Steering high-Q intrinsic chiral quasi-bound states in the continuum via engineered 2.5D phase-change metasurfaces.

High-Q intrinsic quasi-bound states in the continuum (QBICs) require three-dimensional (3D) geometries with both in-plane and out-of-plane mirror symmetry breakings, hindering practical implementations due to the complex architectures. Here we demonstrate that high-Q intrinsic QBICs can be flexibly controlled by using the engineered 2.5D phase-change metasurfaces of GeSbTe (GST). By introducing the additional out-of-plane perturbations of slant angle θ and azimuthal angle φ, highly efficient and high-Q intrinsic circular dichroism (CD) for both reflection and transmission can be realized. The spinning-selected magnetic dipole (MD) is responsible for the high-Q intrinsic chirality. The high-Q intrinsic CD is robust to the variation of structural parameters, and its Q-factor and resonance location can be tuned through the phase transition of GST.