Tunable third-harmonic generation in black phosphorus nanoribbon in the terahertz frequency

Black phosphorus (phosphorene) exhibits strong nonlinear optical properties, making it a promising candidate for applications such as optical switching, low-noise amplification, and harmonic generation. However, these effects are typically weak and require high input power. To overcome this limitation, we propose what we believe to be a novel ultrathin metasurface design based on black phosphorus to enhance third-harmonic generation (THG) in the mid-infrared and terahertz (THz) regimes. The structure supports highly localized plasmon-polaritons due to the anisotropic crystal properties of black phosphorus, leading to strong field confinement along its surface. Introducing a metallic substrate beneath the black phosphorus further sharpens the resonance, resulting in zero transmission and standing wave formation within the intermediate dielectric spacer. This configuration significantly boosts the local field and enhances THG efficiency. Finite-element method (FEM) simulations demonstrate field enhancements up to 224-fold in the armchair direction and 125-fold in the zigzag direction for optimized nanoribbon widths. Peak THG powers of 5.1 W and 3.9 W are achieved at resonant frequencies of 10.9 THz and 10.3 THz, respectively. This tunable nonlinear response is achieved without altering the device geometry. The proposed metasurfaces offer a compact and efficient platform for nonlinear wave generation and could enable advanced applications in THz spectroscopy, imaging, and optoelectronics.