Toroidal dipole bound states in the continuum in asymmetric dimer metasurfaces

Structural symmetry plays a pivotal role in the emergence of symmetry-protected bound states in the continuum (BICs), often observed at the Γ-point within the first Brillouin zone. However, structural symmetry is not an absolute requirement for the formation of BICs at the Γ-point. In this work, we demonstrate that all-dielectric metasurfaces and photonic crystal slabs, made of dimer nanostructures with different sizes and shapes, can sustain BICs at the Γ-point. We show that the nature of these BICs is well preserved, irrespective of the size mismatch/difference, as long as the center-to-center distance between two nanodisks is equal to half of the lattice constants of a superunit cell. The BICs are transformed into quasi-BICs (QBICs) with finite quality (Q) factors by varying the interspacing of dimer nanodisks. Multipole decomposition indicates that this BIC is primarily governed by a toroidal dipole, with a secondary contribution from a magnetic dipole and magnetic quadrupole. Furthermore, we establish that such a BIC is robust against the shape of nanodisks. Notably, we observe that the Q-factor of QBICs for right nanodisks displaced along the y-axis is three orders of magnitude higher than those along the x-axis, suggesting an effective approach to realizing ultrahigh-Q resonances. Finally, we present an experimental demonstration of such a BIC by fabricating silicon dimer metasurfaces and photonic crystal slabs with dimer nanoholes. The trend of measured Q-factors and resonant wavelengths of QBICs shows good agreement with theoretical predictions. The maximum Q-factor is up to 22 633. These results not only advance our understanding of BICs within compound metasurfaces but also hold great promise in enhancing light-matter interactions.