Perfect Nonreciprocal Axion-polaritons

Dynamical axion describes a massive axion quasiparticle that arises from fluctuations of the magnetic order parameter. These fluctuations couple to electromagnetic fields, forming axion-polaritons -- collective modes that are physically magnon-polaritons. We show that under appropriate static external electric and magnetic fields, axion-polaritons acquire a nonreciprocal dispersion, $\omega(\textbf{k}) \neq \omega(-\textbf{k})$, arising intrinsically from direction-dependent axion-photon coupling. Strikingly, we identify a regime of \textit{perfect nonreciprocity} in which the photon propagating in one direction is completely decoupled from the axion while the counter-propagating photon hybridizes strongly. Furthermore, the nonreciprocal dispersion manifests as an optical isolator -- a device transmitting light preferentially in one direction. Our results establish nonreciprocal axion-polaritons as a powerful probe of axion quasiparticles and suggest experimentally accessible routes for their detection.