Micro-transfer printed silicon nitride grating couplers for efficient on-chip light coupling

We demonstrate a new post-processing approach to efficiently couple light in silicon nitride (Si3N4) photonic integrated circuits (PIC) via grating couplers. Thin-film coupons of apodised Si₃N₄ grating couplers functionalized with metallic bottom reflectors and adiabatic couplers are micro-transfer printed onto the input and output of Si₃N₄ waveguides, providing efficient coupling without jeopardizing the rest of the PIC. Two-dimensional FDTD simulations predict coupling efficiencies as large as 70% (-1.5dB) at a wavelength of 785nm. As a proof-of-concept, we report an experimental coupling efficiency of -4.6dB, which is 2.6dB better than the values obtained with standard designs of grating couplers.