Resonance-Enhanced Four-Wave Mixing Imaging for Mapping Defect Regions in Vanadium-Doped WS₂ Monolayers

Defect engineering is crucial for tuning 2D transition metal dichalcogenide properties for quantum and optoelectronic applications. While conventional photoluminescence (PL) and Raman spectroscopies are important characterization tools, their mapping in large area samples can be time-consuming and lacks direct sensitivity for comprehensive defect characterization. Here, we introduce resonance-enhanced four-wave mixing (FWM) imaging for precise imaging and characterization of vanadium-induced defect states in ${\mathrm{WS}}_2$ monolayers. Our multi-modal investigation, integrating hyperspectral PL, Raman, and supported by density functional calculations, reveals nanoscale doping inhomogeneities, their influence on excitonic and vibrational properties. We observe resonance-enhanced FWM signals correlating with vanadium-induced defect regions, evidencing their unique nonlinear optical response. This work establishes FWM as an essential platform for high-resolution, defect-sensitive imaging, advancing defect-engineered excitonic devices and enabling novel nonlinear quantum photonics.