We study the photoluminescence (PL) of a two-dimensional liquid of oriented dipolar excitons in ${\mathrm{In}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ coupled double quantum wells confined to a microtrap. Generating excitons outside the trap and transferring them at lattice temperatures down to $T=240$ mK into the trap we create cold quasiequilibrium bosonic ensembles of some 1000 excitons with thermal de Broglie wavelengths exceeding the excitonic separation. With decreasing temperature and increasing density $n\lesssim 5\times {10}^{10}\frac{1}{{\mathrm{cm}}^2}$ we find an increasingly asymmetric PL line shape with a sharpening blue edge and a broad red tail which we interpret to reflect correlated behavior mediated by dipolar interactions. From the PL intensity $I\left(E\right)$ below the PL maximum at $E_0$ we extract at $T<5$ K a distinct power law $I\left(E\right)\sim {(E_0-E)}^{-\left|\alpha \right|}$ with $-\left|\alpha \right|\approx -0.8$ in the range $E_0-E$ of 1.5–4 meV, comparable to the dipolar interaction energy.

• Received 17 November 2011

DOI:https://doi.org/10.1103/PhysRevB.87.205302