Condensation of Cavity Polaritons in a Disordered Environment

A model for direct two band excitons in a disordered quantum well coupled to light in a cavity is investigated. In the limit in which the exciton density is high, we assess the impact of weak `pair-breaking' disorder on the feasibility of condensation of cavity polaritons. The mean-field phase diagram shows a `lower density' region, where the condensate is dominated by electronic excitations and where disorder tends to close the condensate and quench coherence. Increasing the density of excitations in the system, partially due to the screening of Coulomb interaction, the excitations contributing to the condensate become mainly photon-like and coherence is reestablished for any value of disorder. In contrast, in the photon dominated region of the phase diagram, the energy gap of the quasi-particle spectrum still closes when the disorder strength is increased. Above mean-field, thermal, quantum and fluctuations induced by disorder are considered and the spectrum of the collective excitations is evaluated. In particular, it is shown that the angle resolved photon intensity exhibits an abrupt change in its behaviour, going from the condensed to the non-condensed region.Comment: 17 pages, 9 eps figures; published versio

Models of coherent exciton condensation

That excitons in solids might condense into a phase-coherent ground state was proposed about 40 years ago, and has been attracting experimental and theoretical attention ever since. Although experimental confirmation has been hard to come by, the concepts released by this phenomenon have been widely influential. This tutorial review discusses general aspects of the theory of exciton and polariton condensates, focussing on the reasons for coherence in the ground state wavefunction, the BCS to Bose crossover(s) for excitons and for polaritons, and the relationship of the coherent condensates to standard lasers.Comment: 27 pages, 6 figures. Submitted for a special issue of J. Phys. Cond. Matt. associated with the EU network "Photon-mediated phenomena in semiconductor nanostructures