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Superfluidity of polaritons in semiconductor microcavities

par Paul INDELICATO - 20 septembre 2009

Superfluidity, the ability of a quantum fluid to flow without friction, is one of the most spectacular phenomena occurring in degenerate gases of interacting bosons. Since its first discovery in liquid Helium, superfluidity has been observed in quite different systems, with recent experiments in ultracold trapped atoms being focused on establishing the subtle links between superfluidity and Bose-Einstein condensation.

In solid state systems, polaritons in semiconductor microcavities constitute a weakly interacting two-dimensional bosons system. It has been recently anticipated that they should behave as a novel quantum fluid with particular properties stemming from their intrinsically out-of-equilibrium nature. Polaritons arise in fact from the superposition of excitons (electron-hole bound states in semiconductors) and photons in the regime of strong coupling, and they can be created via laser excitation. After the observation of Bose-Einstein condensation of polaritons in these systems by several groups in the world, one of the challenges in experimental research has been the observation of superfluidity in a dense polariton gas. Very recently, the Quantum Optic team of Laboratoire Kastler Brossel, in collaboration with theoreticians from “Laboratoire MPQ-Paris 7” and the University of Trento, has observed the superfluid propagation of a polariton fluid created by a laser excitation in a semiconductor microcavity. Superfluidity is demonstrated on the basis of the Landau criterion, and it is manifested as the suppression of scattering from defects when the flow velocity is lower than the speed of sound in the fluid. Moreover, if the flow velocity exceeds the speed of sound, the superfluid regime is not accessible any more and a Čerenkov-like effect is observed, characterised by the appearance of linear wave front patterns when the fluid encounters a defect in its flow path. The experimental findings are in excellent quantitative agreement with the predictions based on a generalized Gross-Pitaevskii theory, showing that microcavity polaritons constitute a very rich and flexible system for exploring the physics of out-of-equilibrium quantum fluids.

Figure :
a-I, a-II, a-II- Superfluid regime The images show the real space emission from the microcavity as a function of excitation density, in a region of the sample in which there is a defect present in the polariton flow path. At low density, in the linear regime, the polariton fluid is scattered by the defect, giving rise to parabolic wave fronts, as shown in (a-I). At higher density, the emission is strongly modified by the onset of polariton-polariton interaction (a-II), until the superfluid regime is attained (a-II) : any scattering from the defect is suppressed and no density waves are observed.

b-I, b-II, b-III- Čerenkov regime The images show the real space emission from the microcavity as a function of excitation density, in a region of the sample in which there is a defect present in the polariton flow path. In this case the polariton fluid has been prepared with a flow velocity larger than in the conditions of figure (a). At low excitation density, in the linear regime, polaritons are scattered by the defect giving rise to parabolic wavefronts (b-I). At higher density the onset of polariton-polariton interactions drive the system into the Čerenkov regime, in which the velocity of the polariton fluid is higher than its sound speed. The linear wave-fronts characteristic of this regime are clearly visible in (b-II) and (b-III).