Wednesday, November 7, 2012

1211.1290 (A. Askitopoulos et al.)

Condensation of exciton-polaritons through evaporative cooling    [PDF]

A. Askitopoulos, H. Ohadi, Z. Hatzopoulos, P. G. Savvidis, A. V. Kavokin, P. G. Lagoudakis
Evaporative cooling has led to the coldest temperatures ever observed in the universe: sub-microkelvin temperatures generated in atom traps. It turned out to be the key technique to achieve Bose-Einstein condensation, a long-sought goal in atomic physics. Evaporation describes the process of energetic particles leaving a system with a finite binding energy. This process results in cooling: since the evaporating particles carry away more than their share of thermal energy, the temperature of the system decreases. Here we demonstrate experimentally the condensation of exciton-polaritons through evaporative cooling. A cold fraction of exciton-polaritons condenses in an optically induced two-dimensional trap inside the crystal, decoupled from the decoherence inducing excitonic reservoir. We detect a characteristic Bogolyubov dispersion of the condensate with a sound velocity proportional to the square root of the number of trapped quasi-particles and extract the polariton-polariton interaction strength. In a reference experiment, where the evaporative cooling mechanism is switched off by changing the excitation intensity profile, polariton condensation takes place for excitation densities one order of magnitude higher and the resulting condensate is subject to a much stronger dephasing. These observations pave the way to the realisation of polariton logic devices including optically controlled transistors and ultrafast switches as well as the study of bosonic superfluid effects in semiconductor microcavities.
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