We present experiments on the luminescence of excitons confined in a potential trap at sub-Kelvin temperatures after nanosecond pulsed laser excitation. Analysis of the experimental results with a rate model shows that the so-called Auger decay of yellow excitons, which in previous studies led to a rapid decay of the excitons at high densities and thus prevented reaching the critical density for Bose-Einstein condensation (BEC), is greatly reduced for paraexcitons. We demonstrate that exciton numbers well above 10 10 can be collected in a potential trap, albeit at temperatures in the 10 K range. During their lifetime of about 500 ns the paraexcitons cool down to the temperature of the He bath. This opens up the possibility to observe a BEC of paraexcitons provided that the bath temperature can be reduced to below 100 mK.(1) with 0 being the average oscillator frequency of the trapping potential and ζ the Riemann zeta function, decreases much faster with temperature, T , than in free space.Due to their unique properties, the excitons of the so-called yellow series in the semiconductor cuprous oxide (Cu 2 O) are still considered one of several promising candidates for excitonic BEC (for reviews see [4][5][6]). This is related to the large binding energy of 150 meV, which shifts the Mott density to 10 19 cm −3 at cryogenic temperatures [7,8]. Made up of doubly degenerate valence and conduction bands, the ground state of this series splits into the triply degenerate orthoexciton and the nondegenerate paraexciton, which is the energetically lowest exciton state, = 12.1 meV below the orthoexciton states. Due to the positive parity of the bands, the orthoexciton is only weakly optically allowed (quadrupole transition with oscillator

We present experiments on the luminescence of excitons confined in a potential trap at milli-Kelvin bath temperatures under continuous-wave (cw) excitation. They reveal several distinct features like a kink in the dependence of the total integrated luminescence intensity on excitation laser power and a bimodal distribution of the spatially resolved luminescence. Furthermore, we discuss the present state of the theoretical description of Bose-Einstein condensation of excitons with respect to signatures of a condensate in the luminescence. The comparison of the experimental data with theoretical results with respect to the spatially resolved as well as the integrated luminescence intensity shows the necessity of taking into account a Bose-Einstein condensed excitonic phase in order to understand the behaviour of the trapped excitons.

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We present a theoretical analysis of the emission of localized excitons in GaAs‐AlGaAs quantum wells, which shows a strong nonlinear behavior with increasing excitation. Considering the influence of dynamical screening both on the one‐particle properties of carriers and on the whole spectrum of electron‐hole pair states, we are able to explain the nonlinearity as a transition of the emission from excitonic to electron‐hole pair states in the continuum (electron‐hole plasma). Moreover, our theoretical approach based on the quasi‐particle approximation for the carriers states and quantum kinetic effects in the screening describes the observed changes of the shift of the exciton energy from higher to lower energies at a temperature of T = 10 K (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)