Unbounded Tellegen response in media with multiple resonances

The Tellegen response is a special type of nonreciprocal magnetoelectric coupling which has long remained elusive in photonics and is extremely weak in condensed matter. It is widely accepted that the Tellegen coefficient is restricted by χ²<εμ, where ε and μ are the permittivity and permeability of the material. Here, we demonstrate that this restriction is lifted in the medium with several close resonances, which provides a theoretical foundation for a giant Tellegen response and enhanced nonreciprocal phenomena.