Topologically enhanced giant broadband second-harmonic generation in Weyl semiconductor tellurium

Achieving strong nonlinear optical (NLO) responses in atomically thin layered materials is essential for advanced nanoscale photonic and on-chip integrated optoelectronic technologies. However, most materials with large second-order susceptibility ( ${\chi }^{\left(2\right)}$ ) operate only over narrow wavelength ranges, with rare reports in the mid-infrared (MIR) region. Topological engineering provides a promising strategy to enhance NLO responses. Here, we demonstrate that Weyl semiconductor tellurium (Te) nanoflakes exhibit giant second harmonic generation (SHG) across an ultrabroadband infrared range (1.2-5.0 μm), including the challenging MIR region, achieving conversion efficiency two orders of magnitude greater than GaSe. The extracted ${\chi }^{\left(2\right)}$ spectrum reveals a prominent peak of 5.0 ± 0.4 nm V^-1 at 2.2-μm excitation and two shoulders, attributable to three two-photon resonances with interband transitions near three different Weyl cones, indicating topological enhancement of SHG. The giant, anisotropic, ultrabroadband SHG in Te nanoflakes promises unprecedented versatility for MIR frequency conversion and advanced MIR nonlinear optical devices.