Annealing-free Si3N4 frequency combs for monolithic integration with Si photonics

Abstract

Silicon-nitride-on-insulator (SiNOI) is an attractive platform for optical frequency comb generation in the telecommunication band because of the low two-photon absorption and free carrier induced nonlinear loss when compared with crystalline silicon. However, high-temperature annealing that has been used so far for demonstrating Si3N4-based frequency combs made co-integration with silicon-based optoelectronics elusive, thus reducing dramatically its effective complementary metal oxide semiconductor (CMOS) compatibility. We report here on the fabrication and testing of annealing-free SiNOI nonlinear photonic circuits. In particular, we have developed a process to fabricate low-loss, annealing-free, and crack-free Si3N4 740-nm-thick films for Kerr-based nonlinear photonics featuring a full process compatibility with front-end silicon photonics. Experimental evidence shows that micro-resonators using such annealing-free silicon nitride films are capable of generating a frequency comb spanning 1300–2100 nm via optical parametrical oscillation based on four-wave mixing. This work constitutes a decisive step toward time-stable power-efficient Kerr-based broadband sources featuring full process compatibility with Si photonic integrated circuits on CMOS lines.