Introducing a Symmetry‐Breaking Coupler into a Dielectric Metasurface Enables Robust High‐Q Quasi‐BICs

Dielectric metasurfaces supporting quasibound states in the continuum (quasi‐BICs) exhibit very high‐quality factor resonances and electric field confinement. However, accessing high‐Q end of quasi‐BIC regime usually requires marginally distorting metasurface design from a BIC condition, pushing needed nanoscale fabrication precision to limit. This work introduces novel concept for generating quasi‐BICs, which strongly relaxes this requirement by incorporating relatively large perturbative element close high‐symmetry points an undistorted metasurface, acting as coupler radiation continuum. approach is validated adding ≈100 nm diameter cylinder between two reflection‐symmetry separated 300 gap elliptical disk unit cell, using gallium phosphide dielectric. It found that emerge when cylindrical placed at any position such symmetry points. metasurface's second harmonic generation capability optical range further explored. Displacing much full condition produces record‐breaking normalized conversion efficiencies >10 2 W −1 . The strategy enclosing disruptive multiple can be applied construct robust quasi‐BICs many geometrical designs.