Coherent amplification from multiple scattering in a disordered gain medium
In a random laser, photons scatter from disordered particles while experiencing gain from the active medium.
Below threshold: photons diffuse out (D ~ l*c/3). Above threshold: gain exceeds losses — photons amplified
via stimulated emission, creating coherent lasing modes shaped by disorder. The threshold condition is
g·l_gain = 1 where g is gain coefficient. Multiple scattering provides feedback replacing mirrors.