  Resonant metasurfaces have received extensive attention due to their sharp
spectral feature and extraordinary field enhancement. In this work, by breaking
the in-plane symmetry of silicon nanopillars, we achieve a sharp Fano
resonance. The far-field radiation and near-field distribution of metasurfaces
are calculated and analyzed to further uncover the resonant performance of
metasurfaces. Moreover, the theoretical derivation and simulation exhibit an
inverse quadratic dependence of Q-factors on asymmetry parameters, revealing
that the resonance is governed by the symmetry-protected bound states in the
continuum. Finally we experimentally demonstrate the sharp resonance, and
employ it to effciently boost the third-harmonic generation. This enhancement
can be attributed to the strong optical intensity enhancement inside the
metasurface.
