Second-harmonic generation in NbOI2-integrated silicon nitride microdisk resonators

In recent years, two-dimensional (2D) niobium oxide dihalides (e.g., NbOI2) have garnered significant research interest in nonlinear photonics due to their prominent second-order nonlinear optical properties. Integrating these materials with high-quality-factor optical microcavities represents a crucial approach for developing high-performance on-chip nonlinear optical devices. This work demonstrates NbOI2-integrated silicon nitride (Si3N4) microdisk resonators that achieve second-harmonic generation under low-power (sub-milliwatt) continuous-wave laser pumping, leveraging the superior second-order nonlinearity of NbOI2 and the strong optical field confinement capability of Si3N4 microdisks. The conversion efficiency of the device is calculated to be about 0.024 %/W. The intrinsic lack of inversion symmetry in NbOI2 crystals avoids the laborious layer-number-dependent symmetry screening typically required for other 2D materials, while the developed van der Waals transfer technique provides a universal strategy for integrating niobium oxide dihalides with photonic microcavities. This study not only establishes a material-photon co-design strategy for on-chip nonlinear light sources but also lays a critical foundation for advancing quantum photonic chips and on-chip metrology systems.