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In this study, a generalized guideline is identified to predict the interaction between two-dimensional (2D) layered materials and substrate surfaces. Additionally, the van der Waals (vdW) heterostructures commensurability, the phase formation and the strain relaxation are identified during interface growth. To achieve such a general overview, the case of Ge-Sb-Te (GST) alloys on InAs(111) is studied. In this system, low-lattice mismatch conditions are fulfilled to avoid relaxation due to formation of misfit dislocations and allow to correctly identify vdW epitaxy. At the same time, the substrate can be efficiently prepared into self- and un-passivated surfaces to clarify the role of the surface interaction. Furthermore, the GST epilayer exhibits two different highly ordered 2D structures and a three-dimensional disordered structure, allowing to directly infer the nature of the epitaxy. This study opens the way for the design and mastering of vdW epitaxial growth of 2D heterostructures as well as hybrid 2D and non-layered materials.

The present work displays a route to design strain gradients at the interface between substrate and van der Waals bonded materials. The latter are expected to grow decoupled from the substrates and fully relaxed and thus, by definition, incompatible with conventional strain engineering. By the usage of passivated vicinal surfaces we are able to insert strain at step edges of layered chalcogenides, as demonstrated by the tilt of the epilayer in the growth direction with respect of the substrate orientation. The interplay between classical and van der Waals epitaxy can be modulated with an accurate choice of the substrate miscut. High quality crystalline GexSb2Te3+x with almost Ge1Sb2Te4 composition and improved degree of ordering of the vacancy layers is thus obtained by epitaxial growth of layers on 3–4° stepped Si substrates. These results highlight that it is possible to build and control strain in van der Waals systems, therefore opening up new prospects for the functionalization of epilayers by directly employing vicinal substrates.

The crystallization behavior of amorphous nano regions (20–100 nm in diameter) embedded in a textured epitaxial Ge2Sb2Te5 (GST) 25 nm thick film grown on a Si (1 1 1) substrate has been investigated in situ by transmission electron microscopy (TEM) analysis. The amorphous regions were obtained by irradiation with 30 keV Ge+ at a fluence of 1.5 × 1014 ions cm−2 of masked samples. The adopted configuration simulates the GST structure of a device in the RESET state, it is then of relevance for understanding their data retention characteristics. The in situ TEM analysis indicates that the amorphous to crystal transition for 20 nm dots is characterized by a growth velocity of 3.6 pm s −1 at 75 °C, probably related to the external partially damaged area. At 90 °C annealing crystallization is completed for 20 nm dot. In the case of 50 and 100 nm diameter amorphous dots a growth velocity of about 2.6 pm s −1 was observed at 90 °C and of 170 pm s −1 at 110 °C. The transition is governed only by rearrangement (nucleation is absent) of the atoms located at the boundary of the amorphous dot with the surrounding crystalline regions. In some cases a preferential growth along surfaces normal to the [1 1 1] and to the [1 2 1] directions has been found. The crystallographic characterization of the regrowth crystal indicates a good matching with the zone axis of the surrounding material although the seed, a Si (1 1 1) wafer, at the bottom interface is damaged by the implant.

Dynamic control over the full g-tensor in individual InGaAs/GaAs self-assembled quantum dots is achieved by inducing external strain via a piezoelectric actuator. The full g-tensor is obtained by measuring in different geometries with different angles between an externally applied magnetic field and the quantum dot growth axes. A large decrease in the out-of-plane hole g-factor with strain is observed, whereas the other components are found to be less sensitive. To further investigate this, a numerical model based on eight-band k.p-theory is used and an excellent agreement with the experimental results is established, both qualitatively and quantitatively. Furthermore, the calculations reveal the origin of the observed large change in the out-of-plane hole g-factor to be the increase in heavy-hole light-hole splitting under compressive stress.

GeSbTe (GST) phase-change alloys feature layered crystalline structures made of lamellae separated by van der Waals (vdW) gaps. This work sheds light on the dependence of interlamellae interactions at the vdW gap on film thickness of GST alloys as probed by vibrational spectroscopy. Molecular beam epitaxy is used for designing GST layers down to a single lamella. By combining density-functional theory and Raman spectroscopy, a direct and simple method is demonstrated to identify the thickness of the GST film. The shift of the vibrational modes is studied as a function of the layer size, and the low-frequency range opens up a new route to probe the number of lamellae for different GST compositions. Comparison between experimental and theoretical Raman spectra highlights the precision growth control obtained by the epitaxial technique.

Scalable fabrication of magnetic 2D materials and heterostructures constitutes a crucial step for scaling down current spintronic devices and the development of novel spintronic applications. Here, we report on van der Waals (vdW) epitaxy of the layered magnetic metal Fe3GeTe2 (FGT) - a 2D crystal with highly tunable properties and a high prospect for room temperature ferromagnetism (FM) - directly on graphene by employing molecular beam epitaxy. Morphological and structural characterization confirmed the realization of large-area, continuous FGT/graphene heterostructure films with stable interfaces and good crystalline quality. Furthermore, magneto-transport and x-ray magnetic circular dichroism investigations confirmed a robust out-of-plane FM in the layers, comparable to state-of-the-art exfoliated flakes from bulk crystals. These results are highly relevant for further research on wafer-scale growth of vdW heterostructures combining FGT with other layered crystals such as transition metal dichalcogenides for the realization of multifunctional, atomically thin devices., They also acknowledge the provision of beamtime under the project HC-4068 at the European Synchrotron Radiation Facility (ESRF), located in Grenoble (France). ICN2 researchers acknowledge support from the European Union Horizon 2020 research and innovation programme under Grant Agreement No. 881603 (Graphene Flagship).

In recent years strain engineering is proposed in chalcogenide superlattices (SLs) to shape in particular the switching functionality for phase change memory applications. This is possible in Sb2Te3/GeTe heterostructures leveraging on the peculiar behavior of Sb2Te3, in between covalently bonded and weakly bonded materials. In the present study, the structural and thermoelectric (TE) properties of epitaxial Sb2+xTe3 films are shown, as they represent an intriguing option to expand the horizon of strain engineering in such SLs. Samples with composition between Sb2Te3 and Sb4Te3 are prepared by molecular beam epitaxy. A combination of X-ray diffraction and Raman spectroscopy, together with dedicated simulations, allows unveiling the structural characteristics of the alloys. A consistent evaluation of the structural disorder characterizing the material is drawn as well as the presence of both Sb-2 and Sb-4 slabs is detected. A strong link exists among structural and TE properties, the latter having implications also in phase change SLs. A further improvement of the TE performances may be achieved by accurately engineering the intrinsic disorder. The possibility to tune the strain in designed Sb2+xTe3/GeTe SLs by controlling at the nanoscale the 2D character of the Sb2+xTe3 alloys is envisioned.

We present an experimental and theoretical study of the optical properties of asymmetric strain-free GaAs quantum dots (QDs) fabricated by filling of self-organized nanoholes (NHs) created by local droplet etching. The energy levels are calculated within the effective mass approximation using as input a model anisotropic QD shape with C 2 v symmetry based on atomic force microscopy (AFM) profiles of NHs. The influence of the QD height and shape anisotropy on the exciton emission energy and the s- p x and s- p y energy splittings is studied theoretically. The experimentally observed bound nature of excitonic states is well reproduced by our theoretical approach which includes direct Coulomb energies and correlation effects. We investigate the fine-structure splitting (FSS) of the neutral exciton as a function of dot size. Theoretical calculations of the long-range electron-hole (e-h) exchange interaction predict an increase of the FSS with decreasing QD height, which describes well the experimental trend.

Wide passband interdigital transducers are employed to establish a stable phase-lock between a train of laser pulses emitted by a mode-locked laser and a surface acoustic wave generated electrically by the transducer. The transducer design is based on a multi-harmonic split-finger architecture for the excitation of a fundamental surface acoustic wave and a discrete number of its overtones. Simply by introducing a variation of the transducer's periodicity $p$, a frequency chirp is added. This combination results in wide frequency bands for each harmonic. The transducer's conversion efficiency from the electrical to the acoustic domain was characterized optomechanically using single quantum dots acting as nanoscale pressure sensors. The ability to generate surface acoustic waves over a wide band of frequencies enables advanced acousto-optic spectroscopy using mode-locked lasers with fixed repetition rate. Stable phase-locking between the electrically generated acoustic wave and the train of laser pulses was confirmed by performing stroboscopic spectroscopy on a single quantum dot at a frequency of 320 MHz. Finally, the dynamic spectral modulation of the quantum dot was directly monitored in the time domain combining stable phase-locked optical excitation and time-correlated single photon counting. The demonstrated scheme will be particularly useful for the experimental implementation of surface acoustic wave-driven quantum gates of optically addressable qubits or collective quantum states or for multi-component Fourier synthesis of tailored nanomechanical waveforms., Comment: revised manuscript

We study the effects of the nonlinear piezoelectricity and the In distribution on the exciton energy, the electron-hole electric dipole moment, and the fine-structure splitting in stress-tunable In(Ga)As/GaAs quantum dots integrated onto a piezoelectric actuator. In particular, we investigate in detail the contributions of various elements of the expansion of the electrical polarization in terms of externally induced elastic strain on the latter two important quantum dot properties. Based on the comparison of the effects of first- and second-order piezoelectricity we provide a simple relation to estimate the influence of applied anisotropic stress on the quantum dot dipole moment for quantum dots significantly lattice mismatched to the host crystal.

The present work displays a route to design strain gradients at the interface between substrate and van der Waals bonded materials. The latter are expected to grow decoupled from the substrates and fully relaxed and thus, by definition, incompatible with conventional strain engineering. By the usage of passivated vicinal surfaces we are able to insert strain at step edges of layered chalcogenides, as demonstrated by the tilt of the epilayer in the growth direction with respect of the substrate orientation. The interplay between classical and van der Waals epitaxy can be modulated with an accurate choice of the substrate miscut. High quality crystalline Ge

Deep cooling of electron and nuclear spins is equivalent to achieving polarization degrees close to 100% and is a key requirement in solid-state quantum information technologies. While polarization of individual nuclear spins in diamond and SiC (ref. ) reaches 99% and beyond, it has been limited to 50-65% for the nuclei in quantum dots. Theoretical models have attributed this limit to formation of coherent 'dark' nuclear spin states but experimental verification is lacking, especially due to the poor accuracy of polarization degree measurements. Here we measure the nuclear polarization in GaAs/AlGaAs quantum dots with high accuracy using a new approach enabled by manipulation of the nuclear spin states with radiofrequency pulses. Polarizations up to 80% are observed-the highest reported so far for optical cooling in quantum dots. This value is still not limited by nuclear coherence effects. Instead we find that optically cooled nuclei are well described within a classical spin temperature framework. Our findings unlock a route for further progress towards quantum dot electron spin qubits where deep cooling of the mesoscopic nuclear spin ensemble is used to achieve long qubit coherence. Moreover, GaAs hyperfine material constants are measured here experimentally for the first time.

In forthcoming quantum networks various quantum systems might be involved to accomplish individual tasks, including storage of quantum states, quantum logic operations, error correction, or entanglement distillation. An interface between a single photon emitter and a potential photon storage could provide one fundamental building block of such a hybrid quantum system.

The decrease of thermal conductivity is crucial for the development of efficient thermal energy converters. Systems composed of a periodic set of very thin layers show among the smallest thermal conductivities reported to-date. Here, we fabricate in an unconventional but straightforward way hybrid superlattices consisting of a large number of nanomembranes mechanically stacked on top of each other. The superlattices can consist of an arbitrary composition of n- or p-type doped single-crystalline semiconductors and a polycrystalline metal layer. These hybrid multilayered systems are fabricated by taking advantage of the self-rolling technique. First, differentially strained nanomembranes are rolled into three-dimensional microtubes with multiple windings. By applying vertical pressure, the tubes are then compressed and converted into a planar hybrid superlattice. The thermal measurements show a substantial reduction of the cross-sectional heat transport through the nanomembrane superlattice compared to a single nanomembrane layer. Time-domain thermoreflectance measurements yield thermal conductivity values below 2 W m(-1) K(-1). Compared to bulk values, this represents a reduction of 2 orders of magnitude by the incorporation of the mechanically joined interfaces. The scanning thermal atomic force microscopy measurements support the observation of reduced thermal transport on top of the superlattices. In addition, small defects with a spatial resolution of ∼100 nm can be resolved in the thermal maps. The low thermal conductivity reveals the potential of this approach to fabricate miniaturized on-chip solutions for energy harvesters in, e.g., microautonomous systems.

We demonstrate an all-electrically operated wavelength-tunable on demand single-photon source for the first time. The device consists of a light-emitting diode in the form of a semiconductor nanomembrane containing self-assembled quantum dots integrated onto a piezoelectric crystal. Triggered single photons are generated via injection of ultrashort electrical pulses into the diode, while their energy can be precisely tuned over a broad range by varying the voltage applied to the piezoelectric crystal. High speed operation of this single-photon-emitting diode up to 0.8 GHz is demonstrated. These results represent an important step toward the realization of electrically driven sources of indistinguishable photons on demand.

Independent tuning of emission energy and decay time of neutral excitons confined in single self-assembled In(Ga)As/GaAs quantum dots is achieved by simultaneously employing vertical electric fields and lateral biaxial strain fields. By locking the emission energy via a closed-loop feedback on the piezoelectric actuator used to control the strain in the quantum dot, we continuously decrease the decay time of an exciton from 1.4 to 0.7 ns. Both perturbations are fully electrically controlled and their combination offers a promising route to engineer the indistinguishability of photons emitted from spatially separated single photon sources. © 2017 Author(s).

We show that anisotropic biaxial stress can be used to tune the built-in dipole moment of excitons confined in In(Ga)As quantum dots up to complete erasure of its magnitude and inversion of its sign. We demonstrate that this phenomenon is due to piezoelectricity. We present a model to calculate the applied stress, taking advantage of the so-called piezotronic effect, which produces significant changes in the current-voltage characteristics of the strained diode-membranes containing the quantum dots. Finally, self-consistent k.p calculations reveal that the experimental findings can be only accounted for by the nonlinear piezoelectric effect, whose importance in quantum dot physics has been theoretically recognized although it has proven difficult to single out experimentally., Comment: 6 pages, 4 figures

Scientific Reports 6: Article number: 23843; published online: 01 April 2016; updated: 16 August 2016 In this Article, Stefania Privitera and Emanuele Rimini are incorrectly listed as being affiliated with ‘Paul-Drude-Institut für Festkörperelektronik, Hausvogteiplatz 5-7, 10117 Berlin, Germany’. The correct affiliation is listed below

GaAs/AlGaAs quantum dots grown by in situ droplet etching and nanohole in-filling offer a combination of strong charge confinement, optical efficiency, and high spatial symmetry advantageous for polarization entanglement and spin-photon interface. Here, we study experimentally electron and nuclear spin properties of such dots. We find nearly vanishing electron $g$ factors $({g}_{e}l0.05)$, providing a potential route for electrically driven spin control schemes. Optical manipulation of the nuclear spin environment is demonstrated with nuclear spin polarization up to $65%$ achieved. Nuclear magnetic resonance spectroscopy reveals two distinct types of quantum dots: with tensile and with compressive strain along the growth axis. In both types of dots, the magnitude of strain ${\ensuremath{\epsilon}}_{b}l0.02%$ is nearly three orders of magnitude smaller than in self-assembled dots: On the one hand, this provides a route for eliminating a major source of electron spin decoherence arising from nuclear quadrupolar interactions, and on the other hand such strain is sufficient to suppress nuclear spin diffusion leading to a stable nuclear spin bath with nuclear spin lifetimes exceeding 500 s. The spin properties revealed in this work make this new type of quantum dot an attractive alternative to self-assembled dots for the applications in quantum information technologies.

This paper reviews the recent advances obtained by integrating semiconductor epitaxial films with embedded self-assembled quantum dots (QDs) on top of single-crystal piezoelectric substrates made of lead magnesium niobate–lead titanate (PMN-PT). This combination allows us to study in detail the effects produced by variable strains (up to about ± 0.2%) on the excitonic emission of single QDs and to add a powerful “tuning knob” to QDs. Biaxial stress can be used to reversibly shift the emission wavelength of QDs in a spectral range wider than 10 meV and to modify the relative binding energies of excitonic species. Anisotropic stress has instead a strong influence on the fine structure splitting of neutral excitons. Finally, we present experimental results on the effect of biaxial strain on the optical modes of microring optical resonators and show a simple approach enabling the compensation of piezo-creep via a closed-loop system. Schematic illustration of a QD membrane integrated on top of a PMN-PT substrate. Stress provided by the piezoelectric substrate allows broad range tuning of the emission properties of the overlying dots.

We present an overview and a comparison between three different methods of creating low density lateral In(Ga)As quantum dot molecules (QDMs) embedded in a GaAs matrix. Each of them is based on the use of nanoholes to control the dot nucleation site and generate the QDMs. The three methods used to create suitable nanoholes are: (1) In situ excess gallium droplet etching, where the nanohole shape is modified by overgrowth of a thin GaAs buffer to give QDM nucleation. (2) Ex situ electron-beam lithographic patterning and wet-etching, where the patterned nanohole size is critical for formation of QDMs. (3) In situ strain-selective etching of buried InAs quantum dots by AsBr3. The mechanisms of QDM formation, dependence on growth parameters, advantages and disadvantages of each technique and future challenges are discussed.

Self-assembled quantum dots (QDs) have emerged as attractive quantum systems for the generation of single and entangled photons for quantum information processing [1]. However, owing to the mesoscopic shape, strain and composition anisotropy, self-assembled QDs are subject to an inhomogeneous broadening and an energetic separation between two bright exciton states known as fine-structure-splitting (FSS). These hurdles hamper not only the realization of indistinguishable single photons from different QDs sources, but also the generation of entangled-photon pairs from the biexciton cascade of QDs. Here, we demonstrate a strain-tunable quantum-LED for electrical generation of ultrafast wavelength-tunable single photons and entangled-photon pairs. The device consists of a p-i-n nanomembrane containing InGaAs QDs integrated onto a piezoactuator capable of delivering biaxial or anisotropic strain fields. Triggered single photons are generated via injection of ultra-short electrical pulses into the diode as their energy is tuned over a broad range of ∼ 5meV by varying the voltage applied to the piezoactuator. High speed operation of this single-photon emitting diode up to 0.8 GHz has been demonstrated [2]. Alternatively, application of the anisotropic strain fields enables cancellation of the fine-structure-splitting (FSS) in QDs. We realize a high yield (∼ 30%) of QDs tuned to be suitable for generation of entangled-photon pairs, and the highest speed generation of entangled-photon, that is, 400MHz [3]. The device constitutes an important step towards the practical exploitation of high data-rate quantum applications involving a large number of all-electrically operated indistinguishable single- and entangled-photon sources.

The scalability of quantum dot based non-classical light sources relies on the control over their dissimilar emission energies. Electric fields offer a promising route to tune the quantum dot emission energy through the quantum-confined Stark effect. However, electric fields have been mostly used for tuning the energy of single-photon emission from quantum dots, while electrical control over the energy of entangled-photon emission, which is crucial for building a solid-state quantum repeater using indistinguishable entangled photons, has not been realized yet. Here, we present a method to achieve electrical control over the energy of entangled-photon emission from quantum dots. The device consists of an electrically-tunable quantum diode integrated onto a piezoactuator. We find that, through application of a vertical electric field, the critical uniaxial stress used to eliminate the fine-structure-splitting of quantum dots can be linearly tuned. This allows realization of a triggered source of energy-tunable entangled-photon emission, an important step towards a solid-state quantum repeater application., Comment: 12 pages and 6 figures

A combination of far-infrared and Raman spectroscopy is employed to investigate vibrational modes and the carrier behavior in amorphous and crystalline ordered GeTe-Sb2Te3 alloys (GST) epitaxially grown on Si(111). The infrared active GST mode is not observed in the Raman spectra and vice versa, indication of the fact that inversion symmetry is preserved in the metastable cubic phase in accordance with the F"Equation missing"m3 space group. For the trigonal phase, instead, a partial symmetry break due to Ge/Sb mixed anion layers is observed. By studying the crystallization process upon annealing with both the techniques, we identify temperature regions corresponding to the occurrence of different phases as well as the transition from one phase to the next. Activation energies of 0.43 eV and 0.08 eV for the electron conduction are obtained for both cubic and trigonal phases, respectively. In addition a metal-insulator transition is clearly identified to occur at the onset of the transition between the disordered and the ordered cubic phase.

Scientific reports 6, 23843 (2016). doi:10.1038/srep23843, Phase Change Materials (PCMs) are unique compounds employed in non-volatile random access memory thanks to the rapid and reversible transformation between the amorphous and crystalline state that display large differences in electrical and optical properties. In addition to the amorphous-to-crystalline transition, experimental results on polycrystalline GeSbTe alloys (GST) films evidenced a Metal-Insulator Transition (MIT) attributed to disorder in the crystalline phase. Here we report on a fundamental advance in the fabrication of GST with out-of-plane stacking of ordered vacancy layers by means of three distinct methods: Molecular Beam Epitaxy, thermal annealing and application of femtosecond laser pulses. We assess the degree of vacancy ordering and explicitly correlate it with the MIT. We further tune the ordering in a controlled fashion attaining a large range of resistivity. Employing ordered GST might allow the realization of cells with larger programming windows., Published by Nature Publishing Group, London

Triggered sources of entangled photon pairs are key components in most quantum communication protocols. For practical quantum applications, electrical triggering would allow the realization of compact and deterministic sources of entangled photons. Entangled-light-emitting-diodes based on semiconductor quantum dots are among the most promising sources that can potentially address this task. However, entangled-light-emitting-diodes are plagued by a source of randomness, which results in a very low probability of finding quantum dots with sufficiently small fine structure splitting for entangled-photon generation (∼10−2). Here we introduce strain-tunable entangled-light-emitting-diodes that exploit piezoelectric-induced strains to tune quantum dots for entangled-photon generation. We demonstrate that up to 30% of the quantum dots in strain-tunable entangled-light-emitting-diodes emit polarization-entangled photons. An entanglement fidelity as high as 0.83 is achieved with fast temporal post selection. Driven at high speed, that is 400 MHz, strain-tunable entangled-light-emitting-diodes emerge as promising devices for high data-rate quantum applications., Quantum communications require sources of entangled photons. Electrically triggered sources usually suffer from low entangled-emission efficiency. Here, the authors use piezoelectric strains to tune the fine structure of quantum dot emitters, and increase the entanglement probability and fidelity.

We demonstrate a compact, spectrally-selective, and tunable delay line for single photons emitted by quantum dots. This is achieved by fine-tuning the wavelength of the optical transitions of such "artificial atoms" into a spectral window in which a cloud of natural atoms behaves as slow-light medium. By employing the ground-state fine-structure-split exciton confined in an InGaAs/GaAs quantum dot as a source of single photons at different frequencies and the hyperfine-structure-split $D_1$ transition of Cs-vapors as a tunable delay-medium, we achieve a differential delay of up 2.4 ns on a 7.5 cm long path for photons that are only 60 \mu eV (14.5 GHz) apart. To quantitatively explain the experimental data we develop a theoretical model that accounts for both the inhomogeneously broadening of the quantum-dot emission lines and the Doppler-broadening of the atomic lines. The concept we proposed here may be used to implement time-reordering operations aimed at erasing the "which-path" information that deteriorates entangled-photon emission from excitons with finite fine-structure-splitting., Comment: 29 pages, 5 figures

An electron and a hole trapped in the same quantum dot couple together to form an exciton. Conventionally the hole involved is a heavy hole. Light-hole excitons are now observed by applying elastic stress to initially unstrained gallium arsenide-based dots. The quasiparticles are identified by their optical emission signature, and could be used in future quantum technologies.

Entanglement resources are key ingredients of future quantum technologies. If they could be efficiently integrated into a semiconductor platform, a new generation of devices could be envisioned, whose quantum-mechanical functionalities are controlled via the mature semiconductor technology. Epitaxial quantum dots (QDs) embedded in diodes would embody such ideal quantum devices, but a fine-structure splitting (FSS) between the bright exciton states lowers dramatically the degree of entanglement of the sources and hampers severely their real exploitation in the foreseen applications. In this work, we overcome this hurdle using strain-tunable optoelectronic devices, where any QD can be tuned for the emission of photon pairs featuring the highest degree of entanglement ever reported for QDs, with concurrence as high as 0.75 ± 0.02. Furthermore, we study the evolution of Bell's parameters as a function of FSS and demonstrate for the first time that filtering-free violation of Bell's inequalities requires the FSS to be smaller than 1 μeV. This upper limit for the FSS also sets the tuning range of exciton energies (∼1 meV) over which our device operates as an energy-tunable source of highly entangled photons. A moderate temporal filtering further increases the concurrence and the tunability of exciton energies up to 0.82 and 2 meV, respectively, though at the expense of 60% reduction of count rate.

We demonstrate experimentally the possibility to manipulate the coupling strength in an asymmetric pair of electronically coupled InGaAs quantum dots by using externally induced strain fields. The coupling strength of holes confined in the dots increases linearly with increasing tensile strain. A model based on $\mathbit{k}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbit{p}$ theory explains the effect in terms of modified weight of the light hole component mediating the coupling in the barrier. Our results are relevant to the creation and control of entangled states in optically active quantum dots.

The concept of Fourier synthesis is heavily employed in both consumer electronic products and fundamental research. In the latter, pulse shaping is key to dynamically initialize, probe and manipulate the state of classical or quantum systems. In nuclear magnetic resonance, for instance, shaped pulses have a long-standing tradition and the underlying fundamental concepts have subsequently been successfully extended to optical frequencies and even to implement quantum gate operations. Transferring these paradigms to nanomechanical systems requires tailored nanomechanical waveforms. Here, we report on an additive Fourier synthesizer for nanomechanical waveforms based on monochromatic surface acoustic waves. As a proof of concept, we electrically synthesize four different elementary nanomechanical waveforms from a fundamental surface acoustic wave at $ f_1 \sim 150$ MHz using a superposition of up to three discrete harmonics $f_n$. We employ these shaped pulses to interact with an individual sensor quantum dot and detect their deliberately and temporally modulated strain component via the opto-mechanical quantum dot response. Importantly, and in contrast to the direct mechanical actuation by bulk piezoactuators, surface acoustic waves provide much higher frequencies (> 20 GHz) to resonantly drive mechanical motion. Thus, our technique uniquely allows coherent mechanical control of localized vibronic modes of optomechanical crystals, even in the quantum limit when cooled to the vibrational ground state., 18 pages - final manuscript and supporting material

We investigate the effect of large in-plane strain and vertical electric fields on the binding energies of excitonic complexes confined in single InGaAs/GaAs quantum dots (QDs) and we find that the two independently tunable perturbations modify the interaction energies among electrons and holes in a different manner. By taking advantage of this difference, we frequency-lock the QD fundamental excitation (the neutral exciton) at a predefined value, while the biexciton transition is actively tuned from a binding to an antibinding configuration. Our electrically controlled dual-knob device demonstrates unprecedented control over the electronic properties of the few-particle states in a QD and may be applied to create novel energy-tunable sources of entangled photons using the time-reordering or the time-bin scheme.

We introduce a new class of quantum dot-based devices, in which the semiconductor structures are integrated on top of piezoelectric actuators. This combination allows us on one hand to study in detail the effects produced by variable strains (up to about 0.2%) on the excitonic emission of single quantum dots and on the other to manipulate their electronic- and optical properties to achieve specific requirements for their use in quantum optics experiments and possibly future devices for quantum communication.

International audience; This work presents an experimental method to tune the degree of heavy-hole (HH) and light-hole (LH) mixing of the ground state of quantum dots (QDs). A ferroelectric crystal is used to apply reversible anisotropic biaxial stress to thin nanomembranes, containing GaAs/AlGaAs QDs. The stress-induced modification of the QD anisotropy leads to a change of the relative intensity of the two emission lines produced by the recombination of neutral bright excitonic states. Such a change is ascribed to a variation of the degree of HH-LH mixing. At the same time the modified anisotropy produces a change of the excitonic fine structure splitting (FSS). Model calculations provide a qualitative insight into the relation between strain, HH-LH mixing, and the FSS in epitaxial GaAs/AlGaAs QDs. DOI: 10.1103/PhysRevB.87.075311

Superlattices made of alternating layers of nominal GeTe and Sb2Te3 have been studied by micro-Raman spectroscopy. A structural irreversible transformation into ordered GeSbTe alloy is induced by high power laser light exposure. The intensity ratio of anti-Stokes and Stokes scattering under laser illumination gives a maximum average temperature in the sample of 177 °C. The latter is lower than the growth temperature and of 400 °C necessary by annealing to transform the structure in a GeSbTe alloy. The absence of this configuration after in situ annealing even up to 300 °C evidences an electronic excitation induced-transition which brings the system into a different and stable crystalline state.

International audience; Modifying the shape of nanoholes formed by arsenic debt epitaxy by the overgrowth of a thin GaAs buffer is shown to provide a simple and robust method to grow low density lateral In(Ga)As quantum dot pairs (QDPs). We present here a systematic study of the effect of GaAs buffer thickness. InAs deposition amount, substrate temperature and arsenic overpressure on dot nucleation and QDP formation. A (10-30) nm GaAs buffer over nanoholes initially similar to 10.5 nm deep, (60-80) nm wide results in up to 80% of the nanoholes containing QDPs. The QD pairs are aligned along the [1101 direction and have centre-to-centre separation of similar to 38 nm. These QDPs form following InAs deposition between 1.3 ML and 1.6 ML at 490 degrees C under an arsenic arrival flux of 0.6 ML/s. From the infilling of the hole prior to QD formation, we estimate a net indium surface flux towards the hole of similar to 7 times the incident flux. The substrate temperature does not significantly alter the dot distribution over the range (470-510) degrees C. However, the QDP formation is very sensitive to the arsenic overpressure over the range (0.6-1.2) ML/s because of a partial collapse of the nanohole, due to mass transport as the substrate passes through the (2 x 4) to c(4 x 4) surface reconstruction around 500 degrees C. (C) 2011 Elsevier B.V. All rights reserved.

International audience; The lack of structural symmetry which usually characterizes semiconductor quantum dots lifts the energetic degeneracy of the bright excitonic states and hampers severely their use as high-fidelity sources of entangled photons. We demonstrate experimentally and theoretically that it is always possible to restore the excitonic degeneracy by the simultaneous application of large strain and electric fields. This is achieved by using one external perturbation to align the polarization of the exciton emission along the axis of the second perturbation, which then erases completely the energy splitting of the states. This result, which holds for any quantum dot structure, highlights the potential of combining complementary external fields to create artificial atoms meeting the stringent requirements posed by scalable semiconductor-based quantum technology.

International audience; Very low density growth of GaAs quantum dots in self-assembled nanoholes created by gallium droplet etching is demonstrated. The emission energy of the quantum dots can be accurately controlled by the GaAs deposition amount, from 1.8 to 1.6 eV, independently of the dot density which can be reproducibly controlled over the range 0.2 - 2 x 10(-8) cm(-2) by the gallium deposition rate. The ensemble full-width-half-maximum is

We integrate resonant-cavity light-emitting diodes containing quantum dots onto substrates with giant piezoelectric response. Via strain, the energy of the photons emitted by the diode can be precisely controlled during electrical injection over a spectral range larger than 20 meV. Simultaneously, the exciton fine-structure-splitting and the biexciton binding energy can be tuned to the values required for entangled photon generation.

The evolution of InAs and In(0.85)Mn(0.15)As quantum dots grown at 270 °C is studied as a function of coverage. We show that, in contrast to what occurs at high temperature, the two-dimensional to three-dimensional transition is not abrupt but rather slow. This is due to the finding that part of the deposited material also contributes to the wetting layer growth after quantum dot formation. This aspect is particularly accentuated in In(0.85)Mn(0.15)As deposition. The Voronoi area analysis reveals a significant spatial correlation between islands.

Reversible biaxial strains are used for tuning the emission wavelengths of high quality GaAs/AlGaAs quantum dots (QDs) in the spectral range of the 87Rb D2 lines. The strain is transferred by integrating free standing (Al)GaAs nanomembranes, containing QDs, onto piezoelectric actuators. Narrow excitonic emission lines as sharp as 25 μeV are shown, and a tuning range larger than 5 nm is demonstrated. This range corresponds to an induced anisotropic biaxial strain of the order of 0.15%, as evaluated from the shift in the emission of the GaAs from the nanomembranes. The presented on-chip technology is potentially interesting for future quantum memories based on hybrid semiconductor-atomic interfaces.