Microcavity polaritons are composite half-light half-matter quasi-particles, which have recently been demonstrated to exhibit rich physical properties, such as non-equilibrium Bose-Einstein condensation, parametric scattering and superfluidity. At the same time, polaritons have some important advantages over photons for information processing applications, since their excitonic component leads to weaker diffraction and stronger inter-particle interactions, implying, respectively, tighter localization and lower powers for nonlinear functionality. Here we present the first experimental observations of bright polariton solitons in a strongly coupled semiconductor microcavity. The polariton solitons are shown to be non-diffracting high density wavepackets, that are strongly localised in real space with a corresponding broad spectrum in momentum space. Unlike solitons known in other matter-wave systems such as Bose condensed ultracold atomic gases, they are non-equilibrium and rely on a balance between losses and external pumping. Microcavity polariton solitons are excited on picosecond timescales, and thus have significant benefits for ultrafast switching and transfer of information over their light only counterparts, semiconductor cavity lasers (VCSELs), which have only nanosecond response time

Tunable oscillatory modes of electric-field domains in doped semiconductor superlattices are reported. The experimental investigations demonstrate the realization of tunable, GHz frequencies in GaAs-AlAs superlattices covering the temperature region from 5 to 300 K. The orgin of the tunable oscillatory modes is determined using an analytical and a numerical modeling of the dynamics of domain formation. Three different oscillatory modes are found. Their presence depends on the actual shape of the drift velocity curve, the doping density, the boundary condition, and the length of the superlattice. For most bias regions, the self-sustained oscillations are due to the formation, motion, and recycling of the domain boundary inside the superlattice. For some biases, the strengths of the low and high field domain change periodically in time with the domain boundary being pinned within a few quantum wells. The dependency of the frequency on the coupling leads to the prediction of a new type of tunable GHz oscillator based on semiconductor superlattices.Comment: Tex file (20 pages) and 16 postscript figure

We demonstrate that the tunable potential introduced by a surface acoustic wave on a homogeneous polariton condensate leads to fragmentation of the condensate into an array of wires which move with the acoustic velocity. Reduction of the spatial coherence of the condensate emission along the surface acoustic wave direction is attributed to the suppression of coupling between the spatially modulated condensates. Interparticle interactions observed at high polariton densities screen the acoustic potential, partially reversing its effect on spatial coherence.

We study the momentum distribution and relaxation dynamics of semiconductor microcavity polaritons by angle-resolved and time-resolved spectroscopy. Above a critical pump level, the thermalization time of polaritons at positive detunings becomes shorter than their lifetime, and the polaritons form a quantum degenerate Bose-Einstein distribution in thermal equilibrium with the lattice. DOI: PACS numbers: 71.36.+c, 42.50.ÿp, 78.47.+p, 78.67.ÿn Bose-Einstein condensation (BEC) has been of intense interest to the physics community for decades [1][2][3][4][5]. While atom BEC has been demonstrated since 1995 in various species of atomic gases, no analogue has been established in solid state systems. The outstanding problem for solid state BEC is to have an equilibrium system with high enough density. Earlier works in Cu 2 O excitons showed thermal equilibrium but no quantum degeneracy [6,7]. More recent works on quantum-well excitons showed indirect evidence of quantum degeneracy, but thermal equilibrium could not be inferred [8][9][10][11]. Solid state polariton systems are interesting because they have an effective mass 8 orders of magnitude smaller than the hydrogen atom mass, and 4 orders of magnitude smaller than the exciton mass. Thus the critical temperature of polariton phase transitions range from 1 K to above room temperature. Quantum degeneracy has been demonstrated by many groups in recent years [12 -16], yet thermal equilibrium is never established. In the current work, we obtained for the first time clear and direct evidence of simultaneous thermal equilibrium and quantum degeneracy.In a semiconductor microcavity with embedded quantum wells (QWs), when the confined cavity photon modes strongly couple to the QW excitons, new eigenmodes are formed called the polaritons [17]. As quasiparticles in semiconductors, polaritons have relatively short lifetimes, thus it is generally difficult to cool hot polaritons to the lattice temperature before they decay. On the lower energy branch, polaritons change from excitonlike lower polaritons (ELPs) at large in-plane wave number k to halfexciton half-photon lower polaritons (LPs) at k 0. Correspondingly, their lifetime decreases by 2 orders of magnitude and their energy density of states decreases by 4 orders of magnitude. Hence an energy relaxation bottleneck is commonly observed [18][19][20] at low densities where spontaneous linear phonon-LP scattering is the dominant, yet insufficient, cooling mechanism. At higher densities, however, when the quantum degeneracy condition of N LP 1 is fulfilled, bosonic final state stimulation greatly enhances both the nonlinear LP-LP scattering and the linear LP-phonon scattering [21][22][23]. Recently, a degenerate Bose-Einstein distribution (BED) of LPs has been observed [24], but the LP temperature was T LP 100 K, much higher than the lattice temperature T lat 4:2 K. This suggests that although LP-LP scattering establishes quasiequilibrium among LPs, cooling by the phonon bath is still slower than the decay of the LPs.Fortu...

We perform a Young's double-slit experiment to study the spatial coherence properties of a twodimensional dynamic condensate of semiconductor microcavity polaritons. The coherence length of the system is measured as a function of the pump rate, which confirms a spontaneous build-up of macroscopic coherence in the condensed phase. An independent measurement reveals that the position and momentum uncertainty product of the condensate is close to the Heisenberg limit. An experimental realization of such a minimum uncertainty wavepacket of the polariton condensate opens a door to coherent matter-wave phenomena such as Josephson oscillation, superfluidity, and solitons in solid state condensate systems. PACS numbers:Simple, yet profoundly connected to the foundation of quantum physics, the Young's double-slit experiment has been a benchmark demonstration of macroscopic spatial coherence -off-diagonal long range order (ODLRO) of a macroscopic number of particles [1] -in Bose-Einstein Condensation (BEC) of cold atoms [2,3,4]. Recently, a similar phase transition has been reported for the lower branch of exciton-polaritons (LPs) in planar semiconductor microcavities [5,6,7,8,9,10,11,12], and supporting theoretical frameworks have been developed [13,14,15,16,17,18,19,20]. Interestingly, LPs are free particles in a two dimensional (2D) system where genuine BEC exists only at zero temperature in the thermodynamic limit [21,22]. A quasi-BEC can be defined for a 2D system of a finite size if a macroscopic number of particles occupy a single ground state and if an ODLRO is established throughout the system [23,24]. Yet in the LP experiments to date, the system size is ambiguously defined by the spot size of the pump laser, and there is no quantitative study of the relation between the size and the coherence length of a condensate [10,11]. In this work, we perform a Young's double slit experiment on a LP gas to measure its spatial coherence properties across the phase transition, and compare the measured coherence length with the condensate size. We also measure the position-momentum uncertainty product of the condensate and compare it to the Heisenberg limit.A sketch of the setup is shown in Fig. 1. The microcavity sample is first magnified by a factor of 37.5 and imaged to a plane A, which is in turn imaged by a lens II to a charge-coupled-device (CCD) at plane C for measurement of spatial distribution. For the double-slit experiment, we insert a pair of rectangular slits at plane A, and move the lens II such that the image of plane A (denoted by plane B ) is a distance D behind plane C. Effectively, we observe on the CCD the interference pattern of the LP emission passing through the double- slit. In our experiment, D = 6.7 cm, the width of the slit image at plane B is δ = 53 µm, and the average wavelength of the LP emission is λ ∼ 778.5 nm. Correspondingly, the Fresnel number δ 2 Dλ = 0.05 ≪ 1, thus the far-field condition is satisfied at plane C. When mapped onto the sample surface, the slit width seen by the LPs is ∆r ≈ ...