Unbounded Tellegen Response in Media with Multiple Resonances

Tellegen response is a special type of nonreciprocal magneto-electric coupling which long remained elusive in photonics and extremely weak in condensed matter. It is widely accepted that the Tellegen coefficient is restricted by $\chi^2<\varepsilon\mu$, where $\varepsilon$ and $\mu$ are 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 giant Tellegen response.