Second harmonic generation and third harmonic enhancement in silicon by a composite metasurface and intense terahertz

The compression and integration of nonlinear optical processes to the nanoscale are expected to have significant implications for quantum optics, biology, and medicine. In this work, a composite metasurface consisting of a hollow-bow-tie-shaped metal metasurface and a patterned amorphous silicon metasurface is proposed. An external terahertz (THz) electric field enhanced by the hollow-bow-tie structure is employed to break the centrosymmetry, which allows the generation of optical second harmonic. Meanwhile, the amorphous silicon nanocolumns inside the hollow-bow-tie-shaped structure enhance the optical laser field and improve the conversion efficiency of second and third harmonics. The numerical results indicate that at an incident optical laser intensity of 1.20 GW/cm² and a y-component of the incident THz electric field of 10 kV/cm, the powers of the generated second and third harmonics are 3.96 × 10^-6 W and 7.42 × 10^-6 W, respectively, with conversion efficiencies of 3.93 × 10^-7 and 7.18 × 10^-7. The conversion efficiency of the third harmonic is increased by a factor of 700 compared to that of unpatterned silicon. The configuration proposed in this paper offers a solution for applications that need to take into account both high nonlinear efficiency and ultra-fast tunability.