Broadband and Enhanced Second‐Harmonic Generation in Metallic 3R‐TaS ₂

ABSTRACT 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‐TaS 2 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 MoS 2 , despite the metallic electronic structure of 3R‐TaS 2 . 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.