ENZ medium triggered collapse of Fano resonances and emergence of generalized nonreciprocal Kerker effects by subwavelength hybrid meta-atoms

By delicately designing the subwavelength "meta-atoms," the corresponding scattering properties can be flexibly engineered and one may provide more degrees of freedom for the implementation of custom-made light-matter interaction. In this paper, the magnetic medium loaded subwavelength hybrid meta-atoms (HMAs) are constructed, which exhibit nonreciprocal Fano resonance for the HMA immersed in air background due to the interference of Mie resonances associated with the angular momentum channels (AMCs) m = +1 as well as the combining interaction for the Mie resonances with the AMCs m = +1 and -2. The introduction of the epsilon-near-zero (ENZ) medium as the background medium results in the collapse of nonreciprocal Fano resonance due to the narrowed Mie resonances with much larger quality factors. As a consequence, the multipolar Mie resonances are superimposed solely with the uniform zeroth order mode, where two close intersections exist with one of them behaving as the nearly reciprocal Kerker effects showing the forward scattering and the other one behaving as the generalized nonreciprocal Kerker effects (GNKEs) showing the transverse unidirectional scattering. In addition, the operating frequency can be flexibly regulated by controlling the bias magnetic field, giving rise to the transformation between the reciprocal and nonreciprocal Kerker effects, as well as the frequency reversing of GNKEs with regard to the lower and higher order multipolar Mie resonances. In addition, the GNKE can even be modulated into the unidirectional scattering at nearly arbitrary direction with exceptional contrast by altering the permittivity of the ENZ background medium. By further considering magneto-optical materials the phenomenon can be expected at the optical regime and the research is also instructive for nonreciprocal photonics and potential applications.