Bose-Einstein condensation (BEC) of bosons, i.e. particles with an integer spin, is one of the most fascinating expressions of quantum degeneracy. This thermodynamic phase transition takes place when the de Broglie thermal wavelength λdB = (2p ħ2/m kBT)0.5 becomes comparable to the mean separation between particles. For dilute atomic gases, this critical situation has been recently achieved by lowering the temperature below 10-6 K. In solids, the quest for BEC has been mostly focused on semiconductor excitons for the past three decades. Excitons are composite bosons, consisting of bound electron-hole pairs akin to positronium, but with an effective mass on the order of the free electron mass. They are expected to undergo BEC at temperatures of a few Kelvin, but in spite of intense studies, no conclusive evidence has been reported up to now .
To realize BEC in a solid state system, we have concentrated our efforts on 2D exciton polaritons in semiconductor microcavities. These devices are planar Fabry-Perot cavities with embedded quantum wells. Their optical length is a half-integer multiple of the emission wavelength of quantum well excitons. Polaritons are eigenstates of such a microcavity system which result from the strong coupling between cavity optical modes and quantum well excitons . With respect to excitons, polariton mass and density of state are much smaller, typically by four orders of magnitude, which should favour quantum degeneracy at higher temperature and smaller density. This is a key advantage for polariton BEC since both excitons and polaritons are composite bosons, and behave as bosons only in the dilute density limit. On the other hand, the polariton lifetime is extremely short, on the picosecond time scale, as compared to microseconds measured for “indirect” excitons in coupled quantum wells, and several hundreds of picoseconds for acoustic phonon relaxation times. Thus polaritons cannot be in thermal equilibrium with the host lattice by phonon interaction.
In this presentation we will discuss a recent spectroscopic study at 5 K of a CdTe-based microcavity, using non-resonant and quasi-cw laser excitation. Various observations suggesting BEC of polaritons will be examined: thermalization and condensation, spontaneous linear polarization, temporal and spatial first order correlations.
 D.W. Snoke, When should we say we have observed Bose condensation of excitons ?, Phys. Stat. Sol. (b) 238, 389 (2003).
 C. Weisbuch et al.,Observation of the coupled exciton-photon mode splitting in a semiconductor quantum microcavity, Phys. Rev. Lett. 69, 3314 (1992); A. Kavokin and G. Malpuech, Cavity Polaritons, Elsevier,
Equipe CEA-CNRS “Nanophysique et Semiconducteurs”
Laboratoire de Spectrométrie Physique
Université J. Fourier, Grenoble