Optically-Excited Polariton Condensates

The observation of polariton condensation with the help of high-quality optical microcavities was a long sought goal in solid-state research. Optical pumping of planar microresonator structures consisting of multiple quantum wells as active region offers a common and direct path towards exciton–polariton studies. After obtaining spectral proof of polaritons in the linear regime, which describes the response at low-density excitation, the behaviour of a polariton gas at increased particle densities gets naturally into the focus, where stimulated scattering is expected to set in. The demonstration of a condensate is typically linked to signatures such as a macroscopic ground-state occupation, a change in the photon statistics and a spontaneous build-up of long-range order evidenced as spatial coherence. Indeed, to establish the link to BEC, all aspects must be carefully investigated. These topics are subject of this chapter, which concludes with examples of condensates at elevated temperature, polariton systems with special features and superfluidity studies on polariton condensates.