Optical spectroscopy of single quantum dots at tunable positive, neutral, and negative charge states (original) (raw)
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Physica E: Low-dimensional Systems and Nanostructures, 2002
We investigate semiconductor quantum dots by optically injecting a well controlled unequal number of electrons and holes into an isolated single dot. The injected carriers form charged complexes of many carriers in the dot. Radiative electron-hole pair recombination takes place after the charged complex relaxes to its ground state. We spectrally and temporally resolve the emission and show that it can be used to unambiguously determine the discrete charge states of the emitting quantum dot. In particular, we identify the emission from both negative and positive charge states of the same dot. We show that while negative charging results in red shifted emission energy, compared with a neutral dot, positive charging results in blue shifted emission energy. We explain this observation in terms of the better conÿned wave functions of the holes. Due to their smaller volume, the energy associated with hole-hole repulsion is larger than the combined energy associated with electron-hole attraction and electron-electron repulsion. ? 2002 Published by Elsevier Science B.V.
Spectroscopy of Single Semiconductor Quantum Dots at Negative, Neutral, and Positive Charge States
physica status solidi (a), 2002
We study optically single self-assembled quantum dots embedded within the wide quantum well of a mixed type quantum structure. We compare the steady state and pulsed photoluminescence spectra of these dots to those of previously studied "regular" dots. We unambiguously identify experimentally emission from various discrete charge state of the dots. We provide means for optically tune the charge state of the dot, both negatively and positively. Our observations are used to accurately determine the asymmetry between the quantum dots' confined electron and hole envelope wavefunctions.
Physical Review B, 2012
We present a comprehensive study of the optical transitions and selection rules of variably charged single self-assembled InAs/GaAs quantum dots. We apply high resolution polarization sensitive photoluminescence excitation spectroscopy to the same quantum dot for three different charge states: neutral and negatively or positively charged by one additional electron or hole. From the detailed analysis of the excitation spectra, a full understanding of the single-carrier energy levels and the interactions between carriers in these levels is extracted for the first time.
Physical Review Letters, 2005
We report polarized photoluminescence excitation spectroscopy of the negative trion in single charge tunable InAs/GaAs quantum dots. The spectrum exhibits a p-shell resonance with polarized fine structure arising from the direct excitation of the electron spin triplet states. The energy splitting arises from the axially symmetric electron-hole exchange interaction. The magnitude and sign of the polarization are understood from the spin character of the triplet states and a small amount of quantum dot asymmetry, which mixes the wavefunctions through asymmetric e-e and e-h exchange interactions.
Voltage-Controlled Electron-Hole Interaction in a Single Quantum Dot
Journal of Superconductivity, 2005
The ground state of neutral and negatively charged excitons confined to a single self-assembled InGaAs quantum dot is probed in a direct absorption experiment by high resolution laser spectroscopy. We show how the anisotropic electron-hole exchange interaction depends on the exciton charge and demonstrate how the interaction can be switched on and off with a small dc voltage. Furthermore, we report polarization sensitive analysis of the excitonic interband transition in a single quantum dot as a function of charge with and without magnetic field.
It is demonstrated that the microphotoluminescence ( μ PL) spectrum of a single InAs/GaAs selfassembled quantum dot (QD) undergoes considerable changes when the primary laser excitation is complemented with an additional infrared laser. The primary laser, tuned slightly below the GaAs band gap, provides electron-hole pairs in the wetting layer (WL), as well as excess free electrons from ionized shallow acceptors in the GaAs barriers. An additional IR laser with a fixed energy well below the QD ground state transition generates excess free holes from deep levels in GaAs. The excess electron and hole will experience diffusion separately, due to the time separation between the two events of their generation, to eventually become captured into the QD. Although the generation rates of excess carries are much lower than that of the electron-hole pair generation in the WL, they considerably influence the QD emission at low temperatures. The integrated PL intensity increases by several times as compared to single-laser excitation, and the QD exciton spectrum is redistributed in favor of a more neutral charge configuration. The dependence of the observed phenomenon on the powers of the two lasers and the temperature has been studied and is consistent with the model proposed. The concept of dual excitation could be successfully applied to different low-dimensional semiconductor structures in order to manipulate their charge state and emission intensity.
Emission properties of single InAs/GaAs quantum dot pairs and molecules grown in GaAs nanoholes
Journal of Physics: Conference Series, 2010
We have studied the lateral coupling between InAs/GaAs quantum dot pairs embedded in a field-effect structure. Quantum dot pairs and molecules have been identified by the correlated evolution of the Coulomb blockade features of each QD in the pair. This behaviour is largely distorted in the presence of resonant coupling states in the QD molecule. Single QD voltage evolution shows a crossover in the lineshape profile, which is associated to Spectral Diffusion processes due to residual charged environment.
Coherent Spectroscopy of Optically Gated Charged Single InGaAs Quantum Dots
Physical Review Letters, 2003
The excited states of neutral and charged single InGaAs/GaAs quantum dots are studied using a confocal microspectroscopy technique. Because of their different Coulomb energy shifts, the charged and neutral states of the same quantum dot can be selectively excited. The charge of the quantum dot is controlled by a photo-depletion mechanism. Time-resolved coherent spectroscopy shows that the dephasing time of the excited states is longer when the quantum dot is charged. Rabi oscillation of the excited state of a singly charged quantum dot is demonstrated.
Peculiar many-body effects revealed in the spectroscopy of highly charged quantum dots
Nature Physics, 2007
Coulomb interactions between electrons lead to the observed multiplet structure and breakdown of the Aufbau principle for atomic d and f shells 1 . Nevertheless, these effects can disappear in extended systems. For instance, the multiplet structure of atomic carbon is not a feature of graphite or diamond. A quantum dot is an extended system containing ∼10 6 atoms for which electron-electron interactions do survive and the interplay between the Coulomb energy, J, and the quantization energy, E, is crucial to Coulomb blockade 2-5 . We have discovered consequences of Coulomb interactions in selfassembled quantum dots by interpreting experimental spectra with an atomistic calculation. The Coulomb effects, evident in the photon emission process, are tunable in situ by controlling the quantum dot charge from +6e to −6e. The same dot shows two regimes: J ≤ E for electron charging yet J E for hole charging. We find a breakdown of the Aufbau principle for holes; clear proof of non-perturbative hole-hole interactions; promotion-demotion processes in the final state of the emission process, effects first predicted a decade ago 6 ; and pronounced configuration hybridizations in the initial state. The level of charge control and the energy scales result in Coulomb effects with no obvious analogues in atomic physics.