Non-Hermitian Control of Topological Scattering Singularities Emerging from Bound States in the Continuum

Leveraging topological properties in the response of electromagnetic systems can greatly enhance their potential. Although the investigation of singularity-based electromagnetics and non-Hermitian electronics has considerably increased in recent years in the context of various scattering anomalies, their topological properties have not been fully assessed. In this work, it is theoretically and experimentally demonstrated that non-Hermitian perturbations around bound states in the continuum can lead to singularities of the scattering matrix, which are topologically nontrivial and comply with charge conservation. The associated scattering matrix poles, zeros, and pole-zero pairs delineate extreme scattering events, including lasing, coherent perfect absorption, and absorber-lasers. The presented framework enables a recipe for generation, annihilation, and addition of these singularities in electric circuits, with potential for extreme scattering engineering across abroad range of the electromagnetic spectrum for sensing, wireless power and information transfer, polarization control, and thermal emission devices.