During the past ten years, coupled quantum wells have emerged as a promising system for experiments on Bose condensation of excitons, with numerous theoretical and experimental studies aimed at the demonstration of this effect. One of the issues driving these studies is the possibility of long-range coherent transport of excitons. Excitons in quantum wells typically diffuse only a few micrometres from the spot where they are generated by a laser pulse; their diffusion is limited by their lifetime (typically a few nanoseconds) and by scattering due to disorder in the well structure. Here we report photoluminescence measurements of InGaAs quantum wells and the observation of an effect by which luminescence from excitons appears hundreds of micrometres away from the laser excitation spot. This luminescence appears as a ring around the laser spot; almost none appears in the region between the laser spot and the ring. This implies that the excitons must travel in a dark state until they reach some critical distance, at which they collectively revert to luminescing states. It is unclear whether this effect is related to macroscopic coherence caused by Bose condensation of excitons.

We report on new experiments and theory that unambiguously resolve the recent puzzling observation of large diameter exciton emission halos around a laser excitation spot in two dimensional systems. We find a novel separation of plasmas of opposite charge with emission from the sharp circular boundary between these two regions. This charge separation allows for cooling of initially hot optically generated carriers as they dwell in the charge reservoirs for very long times.

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We report an important step forward for the goal of unambiguous observation of Bose-Einstein condensation of excitons in semiconductors. We have demonstrated a system in which excitons live for microseconds, much longer than their thermalization time, move over distances of hundreds of microns, and can be trapped in a harmonic potential exactly analous to the traps for atomic condensates. We also report recent results of a new method for observing evidence of Bose-Einstein condensation, by angular resolution of the emitted luminescence.

We search for a new light gauge boson, a dark photon, with the D0 experiment. In the model we consider, supersymmetric partners are pair produced and cascade to lightest neutralinos that can decay into the hidden sector state plus either a photon or a dark photon. The dark photon decays through its mixing with a photon into fermion pairs. We therefore investigate a previously unexplored final state that contains a photon, two spatially close leptons, and large missing transverse energy. We do not observe any evidence for dark photons and set a limit on their production.

We found that a novel optically-induced in-plane separation of plasmas of opposite charge is responsible for the large ring emission pattern around a laser excitation spot observed in modulation doped quantum well (QW) structures. The charge separation is a result of an interplay between the electrical field applied perpendicular to the QW and the diffusion of optically generated carriers in the QW plane. Excitonic emission at the sharp boundary between the positive and negative charges forms the ring. The initially hot carriers that are generated optically, cool as they diffuse and are therefore cold before they recombine at the ring. Such separation of charges was only observed when the excitation energy is above the barrier height of the QW. The effect of the lower energy excitation is found to be dramatically different, resulting in a shrinkage and even a total collapse of an existing ring pattern.It was recently reported by several independent groups that an unexpected emission from indirect transitions in modulation doped double quantum well (QW) structures, under a certain range of optical excitation power and electric field (applied along the growth direction), is observed at a large diameter ring far from the excitation spot [1][2][3]. Strikingly, the area in between the center and the ring emission patterns appears dark (as shown by the example in Fig. 1a), raising much debate as to the underlying transport mechanisms of the excited carriers and the nature of the emission. In this paper, we describe new experimental studies that has provided us an insight into the physical mechanism behind those observations. We also present a new and intriguing dependence of the ring radius on the excitation energy. We have performed several different experiments. In the first experiment, we observed similar emission patterns in single QW structures, suggesting that the double well nature and the associated long indirect exciton lifetimes, initially considered crucial, are not important for the formation of the ring. In the second experiment, we found that the ring formation occurs only when the excitation energy is above the barrier of the QW's. For excitation energy smaller than the barrier bandgap, the excitation power needed for the ring formation becomes prohibitively high. This can be seen from Fig. 1b, where the power threshold for the ring formation is plotted as a function of excitation energy for single and double QW samples with different barrier height. Moreover, reducing the excitation energy even further, can result in a shrinkage or a complete collapse of an already existing ring, as will be discussed later in this paper.

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