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

Dielectric metasurfaces supporting quasibound states in the continuum (quasiBICs) exhibit very high-quality factor resonances and electric field confinement. However, accessing the high-Q end of the quasi-BIC regime usually requires marginally distorting the metasurface design from a BIC condition, pushing the needed nanoscale fabrication precision to the limit. This work introduces a novel concept for generating high-Q quasi-BICs, which strongly relaxes this requirement by incorporating a relatively large perturbative element close to highsymmetry points of an undistorted BIC metasurface, acting as a coupler to the radiation continuum. This approach is validated by adding a approximate to 100 nm diameter cylinder between two reflection-symmetry points separated by a 300 nm gap in an elliptical disk metasurface unit cell, using gallium phosphide as the dielectric. It is found that high-Q resonances emerge when the cylindrical coupler is placed at any position between such symmetry points. This metasurface's second harmonic generation capability in the optical range is further explored. Displacing the coupler as much as a full diameter from a BIC condition produces recordbreaking normalized conversion efficiencies > 102W(-1). The strategy of enclosing a disruptive element between multiple high-symmetry points in a BIC metasurface can be applied to construct robust high-Q quasi- BICs in many geometrical designs.

Automated summary

The dielectric metasurfaces supporting high-Q quasi-bound states in the continuum require high fabrication precision, which can be relaxed by incorporating a relatively large perturbative element close to high-symmetry points of an undistorted BIC metasurface. This approach has been validated by adding a cylinder between two reflection-symmetry points in an elliptical disk metasurface unit cell, resulting in high-Q resonances.