Strong electric field enhancement near an amorphous silicon metasurface with non-vertical symmetry.

In this work, we propose a novel method to our knowledge for realizing a leakage mode with a super flat band through the modulation of two Friedrich-Wintgen bound states in the continuum (FW BICs) of a non-vertical symmetry metasurface. This unique mode is formed by partially breaking the BIC generated from the first excited collective magnetic dipole (MD) resonance in a sub-diffraction periodic system, which exhibits a low group velocity of 16 731 m s, while its quality factor ( factor) is 217. We calculate that the near electric field enhancement reached 808 times at an incident angle of 9°22'. Our findings provide unique opportunities for realizing strong-enhanced nonlinear effects in metasurfaces.