Bound State in the Continuum with Dielectric Metasurface

Metasurfaces provide a realistic option for modifying electromagnetic wave parameters such as amplitude, phase, and polarization by customizing the meta-atom. Notably, dielectric metasurfaces have received a lot of attention in recent years due to their low radiative loss. In this work, we demonstrate the concept of bound state in the continuum (BIC) with the Si metasurface. By introducing the asymmetry factor and parameter optimization in our metasurface, we theoretically obtain a resonance with a quality factor (Q-factor) of 650 with a bi-radius four-unit structure. Experimentally, a Q-factor of 120 is successfully induced. We also examine the tunability of the q-BIC resonance with scaling factor. A broadband tunability without sacrificing much the Q-factor can be realized. In addition, by break both in-plane and out-of-plane symmetry, intrinsic chiral metasurfaces with strong circular dichroism (CD) at normal incidence can be realized. We propose a tilted Si structure to overcome the weak chiro-optical responses by inducing chiral bound states in the continuum. A CD close to 1 can be achieved with simulation and we also demonstrate a CD of 0.77 in the experiment. Similar concept can be extended to the visible range by incorporating TiO₂ material. Our studies with fundamental understanding of the BIC in the proposed structure support the potential application of the all-dielectric metasurface including sensing, emission control, and nonlinear optics.