Triplet-singlet relaxation in semiconductor single and double quantum dots

We report on measurements of hole transport through a double quantum dot structure formed by trench isolation from a SiGe:Si heterostructure. A period change in the Coulomb oscillations is observed upon changing a gate bias, which is attributed to the lowering of one of the tunnel barriers, effectively changing the device to a single quantum dot. Accompanying the period change is a significant change in the level of noise associated with the oscillations. This is explained by a carrier energy filtering effect in the double dot compared with the single dot, caused by the slight difference in energy level spacing in each quantum dot.

Summary form only given. Direct observations of the dynamics of single excitonic quantum dots require the use of transient optical spectroscopy. Measurements of the lifetimes of single QD states, Rabi oscillations, biexciton formation and decay, and the development of quantum logic gates are examples of dynamical effects in quantum dots most easily studied through transient nonlinear spectroscopy, but are extremely challenging at the single dot level. We report here the first observation of transient differential transmission from single quantum dots as well as evidence of Rabi oscillations in single quantum dots.Under investigation are quantum dots naturally formed by interface fluctuations in a 42 /spl Aring/ GaAs/Al/sub 0.3/Ga/sub 0.7/As quantum well. Excitons in isolated QD's are probed through 500 nm apertures in an Al mask laid directly onto the sample's surface.

Summary form only given. The excitonic optical response of single quantum dots (QD) has recently been isolated by using high spatial resolution techniques. Photoluminescence (PL) magneto-optical studies of these confined systems have shown interesting physical phenomena such as Zeeman splittings, diamagnetic shifts and the Overhauser effect. Coherent nonlinear optical spectroscopy provides a powerful additional tool for the study of the interaction between magnetic substrates, as has been demonstrated for a single quantum dot. By directly probing the quantum dot resonances under the effect of a magnetic field we can obtain information about spin relaxation times and the presence of dark states. In this paper we show the effect of a magnetic field on single quantum dot resonances (i.e. Zeeman splitting and diamagnetic shift) and report the nonlinear optical response as a function of magnetic field.

The combination of single particle detection and ultrafast laser pulses is an instrumental method to track dynamics at the femtosecond time scale in single molecules, quantum dots and plasmonic nanoparticles. Optimal control of the extremely short-lived coherences of these individual systems has so far remained elusive, yet its successful implementation would enable arbitrary external manipulation of otherwise inaccessible nanoscale dynamics. In ensemble measurements, such control is often achieved by resorting to a closed-loop optimization strategy, where the spectral phase of a broadband laser field is iteratively optimized. This scheme needs long measurement times and strong signals to converge to the optimal solution. This requirement is in conflict with the nature of single emitters whose signals are weak and unstable. Here we demonstrate an effective closed-loop optimization strategy capable of addressing single quantum dots at room temperature, using as feedback observable the two-photon photoluminescence induced by a phase-controlled broadband femtosecond laser. Crucial to the optimization loop is the use of a deterministic and robust-against-noise search algorithm converging to the theoretically predicted solution in a reduced amount of steps, even when operating at the few-photon level. Full optimization of the single dot luminescence is obtained within ~100 trials, with a typical integration time of 100 ms per trial. These times are faster than the typical photobleaching times in single molecules at room temperature. Our results show the suitability of the novel approach to perform closed-loop optimizations on single molecules, thus extending the available experimental toolbox to the active control of nanoscale coherences.

In this thesis, anisotropic etching of graphite and graphene in a hydrogen (H) plasma is investigated. The exposure of graphite flakes at different plasma pressures and sample-plasma distances reveals the existence of two different plasma regimes: the \textit{direct} and the \textit{remote} regime. In the direct regime, high energetic H-ions continuously induce new defects into the graphite surface during the etching process, thus leading to a perforated surface. In the remote plasma regime, on the other hand, well-defined hexagonal etch pits evolve, which grow in size, while their number remains constant. This indicates anisotropic etching, which takes place only at pre-existing defects and edges and leaves the graphite basal plane pristine. In a second step of the experiment, the substrate dependence of single layer graphene etching in the remote plasma regime is investigated. Interestingly, the etching is only anisotropic for hexagonal boron nitride substrates but isotropic if graphene is placed on Si/SiO$_2$. It was previously found that the edges of H plasma defined hexagons on graphite run along the zigzag (ZZ) direction of the crystal lattice. Hence, by inducing artificial defects into a graphene flake, one can tailor diverse graphene nano-structures with presumably well-defined ZZ edges, such as e.g. graphene nano ribbons. However, it is not exactly known how good the quality of as-fabricated graphene edges really is. This open question is addressed in the second work, where the quality of H plasma defined graphene edges is investigated by means of atomic resolution atomic force microscopy (AFM), Raman spectroscopy and low-temperature electronic transport experiments. AFM measurements on hexagons created on graphite surfaces reveal that the edges are aligned to the ZZ direction and the absence of the Raman D-peak suggests that these edges are high quality ZZ edges. In contrast, hexagons created in single layer graphene on hexagonal boron nitride exhibit a relatively large D-peak, pointing towards the presence of edge disorder or armchair segments. Polarization-dependent Raman experiments indicate that the edges consist of a mixture of armchair and ZZ segments. Furthermore, electronic transport measurements, combined with quantum transport simulations, support the findings from the Raman experiments. Hence, H plasma defined edges still suffer from edge disorder and the etching process needs to be further optimized in order to get high quality crystallographic graphene edges. In addition to the graphene experiments, investigations on Ge/Si core/shell nano wires are conducted. In particular, single, double, and triple quantum dots (QDs) of various sizes and with low occupation numbers are formed. In the single QD regime, indications for the last hole state are found. Moreover, Pauli spin blockade is observed in the double QD regime. These results open the door for exploring Ge/Si core/shell nano wires as a potential platform for hole spin-qubit experiments.

STM-photocurrent-spectroscopy on single self-assembled InGaAs quantum dots

We demonstrate that ultraclean single, double, and triple quantum dots (QDs) can be formed reliably in a carbon nanotube (CNT) by a straightforward fabrication technique. The QDs are electrostatically defined in the CNT by closely spaced metallic bottom gates deposited in trenches in SiO2 by sputter deposition of Re. The carbon nanotubes are then grown by chemical vapor deposition (CVD) across the trenches and contacted using conventional resist-based electron beam lithography. Unlike in previous work, the devices exhibit reproducibly the characteristics of ultraclean QDs behavior even after the subsequent electron beam lithography and chemical processing steps. We specifically demonstrate the high quality using CNT devices with two narrow bottom gates and one global back gate. Tunable by the gate voltages, the device can be operated in four different regimes: (i) fully p-type with ballistic transport between the outermost contacts (over a length of 700 nm), (ii) clean n-type single QD behavior where a QD can be induced by either the left or the right bottom gate, (iii) n-type double QD, and (iv) triple bipolar QD where the middle QD has opposite doping (p-type). Our simple fabrication scheme opens up a route to more complex devices based on ultraclean CNTs, since it allows for postgrowth processing.

We report the fabrication of single and double hole quantum dots using a double-layer-gate design on an undoped accumulation mode /GaAs heterostructure. Electrical transport measurements of a single quantum dot show varying addition energies and clear excited states. In addition, the two-level-gate architecture can also be configured into a double quantum dot with tunable inter-dot coupling.

We report on the magneto-optical study of spin polarized energetic fine structures for exciton complex in single CdSe quantum dot (QD) by using micro- photoluminescence (micro-PL) spectroscopy. The zero-field splitting of exciton luminescence peak arisen from the anisotropic exchange interaction of carriers in the QDs was observed. The g-factors for exciton and negatively-charged exciton, i.e. trion in a single QD were determined by fitting the magnetic field dependence of the corresponding PL peaks. By exciting the single QD with circularly polarized light of σ- and σ+ polarization, the spin-up and spin-down trions were selectively generated. The ratio, τ/τsf, of the exciton lifetime and the time constants for the spin-flipping process of trion in a single QD was estimated to be 0.13, which implies a long spin-lifetime in single CdSe QD.

We have grown InP quantum dots in GaInP by gas-source molecular-beam epitaxy on sharp tips etched out of a GaAs substrate. The growth parameters were optimized to produce single quantum dots on the tips, predominantly at the edges. Intensity correlation measurements of the fluorescence from single dots on these tips reveal antibunched photon emision. We propose to use these tip structures with stable single photon emitters at their apex as active probes for further controlled experiments in quantum and nano-optics.

Even as today’s most prominent spin-based qubit technologies are maturing in terms of capability and sophistication, there is growing interest in exploring alternate material platforms that may provide advantages, such as enhanced qubit control, longer coherence times, and improved extensibility. Recent advances in heterostructure material growth have opened new possibilities for employing hole spins in semiconductors for qubit applications. Undoped, strained Ge/SiGe quantum wells are promising candidate hosts for hole spin-based qubits due to their low disorder, large intrinsic spin–orbit coupling strength, and absence of valley states. Here, we use a simple one-layer gated device structure to demonstrate both a single quantum dot as well as coupling between two adjacent quantum dots. The hole effective mass in these undoped structures, m* ∼ 0.08 m0, is significantly lower than for electrons in Si/SiGe, pointing to the possibility of enhanced tunnel couplings in quantum dots and favorable qubit–qubit interactions in an industry-compatible semiconductor platform.

By using a simple model we consider single-channel transmission through a double quantum dot that consists of two single dots coupled by a wire of finite length L . Each of the two single dots is characterized by a few energy levels only, and the wire is assumed to have only one level whose energy depends on the length L . The transmission is described by using S matrix theory and the effective non-Hermitian Hamilton operator H(eff) of the system. The decay widths of the eigenstates of H(eff) depend strongly on energy. The model explains the origin of the transmission zeros of the double dot that is considered by us. Mostly, they are caused by (destructive) interferences between neighboring levels and are of first order. When, however, both single dots are identical and their transmission zeros are of first order, those of the double dot are of second order. First-order transmission zeros cause phase jumps of the transmission amplitude by pi, while there are no phase jumps related to second-order transmission zeros. In this latter case, a phase jump occurs due to the fact that the width of one of the states vanishes when crossing the energy of the transmission zero. The parameter dependence of the widths of the resonance states is determined by the spectral properties of the two single dots.

The results of an x-ray study on freestanding, self-assembled $\mathrm{In}\mathrm{As}∕\mathrm{Ga}\mathrm{As}$ quantum dots grown by molecular beam epitaxy are presented. The studied samples cover the range from statistically distributed single quantum dots to quantum dot bimolecules, and finally to quantum dot quadmolecules. The x-ray diffraction data of the single quantum dots and the bimolecules, obtained in grazing incidence geometry, have been analyzed using the isostrain model. An extended version of the isostrain model has been developed, including the lateral arrangement of the quantum dots within a quantum dot molecule and the superposition of the scattering from different parts of the dots. This model has been applied to the scattering maps of all three samples. Quantitative information about the positions of the dots, the shape, and the lattice parameter distribution of their crystalline core has been obtained. For the single dot and the bimolecule, a strong similarity of the shape and lattice parameter distribution has been found, in agreement with the similarity of their photoluminescence spectra.

We have performed high spatial resolution cathodoluminescence images of single InAs/InP quantum dots. Carrier diffusion lengths in the wetting layer below single dots are evaluated as a function of carrier injection, temperature and sample doping.

A detailed understanding of the population and coherence dynamics in optically driven individual emitters in solids and their signatures in ultrafast nonlinear-optical signals is of prime importance for their applications in future quantum and optical technologies. In a combined experimental and theoretical study on exciton complexes in single semiconductor quantum dots we reveal a detailed picture of the dynamics employing three-beam polarization-resolved four-wave mixing (FWM) micro-spectroscopy. The oscillatory dynamics of the FWM signals in the exciton-biexciton system is governed by the fine-structure splitting and the biexciton binding energy in an excellent quantitative agreement between measurement and analytical description. The analysis of the excitation conditions exhibits a dependence of the dynamics on the specific choice of polarization configuration, pulse areas and temporal ordering of driving fields. The interplay between the transitions in the four-level exciton system leads to rich evolution of coherence and population. Using two-dimensional FWM spectroscopy we elucidate the exciton-biexciton coupling and identify neutral and charged exciton complexes in a single quantum dot. Our investigations thus clearly reveal that FWM spectroscopy is a powerful tool to characterize spectral and dynamical properties of single quantum structures.