Polariton lasers. Hybrid light-matter lasers without inversion

Polariton lasers are coherent emitters in which the fundamental constituents are not photons amplified by a gaining medium but hybrid, part exciton and part photon, quasi-particles named polaritons. In this review we discuss some of the main topics in the field of polariton lasing: we start from an introduction to the concepts of strong coupling regime and polaritons, we then discuss the mechanism of polariton lasing and the main difficulties in achieving it. Some of the main results on polariton lasing reported in the literature, from 2D samples to confined structures, are then reviewed. This latter case will allow us to discuss some of the peculiarities of polariton lasing with respect to traditional lasers. Polariton lasing mostly occurs at cryogenic temperatures, but we will see that it can also be observed at room temperature with a proper choice of materials. To conclude, we will discuss some perspectives for the field.