Superradiant lasing beyond single-mode cavities: mode competition and dipole–dipole interactions

When many atoms couple to a common electromagnetic reservoir, they can spontaneously synchronize their phases and enhance light emission. Under continuous pumping this gives rise to steady-state superradiance, the principle behind superradiant lasers, characterized by intense light with an ultranarrow spectrum. While well understood in single-mode cavities, the fate of superradiant lasing in multimode or cavity-less environments, with mode competition and dipole–dipole interactions, remains unclear. I will discuss how these features affect light intensity and spectrum in two settings: a ring cavity, with two competing modes, and a waveguide, where both mode competition and dipole–dipole interactions are relevant. In the ring cavity, competition yields a bistable steady state: in each decay realization, atomic phases synchronize into one mode, establishing robust global order, and the resulting linewidth shows trends similar to the single-mode superradiant laser with growing atom number. In contrast, in the waveguide, dipole–dipole interactions lead to the coexistence of distinct phase-ordered states that maximize emission into opposite directions, hindering line narrowing in far-field light. These results pave the way for assessing the feasibility of superradiant lasing in cavity-less platforms such as atomic arrays in free space, as well as for designing strategies to restore line narrowing even in the presence of dipole–dipole interactions.