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We investigate, both experimentally and theoretically, the waveguiding properties of a double-trench waveguide in which a conventional single-mode strip waveguide is embedded in a two-dimensional photonic crystal (PhC) slab formed in silicon-on-insulator wafers. We demonstrate that the bandwidth for relatively low-loss (50 dB/cm) waveguiding is significantly expanded to 250 nm, covering almost all the photonic bandgap owing to nearly linear dispersion of the TE-like waveguiding mode. The flat transmission spectrum, however, is interrupted by numerous narrow stop bands. We found that these stop bands can be attributed to anticrossing between TE-like (positive parity) and TM-like (negative parity) modes. This effect is a direct result of the strong asymmetry of the waveguides that have an upper cladding of air and lower cladding of oxide. © 2004 American Institute of Physics.