Bi-Layer Grating Couplers for Hybrid SixNy- Si3N4 Photonics with Sub- Decibel Coupling Efficiency

Over the last twenty years, the research on silicon nitride has had a tremendous impact on the development of chip-scale nonlinear photonics, comb generation and related applications such as sensing, metrology, quantum technologies and communications [1], [2]. The low refractive index contrast between silicon nitride $(\text{Si}_{3}\mathrm{N}_{4}, \mathrm{n}=2.00$ at 1550 nm) and the silica cladding $(\text{SiO}_{2}, \mathrm{n}=1.44$ at 1550 nm), compared to the typical case of silicon waveguides (Si, $\mathrm{n}=3.48$ at 1550 nm), enables achieving extremely low propagation losses and better tolerances in the fabrication process. However, the smaller index contrast makes the design and realization of efficient grating couplers (GCs), which are commonly used to couple light from and to an out-of-plane optical fibre, more difficult than in the case of Silicon-on-Insulator (SOI) devices.