Linear and nonlinear spectroscopic invsetigations of self-assembled quantum dots open up the possibility to study model quantum mechanical systems with potential applications in solid-state based quantum information processing. In this article we review recent advances in our understanding of the physics of self-assembled quantum dots and novel devices based upon them. We focus our attention on four key areas: (i) Investigations of individual quantum dots using photocurrent absorption spectoscopy (ii) The study of coupled pairs of electronically coupled quantum dots with field tunable coupling. (iii) Femtosecond experiments on small ensembles of quantum dots which demonstrate potential for quantum-coherent operations and (iv) applications of quantum dots in novel devices for wavelength selective optical data storage. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We present a structural study of uncapped InxGa1-xAs islands grown on GaAs (001) by molecular beam epitaxy (MBE) with a nominal concentration of x = 0.5. Surface-sensitive grazing incidence x-ray diffraction (GID) is applied to study shape, strain, and interdiffusion in these self-organised grown nanostructures. Contrast variation close to the K edge of As by anomalous GID at the (200) superstructure reflection enhances the chemical sensitivity of the measurement and allows for a direct determination of the InAs concentration as a function of the lateral strain in the quantum dots (QDs). The evaluation of intensity mappings recorded in reciprocal space close to the (200) reflection together with atomic force micrographs (AFM) is used to attribute the strain and the InAs concentration to a certain height in the quantum dots. Thereby, a three-dimensional model of the strain and interdiffusion profile of the InGaAs QDs is reconstructed. (C) 2004 Elsevier Ltd. All rights reserved.

On the basis of a single semiconductor quantum dot incorporated in a photodiode we have succeeded in preparing a two-level system with electric contacts, a setup which was previously not attained on the basis of other two-level systems. By means of such a single quantum dot photodiode we demonstrate the transfer of coherent optical excitations into deterministic photocurrents. Rabi oscillations are shown to be directly reflected in the photocurrent. For optical excitation with pi-pulses we observe a quantitative photocurrent given by I=f /spl times/ e, where f is the repetition frequency of the experiment and e is the elementary charge.

Semiconductor quantum dots can be described as quantum mechanical two-level systems. Under the influence of strong electromagnetic driving fields and in the absence of decoherence such systems exhibit Rabi flopping corresponding to a qubit rotation in the context of quantum computing. Based on a single QD incorporated in a photodiode we have prepared a two-level system with electric contacts. By means of this single QD photodiode we demonstrate the transfer of coherent optical excitations into a deterministic photocurrent. The QD photocurrent directly reflects Rabi flopping of the exciton state. Under pi-pulse excitation the exciton occupation is inverted from 0 to 1 leading ideally to the creation of exactly one exciton per laser pulse. For this condition, the photocurrent is determined by the repetition rate of the experiment f and the elementary charge e, resulting quantitatively in I = f . e.

Quantum mechanical systems, like atoms, molecules, ions, spin-systems and recently also semiconductor quantum dots can be described as two-level systems. Under the influence of strong electromagnetic driving fields and in the absence of decoherence such systems exhibit Rabi oscillations. The Rabi flop of a two-level system by means of an optical π-pulse corresponds to an inversion of the system with respect to its initial state, which is equivalent to a qubit rotation in the context of quantum computing. On the basis of a single semiconductor quantum dot incorporated into a photodiode we have succeeded in preparing a two-level system with electric contacts, a setup which was previously not attained on the basis of other two-level systems, such as atoms. On such a single quantum dot photodiode we perform a photocurrent technique that enables us to monitor the occupation probability of the ground state exciton in a single quantum dot. In a first experiment we show that under the condition of resonant ground state excitation the tunneling-photocurrent saturates for high excitation densities. This photocurrent saturation reflects the incoherent saturation limit of a resonantly driven two-level system (excitonic occupancy =0.5). In a second experiment, we demonstrate the transfer of coherent optical excitations into a deterministic photocurrent. Rabi oscillations are shown to be directly reflected in the photocurrent. For the application of π-pulses we observe a quantitative photocurrent which is given by I=f·e, with the repetition frequency of the experiment f and the elementary charge e.

We report Rabi‐oscillations of the ground state exciton in a single self‐assembled quantum dot (QD). Experimentally, we apply a photocurrent (PC) technique, that enables us to monitor the occupation probability of the exciton state by measuring the dc tunneling current out of the QD. Coherent PC spectrocopy was performed by resonant ps‐excitation of the ground state exciton. Rabi‐oscillations are shown to be directly reflected in the PC. In the case of π‐pulse excitation, we receive a quantitative PC given by I = f × e , with the repetition frequency of the experiment f and the elementary charge e . In our experiment we nearly reach this maximum current showing that our experiment acts as a quantitative proof for Rabi‐flopping in a single QD. In a systematic study of field‐dependence we find Rabi‐oscillations with reduced PC amplitude.

We report about level bleaching in the ground state of a single In0.5Ga0.5As quantum dot, which is observed in terms of a saturation of the photocurrent with increasing excitation power. Our results are explained by a rate equation model for the quantum dot as a two-level system.

Ultrafast relaxation phenomena in self-organized InAs/AlGaAs quantum dots with ground state transition energies of 1.42 eV are analyzed with different methods of fermosecond spectroscopy. A time-correlated luminescence experiment of an ensemble of InAs islands reveals a picosecond relaxation mechanism of carriers within the quantum dots. Moreover, modifications of the quantum dot properties due to excitation of the surrounding wetting layer is found. Employing a shadow-mask technique, a structure containing only a few zero-dimensional objects is studied. Selectively creating electron-hole pairs in excited states of approximately ten quantum dots, we observe characteristic line shifts of their ground state absorption in a two-color fermosecond transmission experiment. This finding demonstrates the feasibility of ultrafast studies of single self-assembled quantum dots.

A method is developed for calculating, in a consistent manner, the realistic electronic structure of three-dimensional (3-D) heterostructure quantum devices under bias and its current density close to equilibrium. The nonequilibrium electronic structure is characterized by local Fermi levels that are calculated self-consistently. We have applied this scheme to predict asymmetric Stark shifts and tunneling of confined electrons and holes in single-dot GaAs/InGaAs photodiodes.

Present-day information technology is based mainly on incoherent processes in conventional semiconductor devices 1 . To realize concepts for future quantum information technologies, which are based on coherent phenomena, a new type of ‘hardware’ is required 2 . Semiconductor quantum dots are promising candidates for the basic device units for quantum information processing. One approach is to exploit optical excitations (excitons) in quantum dots. It has already been demonstrated that coherent manipulation between two excitonic energy levels—via so-called Rabi oscillations—can be achieved in single quantum dots by applying electromagnetic fields 3 , 4 , 5 , 6 , 7 . Here we make use of this effect by placing an InGaAs quantum dot in a photodiode, which essentially connects it to an electric circuit. We demonstrate that coherent optical excitations in the quantum-dot two-level system can be converted into deterministic photocurrents. For optical excitation with so-called π-pulses, which completely invert the two-level system, the current is given by I = fe , where f is the repetition frequency of the experiment and e is the elementary charge. We find that this device can function as an optically triggered single-electron turnstile.

We have fabricated single-quantum-dot photodiodes by embedding InGaAs quantum dots in the intrinsic region of an n-i-Schottky diode combined with near-field shadow masks. As a function of the bias voltage, we study one and the same quantum dot in the two complementary regimes of photocurrent and photoluminescence. The Stark shift of the exciton ground state continues monotonically in both regimes, confirming nicely the observation of the same quantum dot in photoluminescence and photocurrent. In the limit of high electric fields, we observe a broadening of the photocurrent linewidth from which we determine a strongly reduced exciton lifetime of below 1 ps.

We report about optical experiments on single self-assembled quantum dots. In power-dependent low-temperature magneto-photoluminescence experiments we have analysed the emission spectra of single dots for increasing exciton occupation numbers. Decays from different configurations (up to 4 excitons) lead to a renormalization of the emission lines in the region of the s- and p-shell of the dot. Photoluminescence excitation spectroscopy further allows us to explore the absorption properties of a single-quantum dot. Both interband absorption and comparably strong phonon-assisted absorption via InGaAs and GaAs LO phonons are observed, as well as sequential phonon-assisted biexciton generation followed by sequential biexciton decay. Working towards applications of single quantum dots, local charge injection/extraction into/from dots is performed on special p–i–n structures via a STM-tip. STM-induced luminescence from a single-dot results in single-line emission from the quantum dot ground state under the condition of low injection currents. Reverse operation of such a single quantum dot LED allows in addition for spectrally resolved photocurrent experiments. Characteristic sharp peaks in the STM tip current vs. excitation energy are attributed to resonant quantum dot interband absorption processes.

We report level bleaching in the ground state of a single In0.5Ga0.5As quantum dot. This behavior arises from the nonlinear absorption of a single quantum state. The level bleaching is observed in terms of a saturation of the photocurrent with increasing excitation power under the condition of resonant excitation in the quantum dot ground state. Furthermore, the photocurrent saturation is put down to a fundamental rate equation model. The steady-state solutions are in good agreement with the experimentally observed power dependence of the photocurrent.

Summary form only given. Quantum dots, often referred to as artificial atoms, open the field of quantum resolved spectroscopy to semiconductor physics. Using near-field microscopy through electron beam written shadow masks, we have isolated single InGaAs quantum dots for advanced spectroscopic analysis. In power-dependent low-temperature photoluminescence studies, we have investigated the influence of few particle interactions on the emission spectra for increasing exciton occupation numbers. Decays from different configurations (up to 4 excitons) lead to the appearance of specifically renormalized emission lines in the region of the s- and p-shell. Contributions from different shells can thereby be assigned by magneto-photoluminescence experiments.

We report about spatially resolved experiments on self-assembled InGaAs quantum dots. Single quantum dots can be investigated by using STM-induced luminescence spectroscopy. The quantum dot occupancy can be increased via the STM tip current, which results in state filling and therefore in the onset of discrete excited state luminescence. In the limit of low injection currents, a single emission line from the ground state of the dot is observed. Using near-field spectroscopy through shadow masks, we have investigated the optical properties of single self-assembled InGaAs quantum dots as a function of occupancy and magnetic field. This allows us to fully resolve diamagnetic/orbital effects, Zeeman splitting, and to determine manybody-corrections. Photoluminescence excitation spectra further reveal a strong contribution of phonon assisted processes in quantum dot absorption.

Fully confined excitons are observed in natural quantum dots, which are formed by well width fluctuations in narrow GaAs quantum wells. Contributions from a single quantum dot can be isolated by using spatially resolved spectroscopy and resonant charge injection in an electric-field-tunable coupled quantum well structure. In optical and magneto-optical experiments the excitonic and biexcitonic ground states are identified and analyzed.

Quantum mechanical systems, like atoms, molecules, ions, spin-systems, and recently also semiconductor quantum dots can be described in lowest order as two-level systems. Here we report in particular about the properties of excitonic two-level systems defined in the ground state of single self-assembled InGaAs quantum dots. By photocurrent experiments performed on single quantum dot photodiodes we have investigated those two-level systems in the incoherent and coherent regime. Thereby we demonstrate, that a single quantum dot photodiode can be well described as a two-level system with electric contacts. Under the influence of strong electromagnetic driving fields and in the absence of decoherence such systems exhibit Rabi oscillations, as predicted by theory. The Rabi flop of a two-level system by means of an optical π-pulse corresponds to an inversion of the system with respect to its initial state, which is equivalent to a qubit rotation in the context of quantum computing. By means of such a single quantum dot photodiode we demonstrate the transfer of coherent optical excitations into deterministic photocurrents. For optical excitation with π-pulses we observe a quantitative photocurrent given by I=f e, where f is the repetition frequency of the experiment and e is the elementary charge. Most of the key components for information technology are based on semiconductor devices with electronic or opto-electronic functions 1. From physical point of view all those devices rely on incoherent phenomena in view of the associated electronic states in a solid. The use of coherent phenomena for the implementation of quantum information technology is expected to give plenty of room for advanced developments in the future 2. Semiconductor quantum dots (QDs), often referred to as artificial atoms, are suitable entities to implement arrays of qubits for quantum information processing. One possible approach thereby is the use of excitonic excitations in the ground state of a QD as basis for a two-level system (see Fig. 1). Recently coherent population oscillations, so called Rabi oscillations 3, have been demonstrated in the exciton population of single QDs 4-8 . Low temperature dephasing times for excitons in self-assembled QDs have been