Collective fluid dynamics of a polariton condensate in a semiconductor microcavity. Nature 457, 291

Departamento Física de Materiales, Universidad Autonóma de Madrid, 28049 Madrid, Spain.
Nature (Impact Factor: 41.46). 02/2009; 457(7227):291-5. DOI: 10.1038/nature07640
Source: PubMed


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 incoherent or coherent excitation. Condensates of polaritons have been put forward as candidates for superfluidity, and the formation of vortices as well as elementary excitations with linear dispersion 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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    • "First of all, they can form bosonic condensates [9] [10], which implies a macroscopic occupation of a single energy state close to thermal equilibrium [9]. Furthermore, they can enter a superfluid phase [1] [11] [12], possess internal (pseudo-spin) degrees of freedom [13], can be localized by lithographic [2] or optical techniques [14] possibly down to the single-polariton level [15], and their interaction constants are tunable [13]. An ideal trapping technique for the implementation of polariton quantum emulation should combine the following features: (i) the confinement depth should be tunable in a wide range; (ii) intersite coupling should be readily controllable; (iii) surface recombination effects from etching the active medium should be avoided completely. "
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    ABSTRACT: The possibility of investigating macroscopic coherent quantum states in polariton condensates and of engineering polariton landscapes in semiconductors has triggered interest in using polaritonic systems to simulate complex many-body phenomena. However, advanced experiments require superior trapping techniques that allow for the engineering of periodic and arbitrary potentials with strong on-site localization, clean condensate formation, and nearest-neighbor coupling. Here we establish a technology that meets these demands and enables strong, potentially tunable trapping without affecting the favorable polariton characteristics. The traps are based on a locally elongated microcavity which can be formed by standard lithography. We observe polariton condensation with non-resonant pumping in single traps and photonic crystal square lattice arrays. In the latter structures, we observe pronounced energy bands, complete band gaps, and spontaneous condensation at the M-point of the Brillouin zone.
    New Journal of Physics 02/2015; 17(2). DOI:10.1088/1367-2630/17/2/023001 · 3.56 Impact Factor
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    • "The current realization of exciton–polariton BECs [3] [4] [5] has initiated an intense research effort towards observing similar effects in the non-equilibrium system. Phenomena such as superfluidity [6] and vortex formation [7] [8] have been observed experimentally to date, as well as novel effects such as metastable condensation in high energy orbital bands [9] [10]. However, behaviors of dark soliton propagation, collision, and vortex formation in the context of a non-equilibrium exciton–polariton BEC are not well understood. "
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    ABSTRACT: Keywords: Finite-difference time-domain (FDTD) scheme Gross–Pitaevskii equation (GPE) Dark soliton Stability Non-equilibrium condensate Behaviors of dark soliton propagation, collision, and vortex formation in the context of a non-equilibrium condensate are interesting to study. This can be achieved by solving open dissipative Gross–Pitaevskii equations (dGPEs) in multiple dimensions, which are a generalization of the standard Gross–Pitaevskii equation that includes effects of the condensate gain and loss. In this article, we present a generalized finite-difference time-domain (G-FDTD) scheme, which is explicit, stable, and permits an accurate solution with simple computation, for solving the multi-dimensional dGPE. The scheme is tested by solving a steady state problem in the non-equilibrium condensate. Moreover, it is shown that the stability condition for the scheme offers a more relaxed time step restriction than the popular pseudo-spectral method. The G-FDTD scheme is then employed to simulate the dark soliton propagation, collision, and the formation of vortex–antivortex pairs.
    Journal of Computational Physics 02/2015; 282:303-316. DOI:10.1016/ · 2.43 Impact Factor
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    • "On one hand a whole zoology of topological excitations has been revealed: quantized vortices, either pinned to defects [14], resonantly created [15] or engineered in lattices [16], as well as dark [17] and bright [18] solitons. On the other hand propagation of these coherent states has been studied, in particular under resonant excitation, under which conditions evidence of superfluidity has been observed [19] [20]. Due to repulsive polariton–polariton interactions, spontaneous propagation can also be obtained under nonresonant excitation [21], and is observed either in 2D [22] [23] [24] or in 1D geometries [25]. "
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    ABSTRACT: When pumped nonresonantly, semiconductor microcavity polaritons form Bose–Einstein condensates that can be manipulated optically. Using tightly-focused excitation spots, radially expanding condensates can be formed in close proximity. Using high time resolution streak camera measurements we study the time dependent properties of these macroscopic coherent states. By coupling this method with interferometry we observe directly the phase locking of two independent condensates in time, showing the effect of polariton–polariton interactions. We also directly observe fast spontaneous soliton-like oscillations of the polariton cloud trapped between the pump spots, which can be either dark or bright solitons. This transition from dark to bright is a consequence of the change of sign of the nonlinearity which we propose is due to the shape of the polariton dispersion leading to either positive or negative polariton effective mass.
    New Journal of Physics 10/2014; 16(10). DOI:10.1088/1367-2630/16/10/103039 · 3.56 Impact Factor
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