S. Stufler

Fabrication of genuine single-quantum-dot light-emitting diodes

We present a simple approach for the fabrication of genuine single quantum-dot light-emitting diodes. A submicron wide bottom contact stripe is formed by focused ion beam implantation doping into a GaAs buffer layer. Successive overgrowth with a thin intrinsic layer incorporating self-assembled InAs quantum dots, followed by a top contact layer of complementary doping type and standard photolithographic processing, allows for electrical cross sections in the sub-μm2 range. In devices with sufficiently low dot densities, only one single dot is expected to be electrically addressed. Both the observed current versus voltage characteristics and the evolution of the electroluminescence spectra as a function of applied voltage clearly demonstrate that this goal has been achieved.

Power broadening of the exciton linewidth in a single InGaAs∕GaAs quantum dot

We use high-resolution photocurrent spectroscopy to investigate the ground state of a single quantum dot. In the limit of low optical excitation power, we observe a ground state linewidth down to 4μeV. With increasing excitation intensities, the linewidth shows a characteristic power broadening. This effect is a direct consequence of the saturation of the absorption in a two-level system under conditions of high excitation intensities. From a comparison of both effects, we conclude that power-dependent dephasing is negligible in our system.

Coherent and incoherent properties of single quantum dot photodiodes

Abstract 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 .

Coherent optoelectronics with single quantum dots

The optical properties of semiconductor quantum dots are in many respects similar to those of atoms. Since quantum dots can be defined by state-of-the-art semiconductor technologies, they exhibit long-term stability and allow for well-controlled and efficient interactions with both optical and electrical fields. Resonant ps excitation of single quantum dot photodiodes leads to new classes of coherent optoelectronic functions and devices, which exhibit precise state preparation, phase-sensitive optical manipulations and the control of quantum states by electrical fields.

Coherent Properties of Quantum Dot Two-Level Systems

In a single self-assembled InGaAs quantum dot, the one-exciton ground-state transition defines a two-level system, which appears as an extremely narrow resonance of only a few μeV width. The resonant interaction of this two-level system with cw laser fields can be studied in detail by photocurrent spectroscopy, revealing the fine structure of the excitonic ground state as well as the effects of nonlinear absorption and power broadening. For the case of pulsed laser fields and in the absence of decoherence, the two-level system represents a qubit. Excitations with ps laser pulses result in qubit rotations, which appear as Rabi oscillations in photocurrent experiments. Double pulse experiments further allow us to infer the decoherence time and to perform coherent control on a two level system.

Single quantum dot photodiodes: Rabi-flops in a two level system with electric contacts

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.