Merging bound states in the continuum by harnessing higher-order topological charges

Bound states in the continuum(BICs)can confine light with a theoretically infinite Q factor.However,in practical on-chip resonators,scattering loss caused by inevitable fabrication imperfection leads to finite Q factors due to the coupling of BICs with nearby radiative states.Merging multiple BICs can improve the robustness of BICs against fabrication imperfection by improving the Q factors of nearby states over a broad wavevector range.To date,the studies of merging BICs have been limited to fundamental BICs with topological charges±1.Here we show the unique advantages of higher-order BICs(those with higher-order topological charges)in constructing merging BICs.Merging multiple BICs with a higher-order BIC can further improve the Q factors compared with those involving only fundamental BICs.In addition,higher-order BICs offer great flexibility in realizing steerable off-T merging BICs.A higher-order BIC at F can split into a few off-T fundamental BICs by reducing the system symmetry.The split BICs can then be tuned to merge with another BIC,e.g.,an accidental BIC,at an off-Γpoint.When the in-plane mirror symmetry is further broken,merging BICs become steerable in the reciprocal space.Merging BICs provide a paradigm to achieve robust ultrahigh-Q resonances,which are important in enhancing nonlinear and quantum effects and improving the performance of optoelectronic devices.