Bound states in the continuum from a symmetric mode with a dominant toroidal dipole resonance

Bound states in the continuum (BICs), both the symmetry-protected and accidental ones, can be realized in the band of antisymmetric mode in a nanoparticle chain as long as the particles radiate exactly out of phase pairwisely. However, for the symmetric mode, BICs cannot form in this way. Here, we show that accidental BICs can be generated from the symmetric mode, as exemplified by a toroidal dipole mode-induced BIC in a trimerized system. This type of BIC is fully studied by an eigenmode analysis as well as multipole decomposition. It is also demonstrated that the BICs correspond to certain phase singularity of the system, indicating the robust existence of the BICs ensured by their topological nature. The combination of BICs and toroidal dipole resonance may find applications in lasing and subwavelength waveguiding.