  Nonlinear metasurfaces offer new paradigm for boosting optical effect beyond
limitations of conventional materials. In this work, we present an alternative
way to produce pronounced third-harmonic generation (THG) based on nonlinear
field resonances rather than linear field enhancement, which is a typical
strategy for achieving strong nonlinear response. By designing and studying a
nonlinear plasmonic-graphene metasurface at terahertz regime with hybrid guided
modes and bound states in the continuum modes, it is found that a THG with a
narrow bandwidth can be observed, thanks to the strong resonance between
generated weak THG field and these modes. Such strong nonlinear field resonance
greatly enhances the photon-photon interactions, thus resulting in a large
effective nonlinear coefficient of the whole system. This finding provides new
opportunity for studying nonlinear optical metasurfaces.
