The influence of quantum dot (QD) asymmetry on the emission of single three-dimensionally confined biexcitons in II-VI semiconductor nanostructures has been studied by magnetophotoluminescence spectroscopy. Investigating both the biexciton and the single-exciton transition in the same single QD, we obtain a unified picture of the impact of electron-hole exchange interaction on the fine structure and the polarization properties of optical transitions in QDs. The exchange splitting is demonstrated to have a strong influence on the derivation of the biexciton binding energy, which we determine to be about 17 meV, much less than the separation between exciton and biexciton lines (ഠ24 meV) in the spectra.[S0031-9007 (99)08536-1] PACS numbers: 78.66. -w, 71.35.Cc, 71.70.GmIn recent years, optical investigations on single semiconductor quantum dots (QDs), often designated as "artificial atoms," opened a new and exciting field of basic physics studies. In contrast to "real" atoms or molecules, a unique feature of solid state quantum dots is the formation of Wannier excitons giving experimental access to both the Coulomb and the electron-hole (e-h) exchange interaction in three-dimensionally confined solid state systems. Therefore, semiconductor QDs with geometries smaller than or comparable to the bulk exciton Bohr radius can be regarded as a model system in order to study the impact of Coulomb and exchange interaction on the optical properties of zero-dimensional excitons and excitonic complexes [1][2][3][4][5]. Several techniques have been developed to realize semiconductor QDs with high quantum efficiencies. This includes chemically prepared QDs embedded in a matrix [1,6] as well as ensembles of QDs fabricated by means of epitaxy and/or lithography [2,[7][8][9][10][11][12][13][14][15]. A drawback of such QD arrays is a broadening of the optical transitions due to the size dispersion of the dots, which prevents the investigation of, e.g., the fine structure of exciton states [16,17]. In order to suppress the influence of inhomogeneous broadening effects, spectroscopic techniques with a high spatial resolution have been introduced as a powerful experimental tool. This allows one to investigate single quantum dots (SQDs) by means of photoluminescence (PL) spectroscopy [2][3][4][5]10,14,18].In II-VI nanostructures, the e-h exchange interaction is significantly enhanced as compared to the (Ga,In)As system [1,6]. This allows studies of the optical transitions of excitons and multiexcitons without any significant mixing of radiative ("bright") and nonradiative ("dark") excitonic states. Therefore, II-VI materials such as, e.g., CdSe͞ZnSe are ideal systems to study the impact of the dot symmetry and the e-h exchange interaction on the fine structure and the polarization properties of excitons and excitonic complexes in SQDs. Until now, the work on e-h exchange interaction in quantum dots has concentrated on single-exciton states, where, e.g., the energy splitting between dark and bright excitons [1,6] or the splitting of the ...

The decay characteristics of excitons and biexcitons in one single semiconductor quantum dot (QD) are directly monitored using time-and spatially resolved photoluminescence spectroscopy. The experiments are performed on a CdSe͞ZnSe QD, occupied by either one or two excitons at a time, allowing a direct comparison between the radiative lifetime of a biexciton and an exciton confined in the same QD. The rather surprising result of comparable recombination rates for both states is related to the spatial wave function distribution and the spin structure of the particles and their coupling to the photon field, i.e., the superradiance effect.

The quantum-confined Stark effect in a single self-assembled CdSe/ZnSe quantum dot was studied by means of highly spatially resolved photoluminescence spectroscopy. A nanotechnological approach making use of a capacitor-like geometry enabled us to apply a well-defined lateral electric field on the quantum dots. Stark shifts of up to 1.1 meV were obtained, which can be well fitted by a purely quadratic dependence on an electric field. In quite good agreement with theoretical calculations, an exciton polarizability of 4.9×10−3 meV/(kV/cm)2 can be extracted, while the permanent dipole moment in the lateral direction is found to be negligible.

We report on reversible spectral shifts in the emission spectra of self-organized CdSe single quantum dots on a time scale of seconds. Energy shifts of up to 3.5 meV have been observed and can be attributed to the Stark effect caused by fluctuating local electric fields. Most surprisingly, the energy shift turns out to be quasi-periodic with time constants between 70 and 190 s.

The optical properties of CdSe nanostructures grown by migration-enhanced epitaxy of CdSe on ZnSe are studied by time-, energy-, and temperature-dependent photoluminescence and excitation spectroscopy, as well as by polarization-dependent four-wave mixing and two-photon absorption experiments. The nanostructures consist of a coherently strained Zn 1Ϫx Cd x Se/ZnSe quantum well with embedded islands of higher Cd content with sizes of a few nanometer due to strain-induced CdSe accumulation. The local increase in CdSe concentration results in a strong localization of the excitonic wave function, in an increase in radiative lifetime, and a decrease of the dephasing rate. Local LO-phonon modes caused by the strong modulation of the Cd concentration profile are found in phonon-assisted relaxation processes. Confined biexcitons with large binding energies between 20 and 24 meV are observed, indicating the important role of biexcitons even at room temperature. ͓S0163-1829͑99͒16935-2͔