted that photonic crystals active at visible and near-infrared wavelengths would possess a variety of exciting optical properties.1–3 Only in the last several years, however, have experimentalists begun to build materials that realize this potential in the laboratory. This lag between experiment and theory is primarily due to the to the challenges associated with fabricating these unique materials. As the term “crystal” suggests, these samples must consist of highly perfect ordered arrays of solids. However, unlike conventional crystals, which exhibit order on the angstrom length scale, photonic crystals must have order on the submicrometer length scale. In addition, many of the most valuable properties of photonic crystals are only realized when samples possess a “full” photonic bandgap. For such systems, large dielectric contrasts and particular crystal symmetries create a range of frequencies over which light cannot propagate. Realizing the nanoscopic architectures required to form such systems is a challenge for experimentalists. As a result, fabrication schemes that rely on lithographic techniques or spontaneous assembly have been a focus in the development of the field. This review focuses exclusively on a chemical assembly strategy for photoniccrystal production, namely, the use of closepacked arrays of spheres. These systems, termed colloidal crystals, are composed of either silica or polymer colloids; like the natural gemstone opal which they resemble, they diffract visible and near-infrared light as a result of the submicrometer diameters of the colloids. In contrast to topdown fabrication approaches to photonic crystals, which use lithographic or other means to carve periodic structures in monoliths, colloidal crystals spontaneously form under ambient conditions of pres-
MRS BULLETIN/AUGUST 2001
sure and temperature. They are easily prepared without the need for costly instruments, and most important, they offer a clear route toward three-dimensional (3D) photonic crystals with submicrometer periodicity. However, this uncontrolled assembly process can generate more crystal defects than a comparable lithographic procedure. Moreover, standard colloidal crystals do not provide either the crystal symmetry or dielectric contrast required for full photonic bandgaps. Solutions to these problems have been the focus of much research over the past several years. Great strides have been made in developing methods to assemble more perfect colloidal crystals in structures appropriate for optical applications. Replication of these structures into inverse opals of higherindex materials can create systems with full photonic bandgaps. Finally, the simple close-packed motif of these crystals can serve as a starting point for generating the complex periodic architectures needed for integrated photonic-crystal applications.
Colloidal Crystals as Photonic Crystals Long before the term “photonic crystal” was
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