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.

半导体激光器在现代科学技术中无处不在，因为它们结构紧凑、速度快、效率高。它们需要相对较低的功率，因此非常适合信息技术中的应用。然而，在传统的半导体激光器中，达到激光阈值所需的功率具有一个基本的下限，该下限由达到粒子数反转所需的载流子密度决定，或透明度条件。这种限制可以在一种新型激光器——极化激元激光器中克服，它在不同的机制下工作。极化子激光器的相干光发射是由极化子凝聚体产生的，极化子凝聚体是一种相干的、热力学上有利的多体状态，其载流子密度比总体反转密度低得多。此外，由于极化子是通过强耦合形成的物质-光混合模式激子和腔光子，极化激子激光器可以通过光子和激子组件进行控制，从而在调谐和控制模式特性方面提供更大的灵活性。在过去的几十年里，这些前景推动了对极化子激光器的深入研究。在本文中，我们将首先回顾极化子和极化子激光器的基本特性，然后介绍具有非常规特性和功能的极化子激光器的最新进展，以及已证明室温极化子激光器的新材料平台。最后，我们将简要讨论极化子激光器的实际应用前景。