Subwavelength grating couplers for efficient light coupling into silicon nanomembrane based photonic devices

We have investigated the feasibility of multimode polysilicon waveguides to demonstrate the suitability of polysilicon as a candidate for multilayer photonic applications. Solid Phase Crystallization (SPC) with a maximum temperature of 1000°C is used to create polysilicon on thermally grown SiO2. We then measure the propagation losses for various waveguide widths on both polysilicon and crystalline silicon platforms. We find that as the width increases for polysilicon waveguides, the propagation loss decreases similar to crystalline silicon waveguides. The difference in loss between the two platforms for a given waveguide width is due to the scattering from the polysilicon grain boundaries, which excites higher order modes. Depending on the waveguide width, these modes either propagate as higher order modes or are lost as radiation modes. Due to their different propagation constants, the presence of higher order modes is confirmed using sub-wavelength grating couplers. At a waveguide width of 10μm, polysilicon and crystalline silicon waveguides have propagation losses of 0.56dB/cm and 0.31dB/cm, respectively, indicating there is little bulk absorption from the polysilicon. This propagation loss is the lowest for polysilicon demonstrated to date. Modal conversion in multimode waveguides by polysilicon grain boundary scattering are investigated using a sub-wavelength grating coupler and discussed. These results vindicate the use of polysilicon waveguides of varying widths in photonic integrated circuits.