Anisotropic and tunable third-harmonic generation in hybrid black phosphorus–grating metasurfaces

Abstract

We numerically explore third-harmonic generation (THG) in anisotropic black phosphorus (BP) metasurfaces engineered with dielectric grating structures and gold backed reflector, operating in the terahertz (THz) regime. The design supports strong plasmon-polariton resonances and Fabry–Pérot (FP) cavity effects, leading to substantial local field enhancement and efficient nonlinear response. In the armchair (AC) direction, the THG output reaches up to 2.3 W at resonance, with a notable reduction as the d_g (grating depth) increases. The zigzag (ZZ) orientation exhibits lower output power, reflecting BP’s inherent optical anisotropy. This reduction originates from the detuning of the hybrid cavity–plasmon resonance as the grating becomes deeper, since the cavity mode red-shifts while the intrinsic BP plasmon frequency remains nearly fixed. Moreover, the local field maximum shifts deeper inside the metallic grating, reducing the spatial overlap between the enhanced field and the BP layer at the top surface. These results demonstrate the potential of BP-based grating metasurfaces for tunable and high-efficiency nonlinear photonic devices in the far-infrared and THz domains.