An Axion Collider*

Axions, originally proposed to solve the strong CP problem in quantum chromodynamics and now a leading dark matter candidate, have proven elusive experimentally. However, condensed matter analogs of axions appear naturally in some magnetic topological insulators. We propose a novel setup involving counter-propagating electromagnetic waves in a system with dynamical axions, dubbed the `axion collider', and show that the counter-propagating waves are able to resonantly excite the dynamical axion. The dynamical axions not only contribute to the energy loss in the system, which is non-linear in the applied field intensity and strongly dependent upon the relative polarizations of the counter-propagating waves, but provide a finite phase shift to the poynting vector and induce energy transfer between the two electromagnetic waves. We further present a microscopic model for bilayer MnBi₂Te₄, a dynamical axion insulator, and derive the axion coupling as well as the dynamic axion stiffness and mass.