We theoretically study the optical properties of an InAs/GaAs quantum dot (QD) near the area of the second-order resonance between an electron confined in the QD and two longitudinal optical phonons. We present the absorption spectra of an inhomogeneously broadened QD ensemble and show that the minimal model needed for an accurate description of such a system needs to account for 3-phonon states. We study also the influence of the QD height to width ratio on the optical properties of the polaron system. The dependence of the width of the resonance and the position of the second-order resonant feature on the height to width ratio is presented.
The contribution to the biexciton binding energy in quantum dots resulting from the interaction with longitudinal optical phonons is estimated by performing the configuration--interaction calculation of the few-particle states in a simple model of the confining potential and including the phonon corrections by means of a perturbation theory. It is found that the polaron contribution tends to compensate the Coulomb-related biexciton shift (binding energy) and reduces its value by several to even 30%, depending on the material parameters of the system.
We study the second-order polaronic resonance between 2-LO-phonon states and p-shell electron states in a quantum dot. We show that the spectrum in the resonance area can be quantitatively reproduced by a theoretical model using only standard material constants, without any free parameters. We propose also a perturbative approach to the problem based on a quasi-degenerate perturbation theory. This method not only considerably reduces the numerical complexity without considerable loss of accuracy but also gives some insight into the structure and origin of the resonance spectrum.
We calculate the lifetime of conduction band excited states in self-assembled quantum dots by taking into account LO-phonon-electron interaction and various anharmonic phonon couplings. We show that polaron relaxation cannot be accurately described by a semi-classical model. The contributions of different anharmonic decay channels are shown to depend strongly on the polaron energy. We calculate the energy dependence of polaron lifetime and compare our results to available experimental measurements of polaron decay time in InAs/GaAs quantum dots.
We present evidence of magnetic ordering in type-II (Zn, Mn) Te quantum dots. This ordering is attributed to the formation of bound magnetic polarons caused by the exchange interaction between the strongly localized holes and Mn within the dots. In our photoluminescence studies, the magnetic polarons are detected at temperatures up to ~ 200 K, with a binding energy of ~ 40 meV. In addition, these dots display an unusually small Zeeman shift with applied field (2 meV at 10 T). This behavior is explained by a small and weakly temperature-dependent magnetic susceptibility due to anti-ferromagneti…
Magnetic semiconductor quantum dots with a few carriers represent an interesting model system where ferromagnetic interactions can be tuned by voltage. By designing the geometry of a doped quantum dot, one can tailor the anisotropic quantum states of magnetic polarons. The strong anisotropy of magnetic polaron states in disk-like quantum dots with holes comes from the spin splitting in the valence band. The binding energy and spontaneous magnetization of quantum dots oscillate with the number of particles and reflect the shell structure. Due to the Coulomb interaction, the maximum binding ener…
We propose a model of magnetic polaron formation in semiconductor quantum dots doped with magnetic ions. A wetting layer serves as a reservoir of photo-generated holes, which can be trapped by the adjacent quantum dots. For certain hole densities, the temperature dependence of the magnetization induced by the trapped holes is reentrant: it disappears for some temperature range and reappears at higher temperatures. We demonstrate that this peculiar effect is not an artifact of the mean field approximation and persists after statistical spin fluctuations are accounted for. We predict fingerprint…
While time-dependent perturbation theory shows inefficient carrier-phonon scattering in semiconductor quantum dots, we demonstrate that a quantum kinetic description of carrier-phonon interaction predicts fast carrier capture and relaxation. The considered processes do not fulfill energy conservation in terms of free-carrier energies because polar coupling of localized quantum-dot states strongly modifies this picture.
In this work, we present a theoretical study of polaron states in a double quantum dot system. We present realistic calculations which combine 8 band \kp model, configuration interaction approach and collective modes method. We investigate the dependence of polaron energy branches on axial electric field. We show that coupling between carriers and longitudinal optical phonons via Fröhlich interaction leads to qualitative and quantitative reconstruction of the optical spectra. In particular, we study the structure of resonances between the states localized in different dots. We show that $p$-sh…
We present a theoretical investigation of the optical properties of polaronic excitons in stacked self-assembled quantum dots, which is based on the non-adiabatic approach. A parallelepiped-shaped quantum dot is considered as a model for a self-assembled quantum dot in a stack. The exciton-phonon interaction is taken into account for all phonon modes specific for these quantum dots (bulk-like, half-space and interface phonons). We show that the coupling between stacked quantum dots can lead to a strong enhancement of the optical absorption in the spectral ranges characteristic for phonon satel…
We study phonon-assisted dephasing in optically excited semiconductor quantum dots within the frameworks of the independent Boson model and optimal control. Using a realistic description for the quantum dot states and the phonon coupling, we demonstrate that such dephasing has a drastic impact on the coherent optical response. We employ optimal control theory to search for control strategies that allow to fight decoherence, and show that appropriate tailoring of laser pulses allows a complete control of the optical excitation despite the phonon dephasing.
We theoretically examine the effects of polaron formation in quantum dots on the transport properties. When a separation between two electron-levels in a quantum dot matches the energy of the longitudinal optical (LO) phonons, the polarons are strongly formed. The Rabi splitting between the levels is observable in a peak structure of the differential conductance G as a function of the bias voltage. The polaron formation suppresses the peak height of G, which is due to the competition between the resonant tunneling (resonance between a level in the dot and states in the leads) and the polaron f…
Motivated by recent experiments on electric transport through single molecules and quantum dots, we investigate a model for transport that allows for significant coupling between the electrons and a boson mode isolated on the molecule or dot. We focus our attention on the temperature dependent properties of the transport. In the Holstein picture for polaronic transport in molecular crystals the temperature dependence of the conductivity exhibits a crossover from coherent (band) to incoherent (hopping) transport. Here, the temperature dependence of the differential conductance on resonance does…
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…
The size distribution of self-assembled InAs quantum dots grown on (001) InP under the Stranski-Krastanow growth mode is controlled using selective area/chemical beam epitaxy, which allows the formation of quantum dots at specific locations. As the dimensions of the patterned areas are decreased from 1000 nm down to 50 nm or less, scanning electron microscopy reveals a gradual increase in the spatial correlation between quantum dots, which leads to the formation of ordered arrays for dimensions below 200 nm.
Self-assembled quantum dots have remarkable optical, electronic and spintronic properties that make them leading candidates for quantum information technologies. Their characterization requires rapid and local determination of both charge and spin degrees of freedom. We present a way to probe the capacitance of small ensembles of quantum dots at microwave frequencies. The technique employs a capacitance sensor based on a microwave microstrip resonator with sensitivity ~10^(-19) F/rt(Hz), high enough to probe single electrons. The integration of this design in a scanning microscope will provide…
We investigate the electronic transport through a suspended carbon-nanotube quantum dot. In the presence of a magnetic field perpendicular to the nanotube and a nearby metallic gate, two forces act on the electrons: the Laplace and the electrostatic force. They both induce coupling between the electrons and the mechanical transverse oscillation modes. We find that the difference between the two mechanisms appears in the cotunneling current.
We investigate the noise properties of a GaAs-AlAs-GaAs tunneling structure with embedded self-assembled InAs quantum dots in the single-electron tunneling regime. We analyze the dependence of the relative noise amplitude of the shot noise on bias voltage. We observe a non-monotonic behaviour of the Fano-factor $α$ with an average value of $α\approx 0.8$ consistent with the asymmetry of the tunneling barriers. Reproducible fluctuations observed in $α$ can be attributed to the successive participation of more and more InAs quantum dots in the tunneling current.
We investigate how the strain-induced nuclear quadrupole interaction influences the degree of nuclear spin polarization in self-assembled quantum dots. Our calculation shows that the achievable nuclear spin polarization in In_{x}Ga_{1-x}As quantum dots is related to the concentration of indium and the resulting strain distribution in the dots. The interplay between the nuclear quadrupole interaction and Zeeman splitting leads to interesting features in the magnetic field dependence of the nuclear spin polarization. Our results are in qualitative agreement with measured nuclear spin polarizatio…
Here we study the polaronic transport through molecules weakly connected to metallic electrodes in the nonlinear response regime. Molecule itself is treated as a quantum dot with discrete energy levels, its connection to the electrodes is described within the wide-band approximation, while the charging is incorporated by means of the self-consistent potential. Nonperturbative computational scheme, used in this work, is based on the Green's function theory within the framework of mapping technique (GFT-MT). This method transforms the many-body electron-phonon interaction problem into a one-body…
Magnetotunneling spectroscopy was employed to probe the confinement in vertical Si/Ge double-barrier resonant tunneling diodes with regularly distributed Ge quantum dots. Their current-voltage characteristics reveal a step-like behavior in the vicinity of zero bias, indicating resonant tunneling of heavy-holes via three-dimensionally confined unoccupied hole states in Ge quantum dots. Assuming parabolic confinement we extract the strength of the confinement potential of quantum dots.
The knowledge of electron and hole g-factors, their control and engineering are key for the usage of the spin degree of freedom for information processing in solid state systems. The electronic g-factor will be materials dependent, the effect being larger for materials with large spin-orbit coupling. Since electrons can be individually trapped into quantum dots in a controllable manner, they may represent a good platform for the implementation of quantum information processing devices. Here we use self-assembled quantum dots of InAs embedded in GaAs for the g-factor control and engineering.
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.
Polaron dephasing processes are investigated in InAs/GaAs dots using far-infrared transient four wave mixing (FWM) spectroscopy. We observe an oscillatory behaviour in the FWM signal shortly (< 5 ps) after resonant excitation of the lowest energy conduction band transition due to coherent acoustic phonon generation. The subsequent single exponential decay yields long intraband dephasing times of 90 ps. We find excellent agreement between our measured and calculated FWM dynamics, and show that both real and virtual acoustic phonon processes are necessary to explain the temperature dependence of…
Scattering of carriers between spatially separated zero dimensional states has been observed in a system of self-assembled In_{0.55}Al_{0.45}As quantum dots. We believe the interdot tunneling is mediated by localized states below the barrier band edge. The experiment was performed by taking photoluminescence excitation spectra at 4.2K using a near-field scanning optical microscope. Surprisingly, the excitation spectrum from individual dots does not quench to zero at any energy, even when the energy of the exciting light is tuned below the barrier band edge. On top of this continuum, narrow res…
The theory of exciton coupling to photons and LO phonons in quantum dots (QDs) and quantum-dot molecules (QDMs) is presented. Resonant-round trips of the exciton between the ground (bright) and excited (dark or bright) states mediated by the LO-phonon alter the decay time and yield the Rabi oscillation. The initial distributions of the population in the ground and the excited states dominate the oscillating amplitude and frequency. This property provides a detectable signature to the information stored in a qubit made from QD or QDM for a wide range of temperature T. Our results presented here…
We report exact diagonalization studies of inelastic light scattering in few-electron quantum dots under the strong confinement regime characteristic of self-assembled dots. We apply the orthodox (second-order) theory for scattering due to electronic excitations, leaving for the future the consideration of higher-order effects in the formalism (phonons, for example), which seem relevant in the theoretical description of available experiments. Our numerical results stress the dominance of monopole peaks in Raman spectra and the breakdown of selection rules in open-shell dots. The dependence of …
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.
Measuring single-electron charge is one of the most fundamental quantum technologies. Charge sensing, which is an ingredient for the measurement of single spins or single photons, has been already developed for semiconductor gate-defined quantum dots, leading to intensive studies on the physics and the applications of single-electron charge, single-electron spin and photon-electron quantum interface. However, the technology has not yet been realized for self-assembled quantum dots despite their fascinating quantum transport phenomena and outstanding optical functionalities. In this paper, we r…
We investigate the effects of point charge defects on the single particle electronic structure, emission energies, fine structure splitting and oscillator strengths of excitonic transitions in strained In$_{0.6}$Ga$_{0.4}$As/GaAs and strain-free GaAs/Al$_{0.3}$Ga$_{0.7}$As quantum dots. We find that the charged defects significantly modify the single particle electronic structure and excitonic spectra in both strained and strain-free structures. However, the excitonic fine structure splitting, polarization anisotropy and polarization direction in strained quantum dots remain nearly unaffected,…
We present photoluminescence studies of highly excited single self--assembled CdTe quantum dots under continuous--wave and pulsed excitations. We observe appearance of emission bands related to sequential filling of s--, p-- and d--shells. We analyze the inter-shell splitting for five samples, in which the dots were formed from a strained CdTe layer of different width. We find that with increasing the CdTe layer width the inter-shell splitting increases. In a time resolved measurement, we observe a radiative cascade between transitions involving one, two, and more than two excitons.
We have fabricated single electron tunneling structures by forming nanogap metallic electrodes directly upon single self-assembled InAs quantum dots (QDs). The fabricated samples exhibited clear Coulomb blockade effects. Furthermore, a clear Kondo effect was observed when strong coupling between the electrodes and the QDs was realized using a large QD with a diameter of ~ 100 nm. From the temperature dependence of the linear conductance at the Kondo valley, the Kondo temperature TK was determined to be ~ 81 K. This is the highest TK ever reported for artificial quantum nanostructures.
Three-dimensional anisotropy of the Lande g-factor and its electrical modulation are studied for single uncapped InAs self-assembled quantum dots (QDs). The g-factor is evaluated from measurement of inelastic cotunneling via Zeeman substates in the QD for various magnetic field directions. We find that the value and anisotropy of the g-factor depends on the type of orbital state which arises from the three-dimensional confinement anisotropy of the QD potential. Furthermore, the g-factor and its anisotropy are electrically tuned by a side-gate which modulates the confining potential.
We have studied the size dependence of the exciton g-factor in self-assembled InAs/InP quantum dots. Photoluminescence measurements on a large ensemble of these dots indicate a multimodal height distribution. Cross-sectional Scanning Tunneling Microscopy measurements have been performed and support the interpretation of the macro photoluminescence spectra. More than 160 individual quantum dots have systematically been investigated by analyzing single dot magneto-luminescence between 1200nm and 1600 nm. We demonstrate a strong dependence of the exciton g-factor on the height and diameter of the…
We use an atomistic model to consider the effect of shape symmetry breaking on the optical properties of self-assembled InAs/GaAs quantum dots. In particular, we investigate the energy level structure and optical activity of the lowest energy excitons in these nanostructures. We compare between quantum dots with two-fold rotational and two reflections (C2v) symmetry and quantum dots in which this symmetry was reduced to one reflection only (Cs) by introducing a facet between the quantum dots and the host material. We show that the symmetry reduction mostly affects the optical activity of the d…
We demonstrate evidence for a radiative recombination channel of dark excitons in self-assembled quantum dots. This channel is due to a light hole admixture in the excitonic ground state. Its presence was experimentally confirmed by a direct observation of the dark exciton photoluminescence from a cleaved edge of the sample. The polarization resolved measurements revealed that a photon created from the dark exciton recombination is emitted only in the direction perpendicular to the growth axis. Strong correlation between the dark exciton lifetime and the in-plane hole g-factor enabled us to sh…
Self-assembled semiconductor quantum dot is a new type of artificially designed and grown function material which exhibits quantum size effect, quantum interference effect, surface effect, quantum tunneling-Coulumb-blockade effect and nonlinear optical effect. Due to advantages like less crystal defects and relatively simpler fabrication technology, that material may be of important value in future nanoelectronic device researches. In the order of vertical transport, lateral transport and charge storage, this paper gives a brief introduction of recent advances in the electronic properties of t…
We used continuous wave photoluminescence (cw-PL) and time resolved photoluminescence (TR-PL) spectroscopy to compare the properties of magnetic polarons (MP) in two related spatially indirect II-VI epitaxially grown quantum dot systems. In the ZnTe/(Zn,Mn)Se system the holes are confined in the non-magnetic ZnTe quantum dots (QDs), and the electrons reside in the magnetic (Zn,Mn)Se matrix. On the other hand, in the (Zn,Mn)Te/ZnSe system, the holes are confined in the magnetic (Zn,Mn)Te QDs, while the electrons remain in the surrounding non-magnetic ZnSe matrix. The magnetic polaron formation …
We investigate the thermal quenching of the multimodal photoluminescence from InAs/InP (001) self-assembled quantum dots. The temperature evolution of the photoluminescence spectra of two samples is followed from 10 K to 300 K. We develop a coupled rate-equation model that includes the effect of carrier thermal escape from a quantum dot to the wetting layer and to the InP matrix, followed by transport, recapture or non-radiative recombination. Our model reproduces the temperature dependence of the emission of each family of quantum dots with a single set of parameters. We find that the main es…
We measure the dynamics of nuclear spins in a self-assembled quantum dot at a magnetic field of 5 Tesla and identify two distinct mechanisms responsible for the decay of the Overhauser field. We attribute a temperature-independent decay which lasts ~100 seconds to intra-dot diffusion induced by hyperfine-mediated indirect nuclear spin interaction. In addition, we observe a gate-voltage and temperature dependent decay stemming from co-tunneling mediated nuclear spin flip processes. By adjusting the gate-voltage and lowering the electron temperature to ~200 milliKelvin, we prolong the correspond…
We study the leakage dynamics of charge stored in an ensemble of CdTe quantum dots embedded in a field-effect structure. Optically excited electrons are stored and read out by a proper time sequence of bias pulses. We monitor the dynamics of electron loss and find that the rate of the leakage is strongly dependent on time, which we attribute to an optically generated electric field related to the stored charge. A rate equation model quantitatively reproduces the results.
We demonstrate full charge control, narrow optical linewidths, and optical spin pumping on single self-assembled InGaAs quantum dots embedded in a $162.5\,\text{nm}$ thin diode structure. The quantum dots are just $88\,\text{nm}$ from the top GaAs surface. We design and realize a p-i-n-i-n diode that allows single-electron charging of the quantum dots at close-to-zero applied bias. In operation, the current flow through the device is extremely small resulting in low noise. In resonance fluorescence, we measure optical linewidths below $2\,μ\text{eV}$, just a factor of two above the transform l…
We study theoretically the spin-flip relaxation processes for a single electron in a self-assembled InAs/GaAs quantum dot, using an 8-band kp theory in the envelope function approximation. We show that the dominating channel of spin relaxation is spin admixture induced by symmetry-breaking shear strain, which can be mapped onto two effective spin-phonon terms in a conduction band (effective mass) Hamiltonian that have a similar structure and interfere constructively. Unlike the Dresselhaus coupling that dominates spin relaxation in larger, unstrained dots, the shear strain contribution cannot …
The radiative recombination rates of interacting electron-hole pairs in a quantum dot are strongly affected by quantum correlations among electrons and holes in the dot. Recent measurements of the biexciton recombination rate in single self-assembled quantum dots have found values spanning from two times the single exciton recombination rate to values well below the exciton decay rate. In this paper, a Feynman path-integral formulation is developed to calculate recombination rates including thermal and many-body effects. Using real-space Monte Carlo integration, the path-integral expressions f…
We study the formation dynamics of a spontaneous ferromagnetic order in single self-assembled CdMnTe quantum dots. By measuring time-resolved photoluminescence, we determine the formation times for QDs with Mn ion contents x varying from 0.01 to 0.2. At low x these times are orders of magnitude longer than exciton spin relaxation times evaluated from the decay of photoluminescence circular polarization. This allows us to conclude that the direction of the spontaneous magnetization is determined by a momentary Mn spin fluctuation rather than resulting from an optical orientation. At higher x, t…
Luminescence properties of colloidal quantum dots have found applications in imaging, light-sources, and displays. Despite wide interest, several experimental observations from these quantum dots, such as the short lifetime on the scale of microseconds and a zero-longitudinal optical phonon line in low-temperature photoluminescence spectrum, remain unexplained by existing models. Here we propose a model including the effect of solid-state environment on luminescence by capturing coherent and incoherent interactions of band-edge exciton with phonon modes. Our model predicts the formation of dre…
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 report on single InGaAs quantum dots embedded in a lateral electric field device. By applying a voltage we tune the neutral exciton transition into resonance with the biexciton using the quantum confined Stark effect. The results are compared to theoretical calculations of the relative energies of exciton and biexciton. Cascaded decay from the manifold of single exciton-biexciton states has been predicted to be a new concept to generate entangled photon pairs on demand without the need to suppress the fine structures splitting of the neutral exciton.
We studied the formation mechanism of the in-plane nuclear field in single self-assembled In$_{0.75}$Al$_{0.25}$As/Al$_{0.3}$Ga$_{0.7}$As quantum dots. The Hanle curves with an anomalously large width and hysteretic behavior at the critical transverse magnetic field were observed in many single quantum dots grown in the same QD sample. In order to explain the anomalies in the Hanle curve indicating the formation of a large nuclear field perpendicular to the photo-injected electron spin polarization, we propose a new model based on the current phenomenological model for dynamic nuclear spin pol…
We present a theory of the optical control of the spin of an electron in an InAs quantum dot. We show how two Raman-detuned laser pulses can be used to obtain arbitrary single-qubit rotations via the excitation of an intermediate trion state. Our theory takes into account a finite in-plane hole $g$-factor and hole-mixing. We show that such rotations can be performed to high fidelities with pulses lasting a few tens of picoseconds.
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 present a study on the intersublevel spacings of electrons and holes in a single layer of InAs self-assembled quantum dots (SAQDs) using Fourier transform infrared (FTIR) transmission spectroscopy without the application of an external magnetic field. Epitaxial, complementary-doped and semi-transparent electrostatic gates are grown within the ultra high vacuum conditions of molecular beam epitaxy to voltage-tune the device, while a two dimensional electron gas (2DEG) serves as back contact. Spacings of the hole sublevels are indirectly calculated using the photoluminescence spectroscopy alo…
We show here the existence of the indirect coupling of electron and magnetic or nuclear ion spins in self-assembled quantum dots mediated by electron-electron interactions. With a single localized spin placed in the center of the dot, only the spins of electrons occupying the zero angular momentum states couple directly to the localized spin. We show that when the electron-electron interactions are included, the electrons occupying finite angular momentum orbitals interact with the localized spin. This effective interaction is obtained using exact diagonalization of the microscopic Hamiltonian…
We report the measurement of extremely slow hole spin relaxation dynamics in small ensembles of self-assembled InGaAs quantum dots. Individual spin orientated holes are optically created in the lowest orbital state of each dot and read out after a defined storage time using spin memory devices. The resulting luminescence signal exhibits a pronounced polarization memory effect that vanishes for long storage times. The hole spin relaxation dynamics are measured as a function of external magnetic field and lattice temperature. We show that hole spin relaxation can occur over remarkably long times…
We investigate the electron and hole spin relaxation in an ensemble of self-assembled InAs/In$_{0.53}$Al$_{0.24}$Ga$_{0.23}$As/InP quantum dots with emission wavelengths around $1.5$~$μ$m by pump-probe Faraday rotation spectroscopy. Electron spin dephasing due to the randomly oriented nuclear Overhauser fields is observed. At low temperatures we find a sub-microsecond longitudinal electron spin relaxation time $T_1$ which unexpectedly strongly depends on temperature. At high temperatures the electron spin relaxation time is limited by optical phonon scattering through spin-orbit interaction de…
We have investigated the polarization dependence of the emission spectra of p-shell multiexcitons of a quantum dot when the single particle level spacing is larger than the characteristic energy of the Coulomb interactions. We find that there are many degenerate multiexciton states. The emission intensities depend on the number of degenerate initial and final states of the optical transitions. However, unlike the transition energies, they are essentially independent of the strength of the Coulomb interactions. In the presence of electron-hole symmetry the independence is exact.
Quantum dots are arguably one of the best platforms for optically accessible spin based qubits. The paramount demand of extended qubit storage time can be met by using quantum-dot-confined dark exciton: a longlived electron-hole pair with parallel spins. Despite its name the dark exciton reveals weak luminescence that can be directly measured. The origins of this optical activity remain largely unexplored. In this work, using the atomistic tight-binding method combined with configuration-interaction approach, we demonstrate that atomic-scale randomness strongly affects oscillator strength of d…
We report on a high resolution x-ray diffraction study unveiling the effect of carriers optically injected into (In,Ga)As quantum dots on the surrounding GaAs crystal matrix. We find a tetragonal lattice expansion with enhanced elongation along the [001] crystal axis that is superimposed on an isotropic lattice extension. The isotropic contribution arises from excitation induced lattice heating as confirmed by temperature dependent reference studies. The tetragonal expansion on the femtometer scale is attributed to polaron formation by carriers trapped in the quantum dots.
We investigate polarization properties of neutral exciton emission in single self-assembled InAs/GaAs quantum dots. The in-plane shape and strain anisotropy strongly couple the heavy and light hole states and lead to large optical anisotropy with non-orthogonal linearly polarized states misaligned with respect to the crystallographic axes. Owing to a waveguiding experimental configuration, luminescence polarization along the growth axis has been observed revealing the presence of shear components of the deformation tensor out of the growth plane. Resonant luminescence experiments allowed deter…
The optical spectrum of a quantum dot is typically dominated by the fundamental transition between the lowest-energy configurations. However, the radiative Auger process can result in additional red-shifted emission lines. The origin of these lines is a combination of Coulomb interaction and symmetry-breaking in the quantum dot. In this paper, we present measurements of such radiative Auger lines for a range of InGaAs/GaAs self-assembled quantum dots and use a tight-binding model with a configuration interaction approach to explain their appearance. Introducing a composition fluctuation cluste…
