Using optical two-dimensional Fourier transform spectroscopy, we report temperature- and excitation-density-dependent measurements of the homogeneous linewidth of the exciton ground-state transition in a single layer of interfacial GaAs quantum dots (QDs). We show that the homogeneous linewidth increases nonlinearly with temperature from 6 to 50 K and that the thermal broadening is well described by an activation term and offset. The absence of a phonon-activation peak in the two-dimensional spectra reveals that elastic scattering of excitons with acoustic phonons via virtual transitions between the ground and excited states significantly contributes to the thermal broadening. We find that the combination of increasing virtual activation energy and exciton-phonon coupling strength with decreasing QD size results in greater thermal broadening for excitons localized in smaller QDs. The homogeneous linewidth also exhibits a strong excitation-density dependence and is shown to increase linearly as the photon density increases from $2\times {10}^{11}$ to $1\times {10}^{12}$ photons pulse${}^{-1}$ cm${}^{-2}$ at 6 K. This trend is attributed to strong coupling of excitons within the same QD and is independent of the quantum-well exciton population density.

• Received 2 August 2010

DOI:https://doi.org/10.1103/PhysRevB.83.115324