Silicon nitride waveguides with directly grown WS2 for efficient second-harmonic generation

Different functions can be directly realized by silicon (Si) in integrated electronic circuits. Although Si and silicon nitride (Si3N4) photonics have shown great potential in integrated optoelectronic devices, different functions such as light generation, transparency for guided light, and light detection can't be simultaneously achieved only by Si or Si3N4. Second-order nonlinearity is another optical property they don't possess due to their centrosymmetric property. Several kinds of 2D materials are emerging recently and transferred to specified photonic devices aiming to improve their nonlinear performance. However, the transferring methods are time-consuming, unable to achieve large-scale production, and will inevitably cause material damage and introduce impurities at the interface. Herein, we demonstrate direct growth of large-area homogeneous monolayer WS2 by physical vapor deposition method onto Si3N4 waveguides. The WS2 growth can be controlled and mainly along the Si3N4 waveguides and the waveguides show obvious enhancement of second-harmonic generation with elongated WS2 coverage. The direct growth of WS2 endow Si3N4 integrated photonics with new nonlinear optical properties. As an alternative method of transferring 2D materials, the method we present here is compatible with large-scale integrated photonic fabrication, which lays a foundation for on-chip integrated optical fabrication and applications.