Bound states in the continuum in all-dielectric metasurfaces with scaled lattice constants

Bound states in the continuum (BICs) have emerged as an efficient tool for trapping light at the nanoscale, promising several exciting applications in photonics. Breaking the structural symmetry has been proposed as an effective way of exciting quasi-BlCs (QBICs) and generating high-Q resonances. Herein, we demonstrate that QBICs can be excited in an all-dielectric metasurface by scaling the lattice of the metasurface, causing translational symmetry breaking. The corresponding BICs arise from band folding from the band edge to the Gamma point in the first Brillouin zone. Multipole analysis reveals that the toroidal dipole dominates these QBICs. Furthermore, scaling the lattice along different directions provides additional freedom for tailoring QBICs, enabling polarization-dependent or -independent QBICs. In addition, this allows the realization of two QBICs at different wavelengths using plane-wave illumination with different polarizations on the metasurface. We experimentally demonstrated the existence of these BICs by fabricating silicon metasurfaces with scaled lattices and measuring their transmission spectra. The vanished resonant linewidth identifies BICs in the transmission spectrum, and the QBICs are characterized by high-Q Fano resonances with the Q-factor reaching 2000. Our results have potential applications in enhancing light-matter interaction, such as laser, nonlinear harmonic generation, and strong coupling.