Scattering singularities in Fano-resonant multilayer grating structure with phase-change materials

In non-Hermitian systems, the singularities refer to poles or zeros of the scattering and transfer matrices. Here, we investigate the kinds of singularities in a non-parity-time-symmetric multilayer structure which mainly consists of double layers of silver grating and one layer of phase-change material Ge₂Sb₂Te₅ (GST). When the GST is in its crystalline phase, a unidirectional zero-reflection singularity, i.e., exceptional point (EP) is achieved with the aid of Fano resonance. Interestingly, a coherent perfect absorber (CPA)-laser singularity adjacent to the EP is observed in the Fano peak. Meanwhile, a few CPA singularities exist at other wavelengths. By switching the GST from its crystalline to amorphous phase, the CPA-laser singularity will be absent, and the CPA singularity can be switched to a laser singularity. The scattering spectra near the singularities are sensitive to a tiny perturbation of the gain material and the environmental refractive index. The study is beneficial for making switchable active optical implementations.