Hybrid integration of a SESAM with a silicon nitride photonic integrated circuit
The realization of high-performance femtosecond lasers on photonic integrated circuits (PICs) requires robust, low-loss coupling between a saturable absorber and a photonic waveguide. In this work, we demonstrate the hybrid integration of a semiconductor saturable absorber mirror (SESAM) with a silicon nitride (SiN) PIC by micro-assembly. This micro-assembly strategy transfers the non-linear functionality of the absorber onto the PIC. We characterize the linear and non-linear performance of an unbonded SESAM and of a SESAM bonded to inverse-tapered SiN waveguides. At 1560~nm, the bonded device shows a waveguide-coupled modulation depth of 5.2\%, a maximum reflectivity of 37.2\%, and a saturation fluence of 47~\textmu J/cm$^2$. This work demonstrates that SESAMs can be integrated directly onto silicon nitride-based PIC platforms and provides a pathway towards fully integrated, self-starting, chip-scale ultrafast lasers.