Optical scattering has long been considered detrimental to laser as it induces additional loss. However, strong scattering increases the path length or dwell time of light inside the gain medium, thus enhances optical amplification. We showed that laser cavities could be formed through the process of light scattering and interference in random media. In contrast to the conventional lasers whose cavities are formed by mirrors, cavities of random lasers are "self-formed" in disordered materials. Therefore, random laser is also called mirrorless laser. Optical scattering not only provides feedback for lasing, but also leads to spatial confinement of light on the micro-scale. We invented a new type of microlaser that is made of disordered medium. The fabrication of such micro random laser is much easier and cheaper than that for most microlasers. The random lasers have many potential applications, e.g., optical tagging and encoding, multi-color display, bio-chemical sensing, tumor detection, photodynamic therapy.

A micro random laser. (a) Scanning electron micrograph of a micron-sized cluster of ZnO nanoparticles. Optical excitation of ZnO nanoparticles leads to lasing in the cluster. (b) Spatial distribution of laser emission in the cluster. (c) Intensity of emission from the ZnO cluster as a function of the incident pump pulse energy, showing the threshold behavior. (d) The emission spectrum with two lasing peaks.