Manipulation of the multiple bound states in the continuum and slow light effect in the all-dielectric metasurface

All-dielectric metasurface with ultra-high quality resonances underpinned by bound states in the continuum (BICs) have attracted lots of attention in recent years for they enable new methods of wavefront control and light focusing. We study a metasurface composed of one transverse nanohole (TNs) and two identical vertical nanoholes (VNs) in one lattice, which supports both symmetry-protected and accidental BICs (at-& UGamma; and off-& UGamma; BICs). Based on the destructive interference between the surface states from the TN element and the identical VNs element, two at-& UGamma; BICs emerge, and they turn into quasi-BICs by rotating the electric field polarization direction of the incident plane wave from x to y. The off-& UGamma; BICs come from destructive interference from different radiation channels, which are influenced by the in-plane structural parameters symmetry insignificantly. Two at-& UGamma; BICs and one off-& UGamma; BIC of the metasurface all have ultra-high Q-factors (exceeding 10(6), 10(4), and 10(6), respectively), which means much in the application of biosensors. Especially, this nanostructure has outstanding ultra-slow light properties at BICs, with a group index about 10(6), which underpin a new generation of flat-optics slow light devices.