Bound States in the Continuum in Asymmetric Dielectric Metasurfaces

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 Gamma-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.