We present strong enhancement of third harmonic generation in an amorphous silicon metasurface consisting of elliptical nano resonators.We show that this enhancement originates from a new type of multi-mode Fano mechanism.These‘Super-Fano’resonances are investigated numerically in great detail using full-wave simulations.The theoretically predicted behavior of the metasurface is experimentally verified by linear and nonlinear transmission spectroscopy.Moreover,quantitative nonlinear measurements are performed,in which an absolute conversion efficiency as high as η_(max)≈2.8×10^(-7) a peak power intensity of 1.2 GW cm^(-2) is found.Compared to an unpatterned silicon film of the same thickness amplification factors of up to ~900 are demonstrated.Our results pave the way to exploiting a strong Fano-type multi-mode coupling in metasurfaces for high THG in potential applications.
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