We measure tunnelling currents through electrostatically defined quantum dots in a GaAs/AlGaAs heterostructure connected to two leads. For certain tunnelling barrier configurations and high sample bias we find a pronounced resonance associated with a Fermi edge singularity. This many-body scattering effect appears when the electrochemical potential of the quantum dot is aligned with the Fermi level of the lead less coupled to the dot. By changing the relative tunnelling barrier strength we are able to tune the interaction of the localised electron with the Fermi sea.
We study the electronic and optical properties of laterally coupled InGaAs/GaAs quantum dot molecules under lateral electric field. We find that electrons perceive the double-dot structure as a compound single object, while the holes discern two well separated dots. Through a combination of predictive atomistic modeling, detailed morphology studies, and single object micro-photoluminescence measurements, we show that this peculiar confinement results in an unusual heterogeneous behavior of electrons and holes with profound consequences on optical properties.
Free standing beams containing a two-dimensional electron system are shaped from a GaAs/AlGaAs heterostructure. Quantum point contacts and (double) quantum dots are laterally defined using metal top gates. We investigate the electronic properties of these nanostructures by transport spectroscopy. Tunable localized electron states in freely suspended nanostructures are a promising tool to investigate the electron-phonon-interaction.
We used AlGaSb/AlGaAs material system for a theoretical study of photovoltaic performance of the proposed GaAs-based solar cell in which the type-II quantum dot (QDs) absorber is spatially separated from the depletion region. Due to inelastic scattering of photoelectrons on QDs and proper doping of both QDs and their spacers, concentrated sunlight is predicted to quench recombination through QDs. Our calculation shows that 500-sun concentration can increase the Shockley-Queisser limit from 35% to 40% for GaAs single-junction solar cells.
We report the observation of ballistic commensurability oscillations and positive magnetoresistance in a high-mobility, (001) GaAs two-dimensional hole system with a unidirectional, surface-strain-induced, periodic potential modulation. The positions of the resistivity minima agree well with the electrostatic commensurability condition. From an analysis of the amplitude of the oscillations we deduce a ballistic scattering time and an effective magnitude for the induced periodic potential seen by the two-dimensional holes.
We report a new transport feature in a GaAs lateral double quantum dot that emerges only for magnetic field sweeps and shows hysteresis due to dynamic nuclear spin polarization (DNP). This DNP signal appears in the Coulomb blockade regime by virtue of the finite inter-dot tunnel coupling and originates from the crossing between ground levels of the spin triplet and singlet extensively used for nuclear spin manipulations in pulsed gate experiments. The unexpectedly large signal intensity is suggestive of unbalanced DNP between the two dots, which opens up the possibility of controlling electron…
Efforts to achieve efficient injection of spin-polarized electrons into a semiconductor, a key prerequisite for developing electronics that exploit the electron's spin degree of freedom, have so far met with limited success. Here we report experimental studies of lateral spin injection and detection through electrodeposited Fe/GaAs tunnel contacts. We demonstrate spin injection efficiencies two orders of magnitude higher than for state-of-the-art contacts fabricated via ultra-high-vacuum methods, including those with MgO or Al2O3 tunnel barriers. To account for this enhancement, we propose tha…
We report on a new approach for positioning of self-assembled InAs quantum dots on (110) GaAs with nanometer precision. By combining self-assembly of quantum dots with molecular beam epitaxy on in-situ cleaved surfaces (cleaved-edge overgrowth) we have successfully fabricated arrays of long-range ordered InAs quantum dots. Both atomic force microscopy and micro-photoluminescence measurements demonstrate the ability to control size, position, and ordering of the quantum dots. Furthermore, single dot photoluminescence investigations confirm the high optical quality of the quantum dots fabricated…
Molecular beam epitaxy is employed to manufacture self-assembled InAs/GaAs quantum dot Schottky resonant tunneling diodes. By virtue of a thin AlAs insertion barrier, the thermal current is effectively reduced and electron resonant tunneling through quantum dots under both forward and reverse biased conditions is observed at relatively high temperature of 77K. The ground states of quantum dots are found to be at ~0.19eV below the conduction band of GaAs matrix. The theoretical computations are in conformity with experimental data.
Detailed theoretical studies of the electronic structure of (InGa)(AsSb)/GaAs/GaP quantum dots are presented. This system is unique since it exhibits concurrently direct and indirect transitions both in real and momentum space and is attractive for applications in quantum information technology, showing advantages as compared to the widely studied (In,Ga)As/GaAs dots. We proceed from the inspection of the confinement potentials for ${\bf k}\neq 0$ and ${\bf k}= 0$ conduction and ${\bf k}= 0$ valence bands, through the formulation of ${\bf k}\cdot{\bf p}$ calculations for ${\bf k}$-indirect tra…
Strong spin-orbit interaction characteristic for p-type GaAs systems, makes such systems promising for the realization of spintronic devices. Here we report on transport measurements in nanostructures fabricated on p-type, C-doped GaAs heterostructures by scanning probe oxidation lithography. We observe conductance quantization in a quantum point contact, as well as pronounced Coulomb resonances in two quantum dots with different geometries. Charging energies for both dots, extracted from Coulomb diamond measurements are in agreement with the lithographic dimensions of the dots. The absence of…
We report on lateral spin-caloric transport (LSCT) of electron spin packets which are optically generated by ps laser pulses in the non-magnetic semiconductor n-GaAs at $T\leq35$K. LSCT is driven by a local temperature gradient induced by an additional cw heating laser. The spatio-temporal evolution of the spin packets is probed using time-resolved Faraday rotation. We demonstrate that the local temperature-gradient induced spin diffusion is solely driven by a non-equilibrium hot spin distribution, i.e. without involvement of phonon drag effects. Additional electric field-driven spin drift exp…
Time-resolved microphotoluminescence study is presented for quantum dots which are formed in the InAs/GaAs wetting layer. These dots are due to fluctuations of In composition in the wetting layer. They show spectrally sharp luminescence lines with a low spatial density. We identify lines related to neutral exciton and biexciton as well as trions. Exciton emission antibunching (second order correlation value of g^2(0)=0.16) and biexciton-exciton emission cascade prove non-classical emission from the dots and confirm their potential as single photon sources.
We study experimentally the dependence of dynamic nuclear spin polarization on the power of non-resonant optical excitation in two types of individual neutral semiconductor quantum dots: InGaAs/GaAs and GaAs/AlGaAs. We show that the mechanism of nuclear spin pumping via second order recombination of optically forbidden (''dark'') exciton states recently reported in InP/GaInP quantum dots [Phys. Rev. B 83, 125318 (2011)] is relevant for material systems considered in this work. In the InGaAs/GaAs dots this nuclear spin polarization mechanism is particularly pronounced, resulting in Overhauser s…
Elimination of the acoustoelectric domain and increasing of the emission intensity by means of shunting the lateral current in the InGaAs/GaAs heterostructures
Spin-flip rates in GaAs quantum dots can be quite slow, thus opening up the possibilities to manipulate spin states in the dots. We present here estimations of inelastic spin-flip rates mediated by hyperfine interaction with nuclei. Under general assumptions the nucleus mediated rate is proportional to the phonon relaxation rate for the corresponding non-spin-flip transitions. The rate can be accelerated in the vicinity of a singlet-triplet excited states crossing. The small proportionality coefficient depends inversely on the number of nuclei in the quantum dot. We compare our results with kn…
We describe the optical emission and the carrier dynamics of an ensemble of self-assembled GaAs quantum dots embedded in GaP(001). The QD formation is driven by the 3.6 % lattice mismatch between GaAs and GaP in Stranski-Krastanow mode after deposition of more than 1.2 monolayers of GaAs. The quantum dots have an areal density between 6 and 7.6x1010 per cm-2 and multimodal size distribution. The luminescence spectra show two peaks in the range of 1.7 and 2.1 eV. The samples with larger quantum dots have red emission and show less thermal quenching compared to the samples with smaller QDs. The …
We report on the photoluminescence characterization of GaAs quantum dots embedded into AlGaAs nano-wires. Time integrated and time resolved photoluminescence measurements from both an array and a single quantum dot/nano-wire are reported. The influence of the diameter sizes distribution is evidenced in the optical spectroscopy data together with the presence of various crystalline phases in the AlGaAs nanowires.
The electronic structure of InAs quantum dots covered with the GaAs(1-y)Sb(y) strain reducing layer has been studied using the k.p theory. We explain previous experimental observations of the red shift of the photoluminescence emission with increasing y and its blue shift with increasing excitation power. For y>0.19 type-II dots are formed with holes localized in the GaAsSb close to the dot base; two segments at opposite sides of the dot, forming molecular-like states, result from the piezoelectric field. We also propose an experiment that could be used to identify the hole localization using …
We experimentally determine isotropic and anisotropic g-factor corrections in lateral GaAs single-electron quantum dots. We extract the Zeeman splitting by measuring the tunnel rates into the individual spin states of an empty quantum dot for an in-plane magnetic field with various strengths and directions. We quantify the Zeeman energy and find a linear dependence on the magnetic field strength which allows us to extract the g-factor. The measured g-factor is understood in terms of spin-orbit interaction induced isotropic and anisotropic corrections to the GaAs bulk g-factor. Because this imp…
Vertically stacked and coupled InAs/GaAs self-assembled quantum dots (SADs) are predicted to exhibit a strong non-parabolic dependence of the interband transition energy on the electric field, which is not encountered in single SAD structures nor in other types of quantum structures. Our study based on an eight-band strain-dependent ${\bf k}\cdot{\bf p}$ Hamiltonian indicates that this anomalous quantum confined Stark effect is caused by the three-dimensional strain field distribution which influences drastically the hole states in the stacked SAD structures.
The dynamics of an exciton-LO-phonon system after an ultrafast optical excitation in an InAs/GaAs quantum dot is studied theoretically. Influence of anharmonic phonon damping and its interplay with the phonon dispersion is analyzed. The signatures of the zone-edge decay process in the absorption spectrum and time evolution are highlighted, providing a possible way of experimental investigation on phonon anharmonicity effects.
I present a systematic study of self-assembled InAs/InP and InAs/GaAs quantum dots single particle and many body properties as a function of quantum dot-surrounding matrix valence band offset. I use an atomistic, empirical tight-binding approach and perform numerically demanding calculations for half-million atom nanosystems. I demonstrate that the overall confinement in quantum dots is a nontrivial interplay of two key factors: strain effects and the valence band offset. I show that strain effects determine both the peculiar structure of confined hole states of lens type InAs/GaAs quantum dot…
We have investigated micro-photoluminescence ($μ$-PL) and excitation ($μ$-PLE) spectra of a single GaAs/AlGaAs quantum dot grown on a GaAs (411)A surface. We observed sharp resonant lines in both $μ$-PL and $μ$-PLE spectra, corresponding to the discrete energy levels of the dot. When the sample was excited at one of the resonant lines, resonant luminescence lines appear not only in Stokes side but also in anti-Stokes side. We discuss the possible origins of the anomalus anti-Stokes luminescence.
We report magnetotransport measurements of two-dimensional holes in open quantum dots, patterned either as a single-dot or an array of dots, on a GaAs quantum well. For temperatures $T$ below 500 mK, we observe signatures of coherent transport, namely, conductance fluctuations and weak antilocalization. From these effects, the hole dephasing time $τ_ϕ$ is extracted using the random matrix theory. While $τ_ϕ$ shows a $T$-dependence that lies between $T^{-1}$ and $T^{-2}$, similar to that reported for electrons, its value is found to be approximately one order of magnitude smaller.
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 reaches 99% and beyond, it has been limited to 60-65% 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 …
We investigate the optical properties of InAs quantum dots grown by molecular beam epitaxy on GaAs(110) using Bi as a surfactant. The quantum dots are synthesized on planar GaAs(110) substrates as well as on the {110} sidewall facets of GaAs nanowires. At 10 K, neutral excitons confined in these quantum dots give rise to photoluminescence lines between 1.1 and 1.4 eV. Magneto-photoluminescence spectroscopy reveals that for small quantum dots emitting between 1.3 and 1.4 eV, the electron-hole coherence length in and perpendicular to the (110) plane is on the order of 5 and 2 nm, respectively. T…
The spin degree of freedom in solids offers opportunities beyond charge-based electronics and is actively investigated for both spintronics and quantum computation. However, the interplay of these spins with their native environment can give rise to detrimental effects such as spin relaxation and decoherence. Here, we use an all-electrical, lateral GaAs spin valve to manipulate and investigate the inherent nuclear spin system. Hanle satellites are used to determine the nuclear spin relaxation rates for the previously unexplored temperature range down to 100 mK, giving T1 times as long as 3 hou…
Single and multi-band (Burt-Foreman) k.p Hamiltonians for GaAs crystal phase quantum dots are developed and used to assess ongoing experimental activity on the role of such factors as quantum confinement, spontaneous polarization, valence band mixing and exciton Coulomb interaction. Spontaneous polarization is found to be a dominating term. Together with the control of dot thickness [Vainorious Nano Lett. 15, 2652 (2015)] it enables wide exciton wavelength and lifetime tunability. Several new phenomena are predicted for small diameter dots [Loitsch et al. Adv. Mater. 27, 2195 (2015)], includin…
The effect of an electric field on spin precession in In0.5Ga0.5As/GaAs self-assembled quantum dots is calculated using multiband real-space envelope-function theory. The dependence of the Lande' g tensor on electric fields should permit high-frequency g tensor modulation resonance, as well as direct, nonresonant electric-field control of the hole spin. Subharmonic resonances have also been found in g tensor modulation resonance of the holes, due to the strong quadratic dependence of components of the hole g tensor on the electric field.
We introduce a high-temperature droplet epitaxy procedure, based on the control of the arsenization dynamics of nanoscale droplets of liquid Ga on GaAs(111)A surfaces. The use of high temperatures for the self-assembly of droplet epitaxy quantum dots solves major issues related to material defects, introduced during the droplet epitaxy fabrication process, which limited its use for single and entangled photon sources for quantum photonics applications. We identify the region in the parameter space which allows quantum dots to self-assemble with the desired emission wavelength and highly symmet…
The control over the spectral broadening of an ensemble of emitters, mainly attributable to the size and shape dispersion and the homogenous broadening mechanisms, is crucial to several applications of quantum dots. We present a convenient self-assembly approach to deliver strain-free GaAs quantum dots with size distribution below 15%, due to the control of the growth parameters during the preliminary formation of the Ga droplets. This results in an ensemble photoluminescence linewidth of 19 meV at 14 K. The narrow emission band and the absence of a wetting layer promoting dot-dot coupling all…
We report strong heavy hole-light mixing in GaAs quantum dots grown by droplet epitaxy. Using the neutral and charged exciton emission as a monitor we observe the direct consequence of quantum dot symmetry reduction in this strain free system. By fitting the polar diagram of the emission with simple analytical expressions obtained from k$\cdot$p theory we are able to extract the mixing that arises from the heavy-light hole coupling due to the geometrical asymmetry of the quantum dot.
Carrier dynamics of submonolayer (SML) InGaAs/GaAs quantum dots (QDs) were studied by micro-photoluminecence (MPL), selectively excited photoluminescence (SEPL), and time-resolved photoluminescence (TRPL). MPL and SEPL show the coexistence of localized and delocalized states, and different local phonon modes. TRPL reveal shorter recombination lifetimes and longer capture times for the QDs with higher emission energy. This suggests that the smallest SML QDs are formed by perfectly vertically correlated 2D InAs islands, having the highest In content and the lowest emission energy, while a slight…
We have fabricated a quantum dot single electron transistor based on an AlGaAs/GaAs heterostructure without any modulation doping. Our device is very stable from an electronic perspective, with clear Coulomb blockade oscillations, and minimal drift in conductance when the device is set to the midpoint of a Coulomb blockade peak and held at constant gate bias. Bias spectroscopy measurements show typical Coulomb 'diamonds' free of any significant charge fluctuation noise. We also observe excited state transport in our device.
We study the influence of quantum dot symmetry on the Rabi frequency and phonon induced spin relaxation rate in a single electron GaAs spin qubit. We find that anisotropic dependence on the magnetic field direction is independent of the choice of the gating potential. Also, we discover that relative orientation of the quantum dot, with respect to the crystallographic frame, is relevant in systems with ${\bf C}_{1{\rm v}}$, ${\bf C}_{2{\rm v}}$, or ${\bf C}_{n}$ ($n\neq4r$) symmetry. To demonstrate the important impact of the gating potential shape on the spin qubit lifetime, we compare the eff…
Phonon-induced spin relaxation in coupled lateral quantum dots in the presence of spin-orbit coupling is calculated. The calculation for single dots is consistent with experiment. Spin relaxation in double dots at useful interdot couplings is dominated by spin hot spots that are strongly anisotropic. Spin hot spots are ineffective for a diagonal crystallographic orientation of the dots with a transverse in-plane field. This geometry is proposed for spin-based quantum information processing.
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 InGaAs/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…
GaAs/AlGaAs quantum dots grown by in-situ droplet etching and nanohole infilling offer a combination of strong charge confinement, optical efficiency, and spatial symmetry required for polarization entanglement and spin-photon interface. Here we study spin properties of such dots. We find nearly vanishing electron $g$-factor ($g_e<0.05$), providing a route for electrically driven spin control schemes. Optical manipulation of the nuclear spin environment is demonstrated with nuclear spin polarization up to $60\%$ achieved. NMR spectroscopy reveals the structure of two types of quantum dots and …
High-resolution spectral hole burning (SHB) in coherent nondegenerate differential transmission spectroscopy discloses spin-trion dynamics in an ensemble of negatively charged quantum dots. In the Voigt geometry, stimulated Raman spin coherence gives rise to Stokes and anti-Stokes sidebands on top of the trion spectral hole. The prominent feature of an extremely narrow spike at zero detuning arises from spin population pulsation dynamics. These SHB features confirm coherent electron spin dynamics in charged dots, and the linewidths reveal spin spectral diffusion processes.
The magnetic field dependence of the excitonic states in unstrained GaAs/AlGaAs quantum dots is investigated theoretically and experimentally. The diamagnetic shift for the ground and the excited states are studied in magnetic fields of varying orientation. In the theoretical study, calculations are performed within the single band effective mass approximation, including band nonparabolicity, the full experimental three-dimensional dot shape and the electron-hole Coulomb interaction. These calculations are compared with the experimental results for both the ground and the excited states in fie…
We study the dynamics of excitons in GaAs/(Al,Ga)As core-shell nanowires by continuous-wave and time-resolved photoluminescence and photoluminescence excitation spectroscopy. Strong Al segregation in the shell of the nanowires leads to the formation of Ga-rich inclusions acting as quantum dots. At 10 K, intense light emission associated with these shell quantum dots is observed. The average radiative lifetime of excitons confined in the shell quantum dots is 1.7 ns. We show that excitons may tunnel toward adjacent shell quantum dots and nonradiative point defects. We investigate the changes in…
We evaluate the Lande g factor of electrons in quantum dots (QDs) fabricated from GaAs quantum well (QW) structures of different well width. We first determine the Lande electron g factor of the QWs through resistive detection of electron spin resonance and compare it to the enhanced electron g factor determined from analysis of the magneto-transport. Next, we form laterally defined quantum dots using these quantum wells and extract the electron g factor from analysis of the cotunneling and Kondo effect within the quantum dots. We conclude that the Lande electron g factor of the quantum dot is…
We calculate the emission spectrum of neutral and charged excitons in a pair of laterally coupled InGaAs quantum dots with nearly degenerate energy levels. As the interdot distance decreases, a number of changes take place in the emission spectrum which can be used as indications of molecular coupling. These signatures ensue from the stronger tunnel-coupling of trions as compared to that of neutral excitons.
The effects of interplay between spin-orbit coupling and Zeeman splitting on weak localisation and universal conductance fluctuations in lateral semiconductor quantum dots are analysed: All possible symmetry classes of corresponding random matrix theories are listed and crossovers between them achievable by sweeping magnetic field and changing the dot parameters are described. We also suggest experiments to measure the spin-orbit coupling constants.
We demonstrate charge tuning in strain free GaAs/AlGaAs quantum dots (QDs) grown by droplet epitaxy on a GaAs(111)A substrate. Application of a bias voltage allows the controlled charging of the QDs from $-3|e|$ to $+2|e|$. The resulting changes in QD emission energy and exciton fine-structure are recorded in micro-photoluminescence experiments at T=4K. We uncover the existence of excited valence and conduction states, in addition to the s-shell-like ground state. We record a second series of emission lines about 25meV above the charged exciton emission coming from excited charged excitons. Fo…
Recently, longitudinal acoustic phonons have been identified as the main source of the intensity damping observed in Rabi rotation measurements of the ground-state exciton of a single InAs/GaAs quantum dot. Here we report experiments of intensity damped Rabi rotations in the case of detuned laser pulses, the results have implications for the coherent optical control of both excitons and spins using detuned laser pulses.
A microscopic study of the individual annealed (In,Ga)As/GaAs quantum dots is done by means of high-resolution transmission electron microscopy. The Cauchy-Green strain-tensor component distribution and the chemical composition of the (In,Ga)As alloy are extracted from the microscopy images. The image processing allows for the reconstruction of the strain-induced electric-field gradients at the individual atomic columns extracting thereby the magnitude and asymmetry parameter of the nuclear quadrupole interaction. Nuclear magnetic resonance absorption spectra are analyzed for parallel and tran…
We demonstrate tuning of single quantum dot emission lines by the combined action of the dynamic acoustic field of a radio frequency surface acoustic wave and a static electric field. Both tuning parameters are set all-electrically in a LiNbO$_{3}$-GaAs hybrid device. The surface acoustic wave is excited directly on the strong piezoelectric LiNbO$_{3}$ onto which a GaAs-based p-i-n photodiode containing a single layer of quantum dots was epitaxially transferred. We demonstrate dynamic spectral tuning with bandwidths exceeding 3 meV of single quantum dot emission lines due to deformation potent…
We consider the hyperfine-mediated transition rate between Zeeman split spin states of the lowest orbital level in a GaAs quantum dot. We separate the hyperfine Hamiltonian into a part which is diagonal in the orbital states and another one which mixes different orbitals. The diagonal part gives rise to an effective (internal) magnetic field which, in addition to an external magnetic field, determines the Zeeman splitting. Spin-flip transitions in the dots are induced by the orbital mixing part accompanied by an emission of a phonon. We evaluate the rate for different regimes of applied magnet…
Semiconductor quantum dots are converging towards the demanding requirements of photonic quantum technologies. Among different systems, quantum dots with dimensions exceeding the free-exciton Bohr radius are appealing because of their high oscillator strengths. While this property has received much attention in the context of cavity quantum electrodynamics, little is known about the degree of indistinguishability of single photons consecutively emitted by such dots and on the proper excitation schemes to achieve high indistinguishability. A prominent example is represented by GaAs quantum dots…
We report both zinc-blende (ZB) and wurtzite (WZ) crystal phase self-assembled GaAs quantum dots (QDs) embedding in a single GaAs/AlGaAs core-shell nanowires (NWs). Optical transitions and single-photon characteristics of both kinds of QDs have been investigated by measuring photoluminescence (PL) and time-resolved PL spectra upon application of hydrostatic pressure. We find that the ZB QDs are of direct band gap transition with short recombination lifetime (~1 ns) and higher pressure coefficient (75-100 meV/GPa). On the contrary, the WZ QDs undergo a direct-to-pseudodirect bandgap transition …
Making use of droplet epitaxy, we systematically controlled the height of self-assembled GaAs quantum dots by more than one order of magnitude. The photoluminescence spectra of single quantum dots revealed the strong dependence of the spectral linewidth on the dot height. Tall dots with a height of ~30 nm showed broad spectral peaks with an average width as large as ~5 meV, but shallow dots with a height of ~2 nm showed resolution-limited spectral lines (<120 micro eV). The measured height dependence of the linewidths is in good agreement with Stark coefficients calculated for the experimental…
We show how the in-plane magnetic field, which breaks time-reversal and rotational symmetries of the orbital motion of electrons in a heterostructure due to the momentum-dependent inter-subband mixing, affects weak localisation correction to conductance of a large-area chaotic lateral quantum dot and parameteric dependences of universal conductance fluctuations in it.
Single photon sources, which are compatible with quantum memories are an important component of quantum networks. In this article, we show optical investigations on isolated GaAs/Al$_{0.25}$Ga$_{0.75}$As quantum dots grown via droplet epitaxy, which emit single photons on resonance with the Rb-87-D$_2$ line (780 nm). Under continuous wave resonant excitation conditions, we observe bright, clean and narrowband resonance fluorescence emission from such a droplet quantum dot. Furthermore, the second-order correlation measurement clearly demonstrates the single photon emission from this resonantly…
The electronic structure of strain-engineered single InGaAs/GaAs quantum dots emitting in the telecommunication O band is probed experimentally by photoluminescence excitation spectroscopy. Observed resonances can be attributed to p-shell states of individual quantum dots. The determined energy difference between s-shell and p-shell shows an inverse dependence on the emission energy. The experimental data are compared with the results of confined states calculations, where the impact of the size and composition in the investigated structures is simulated within the 8-band $\mathbf{k}\cdot\math…
For InAs/GaAs(001) quantum dot (QD) system, the wetting layer (WL) evolution and its temperature dependence were studied using reflectance difference spectroscopy (RDS) and analyzed with a rate equation model. The WL thicknesses showed a monotonic increase at relatively low growth temperatures but a first increase and then decrease at higher temperatures, which were unexpected from the thermodynamic understanding. By adopting a rate equation model, the temperature dependence of QD growth was assigned as the origin of different WL evolutions. A brief discussion on the indium desorption was also…
We report quantum dots fabricated on very shallow 2-dimensional electron gases, only 30 nm below the surface, in undoped GaAs/AlGaAs heterostructures grown by molecular beam epitaxy. Due to the absence of dopants, an improvement of more than one order of magnitude in mobility (at 2E11 /cm^2) with respect to doped heterostructures with similar depths is observed. These undoped wafers can easily be gated with surface metallic gates patterned by e-beam lithography, as demonstrated here from single-level transport through a quantum dot showing large charging energies (up to 1.75 meV) and excited s…
We describe the electronic properties of a double dot for which the lateral coupling between the two vertical dots can be controlled in-situ with a center gate voltage (Vc) and the current flows through the two dots in series. When Vc is large and positive, the two dots merge. As Vc is made less positive, two dots are formed whose coupling is reduced. We measure charging diagrams for positive and negative source-drain voltages in the weak coupling regime and observe current rectification due to the Pauli spin blockade when the hyperfine interaction between the electrons and the nuclei is suppr…
External control over the electron and hole wavefunctions geometry and topology is investigated in a p-i-n diode embedding a dot-in-a-well InAs/GaAsSb quantum structure with type II band alignment. We find highly tunable exciton dipole moments and largely decoupled exciton recombination and ionization dynamics. We also predicted a bias regime where the hole wavefunction topology changes continuously from quantum dot-like to quantum ring-like as a function of the external bias. All these properties have great potential in advanced electro-optical applications and in the investigation of fundame…
