Broadband and Enhanced Second-Harmonic Generation in Metallic 3R-TaS₂

Second-harmonic generation (SHG) serves as a fundamental mechanism for frequency conversion and light manipulation in photonic devices, as well as a powerful probe for exploring nonlinear optical properties in 2D materials. However, in metallic 2D systems, strong free-carrier absorption and shallow optical penetration depth usually suppress the nonlinear response. Realizing pronounced SHG in such highly absorptive materials thus remains a major challenge in nonlinear photonics. In this work, we systematically investigate the SHG response of non-centrosymmetric metallic 3R-TaS₂ and demonstrate strong broadband SHG across a wide emission wavelength range of 510-705 nm. By analyzing the wavelength-dependent optical constants and dielectric function, we show that this broadband SHG arises from the combined effects of the non-centrosymmetric crystal structure, a dielectric-like optical response in the visible range, and the absence of sharp resonance features, together with free-carrier-mediated optical damping. Furthermore, substantial SHG enhancement is achieved using a reflective gold substrate, with the enhancement factor increasing from 2.7 to 45.5 as the flake thickness decreases from 123 to 68 nm. The extracted effective second-order nonlinear susceptibility is comparable to that of MoS₂, despite the metallic electronic structure of 3R-TaS₂. These results demonstrate that strong, broadband nonlinear optical responses can persist in metallic layered materials and provide new insights into the interplay among metallicity, optical response, and nonlinear light-matter interactions in van der Waals systems.