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, cannot be simultaneously achieved only by Si or Si₃N₄. Second-order nonlinearity is another optical property they do not possess due to their centrosymmetric properties. Several kinds of 2D materials emerged recently and were transferred to specified photonic devices aimed at improving their nonlinear performance. However, the transferring methods are time-consuming, unable to achieve large-scale production, and will inevitably cause materials damage and introduce impurities at the interface. Herein, we demonstrate the direct growth of large-area homogeneous monolayer WS₂via a physical vapor deposition method onto Si₃N₄ waveguides. The WS₂ growth can be controlled mainly along the Si₃N₄ waveguides and the waveguides show an obvious enhancement of second-harmonic generation with the elongated WS₂ coverage. The direct growth of WS₂ endows 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 the foundation for on-chip integrated optical fabrication and applications.