First-order spatial coherence of indirect excitons in coupled quantum wells

We discuss a k(parallel to)-filtering effect for the optical coherence function of indirect excitons in coupled quantum wells. In far-field optical experiments the light collected from the low-energy excitons inside the radiative zone produces interference patterns due to the very small difference between their momenta. This small fraction of the exciton distribution is further reduced due to the angular aperture of the experimental apparatus which corresponds to a maximum wavevector and results in a damped oscillatory behavior of the coherence function. Calculation of the optical coherence function above the phase transition temperature yields a coherence length C, consistent with the experiments, in contrast to the actual coherence length of indirect excitons which is smaller by an order of magnitude. The k(parallel to)-filtering effect can naturally explain the recently observed large coherence lengths without the assumption of a condensed or superfluid phase. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim