Nonlinear graphene metasurface to enhance third harmonic generation at terahertz frequencies

Nonlinear processes have an extremely weak nature at terahertz (THz) frequencies, which is detrimental to several potential applications, such as nonlinear THz spectroscopy and noninvasive THz imaging. To alleviate this severe limitation, we propose an ultrathin nonlinear THz metasurface composed of patterned graphene micro-ribbons to significantly enhance third harmonic generation (THG). The nonlinear THG conversion efficiency can be increased by several orders of magnitude benefiting from the large nonlinear conductivity of graphene, as well as, the enhancement of the incident THz field due to the excitation of localized plasmons. In addition, the nonlinear response of the proposed metasurface can be dynamically tuned by varying the Fermi energy of graphene without changing its geometry. The proposed nonlinear metasurface provides a new way to realize compact and efficient nonlinear chip-scale THz devices that can be used as tunable and adaptive THz sources or wave mixers.