Carrier relaxation and recombination in self-organized InAs/GaAs quantum dots ͑QD's͒ is investigated by photoluminescence ͑PL͒, PL excitation ͑PLE͒, and time-resolved PL spectroscopy. We demonstrate inelastic phonon scattering to be the dominant intradot carrier-relaxation mechanism. Multiphonon processes involving up to four LO phonons from either the InAs QD's, the InAs wetting layer, or the GaAs barrier are resolved. The observation of multiphonon resonances in the PLE spectra of the QD's is discussed in analogy to hot exciton relaxation in higher-dimensional semiconductor systems and proposed to be intricately bound to the inhomogeneity of the QD ensemble in conjunction with a competing nonradiative recombination channel observed for the excited hole states. Carrier capture is found to be a cascade process with the initial capture into excited states taking less than a few picoseconds and the multiphonon ͑involving three LO phonons͒ relaxation time of the first excited hole state being 40 ps. The ͉001͘ hole state presents a relaxation bottleneck that determines the ground-state population time after nonresonant excitation. For the small self-organized InAs/GaAs QD's the intradot carrier relaxation is shown to be faster than radiative ͑Ͼ1 ns͒ and nonradiative ͑Ϸ100 ps͒ recombination explaining the absence of a ''phonon bottleneck'' effect in the PL spectra. ͓S0163-1829͑97͒09340-5͔

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The first measurement of transverse-spin-dependent azimuthal asymmetries in the pion-induced Drell-Yan (DY) process is reported. We use the CERN SPS 190  GeV/c π^{-} beam and a transversely polarized ammonia target. Three azimuthal asymmetries giving access to different transverse-momentum-dependent (TMD) parton distribution functions (PDFs) are extracted using dimuon events with invariant mass between 4.3  GeV/c^{2} and 8.5  GeV/c^{2}. Within the experimental uncertainties, the observed sign of the Sivers asymmetry is found to be consistent with the fundamental prediction of quantum chromodynamics (QCD) that the Sivers TMD PDFs extracted from DY have a sign opposite to the one extracted from semi-inclusive deep-inelastic scattering (SIDIS) data. We present two other asymmetries originating from the pion Boer-Mulders TMD PDFs convoluted with either the nucleon transversity or pretzelosity TMD PDFs. A recent COMPASS SIDIS measurement was obtained at a hard scale comparable to that of these DY results. This opens the way for possible tests of fundamental QCD universality predictions.

The quantum confined Stark effect is observed for quantum dots (QD's) exposed to randomly fluctuating electric fields in epitaxial structures. These fields, attributed to charges localized at defects in the vicinity of the QD's, lead to a jitter in the emission energies of individual QD's. This jitter has typical frequencies of below about 1 Hz and is characteristic for each QD thus providing a unique means to unambiguously identify the emission spectra of single QD's. Up to eight lines are identified for individual QD's and attributed to excitonic, biexcitonic, and LO-phonon-assisted transitions. The intensity of the LO-phonon replica is surprisingly large corresponding to Huang-Rhys factors of about one

Excited states and energy relaxation processes are studied for stacked InAs/GaAs QD's with GaAs cap layers grown by migration enhanced epitaxy. Photoluminescence excitation ͑PLE͒ spectra reveal the excited state spectrum as a function of size for self-assembled InAs QD's in multilayered samples with 36-ML spacers. The observed energy shifts and splittings are consistent with those of hole states numerically calculated for pyramidal QD's supporting assignment to the transition between the electron ground ͉000͘ and the ͉001͘ excited hole state. The optical results suggest the island shape uniformity to improve in multilayered samples, which is attributed to the contribution of the buried islands to the surface strain altering the island formation kinetics and energetics that also underlie vertical self-organization. Time-resolved photoluminescence ͑TRPL͒ results yield a lifetime of 40 ps for the first excited ͉001͘ hole state, attributed to multiphonon relaxation processes bridging the approximately 100 meV level separation, and ground-state lifetimes around 700 ps independent of the detection energy. At high excitation densities saturation of QD states leads to long-living excited-state PL and up to 1 ns delay in the ground-state PL decay, showing radiative decay to be the dominant recombination process in the QD's. The results presented contribute to the understanding of PLE spectra of an inhomogeneous QD ensemble, which is argued to be sensitive to the shape uniformity, the excited-state spectrum, and competing recombination processes. ͓S0163-1829͑98͒03115-4͔

We identify fundamental mechanisms of electron escape from self-organized InAs quantum dots ͑QD's͒ in a vertical electric field by time-resolved capacitance spectroscopy. Direct tunneling and a thermally activated escape process are observed. The QD electron ground and first-excited states are concluded to be located ϳ190 and ϳ96 meV below the GaAs matrix conduction band, respectively. Our experimental results and their interpretation are in good agreement with eight-band k•p calculations and demonstrate the importance of tunnel processes.