  Tellegen medium has long been a topic of debate, with its existence being
contested over several decades. It was first proposed by Tellegen in 1948 and
is characterized by a real-valued cross coupling between electric and magnetic
responses, distinguishing it from the well-known chiral medium that has
imaginary coupling coefficients. Significantly, Tellegen responses are closely
linked to axion dynamics, an extensively studied subject in condensed matter
physics. Here, we report the realization of Tellegen metamaterials in the
microwave region through a judicious combination of subwavelength metallic
resonators, gyromagnetic materials, and permanent magnet discs. We observe the
key signature of the Tellegen response, i.e. a Kerr rotation for reflected
wave, while the polarization remains the same in the transmission direction.
The retrieved effective Tellegen parameter is several orders of magnitude
greater than that of natural materials. Our work opens door to a variety of
nonreciprocal photonic devices and may provide a platform for studying axion
physics.
