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Materials Science

Aspects of Photonic Crystals

Albert Polman and Pierre Wiltzius, Guest Editors The electronics revolution of the past 50 years has its roots in two scientific and technological areas. On the one hand, there have been tremendous advancements in our understanding of the physics of metals, dielectrics, and semiconductors, leading to the development of devices such as the transistor. On the other hand, a variety of processing techniques such as thin-film growth and deposition, ion implantation, and photolithography have allowed the massive integration of electronic functionality within a very small area, leading to microprocessors and high-density memory, among other innovations. Our ability to control photons is in many ways in its infancy, compared with how we can manipulate electrons. Passive devices such as optical fibers, waveguides, splitters, and multiplexers are well developed. But as soon as more complex functionality or integration is required, the optical solutions do not yet exist. For example, all-optical switches are still very rudimentary and bulky, and the size of an optical integrated circuit (IC) is most often in the millimeter or centimeter range rather than the submicrometer dimensions common in electronic technology. Nevertheless, there is a clear need to develop new materials and concepts with increased optical functionality for a variety of applications. The global telecommunications market is on an extraordinarily steep growth curve, driven largely by the explosion of the Internet, which plays an increasingly pervasive role in our daily life. The demand for broadband communications networks is expected to grow for many years to come. New approaches for the manipulation of photons will have to be developed to realize the more advanced optical elements needed for networks in the coming decade. Photonic crystals may play an important role in this development.

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A photonic crystal is a regularly structured material that exhibits strong interaction with light. The conceptually simplest example of such a material is a multilayer stack of alternating high- and lowdielectric-constant materials. Strong interaction with light occurs in such a material because of interference between the light beams that are reflected and refracted at all interfaces inside the material. The final optical response is determined by the coherent superposition of all of these optical waves. It has long been known that such multilayer stacks can be engineered to have, for example, nearly perfect reflection over a (narrow or broad) wavelength range, a so-called stop band.1 Thin-film deposition techniques have made such structures widely available. Well-known examples of such “one-dimensional” (1D) photonic crystals are dielectric mirrors, filters, fiber gratings, distributed-feedback structures, and vertical-cavity surface-emitting lasers. Research is also being focused on “omnidirectional” mirrors that reflect light over a well-defined wavelength range in all directions, again using an alternating array o