Mirror‐Induced Electromagnetic Asymmetry Enables Greatly Enhanced Second‐Harmonic Generation From 2H‐Phase WS 2 Nanoparticles

ABSTRACT Second‐harmonic generation (SHG) in centrosymmetric materials is forbidden within the electric‐dipole (ED) approximation, posing a fundamental challenge for nonlinear nanophotonics. Here, we demonstrate an effective approach to activate and enhance SHG from centrosymmetric WS 2 nanoparticles using a nanoparticle‐on‐mirror architecture. By placing hexagonal WS 2 nanoparticles onto a metallic film, mirror‐induced field asymmetry and strong nanoscale field confinement create an inversion‐asymmetric local electromagnetic environment, enabling pronounced SHG emission from individual nanoparticles. Excitation of the ED‐dominated hybrid nanoparticle–mirror resonance leads to a maximum SHG enhancement factor of approximately 2000 compared with bulk WS 2 under identical excitation conditions. Polarization‐, power‐, and wavelength‐dependent measurements confirm the second‐order nonlinear nature of the emission and reveal a clear correlation between linear optical resonances and SHG enhancement. Numerical simulations further reveal that the metallic mirror induces strong asymmetric electric‐field localization at the WS 2 –substrate interface, providing a physical origin for the observed nonlinear response. These results establish mirror‐induced electromagnetic asymmetry as an effective strategy for enabling nonlinear optical processes in centrosymmetric two‐dimensional materials and provide a versatile platform for designing efficient nonlinear nanophotonic devices through substrate engineering and resonant mode control.