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 ...

We present an analysis and demonstration of few-cycle ultrashort laser pulse characterization using second-harmonic dispersion scans and numerical phase retrieval algorithms. The sensitivity and robustness of this technique with respect to noise, measurement bandwidth and complexity of the measured pulses is discussed through numerical examples and experimental results. Using this technique, we successfully demonstrate the characterization of few-cycle pulses with complex and structured spectra generated from a broadband ultrafast laser oscillator and a high-energy hollow fiber compressor.

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.

Room-temperature recombination dynamics has been investigated in a large set of different Cu(In,Ga)Se2 absorber films and compared to the electrical device characteristics of the respective solar cell modules. For a given cell preparation process, a characteristic relation between the low-injection minority-carrier lifetime of the absorber layers and the conversion efficiency of the solar cells is observed: Long lifetimes correlate with high open circuit voltages and conversion efficiencies, while no significant influence of the lifetime on the short circuit current is found.

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.