  Strong coupling between cavity photons and various excitations in condensed
matters boosts the field of light-matter interaction and generates several
exciting sub-fields, such as cavity optomechanics and cavity magnon polariton.
Axion quasiparticles, emerging in topological insulators, were predicted to
strongly couple with the light and generate the so-called axion polariton.
Here, we demonstrate that there arises a gapless level attraction in cavity
axion polariton of antiferromagnetic topological insulators, which originates
from a nonlinear interaction between axion and the odd-order resonance of
cavity. Such a novel level attraction is essentially different from
conventional level attractions with the mechanism of either a linear coupling
or a dissipation-mediated interaction, and also different from the level
repulsion induced by the strong coupling in common polaritons. Our results
reveal a new mechanism of level attractions, and open up new roads for
exploring the axion polariton with cavity technologies. They have potential
applications for quantum information and dark matter research.
