Optical spectroscopy of a single naturally formed GaAs quantum dot

Summary form only given. With fine spatial and spectral resolution we measure the homogeneous spectra of single naturally formed GaAs quantum dots. The origins of the measured linewidths and of very-fine-structure splitting are discussed in terms of the dynamics and energetics of the exciton in zero dimensions. Semiconductor quantum dots (Qdots) are electronic structures that localize the exciton in all three spatial dimensions and impose a complete quantization of the exciton energy spectrum. Until recently optical experiments on quantum dots have been hampered by the dominance of extrinsic effects, most notably dot-size fluctuations, which result in inhomogeneous broadening of the spectral features, and surface defects, which decrease charge carrier lifetimes. Recently solutions to these problems have been found and experiments are now beginning to probe the intrinsic optical properties of these zero-dimensional systems. In our experiments we use a novel near-field optical spectroscopic technique to perform photoluminescence (PL) excitation measurements on single GaAs quantum dots that are formed naturally from the monolayer-sized interface fluctuations in a thin GaAs/AlGaAs quantum well. By reduction of the area of the sample excited down to the optical near-field regime, only a few dots are probed. With resonant excitation the excited-state spectra of a single quantum dot is measured. Unexpectedly, we find that each of the spectral lines is linearly polarised with a fine structure splitting that varies in sign and magnitude.