Multipole generalization of the Witten effect in Mie-resonant photonics

We present a generalization of the Witten effect on the case of oscillating multipole sources exciting a nonreciprocal sphere with an effective axion response. We find that the fields outside of the sphere are presented as a superposition of in-phase electric and magnetic multipoles, which is an indicator of T-violation characteristic for nonreciprocal bianisotropic media. In addition to the appearance of a cross-polarized component in the radiation, Mie resonances of the system hybridize with each other, exhibiting characteristic double peaks in Mie spectra observed especially clearly for higher-order multipole resonances. Given the spherical symmetry, this characteristic feature provides a sensitive probe of axion-type nonreciprocal responses in Mie-resonant photonics.