Laser under ultrastrong electromagnetic interaction with matter

The conventional picture of the light amplification by stimulated emission of radiation (laser) is broken under the ultrastrong interaction between the electromagnetic fields and matter, and distinct dynamics of the electric field and of the magnetic one make the "laser" qualitatively different from the conventional laser, which has been described simply without the distinction. The "laser" in the ultrastrong regime can show a rich variety of behaviors with spontaneous appearance of coherence. We found that the "laser" generally accompanies odd-order harmonics of the electromagnetic fields both inside and outside the cavity and a synchronization with an oscillation of atomic population. A bistability is also demonstrated in a simple model under two-level and single-mode approximations.