Sub-wavelength nanostructures for engineering the effective index of silicon-on-insulator waveguides

Silicon-on-insulator (SOI) is considered the most promising high-index-contrast waveguide platform for fabricating microphotonic devices with high integration density. However, there are fundamental challenges with this waveguide system related to the fixed values of the refractive indices of the constituent materials (Si and SiO₂). In this paper, we present a method that can potentially circumvent this limitation by using the sub-wavelength grating effect in SOI waveguides to effectively engineer materials with intermediate refractive indices by standard lithographic patterning. The first implementations of the SWG effect in SOI waveguides are discussed, including our latest results in fiber-chip couplers, anti-reflective and high-reflectivity waveguide facets, mode transformers between waveguides of different geometries, and microphotonic waveguide crossings.