Magnetization‐Induced Scattering Evolutions for Nonreciprocal Scatterers

Electromagnetic scatterers support quasi‐normal modes (QNMs), and when interacting with external sources, the scattered waves can be expanded as coherent superpositions of radiations by all QNMs excited. For reciprocal scatterers, if there exists a direction along which the QNM radiation polarizations are identical, for plane waves incident opposite to this direction, it is revealed that: i) the angular distributions of scattering intensities and polarizations are independent of the incident polarizations and ii) the total scattering strength (scattering cross section C sca ) is solely decided by the geodesic distance between two points on the unit sphere that represent respectively the QNM radiation polarization and the incident polarization: C sca ∝1 + cos ( g ). When reciprocal scatterers are extended to nonreciprocal magneto‐optical scatterers magnetized by external static magnetic fields, scatterings evolve under external magnetizations, with the aforementioned reciprocity‐secured principles being both broken by nonreciprocity. The study can potentially stimulate new explorations in magneto‐optics from the unique perspective of QNM excitations and radiations, which may empower disruptive applications based on nonreciprocal photonic elements and devices.