Few-particle Effects Research Articles

A study of nitrogen incorporation in pyramidal site-controlled quantum dots

We present the results of a study of nitrogen incorporation in metalorganic-vapour-phase epitaxy-grown site-controlled quantum dots (QDs). We report for the first time on a significant incorporation (approximately 0.3%), producing a noteworthy red shift (at least 50 meV) in some of our samples. Depending on the level of nitrogen incorporation/exposure, strong modifications of the optical features are found (variable distribution of the emission homogeneity, fine-structure splitting, few-particle effects). We discuss our results, especially in relation to a specific reproducible sample which has noticeable features: the usual pattern of the excitonic transitions is altered and the fine-structure splitting is suppressed to vanishing values. Distinctively, nitrogen incorporation can be achieved without detriment to the optical quality, as confirmed by narrow linewidths and photon correlation spectroscopy.

Many-particle effects in self-organized quantum dots

The influence of spectator particles on the electronic and optical properties is investigated for MOCVD grown In(Ga)As/GaAs quantum dots (QDs). The correlation between optical and structural properties allows for an analysis of the structural-dependent few-particle effects in single quantum dots, demonstrating the importance of correlation in self-organized quantum dots. For the investigation of the ground and excited QD states in view of Coulomb interaction, photoluminescence excitation and high-excitation photoluminescence experiments are used in presence of the maximum possible number of spectator electrons, holes and excitons. The results allow on the one hand to identify the electron and hole levels participating at the observed QD transitions and on the other hand to quantify clearly the differences between single-exciton and many-particle transitions.

Few-Particle Effects in Self-Organized Quantum Dots

Few-particle effects of zero-dimensional charge carriers in self-organized quantum dots (QDs) are investigated both experimentally and theoretically. The actual three-dimensional confinement potential is determined by the real-space geometry and chemical composition of a realistic QD, reflecting its low symmetry, the influence of inhomogeneous strain on the band structure, and piezoelectric effects. Calculations based on the eight-band k · p model combined with the configuration-interaction method (CI) are presented, yielding insight into the complex interplay of confinement potential and electronic structure. For the InAs/GaAs model system, the existence of both binding and anti-binding biexciton states is predicted and verified experimentally. For CdZnSe/ZnSSe QDs, model calculations are used to identify experimentally observed emission lines from the decay of neutral as well as charged exciton (X) and biexciton (XX) states. Here the variation of the binding energy and the finestructure splitting is attributed to a variation of material composition, i.e. Zn–Cd intermixing and the shape of the probed QDs, respectively.

Few-particle effects in single CdTe quantum dots

Optical emission from individual, n-type modulation-doped CdTe quantum dots (QD's) is presented. Magnetomicrophotoluminescence and power-excitation-dependent measurements allows one to identify the neutral exciton, the negatively charged exciton, and the biexciton. All these few-particle species are exposed to randomly fluctuating electric fields attributed to charges trapped in the vicinity of the QD. The resulting quantum confined Stark effect, characteristic for each quantum dot, is analyzed in detail. The correlations between the emission energies of the neutral and charged exciton as well as between their integrated intensities confirm the identification of the charged species. The binding energies of the biexciton and charged exciton decrease sharply with the magnitude of the local electric field.

State-filling and renormalization in charged InGaAs/GaAs quantum dots

Few-particle effects have been studied for charged InGaAs/GaAs quantum dots (QDs) by means of size-selective photoluminescence excitation spectroscopy (PLE). The effects of spectator electrons on the electronic properties, in particular exciton transitions in the near infrared spectral region, are investigated. The characteristic quenching of excitation channels due to state-filling with increasing charging allows to identify excited exciton transitions and to correlate the observed renormalization of the transition energies to the QD population.

Calorimetric investigation of intersublevel transitions in charged quantum dots

Intersublevel transitions in InGaAs quantum dots (QD's) are observed by means of midinfrared Fourier transform calorimetric absorption spectroscopy. This technique permits one to measure spectrally resolved the heating ofa sample illuminated by a midinfrared radiation source. Thus, effectively a QD bolometer for the midinfrared energy range has been realized, allowing for sensitive absorption studies. Absorption peaks observed in the 70 to 110 meV range are attributed to electron intersublevel transitions based on a comparison to exciton properties derived from photoluminescence and photoluminescence-excitation measurements. For charge-tunable QD's in a space charge region, the population of ground and excited electron sublevels can be changed by the applied reverse bias, allowing for an identification of the observed intersublevel transitions on the basis of their charging dependence. Charging-dependent energy shifts and intensity changes of the intersublevel absorption peaks are ascribed to few-particle effects and to Pauli blockade effects, respectively. The QD intersublevel absorption cross section is estimated to he of the order of 10 - 1 3 cm 2 , the according absorption coefficient is comparable to that of QD interband (exciton) transitions.

Role of Coulomb Correlations in the Optical Spectra of Semiconductor Quantum Dots

We present a consistent theoretical description of few-particle effects in the optical spectra of semiconductor quantum dots, based on a direct-diagonalization approach. We show that, because of the strong Coulomb interaction among electrons and holes, each configuration of the confined few-particle system leads to its characteristic signature in the optical spectra. We discuss quantitative predictions and comparison with experiments for both absorption and luminescence.

Few-particle effects in semiconductor quantum dots: observation of multicharged excitons

We investigate experimentally and theoretically few-particle effects in the optical spectra of single quantum dots (QDs). Photodepletion of the QD together with the slow hopping transport of impurity-bound electrons back to the QD are employed to efficiently control the number of electrons present in the QD. By investigating structurally identical QDs, we show that the spectral evolutions observed can be attributed to intrinsic, multi-particle-related effects, as opposed to extrinsic QD-impurity environment-related interactions. From our theoretical calculations we identify the distinct transitions related to excitons and excitons charged with up to five additional electrons, as well as neutral and charged biexcitons.

Optical Spectra of Single Quantum Dots: Influence of Impurities and Few-Particle Effects

The evolution of photoluminescence (PL) spectra of single GaAs/AlGaAs quantum dots (QD) is studied as a function of laser excitation power and temperature. At very low powers, where multi-exciton occupation of the QD can be excluded, an unexpected and pronounced spectral evolution is observed (large energy shifts and appearance of multiple emission lines). A similar evolution is observed at low excitation powers with increasing temperature. A model, taking into account the influence of the shallow, residual impurities in the environment of each QD, explains the observed spectral evolutions in terms of photo-depletion of the QD and hopping of impurity-bound carriers back into the QD. Theoretical calculations of the PL due to Nelectrons + 1 hole (Ne + 1h) QD states allow us to attribute the ≈︂2 meV spaced lines in the experimental spectra to the different charge states Ne + 1h, (N — 1) e + 1h, … of the QD.

Excitonic and biexcitonic effects in the coherent optical response of semiconductor quantum dots

We analyze few-particle effects in the coherent optical spectra of semiconductor quantum dots using a density-matrix approach that explicitly accounts for exciton–exciton interactions. A consistent description of additional peaks appearing at high photoexcitation density is given.

Few-particle effects in the optical spectra of semiconductor quantum dots

We analyze few-particle effects in the optical spectra of semiconductor quantum dots using a density-matrix approach that explicitly accounts for exciton–exciton, as well as exciton–carrier interactions. We give a consistent description of additional peaks appearing at high photoexcitation density, and predict that a strong polarization dependence should be characteristic of few-particle features.

Few-Particle Effects in Nonlinear Optical Spectra of Semiconductor Quantum Dots

ABSTRACTWe present a density-matrix approach for the description of nonequilibrium carrier dynamics in optically excited semiconductor quantum dots, that explicitly accounts for exciton-exciton as well as exciton-carrier interactions. Within this framework, we analyze few-particle effects in the optical spectra and provide a consistent description of additional peaks appearing at high photoexcitation density. We discuss possible applications of such optical nonlinearities in future coherent-control experiments.

Optical studies of individual InAs quantum dots in GaAs: few-particle effects

Optical emission from individual strained indium arsenide (InAs) islands buried in gallium arsenide (GaAs) was studied. At low excitation power density, the spectra from these quantum dots consist of a single line. At higher excitation power density, additional emission lines appeared at both higher and lower energies, separated from the main line by about 1 millielectron volt. At even higher excitation power density, this set of lines was replaced by a broad emission peaking below the original line. The splittings were an order of magnitude smaller than the lowest single-electron or single-hole excited state energies, indicating that the fine structure results from few-particle interactions in the dot. Calculations of few-particle effects give splittings of the observed magnitude.

Hartree-Fock states in the thermodynamic limit. III. Low-density clustering effects

We present an orthonormal set of single-particle orbital functions explicitly satisfying the Hartree-Fock (HF) equations for occupied states, and which give low-density few-particle clustering effects in a many-fermion system. The initial study given here is in one and three dimensions for attractive $\ensuremath{\delta}$ pair interactions. In one dimension the numerical results compare favorably with the exact (lowest) HF state at zero density. The results turn out to be superior to the classic Overhauser HF orbitals, as well as to some more recent ones.