Multi-frequency and wide-angle nonreciprocity arising from non-uniform magnetic field

We investigate the effect of non-uniform external static magnetic field on the scattering nonreciprocity of a deep-subwavelength gyrotropic particle system. Based on a scattering matrix approach, we show analytically that asymmetrical extinction cross section of the scattering system for opposite illumination directions can be accessed at multi-frequency and wide-angle regimes, in the presence of magnetic field with weak spatial non-uniformity. A simplified dual-dipole model is further employed to capture the generic physics of nonreciprocal scattering, essentially attributed to the asymmetrical magneto-electric cross interactions. In addition, typical examples are provided with full-wave numerical verifications. Our finding may open a straightforward and efficient pathway to manipulate nonreciprocal scattering at the subwavelength scale.