Bound States in the Continuum in Asymmetric Dielectric Metasurfaces

Abstract It is well established that for symmetry‐protected bound states in the continuum (BICs), introducing the broken geometry symmetry in a dielectric metasurface transforms such a BIC into a quasi‐BIC (QBIC) with high‐quality factor (Q‐factor). Typically, the smaller the asymmetry parameter, the larger the Q‐factor. However, it is very challenging to fabricate such nanostructures with an ultra‐small asymmetry parameter, thus limiting the measured Q‐factor of QBIC. In this work, the authors demonstrated that BICs can be sustained at Γ‐point in an asymmetric dielectric metasurface, whose unit cell is composed of a dielectric cuboid with an off‐centre hole inside it. Multipole decompositions and near‐field distributions indicate that the toroidal dipole dominates the nature of such a QBIC. Furthermore, the authors found that such a BIC is robust against the shape of the hole. Besides, two BICs at different wavelengths can be achieved by applying either a rectangular hole or a rectangular lattice. Finally, the authors presented experimental verifications of BIC types by fabricating asymmetric silicon metasurfaces. Measurement results show that the Q‐factor of QBIC can reach almost 5,000. The results may enrich the library of BICs and find exciting applications in developing high‐performance photonics devices, such as nanolasers, biosensors and enhanced nonlinear harmonic generation.