Photonic crystal slab structures are constructed by introducing strong two-dimensionally periodic index contrast into a high-index dielectric guiding slab. With sufficient index contrast in the vertical direction, such structures support an in-plane photonic band gap that lies below the light line, which allows them to function as a fundamental substrate for large-scale integrated micro-photonic circuit applications. For photonic integrated circuits, an essential building block is the waveguide structure. In order to function as an effective information carrying channel, the waveguide should possess several necessary properties: It should have its dispersion curve lying within the gap region below the light line to ensure low loss propagation within the guide and around sharp comers. The waveguide should also be single-moded, possess sufficient bandwidth to accommodate the incoming signal, and display minimal dispersion within the signal bandwidth. In a photonic crystal slab, a waveguide is typically created introducing a line defect into the slab structure. These structures have been studied extensively with experiments and three-dimensional simulations. However, many of the proposed waveguide structures exhibit relatively small guiding bandwidth and large group velocity dispersion. Developing ways to enlarge the waveguide bandwidth is therefore an important direction of research in photonic crystal structures.
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