Silica-embedded silicon photonic crystal waveguides.

We report on the fabrication and characterization of silicon photonic crystal waveguides completely embedded in silica. These waveguides offer a robust alternative to air-membranes and are fully compatible with monolithic integration. Despite the reduced refractive index contrast compared to the air-membranes, these waveguides offer a considerable operating range of approximately 10 nm in the 1550 nm window. While the reduced index contrast weakens the perturbations due to surface roughness, we measure losses of 35 +/- 3dB/cm compared to 12 +/- 3 dB/cm for nominally identical air-membranes. Numerical analysis reveals that the difference in loss results from the different mode distribution and group index of the respective waveguide modes. Radius disorder is used as a fitting parameter in the numerical simulations with the best fits found for disorder levels of 1.4 - 1.7 nm RMS, which attest to the high quality of our structures.