Topologically Enhanced Giant Broadband Second-Harmonic Generation in Weyl Semiconductor Tellurium

Realizing strong nonlinear optical (NLO) responses in atomically thin layered materials is essential for the advancement of nanoscale photonic and on-chip integrated optoelectronic devices. However, the operation range of layered materials with large second-order susceptibility \(\:{\chi\:}^{\left(2\right)}\) is often limited to very narrow wavelength range, and reports in the mid-infrared (MIR) region are rare. The topological engineering of materials to enhance nonlinear optical response provides an alternative route for such purpose. In this study, we demonstrate that Weyl semiconductor tellurium (Te) nanoflakes exhibit giant second harmonic generation (SHG) responses over ultrabroadband infrared wavelength range (1.2–4.9 µm) including the challenging MIR wavelength, with a conversion efficiency two orders of magnitude greater than that of GaSe. The extracted \(\:{\chi\:}^{\left(2\right)}\) spectrum reveals a significant peak of 5.0 \(\:\pm\:\) 0.4 nm V ^-1 at 2.2-µm excitation and two shoulders, which can be clearly attributed to three different two-photon resonances with interband transitions in the vicinity of three different Weyl cones, consistent with the topological enhancement of SHG. The intrinsic giant, highly anisotropic, and ultrabroadband SHG response of Te nanoflakes promises unprecedented versatility and efficiency in MIR frequency conversion. Our results also highlight the potential of enhancing NLO responses by engineering Berry phase in topological materials and underscore the practical applications of Te nanoflakes in advanced MIR nonlinear optical devices.