Overcoming stress limitations in SiN nonlinear photonics via a bilayer waveguide

Silicon nitride (SiN) formed via low pressure chemical vapor deposition (LPCVD) is an ideal material platform for on-chip nonlinear photonics owing to its low propagation loss and competitive nonlinear index. Despite this, LPCVD SiN is restricted in its scalability due to the film stress when high thicknesses, required for nonlinear dispersion engineering, are deposited. This stress in turn leads to film cracking and makes integrating such films in silicon foundries challenging. To overcome this limitation, we propose a bilayer waveguide scheme comprised of a thin LPCVD SiN layer underneath a low-stress and low-index PECVD SiN layer. We show group velocity dispersion tuning at 1,550 nm without concern for film-cracking while enabling low loss resonators with intrinsic quality factors above 1 million. Finally, we demonstrate a locked, normal dispersion Kerr frequency comb with our bilayer waveguide resonators spanning 120 nm in the c-band with an on-chip pump power of 350 mW.