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A Random Laser (RL) is a laser in which optical feedback is provided by scattering particles. As in conventional lasers, a gain medium is required for optical amplification. However, opposite to Fabry-Perot cavities and Distributed FeedBack lasers, no reflective surfaces neither distributed periodic structures are used in RLs, as light is confined in active region by diffusive elements, that can be or not spatially distributed inside the gain medium.

Random lasing have been reported from a large variety of materials, e.g. colloidal solutions of dye and scattering particles, semiconductor powders , optical fibers and polymers. Due to the output emission with low spatial coherence and laser-like energy convertion efficiency, RLs are attractive devices for energy efficient illumination application.

Hystory
The first proposal of a lasing device in which optical feedback was provided by a scattering medium appeared shortly after the first demonstration of the Ruby laser: a gain medium was placed between a reflective mirror and a scattering surface and the "non-resonant" feedback was intended to provide emission only related to the active material energy transitions. Some years later, researchers obtained lasing action from a solution of dye and highly scattering particles of titanium dioxide. The emission they observed was characterized by a single peaked spectrum with few nanometers linewidth. Due to the absence of sharper resonances, typical of classical resonators, this kind of emission was named as "non-resonant", or "incoherent", or "amplitude-only" emission, considering that no phase correlation was involved into the lasing mechanism. After this result, another kind of emission from a scattering active medium was observed. Sharp peaks with sub-nanometer linewidths and randomly placed in frequency were observed from optically pumped, nanocristals of zync oxide. The term random laser appeared after these results and this kind of emission was labelled as "resonant", or "coherent", random lasing, as the narrow modes

Principle of operation
Different theoretical frameworks have been proposed for understanding random lasing action, however