Seidt, LevinLevinSeidtWeber, ThomasThomasWeberSeredin, Albert A.Albert A.SeredinPossmayer, ThomasThomasPossmayerSavelev, RomanRomanSavelevPetrov, Mihail I.Mihail I.PetrovMaier, Stefan A.Stefan A.MaierTittl, AndreasAndreasTittlMenezes, Leonardo de S.Leonardo de S.MenezesSortino, LucaLucaSortino2026-03-122026-03-122025-09-02Npj Nanophotonics 2 (1): 37 (2025)https://hdl.handle.net/11420/62025Second-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 C₃ᵥ 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.en#PLACEHOLDER_PARENT_METADATA_VALUE#20251Technology::600: TechnologyJournal Article10.1038/s44310-025-00083-4Journal Article