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Exciton-polaritons are mixed light-matter quasi-particles arising from the strong coupling between photons and excitons in a micrometer sized cavity with embedded quantum wells. They have been studied extensively since the discovery of strong light-matter coupling in these systems in 1992 [1].**

Polaritons are bi-dimensional composite bosons that can exhibit macroscopic quantum coherence effects at high temperatures (5-300K) due to their very low mass (~10-4 times that of the electron, inherited from their photonic component). In particular, polaritons behave as a quantum fluid with specific properties coming from their out of equilibrium nature, determined by their short lifetime (few picoseconds).

In this talk I will first discuss the observation of the superfluid propagation [2, 3] of a polariton fluid directly created by resonant laser excitation in a InGaAs semiconductor microcavity. The superfluidity manifests itself by the suppression of the Rayleigh scattering on the natural defects present in the microcavity when the speed of fluid is less than the speed of sound. In the opposite case, when the flow is supersonic, the Cerenkov regime is clearly observed.

These findings are in excellent quantitative agreement with a generalized Gross- Pitaevskii theory allowing the description of the polariton superfluidity in terms of the Landau criterion, originally developed to explain the results obtained with liquid helium and recently employed to demonstrate the superfluidity in atomic BECs.

A remarkable situation is realised when the polariton superfluid flows with high speed against an obstacle whose size is larger than the superfluid healing length: the superfluidity is then broken and a rich variety of phenomena like as quantised vortex and soliton formation are expected. I will present our recent results demonstrating the transition from the superfluid regime to the turbulent vortex generation and dark soliton nucleation due to the interaction of a polariton fluid with a defect [4, 5].

All these results show that polaritons constitute an ideal system for the study of the quantum fluids properties.

In the last part of the talk I will present a recently developed flexible technique allowing generating optically controlled potential barriers and traps for polaritons fluids and I will discuss the perspectives opened for the observation of vortex lattices in these systems.

References[1] C. Weisbuch, M. Nishioka, A. Ishikawa, Y. Arakawa, Phys. Rev. Lett. 69, 3314 (1992)

[2] A. Amo, D. Sanvitto, F. P. Laussy, D. Ballarini, et al., Nature 457, 291 (2009)

[3] A. Amo, J. Lefre?re, S. Pigeon, C. Adrados, C. Ciuti, I. Carusotto, R. Houdre?, E. Giacobino, A. Bramati, Nature Physics 5, 805 (2009)

[4] A. Amo, S. Pigeon, D. Sanvitto, V. G. Sala, R. Hivet, I. Carusotto, F. Pisanello, G. Leme?nager, R. Houdre?, E. Giacobino, C. Ciuti and A. Bramati, Science, 332,1167 (2011)

[5] D. Sanvitto, S. Pigeon, A. Amo, D. Ballarini, M. De Giorgi, I. Carusotto, R. Hivet, F. Pisanello, V. G. Sala, P. S. S. Guimaraes, R. Houdre?, E. Giacobino, C. Ciuti, A. Bramati & G. Gigli, Nature Photonics, 5, 610 (2011)

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