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The purpose of this chapter is to introduce the concept of a random laser and show how strongly the properties of random lasers depend on the strength of light scattering in a gain medium. Random lasers, theoretically predicted forty years ago and experimentally demonstrated more than twenty years ago, have developed into a large area of research. It is not the goal of this chapter to provide a comprehensive review of all of the results and phenomena known in the field, many of which are discussed in Refs. 5-€“7. Instead, using a limited number of examples, I attempt to identify the major regimes of operation of random lasers and relate them to the strength of scattering or character of feedback. The chapter is organized as follows. The concept of regular lasers is briefly discussed in Sec. 12.2, and the idea of a laser with nonresonant feedback and a random laser are introduced in Sec. 12.3. Photon migration and localization in scattering media, their theoretical description, and applications to random lasers are discussed in Sec. 12.4. The experimental behavior of random lasers with nonresonant feedback, exemplified by neodymium random lasers, is described in detail in Sec. 12.5. Stimulated emission in ZnO random lasers with resonant feedback is presented in Sec. 12.6. Different strengths of scattering, regimes of the stimulated emission feedback, and their effect on the coherence of random laser operation are discussed in Sec. 12.7. Finally, the major regimes of the operation of random lasers are summarized in Sec. 12.8.
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