Ultrafast all-optical switching in nonlinear 3R-MoS₂ van der Waals metasurfaces

Second-order nonlinear optical processes are fundamental to photonics, spectroscopy, and information technologies, with material platforms playing a pivotal role in advancing these applications. Here, we demonstrate the exceptional nonlinear optical properties of the van der Waals crystal 3R-MoS₂, a rhombohedral polymorph exhibiting high second-order optical susceptibility (χ ⁽²⁾) and remarkable second-harmonic generation (SHG) capabilities. By designing high quality factor resonances in 3R-MoS₂ metasurfaces supporting quasi-bound states in the continuum (qBIC), we first demonstrate SHG efficiency enhancement exceeding 10². Additionally, by using degenerate pump-probe spectroscopy, we harness the C _3v system's symmetry to realize ultrafast SHG polarization switching with near-unity modulation depth. The operation speeds are limited only by the excitation pulse duration, allowing its characterization via the nonlinear autocorrelation function. These findings establish 3R-MoS₂ as a transformative platform for nanoscale nonlinear optics, offering large conversion efficiencies and ultrafast response times for advanced pulse measurement devices, integrated photonics, and quantum technologies.