Enhanced terahertz third-harmonic generation in graphene-metal metasurface with bound states in the continuum

We propose a graphene-metal metasurface with bound states in the continuum (BICs) for the enhancement of terahertz (THz) third-harmonic generation (THG). With the adjustment of graphene Fermi level, the parameter-tuned BICs produced by the graphene-metal metasurface can make the transition to distinct quasi-BICs. It enables strong confinement of the localized field in monolayer graphene. By exploiting the large third-order nonlinearity of graphene, the efficient THG process at the quasi-BICs can be boosted under low pump threshold. With the 50 kW/cm2 of fundamental THz incidence, a THG conversion efficiency as high as ∼3% is achievable at the quasi-BICs regime. This renders the BIC metasurfaces very attractive for THz nonlinear photonics.