Enhancing third-order harmonic generation in a graphene-gold grating structure with MgF₂ substrate at terahertz frequencies

Abstract In this work, we investigate the third-order harmonic generation (THG) in graphene using plasmonic structures based on gold gratings with an MgF 2 substrate. Graphene stands out as a key material in this process due to its significant third-order nonlinear properties and flexibility in tuning Fermi energy. The use of a gold grating as a plasmonic structure significantly enhances the optical field near the graphene surface in the terahertz frequency range, thereby improving THG efficiency. In this configuration, the incident fundamental light is confined and amplified along the graphene surface, a phenomenon attributed to the generation of highly localized surface plasmon resonances. Furthermore, we comprehensively examine the tuning of structural parameters, such as Fermi energy, gold grating height and width, input intensity, and incident angle, which results in a shift of the THG peak frequencies toward higher values and enhances system performance. Additionally, the input light intensity nonlinearly influences the THG output intensity, a factor of critical importance for advanced photonic applications. Our results demonstrate a 440-fold enhancement in THG intensity at 31.8 THz with an optimized Fermi energy of 0.4 eV. High potential for application in optical systems such as adjustable light sources, ultrafast optical switches, and optical sensors is demonstrated by this hybrid graphene-gold grating structure. Given the favorable response of this structure to control parameters and its significant enhancement of the field, ultimately leading to improved conversion efficiency, such systems can serve as promising platforms for the development of emerging and photonic technologies in the future.