Overcoming stress limitations in SiN nonlinear photonics via a bilayer waveguide

TL;DR

This paper introduces a bilayer SiN waveguide design that mitigates stress-related cracking, enabling high-quality nonlinear photonic devices with broad frequency combs at low pump power.

Contribution

The study presents a novel bilayer waveguide scheme combining LPCVD and PECVD SiN layers to overcome stress limitations in SiN photonics.

Findings

Achieved group velocity dispersion tuning at 1550nm

Produced high-Q resonators with quality factors above 1 million

Demonstrated a 120nm span Kerr frequency comb in the C-band

Abstract

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 1550nm without concern for filmcracking 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 120nm in the c-band with an on-chip pump power of 350mW.