Recent developments on polariton lasers

Semiconductor lasers are ubiquitous in modern science and technology for they are compact, fast, and efficient. They require relatively low power and thus are well suited for applications in the information technology. However, in conventional semiconductor lasers, the power required to reach the lasing threshold has a fundamental lower bound determined by the carrier density required to reach population inversion, or the transparency condition. This limitation can be overcome in a new type of laser, a polariton laser, which operates under a different mechanism. Coherent light emission from a polariton laser results from a polariton condensate, which is a coherent, thermodynamically favored many-body state, formed at a much lower carrier density than the population inversion density. Furthermore, since polaritons are matter-light hybrid modes formed via strong coupling between excitons and cavity photons, polariton lasers can be controlled via both the photon and exciton components, allowing greater flexibility in tuning and controlling the mode properties. These prospects have propelled intense research effort on polariton lasers in the past few decades. In this article, we will first review the essential properties of polaritons and polariton lasers, followed by recent developments on polariton lasers with unconventional properties and functionalities, and on new material platforms where room temperature polariton lasers have been demonstrated. We will conclude with a brief discussion on prospects of practical applications of polariton lasers.