Broadband and efficient third-harmonic generation from black phosphorus–hybrid plasmonic metasurfaces in the mid-infrared

Black phosphorus (BP), with a mid-infrared (MIR) bandgap of 0.34 eV, presents itself as a promising material for MIR nonlinear optical applications. We report the realization of MIR third-harmonic generation (THG) in both BP and BP-hybrid plasmonic metasurfaces (BPM). BP exhibits a high third-order nonlinear susceptibility (${\chi }^{\left(3\right)}$) exceeding 10⁻¹⁸ m²/V² in the MIR region with a maximum value of 1.55 × 10⁻¹⁷ m²/V² at 5000 nm. The BP flake achieves a THG conversion efficiency of 1.4 × 10⁻⁵, surpassing that of other 2D materials by over one order of magnitude. To further enhance this nonlinear performance, a BPM is designed and fabricated to achieve a two-order-of-magnitude enhancement in THG, leading to a record conversion efficiency of 6.5 × 10⁻⁴, exceeding the performance of previously reported metasurfaces by more than one order of magnitude. These findings establish BP as a promising platform for next-generation MIR nonlinear optical devices.