We have succeeded in generating highly charged excitons in InAs self-assembled quantum dots by embedding the dots in a field-effect heterostructure. We discover an excitonic Coulomb blockage: over large regions of gate voltage, the exciton charge remains constant. We present here a summary of the emission properties of the charged excitons.

We describe the design and performance of a fiber-based confocal microscope for cryogenic operation. The microscope combines positioning at low temperatures along three space coordinates of millimeter translation and nanometer precision with high stability and optical performance at the diffraction limit. It was successfully tested under ambient conditions as well as at liquid nitrogen (77 K) and liquid helium (4 K) temperatures. The compact nonmagnetic design provides for long term position stability against helium refilling transfers, temperature sweeps, as well as magnetic field variation between -9 and 9 T. As a demonstration of the microscope performance, applications in the spectroscopy of single semiconductor quantum dots are presented.

Magnetic field and temperature dependent photoluminescence studies on neutral and charged excitons in individual InAs quantum dots allow us to uncover different mechanisms by which the discrete quantum dot states are coupled to delocalized continuum states in a quantum well (the wetting layer). The behavior of the neutral and singly charged excitons can be explained taking only discrete quantum dot states into account. For doubly and triply charged excitons we have to consider spin dependent coherent and incoherent interactions between discrete quantum dot states and delocalized wetting layer states.

By embedding a layer of self-assembled quantum dots into a field-effect structure, we are able to control the exciton charge in a single dot. We present the results of photoluminescence experiments as a function of both charge and magnetic field. The results demonstrate a hierarchy of energy scales determined by quantization, the direct Coulomb interaction, the electron–electron exchange interaction, and the electron–hole exchange interaction. For excitons up to the triply charged exciton, the behavior can be understood from a model assuming discrete levels within the quantum dot. For the triply charged exciton, this is no longer the case. In a magnetic field, we discover a coherent interaction with the continuum states, the Landau levels associated with the wetting layer.

We review the optics of negatively charged excitons in self assembled InAs/GaAs quantum dots. The charging state of a quantum dot was tuned electrostatically in a field effect structure. We investigated in this way the neutral exciton as well as the negatively singly, doubly and triply charged excitons, namely the X0, X1–, X2– and X3– excitons. A Zeeman splitting and a diamagnetic shift in the photoluminescence emission line is clearly observed for X0, X1– and X2– exciton as a function of magnetic field. In contrast the triply negatively charged exciton shows a completely different magnetic behaviour. Here, under the influence of a high magnetic field, the quantum dot electronic states are found to be hybridized with the continuum of states in the wetting layer. Such a hybridization turns out to be induced by photon emission of the X3– exciton. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report on optical spectroscopy of self-assembled InAs quantum dots in a magnetic field. We describe how we measure the emission characteristics of a single quantum dot (QD) in high magnetic fields at low temperature using a miniature, fiber-based confocal microscope. Example results are presented on a QD whose charge can be controlled using a field-effect device. For the uncharged, singly and doubly charged excitons we find a diamagnetism and the spin Zeeman effect. In contrast, for the triply-charged exciton we find a fundamentally different behavior. Anti-crossings in magnetic field imply that confined states of the QD are hybridized with Landau-like levels associated with the two-dimensional continuum.

The self-assembly of semiconductor quantum dots has opened up new opportunities in photonics. Quantum dots are usually described as ‘artificial atoms’, because electron and hole confinement gives rise to discrete energy levels. This picture can be justified from the shell structure observed as a quantum dot is filled either with excitons1 (bound electron–hole pairs) or with electrons2. The discrete energy levels have been most spectacularly exploited in single photon sources that use a single quantum dot as emitter3,4,5,6. At low temperatures, the artificial atom picture is strengthened by the long coherence times of excitons in quantum dots7,8,9, motivating the application of quantum dots in quantum optics and quantum information processing. In this context, excitons in quantum dots have already been manipulated coherently10,11,12. We show here that quantum dots can also possess electronic states that go far beyond the artificial atom model. These states are a coherent hybridization of localized quantum dot states and extended continuum states: they have no analogue in atomic physics. The states are generated by the emission of a photon from a quantum dot. We show how a new version of the Anderson model that describes interactions between localized and extended states can account for the observed hybridization.

A summary is presented of optical spectroscopy measurements on self-assembled quantum rings. The rings are unusual quantum dots, having a pronounced hole in the centre, and can be self-assembled during growth using a modified Stranski-Krastanow procedure. We embed the quantum rings in a capacitor-like structure which enables us to load the quantum rings with electrons and to apply a vertical electric field. The results of these experiments, performed on a large number of individual quantum rings at low temperature, reveal pronounced Coulomb interactions and an excitonic Stark effect. Additionally, we have applied a magnetic field, primarily to probe the form of the lateral wave functions, but we find that this too yields insights into the Coulomb interactions. Model calculations are used to interpret these results.

We report on low temperature (4.2 K) magneto-luminescence measurements performed on charged tuneable GaInAs self-assembled quantum dots. We mapped the magnetic field dispersion (0-9 Tesla) of the exciton with excess electron charges set from 0 to 3. For the doubly and triply charged excitons the emission line shows a doublet corresponding to the singlet and triplet configuration of the quantum dot in the final state after photon emission. We map here for the first time the dispersion of the singlet state of the doubly charged exciton and show that the triply charged exciton undergoes a transition resulting from a magnetic field induced frustration of the Hund's rule.

We have measured the vertical Stark effect of excitons confined to individual self-assembled ring-shaped quantum dots. We find that the excitons have very large permanent dipole moments corresponding to electron-hole separations up to 2.5 nm, comparable to the nanostructures' physical height. We find a trend of both permanent dipole moment and polarizability on the emission energy, but a very strong correlation between the permanent dipole moment and the polarizability.

We present results on the influence of a magnetic field on excitons in semiconductor quantum dots, concentrating on the diamagnetic curvature. We use samples with a bimodal ensemble photoluminescence (PL) and we find that for the low-energy PL branch, the diamagnetic curvature is independent of charge, yet for the high-energy branch, the diamagnetic curvature is strongly reduced with excess charge. Guided by model calculations, we interpret the two classes as typical of the strong and intermediate confinement regimes. In the light of this, we predict that in the weak confinement regime the excitonic diamagnetic shift is strongly dependent on surplus charge, corresponding to a reversal in sign of the conventional diamagnetic shift for neutral excitons.

We report on measurements of the magneto-optical properties of excitons confined in ring-shaped self-assembled semiconductor quantum dots. The rings are embedded in a field-effect structure that allows the number of confined electrons to be set electrostatically. In addition, electron-hole pairs are generated optically. The resulting photoluminescence spectra of neutral, singly and doubly charged excitons were measured at 4.2K as a function of the applied magnetic field (0–9T). The emission energy shows a diamagnetic shift as well as a Zeeman splitting. We measured the emission of different charge states of the exciton in many individual dots. In a few of the measured rings, a new behavior was observed, namely a clear departure from the low field diamagnetic dispersion for fields larger than 6T.