Heterogeneous integration of silicon nitride photonics and CVD-grown WS₂ for second harmonic generation

Silicon nitride photonics has emerged as a promising integrated optical platform due to its broad transparency window, low optical loss, and mature fabrication technology. However, the inherent centrosymmetric crystal structure of silicon nitride fundamentally restricts its applications in second-order nonlinear optical processes. Monolayer transition metal dichalcogenides, particularly tungsten disulfide (WS₂), exhibit strong second-order nonlinear responses, making them ideal candidates for nonlinear photonic applications. Herein, we demonstrate a heterogeneously integrated platform combining silicon nitride waveguides with chemical vapor deposition (CVD)-grown monolayer WS₂, enabling second harmonic generation. A specially designed silica cladding featuring gentle-slope profile on silicon nitride strip waveguides facilitates the integration of centimeter-scale WS₂ film with photonic circuits. This approach provides a robust solution for incorporating second-order nonlinearity into silicon nitride photonic systems. The demonstrated platform holds significant potential for advancing quantum networks, visible-light lasers, and integrated optical modulation/detection systems.