Broad bandwidth double-trench waveguides in silicon-on-insulator photonic crystal slabs

We investigate both experimentally and theoretically the waveguiding properties of the novel design of channel waveguides in silicon-on-insulator (SOI) photonic crystal slabs. It is known that the channel waveguides defined by a missing of one row of holes in a triangular-lattice photonic crystal are characterized by a very narrow transmission bandwidth limited by large group velocity dispersion. In order to increase the bandwidth we investigate an alternative design, where the conventional single-mode strip waveguide is embedded into a photonic crystal slab-a so-called double-trench waveguide. Such a design is intended to combine the best features of photonic crystal slabs, such as suppression of radiation losses at bends and imperfections, with broad bandwidth and small group velocity dispersion. We report the successful demonstration of this broad-bandwidth photonic crystal waveguide with propagation losses as low as 35 dB/cm, which are among the lowest reported in the literature. Furthermore, we found that the modes of positive (quasi-TE) and negative (quasi-TM) parity significantly interact in our structures due to the absence of the oxide layer on top of the SOI slab and the resulting asymmetry. As a result of this interaction multiple mini-stopbands appear in the areas of anti-crossing of the positive and negative parity modes. The results are successfully modeled by the plane-wave calculations confirming the nature of the experimentally observed mini-stopbands. To the best of our knowledge this is the first demonstration of the effects of asymmetry on the transmission characteristics of the photonic crystal slabs.