Silica-embedded silicon photonic crystal waveguides

Photonic crystal waveguides are currently of great interest for the transfer and processing of information using light. A crucial issue in this respect is that of propagation losses. This paper reports on the fabrication and characterization of silicon photonic crystal waveguides completely embedded in silica. These waveguides offer a robust alternative to silicon membranes and are fully compatible with monolithic integration. Despite the reduced refractive index contrast compared to self-standing membranes, these waveguides offer a considerable operating range of about 10 nm in the third telecom window (i.e., around 1550 nm wavelength). 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 silicon 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 root-mean-square deviation, which attest to the high quality of our structures. The paper arises from a collaboration between the Universities of Pavia and St. Andrews (United Kingdom).