Semiconductor microcavities offer unique systems in which to investigate the physics of weakly interacting bosons. Their elementary excitations, polaritons—mixtures of excitons and photons—can accumulate in macroscopically degenerate states to form various types of condensate in a wide range of experimental configurations, under either incoherent1,2 or coherent3,4 excitation. Condensates of polaritons have been put forward as candidates for superfluidity5,6, and the formation of vortices7 as well as elementary excitations with linear dispersion8 are actively sought as evidence to support this. Here, using a coherent excitation triggered by a short optical pulse, we have created and set in motion a macroscopically degenerate state of polaritons that can be made to collide with a variety of defects present in the microcavity. Our experiments show striking manifestations of a coherent light–matter packet, travelling at high speed (of the order of one per cent of the speed of light) and displaying collective dynamics consistent with superfluidity, although one of a highly unusual character as it involves an out-of-equilibrium dissipative system. Our main results are the observation of a linear polariton dispersion accompanied by diffusionless motion; flow without resistance when crossing an obstacle; suppression of Rayleigh scattering; and splitting into two fluids when the size of the obstacle is comparable to the size of the wave packet. This work opens the way to the investigation of new phenomenology of out-of-equilibrium condensates.
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We thank I. Carusotto, M. Wouters and N. Berloff for discussions and D. Steel for a critical reading of the manuscript. This work was partially supported by the Spanish Ministerio de Educación y Ciencia (MEC) (MAT2005-01388, NAN2004-09109-C04-04 & QOIT-CSD2006-00019), the Comunidad Autónoma de Madrid (S-0505/ESP-0200) and the IMDEA-Nanociencia. D.B. and E.d.V. acknowledge a scholarship (Formacion de Profesorado Universitario) of the Spanish MEC. D.S. and M.D.M. thank the Ramón y Cajal Programme.
Supplementary Video 9 shows polaritons colliding with a large point defect in reciprocal space, corresponding to the images shown in Fig.5IIb. This is the k-space counterpart of Supplementary Video 8.
Supplementary Video 1 shows appearance of the Rayleigh scattering circle in reciprocal space for a moving polariton droplet created with low pump power.
Supplementary Video 2 shows polaritons, created with low pump power, getting trapped in a local defect. This is the real space counterpart of Supplementary Video 1.
Supplementary Video 3 shows free polariton motion in real space, corresponding to the images shown in Fig.3a.
Supplementary Video 4 shows free polariton motion in reciprocal space, corresponding to the images shown in Fig.3b. This is the k-space counterpart of Supplementary Video 3.
Supplementary Video 5 shows simulation of a free polariton motion in real and k-space, corresponding to the image shown in Fig.4a
Supplementary Video 6 shows polaritons colliding with a small point defect in real space, corresponding to the images shown in Fig.5Ia.
Supplementary Video 7 shows polaritons colliding with a small point defect in reciprocal space, corresponding to the images shown in Fig.5Ib. This is the k-space counterpart of Supplementary Video 6.
Supplementary Video 8 shows polaritons colliding with a large point defect in real space, corresponding to the images shown in Fig.5IIa.
About this article
Nature Communications (2017)