Enhanced terahertz third-harmonic generation at critical coupling by the quasi-bound states in the continuum

We propose a metasurface framework for enhancing terahertz (THz) third-harmonic generation (THG) using quasi-bound states in the continuum (quasi-BICs) at critical coupling. The THz nonlinear metasurface comprises a silicon rod dimer structure integrated with monolayer graphene. By introducing an out-of-plane perturbation to break the structural symmetry, the symmetry-protected BICs supported by the silicon rod dimer metasurface are converted into quasi-BICs. The integration of monolayer graphene enables the system to satisfy critical coupling between radiative and nonradiative processes; the quasi-BICs achieve theoretical maximum absorption, thereby significantly enhancing the THz THG response. A high THG conversion efficiency of 0.3 is achieved at the quasi-BICs under an incident intensity of 50 kW/cm². This approach demonstrates the potential of metasurfaces for on-chip high-efficiency nonlinear optical devices in the THz regime and can be extended to other loss-dominated systems.