Complexity and coherence in Random Lasers

In a nutshell, a random laser (RL) is a disordered amplifying cavity emitting coherent radiation. In the early developments, the theoretical framework at the basis of the RL has relied on light diffusion and localization. Various authors also reported on the emerging of many coupled oscillation modes while increasing the pump energy and the consequent non-trivial dynamics of the resulting optical signals. Coupling of modes were also reported. When considered from a semi-classical perspective, a multi-mode random laser strikingly displays those ingredients which are typical of the physics of complexity: i.e., randomness and nonlinearity. The latter is due to typical mode-interaction processes, like mode-competition and mode-locking, which have been studied since the beginning of laser theory.