Robust ultrahigh- Q symmetry-protected quasi-bound states in the continuum and radiation unidirectionality enabled by stacked dielectric metasurfaces

Bound states in the continuum (BICs) are special optical modes embedded into the spectrum of free space waves but decoupled from the radiation continuum, being widely used to develop high quality ( Q ) factor photonic devices with different optical functionalities. Breaking the geometrical symmetry of optical systems is the most commonly used approach to access high- Q resonances termed symmetry-protected quasi-BICs. Nevertheless, the sharp decrease of the Q factor in the parameter space of asymmetries restricts the efficiency of resonators as functional devices. Here, using stacked dielectric metasurfaces, we enhance the robustness of symmetry-protected quasi-BICs against the symmetry breaking. In such designs, the coupling between a bright quasi-BIC and a dark BIC leads to dramatic enhancement of the Q factor in a wide range of asymmetry parameters, with values 4 or even 8 times as high as those obtained from conventional designs. Furthermore, engineered interlayer distance induces unidirectional quasi-BIC resonances that can continuously exist across nearly the entire parameter space, making our designs a potentially attractive platform for ultranarrow-band perfect absorption. We envision this kind of system of stacked metasurfaces as a prospective platform for suppressing out-of-plane radiation losses and radiation manipulation.