Silicon nitride passive and active photonic integrated circuits: trends and prospects

The use of silicon nitride in integrated photonics has rapidly progressed in recent decades. Ultra-low-loss waveguides based on silicon nitride are a favorable platform for the research of nonlinear and microwave photonics and their application to a wide variety of fields, including precision metrology, communications, sensing, imaging, navigation, computation, and quantum physics. In recent years, the integration of Si and III-V materials has enabled new large-scale, advanced silicon nitride-based photonic integrated circuits with versatile functionality. In this perspective article, we review current trends and the state-of-the-art in silicon nitride-based photonic devices and circuits. We highlight the hybrid and heterogeneous integration of III-V with silicon nitride for electrically pumped soliton microcomb generation and ultra-low-noise lasers with fundamental linewidths in the tens of mHz range. We also discuss several ultimate limits and challenges of silicon nitride-based photonic device performance and provide routes and prospects for future development.