High-efficiency grating coupler for an ultralow-loss Si₃N₄-based platform

Integrated silicon nitride waveguides of 100 nm height can achieve ultralow propagation losses below 0.1 dB/cm at the 1550 nm wavelength band but lack the scattering strength to form efficient grating couplers. An enhanced grating coupler design based on an amorphous silicon layer on top of silicon nitride is proposed and demonstrated to improve the directionality of the coupler. The fabrication process is optimized for a self-alignment process between the amorphous silicon and silicon nitride layers without increasing waveguide losses. Experimental coupling losses of 5 dB and a 3 dB bandwidth of 75 nm are achieved with both regular and focusing designs.