Graphene-tuned plasmonic metasurface based on split-ring resonators for enhanced second harmonic generation in the mid-infrared

The second harmonic generation (SHG) plays a crucial role in a wide range of optical and photonic applications. In this study, we propose and optimize a novel plasmonic nanoantenna based on a split-ring resonator (SRR) configuration engineered to enhance SHG efficiency. The designed metasurface consists of a quadruple SRR arrangement, in which both near-field and far-field responses confirm the presence of strong second-harmonic radiation. The orthogonal orientation of the SRR elements enables polarization-orthogonal field coupling, resulting in SHG resonances at 60 THz and 130 THz. Due to the symmetric layout of the nanoparticle array, the structure exhibits polarization-independent behavior. Furthermore, a graphene layer is integrated into the metasurface to provide dynamic tunability of the resonance frequencies through adjustment of its chemical potential. To validate the effectiveness of the SRR configuration, a comparison is made with an array of rectangular nanoparticles lacking split gaps. The results indicate that the introduction and placement of gaps are essential for controlling the near-field localization and the corresponding far-field radiation pattern. These engineered gaps facilitate the excitation and manipulation of dipolar and quadrupolar plasmonic modes, thereby enabling and enhancing SHG. Overall, the proposed metasurface demonstrates the ability to tailor nonlinear optical responses through geometric design and material integration.