Graphene Nanosheet-Gold Nanopatch Metasurfaces for Nonlinear Response at THz Frequencies

Graphene's exceptional nonlinear optical properties combined with resonant photonic structures offer a promising pathway for efficient nonlinear applications at terahertz (THz) frequencies. In this work, we propose and demonstrate a fabrication-friendly hybrid nonlinear metasurface composed of gold patches integrated with uniform graphene, circumventing the need for complex graphene patterning. The structure supports strong localized resonances that enhance nonlinear interactions. By exploiting resonant enhancement at both the fundamental and third harmonic frequencies, we predict via simulations third-harmonic generation efficiencies as high as -15 dB (3.2%) under continuous-wave excitation at modest intensities (0.1 MW/cm²). The metasurface is fabricated via electron-beam lithography and experimentally characterized under pulsed excitation using THz time-domain spectroscopy. The broadband excitation spectrum prevents unambiguous isolation of a third-harmonic signal; however, we experimentally observe pronounced nonlinear frequency shifts up to 0.5 THz (12.5% fractional change), driven by self-phase modulation, consistent with simulation results. Our findings highlight the potential of tailored graphene-based metasurfaces for efficient nonlinear THz photonic devices.