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Long-range transport in excitonic dark states in coupled quantum wells


During the past ten years, coupled quantum wells have emerged as a promising system for experiments on Bose condensation of excitons, with numerous theoretical1,2,3,4,5,6 and experimental7,8,9,10,11,12 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.

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Figure 1: The band structure of the coupled quantum wells in the presence of electric field.
Figure 2: Spectral images of the luminescence from indirect excitons for various laser powers.
Figure 3: The critical excitation density for appearance of the luminescence ring, as a function of the bath temperature in a variable-temperature cryostat.
Figure 4: Luminescence profile and intensity.


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This work has been supported by the National Science Foundation and by the Department of Energy. We thank V. Negoita for early contributions to these experiments.

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Correspondence to D. Snoke.

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Snoke, D., Denev, S., Liu, Y. et al. Long-range transport in excitonic dark states in coupled quantum wells. Nature 418, 754–757 (2002).

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