Correlated photon emission from semiconductor quantum dots grown in inverted pyramids

We demonstrate photon antibunching and deterministic single-photon operation using single InGaAs/AlGaAs semiconductor quantum dots grown on pre-patterned substrates. Additionally, we observe several types of single-photon cascades involving biexcitonic and other excitonic complexes and have modeled this behavior using numerical Monte-Carlo simulations. This method allows us to determine different non-radiative mechanisms otherwise not directly accessible via conventional spectroscopic methods.     

Theory of excitons, charged excitons, exciton fine-structure and entangled excitons in self-assembled semiconductor quantum dots

We show how the atomistic pseudopotential many-body theory of InGaAs/GaAs addresses some important effects, including (i) the fine-structure splittings (originating from interband spin exchange), (ii) the optical spectra of charged quantum dots and (iii) the degree of entanglement in a quantum dot molecule.     

Microscopic approach to many-exciton complexes in self-assembled InGaAs/GaAs quantum dots

We present a numerical calculation of many-exciton complexes in self-assembled InAs/GaAs quantum dots. We apply continuum elasticity theory and atomistic valence-force-field method to calculate strain distribution, and make use of various methods, ranging from a quasi-atomistic tight-binding approach to the single-band effective-mass approximation, to obtain single-particle energy levels. The effect of strain is incorporated by the deformation potential theory. We expand multiexciton states in the basis of Slater determinants and solve the many-body problem by the configuration-interaction method. The dynamics of multiexcitons is studied by solving the rate equations, from which the excitation–power dependence of emission spectrum is obtained. The emission spectra calculated by the microscopic tight-binding approach are found to be in good agreement with those obtained by the simple effective-mass method.     

Optically induced mechanical interaction between semiconductor quantum dots under an electronic resonance condition

Assuming an electronic resonance condition, we study the shape-dependence of the radiation force (RF) on a semiconductor quantum dot (QD) floating in medium and the optically induced mechanical interaction (OIMI) between two QDs with Maxwell stress tensor (MST) method, where the response fields are calculated by the improved discrete dipole approximation (DDA). Main results are as follows: (1) Properties of the RF on an isolated QD drastically change due to its shape and polarization of an incident light, which can be used for shape-selective manipulation. (2) Anomalous OIMI between two QDs arises depending on the spatial structures of internal fields, which results from the interaction between polarizations in respective QDs when they are near each other.     

Selecting wave function states in open quantum dots

We study how wave function scarring in an open quantum dot is influenced as the strength of its environmental coupling is varied and show evidence for groups of wave function scars that recur periodically with gate voltage. The precise form of these scars is found to evolve with gate voltage, which we discuss in terms of the properties of the semi-classical orbits that give rise to the scars. We also provide convincing experimental evidence for a correlation between the scars and the oscillations observed in the conductance when the gate voltage is varied.     

Confined states in two-dimensional flat elliptic quantum dots and elliptic quantum wires

The energy spectrum and corresponding wave functions of a flat quantum dot with elliptic symmetry are obtained exactly. A detailed study is made of the effect of ellipticity on the energy levels and the corresponding wave functions. The analytical behavior of the energy levels in certain limiting cases is obtained.     

Zeeman effect and magnetic anomalies in narrow-gap semiconductor quantum dots

We present a systematic theoretical study, based on the Kane–Weiler 8×8 k·p model, of the linear Zeeman splitting introduced by the interaction between the angular momentum and the magnetic field which can give a measure of the non-linear Zeeman effect associated with interband coupling and diamagnetic contributions. The conduction and valence bands g-factors are calculated for InSb spherical and semi-spherical quantum dots. The calculations of the g-factors showed an almost linear dependence, for the ground state, on the magnetic field. We have also found that the strong magnetic field dependence as well as the dependence on the dot size of the effective spin splitting can be unambiguously attributed to the strength of the inter-level mixing.     

Few-electron semiconductor quantum dots with Gaussian confinement

We have performed Hartree-Fock calculations of the electronic structure of N ≤ 10 electrons in a quantum dot modeled with a confining Gaussian potential well. We discuss the conditions for the stability of N bound electrons in the system. We show that the most relevant parameter determining the number of bound electrons is V ₀ R ². Such a feature arises from widely valid scaling properties of the confining potential. Gaussian Quantum dots having N = 2, 5, and 8 electrons are particularly stable in agreement with the Hund rule. The shell structure becomes less and less noticeable as the well radius increases.      

Long-range radiative interaction between semiconductor quantum dots

We model the resonant excitation transfer between semiconductor quantum dots, accounting for the radiative nature of the electromagnetic field. The model based on Maxwell equations and on a non-local linear susceptibility accounts both for the instantaneous dipole–dipole coupling, decaying as R⁻³, and for retardation effects, decaying as R⁻¹. The coupling is strongly resonant and its spatial range is of the order of the wavelength, due to the radiative nature of the retarded contribution.     

Magneto-optical study of nonmagnetic quantum dots coupled to a magnetic semiconductor quantum well

We have investigated a series of double-layer structures consisting of a layer of self-assembled non-magnetic CdSe quantum dots (QDs) separated by a thin ZnSe barrier from a ZnCdMnSe diluted magnetic semiconductor (DMSs) quantum well (QW). In the series, the thickness of the ZnSe barrier ranged between 12 and 40 nm. We observe two clearly defined photoluminescence (PL) peaks in all samples, corresponding to the CdSe QDs and the ZnCdMnSe QW, respectively. The PL intensity of the QW peak is observed to decrease systematically relative to the QD peak as the thickness of the ZnSe barrier decreases, indicating a corresponding increase in carrier tunneling from the QW to the QDs. Furthermore, polarization-selective PL measurements reveal that the degree of polarization of the PL emitted by the CdSe QDs increases with decreasing thickness of the ZnSe barriers. The observed behavior is discussed in terms of anti-parallel spin interaction between carriers localized in the non-magnetic QDs and in the magnetic QWs.     

Nano-laser on silicon quantum dots

A new conception of nano-laser is proposed in which depending on the size of nano-clusters (silicon quantum dots (QD)), the pumping level of laser can be tuned by the quantum confinement (QC) effect, and the population inversion can be formed between the valence band and the localized states in gap produced from the surface bonds of nano-clusters. Here we report the experimental demonstration of nano-laser on silicon quantum dots fabricated by nanosecond pulse laser. The peaks of stimulated emission are observed at 605 nm and 693 nm. Through the micro-cavity of nano-laser, a full width at half maximum of the peak at 693 nm can reach to 0.5 nm. The theoretical model and the experimental results indicate that it is a necessary condition for setting up nano-laser that the smaller size of QD (d < 3 nm) can make the localized states into band gap. The emission energy of nano-laser will be limited in the range of 1.7-2.3 eV generally due to the position of the localized states in gap, which is in good agreement between the experiments and the theory.     

Creation of entangled states of excitons in coupled quantum dots

We present two methods for the creation of two-particle entangled states of excitons in a coupled quantum dot system. The system contains two identical quantum dots that are coupled by an inter-dot hopping process. The manipulation of the system is succeeded by proper application of an external laser field.     

Transport through quasi-degenerate states in coupled quantum dots

By means of a general method for treating mesoscopic systems with strong internal correlations, transport properties through a set of quasi-degenerate transitions in the interacting region, or active element (AE), are considered. It is shown that the behaviour of the AE drastically changes as the couplings to the contacts are varied from the strong to the weak coupling limit. These changes strongly influence the transport properties of the system, from a single increase of the current to a staircase form with unequally large steps. In the present study, kinematic interactions, non-equilibrium populations numbers and dependence on the bias voltage has been included in the treatment of the local properties of the AE. Analytical results for the equilibrium situation are presented as well as a derivation of the corresponding non-equilibrium quantities. Results from self-consistent numerical calculations of the considered case are presented.     

Measuring the energy level repulsion in quantum dots

Energy level repulsion is one of the remnants of classical chaos in quantum mechanics. Measurements of the distribution of nearest neighbor spacings in quantum dots reveal, in contrast to other classically chaotic systems, deviations from the predictions made by random matrix theory. Here, we survey possible contributions to these deviations from experimental peculiarities present in measurements on quantum dots, and discuss the methods to eliminate or reduce such distortions.     

Optical spectroscopy of single InAs/InP quantum dots around λ=1.55 μm

We discuss the preparation and spectroscopic characterisation of a single InAs/InP quantum dot suitable for long-distance quantum key distribution applications around λ=1.55 μm. The dot is prepared using a site-selective growth technique which allows a single dot to be deposited in isolation at a controlled spatial location. Micro-photoluminescence measurements as a function of exciton occupation are used to determine the electronic structure of the dot. Biexciton emission, shell filling and many-body re-normalization effects are observed for the first time in single InAs/InP quantum dots.     

Fano effect and Andreev bound states in T-shape double quantum dots

In this Letter, we investigate the transport through a T-shaped double quantum dot coupled to two normal metal leads left and right and a superconducting lead. Analytical expressions of Andreev transmission and local density of states of the system at zero temperature have been obtained. We study the role of the superconducting lead in the quantum interferometric features of the double quantum dot. We report for first time the Fano effect produced by Andreev bound states in a side quantum dot. Our results show that as a consequence of quantum interference and proximity effect, the transmission from normal to normal lead exhibits Fano resonances due to Andreev bound states. We find that this interference effect allows us to study the Andreev bound states in the changes in the conductance between two normal leads.     

Surface integral determination of built-in electric fields and analysis of exciton binding energies in nitride-based quantum dots

We present a simple analytical approach to calculate the built-in strain-induced and spontaneous piezoelectric fields in nitride-based quantum dots (QDs) and then apply the method to describe the variation of exciton, biexciton and charged exciton energy with dot size in GaN/AlN QDs. We first present the piezoelectric potential in terms of a surface integral over the QD surface, and confirm that, due to the strong built-in electric field, the electrons are localised near the QD top and the holes are localised in the wetting layer just below the dot. The strong localisation and smaller dielectric constant results in much larger Coulomb interactions in GaN/AlN QDs than in typical InAs/GaAs QDs, with the interaction between two electrons, J_ee, or two holes, J_hh, being about a factor of three larger. The electron–hole recombination energy is always blue shifted in the charged excitons, X⁻ and X⁺, and the biexciton, and the blue shift increases with increasing dot height. We conclude that spectroscopic studies of the excitonic complexes should provide a useful probe of the structural and piezoelectric properties of GaN-based QDs.     

Photoluminescence quantum and surface states of excitons in ZnSe and CdS nanoclusters

The optical spectra of quantum dots (QDs) of CdS and ZnSe grown in borosilicate glass by the sol-gel method are obtained and analyzed. It is found that at concentrations of the two semiconductors x<0.06% the emission spectra are due to annihilation of free (internal) excitons in quantum states. The mean size of the quantum dots for a given concentration of ZnSe and CdS is calculated and found to be in good agreement with the X-ray data, and the exciton binding energy is calculated with allowance for the dielectric mismatch between the semiconductor and matrix. It is proposed that this mismatch may be the cause giving rise to the exciton percolation level that is observed in QD arrays for both systems at x>0.06%. The emission from the surface level of CdS QDs in the region ~2.7 eV, formed by the outer atoms with dangling bonds, is observed for the first time, as is the emission band from surface localized states. The relation between the position of the maximum of this band and the energy of the 1S state of the free exciton is established. It is shown that the properties of surface localized states are largely similar to the analogous properties of localized states of 3D (amorphous semiconductors, substitutional solid solutions of substitution) and 2D (quantum wells and superlattices) structures.