Efficient small grating couplers for low-index difference waveguide systems

Due to the small coupling strength of waveguide grating couplers in low index contrast material systems such as polymers, the efficient coupling to single-mode waveguides via surface gratings represents a severe challenge. In this work, we demonstrate that the coupling strength of grating couplers in low-index difference waveguide systems can be strongly enhanced by the application of a thin high-index coating (HIC) on top of surface gratings. This allows reducing the grating coupler aperture size without sacrificing efficiency by up to more than an order of magnitude, which enables low-loss lateral tapering to single-mode waveguides.