Time-resolved resonance fluorescence (RF) is used to analyse electron tunneling between a single self-assembled quantum dot (QD) and an electron reservoir. In equilibrium, the RF intensity reflects the average electron occupation of the QD and exhibits a gate voltage dependence that is given by the Fermi distribution in the reservoir. In the time-resolved signal, however, we find that the relaxation rate for electron tunneling is, surprisingly, independent of the occupation in the charge reservoir -in contrast to results from all-electrical transport measurements. Using a master equation approach, which includes both the electron tunneling and the optical excitation/recombination, we are able to explain the experimental data by optical blocking, which also reduces the electron tunneling rate when the QD is occupied by an exciton.PACS numbers: 73.21. La, 73.23.Hk, 73.63.Kv, 78.67.Hc Electron tunneling into semiconductor quantum dots (QDs) has been used to study Coulomb [1] and exchange interaction [2], as well as to prepare, read-out, and manipulate spin states [3,4]. It has also been employed to study shot noise [5,6] and reveal the Fano factor [7] in mesoscopic systems. Most of these transport measurements have been performed on semiconductor QDs, which were defined in a two-dimensional electron gas (2DEG) by lithography techniques [8]. Another QD system, which is highly interesting for optical purposes, are epitaxially-grown self-assembled QDs [9], where the optical excitonic transitions can be coupled to a photon light field to study quantum optics [10], e. g. in resonance fluorescence [11,12]. They are also extensively studied for optical devices, like single photon sources [13][14][15], QD lasers [16,17] or optical amplifiers [18].We use here resonance fluorescence (RF) as an optical probe to study for the first time the transport tunneling dynamics between an electron reservoir and a single selfassembled QD. Using voltage pulses and a time-resolved RF detection scheme, we are able to map the Fermi distribution in the electron reservoir and measure the relaxation rate for tunneling between the QD and the charge reservoir. We find clear evidence that the optical excitation of the QD reduces this rate, effectively leading to an optical blocking of single electron tunneling.The investigated sample was grown by molecular beam epitaxy (MBE) and resembles a field-effect-transistor structure [1,19] containing a layer of self-assembled InAs QDs (see also [20]).We use a confocal microscope setup in a bath-cryostat at a temperature of 4.2 K (see also [21]). RF spectroscopy of the exciton X and trion X − resonances at different gate voltages and laser frequencies shows a transition region between 0.26 V and 0.27 V (outlined in red in Fig. 1), where both transitions are simultaneously visible. Because of the thermally broadened distribution of electrons in the back contact, in this range of gate voltages, the QD is occupied by a single electron with a probability P , giving rise to the X − transition. Correspondingl...

We present capacitance-voltage spectra for the conduction band states of InAs quantum dots obtained under continuous illumination. The illumination leads to the appearance of additional charging peaks that we attribute to the charging of electrons into quantum dots containing a variable number of illumination-induced holes. By this we demonstrate an electrical measurement of excitonic states in quantum dots. Magnetocapacitance-voltage spectroscopy reveals that the electron always tunnels into the lowest electronic state. This allows us to directly extract, from the highly correlated many-body states, the correlation energy. The results are compared quantitatively to state of the art atomistic configuration interaction calculations, showing very good agreement for a lower level of excitations and also limitations of the approach for an increasing number of particles. Our experiments offer a rare benchmark to many-body theoretical calculations.

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Ultrasonic non-destructive testing (NDT) techniques are widely used in industry to evaluate properties of a component, material or system without causing any damage. The time of flight diffraction method (TOFD) is regarded as a valuable procedure for the detection of welding defects. We propose an innovative concept for NDT relying on tangential homology. The novel technique allows us to analyze topological properties of manifolds from discrete data. This in turn provides an automated classification of TOFD defects which does not require any interactions with a human operator.