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

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.