Binding energies of an exciton in a Gaussian potential quantum dot

In this paper, an exciton trapped by a Gaussian confining potential quantum dot has been investigated. Calculations are made by using the method of numerical diagonalization of Hamiltonian in the effective-mass approximation. The dependences of binding energies of the ground state and the first excited state on the size of the confining potential and the strength of the magnetic field are analysed explicitly.     

Dynamical localization effect in a coupled quantum dot array driven by an AC magnetic field

We have studied the transport properties of a ring-coupled quantum dot array driven by an AC magnetic field, which is connected to two leads, and we give the response of the transport current to the dynamical localization. We found that when the ratio of the magnetic flux to the total quantum dots number is a root of the zeroth order Bessel function, dynamical localization and collapse of quasi-energy occurs and importantly, the transport current displays a dip which is the signal of dynamical localization. The dynamical localization effect is strengthened as a result of the increase of the quantum dot number, and it is weakened on account of the increase of the dots-lead hopping rate.     

Controllable gain, absorption and dispersion properties of an asymmetric double quantum dot nanostructure

We propose a method to control the gain, absorption and dispersion properties in an asymmetric double quantum dot nanostructure interacting with four optical fields. From numerical results, it is found that the gain, zero-absorption (transparency), normal dispersion and the anomalous dispersion can be achieved separately by modulating the intensities of two control fields or the relative phase of the applied fields. This approach allows substantial flexibility in the manipulation of light propagation between subluminal and superluminal.     

磁场和量子点尺寸对激子性质的影响

在In_0.6Ga_0.4As/GaAs量子点中,采用一维等效势模型和有限差分法理论计算了激子态的性质,得到了激子跃迁能和束缚能随磁场、横向束缚强度以及量子点尺寸的变化关系.结果表明:加入磁场后,Zeeman效应使得激子的能级简并度解除,激子的基态跃迁能与实验符合得很好;横向束缚强度或磁场强度的增加使得激子的束缚增强;量子点的尺寸对激子的束缚产生重要的影响;通过电子-空穴间平均距离以及激子体系波函数分布图像分析了其产生的物理机制.     

Another model for a multiexcitonic quantum dot in an optical microcavity

Very recently, a multiexcitonic quantum dot in an optical microcavity have been theoretically studied [Herbert Vincka, Boris A. Rodriguez, and Augusto Gonzalez, Physica E, 2006, 35: 99–102]. However, due to the inevitable damping losses through the microcavity, in this work, we will present a more precise and sound model in the Lindblad form master equation to investigate the photonic properties of a single quantum dot (QD) in an optical microcavity system, in which the QD may confine the multiexcitons and be in resonant interaction with a single photonic mode of an optical microcavity. The excitation energies, and the properties of the emission photon from the QD microcavity are computed as functions of the exciton-photon coupling strength, detuning, and pump rate. We further compare our results with their results, and find that the calculated intensity of the emitted photon and the spectra crucially depend on the exciton-photon coupling strength g, the photon detuning, and the number of excitons in the QD. Finally, we will give a physical mechanism of the dressed-state picture for the strong coupling between the single mode of an optical microcavity and the QD emitters to explain the details of the emission photon spectra. Our study establishes useful guidelines for the experimental study of such multiexcitonic quantum dot in an optical microcavity system.      

Control of indirect exciton population in an asymmetric quantum dot molecule

We analyze the problem of coherent population transfer to the indirect exciton state in an asymmetric double semiconductor quantum dot molecule that interacts with an external electromagnetic field. Using the controlled rotation method, we obtain analytical solutions of the time-dependent Schrödinger equation and determine closed-form conditions for the parameters of the applied field and the quantum system that lead to complete population transfer to the indirect exciton state, in the absence of decay effects. Then, by numerical solution of the relevant density matrix equations we study the influence of decay mechanisms to the efficiency of population transfer.     

耦合量子阱中激子凝聚的研究进展

近年来，半导体量子阱中激子的玻色一爱因斯坦凝聚研究取得了很大进展．实验上利用耦合量子阱间接激子中电子和空穴在空间上的分离，显著提高了激子的冷却速度和寿命，成功地把激子冷却到1K以下，观察到了激子的准凝聚状态，并且在强激光照射下，发现了随光照强度增强而增大的激子发光环和环上形成的有规则斑点图案，引起了广泛的兴趣和重视．理论研究表明，发光环的出现是电子和空穴在量子阱中的反常输运行为造成的，但环上形成规则斑点的物理机理目前尚不清楚．文章介绍了这方面的实验背景和形成激子环的物理图像，指出了理论研究中存在的问题，并对解决问题的方案进行了讨论．     

Symmetry and size effects on energy and entanglement of an exciton in coupled quantum dots

We study theoretically the essential properties of an exciton in vertically coupled Gaussian quantum dots in the presence of an external magnetic field. The ground state energy of a heavy-hole exciton is split into four energy levels due to the Zeeman effect. For the symmetrical system, the entanglement entropy of the exciton state can reach a value of 1. However, for a system with broken symmetry, it is close to zero. Our results are in good agreement with previous studies.     

Carrier tunneling in asymmetric coupled quantum dots

Exciton spin relaxation at low temperatures in InAlAs–InGaAs asymmetric double quantum dots embedded in AlGaAs layers has been investigated as a function of the barrier thickness by the time-resolved photoluminescence measurements. With decreasing the thickness of the AlGaAs layer between the dots, the spin relaxation time change from 3 ns to less than 500 ps. The reduction in the spin relaxation time was considered to originate from the spin-flip tunneling between the ground state in InAlAs dot and the excited states in InGaAs dot, and the resultant tunneling leads to the spin depolarization of the ground state in InGaAs dot.     

Quantum control of two interacting electrons in a coupled quantum dot

Quantum-state engineering, i.e. active manipulation over the coherent dynamics of suitable quantum-mechanical systems, has become a fascinating prospect of modern physics. Here we discuss the dynamics of two interacting electrons in a coupled quantum dot driven by an external electric field. The results show that the two quantum dots can be used to prepare a maximally entangled Bell state by changing the strength and duration of an oscillatory electric field. Different from the suggestion made by Loss \textit et al (1998 Phys. Rev. A 57 120, the present entanglement involves the spatial degree of freedom for the two electrons. We also find that the coherent tunnelling suppression discussed by Grossmann \textit et al (1991 Phys. Rev. Lett. 67 516 persists in the two-particle case: i.e. two electrons initially localized in one dot can remain dynamically localized, although the strong Coulomb repulsion prevents them from behaving so. Surprisingly, the interaction enhances the degree of localization to a large extent compared with that in the non-interacting case. This phenomenon is referred to as the Coulomb-enhanced dynamical localization.     

Spectroscopy of molecular states in a few-electron double quantum dot

Semiconductor quantum dots, so-called artificial atoms, have attracted considerable interest as mesoscopic model systems and prospective building blocks of the “quantum computer”. Electrons are trapped locally in quantum dots, forming controllable and coherent mesoscopic atom- and moleculelike systems. Electrostatic definition of quantum dots by use of top gates on a GaAs/AlGaAs heterostructure allows wide variation of the potential in the underlying two-dimensional electron gas. By distorting the trapping potential of a single quantum dot, a strongly tunnel-coupled double quantum dot can be defined. Transport spectroscopy measurements on such a system charged with N=0,1,2,… electrons are presented. In particular, the tunnel splitting of the double well potential for up to one trapped electron is unambiguously identified. It becomes visible as a pronounced level anticrossing at finite source drain voltage. A magnetic field perpendicular to the two-dimensional electron gas also modulates the orbital excitation energies in each individual dot. By tuning the asymmetry of the double well potential at finite magnetic field the chemical potentials of an excited state of one of the quantum dots and the ground state of the other quantum dot can be aligned, resulting in a second level anticrossing with a larger tunnel splitting. In addition, data on the two-electron transport spectrum are presented.     

External electric field effect on the optical rectification coefficient of an exciton in a spherical parabolic quantum dot

External electric field effects on the optical rectification coefficient of an exciton confined in a spherical parabolic quantum dot are theoretically investigated. To this end, energy eigenvalues and eigenfunctions of the system are calculated, using the direct matrix diagonalization method. The compact-density matrix approach and an iterative method are used to find the optical rectification coefficient of a typical GaAs parabolic quantum dot. The results show that the optical rectification coefficient strongly depends on the confinement frequency and the magnitude of the electric field. Moreover, the peak value of this optical quantity is shifted to the aspect of high energy when the influence of the electric field is considered.     

Metal–insulator transition in quantum dot arrays

We present evidence for a re-entrant metal–insulator transition that arises in quantum dot arrays as the gate voltage is used to sweep their density of states past the Fermi level. The form of the temperature variation of the conductance observed in these arrays can be accounted for using a functional form derived from studies of the metal–insulator transition in two dimensions, although the values obtained for the fit parameters suggest that the behavior we observe here may be quite distinct to that found in two dimensions.     

Optical properties of exciton confinement in wurtzite ZnO/MgxZn1−xO coupled quantum dots

Based on the framework of effective-mass approximation and variational approach, optical properties of exciton are investigated theoretically in ZnO/Mg_xZn_1−xO vertically coupled quantum dots (QDs), with considering the three-dimensional confinement of electron and hole pair and the strong built-in electric field effects due to the piezoelectricity and spontaneous polarization. The exciton binding energy, the emission wavelength and the oscillator strength as functions of the different structural parameters (the dot height and the barrier thickness between the coupled wurtzite ZnO QDs) are calculated with the built-in electric field in detail. The results elucidate that structural parameters have a significant influence on the exciton state and optical properties of ZnO coupled QDs. These results show the optical and electronic properties of the quantum dot that can be controlled and also tuned through the nanoparticle size variation.     

LO-phonon assisted tunneling from spatially direct to indirect exciton in semimagnetic double quantum well structures

Relaxation from spatially direct to the spatially indirect exciton through ZnSe barriers of different thicknesses is investigated in (ZnCdMn)Se/ZnSe/(ZnCd)Se asymmetric double quantum wells by use of magneto-optical steady-state photoluminescence (PL) and PL excitation (PLE) experiments. The 1-LO-phonon scattering has been found to be the relevant mechanism for effective electron and hole tunneling.     

Effects of bias on dynamics of an AC-driven two-electron quantum-dot molecule

The effects of bias on the dynamical localization of two interacting electrons in a pair of coupled quantum dots driven by external AC fields have been numerically investigated. With an effective two-site model and Floquet formalism,the time-dependent Schroedinger equation is numerically solved and the Pmin, the minimum of the population evolution of the initial state within a certain time period, is used to quantify the degree of the dynamical localization. Results indicate that the bias can change the energy of the initial state and break the dynamical symmetry of the system with a pure AC field. And the amplitude of the AC field with dynamical localization phenomenon changes with bias. All the numerical results are explained by the perturbation theory and two-level approximation.     

Effect of electron–electron interactions in a quantum dot with a tapered constriction

We study the effect of electron–electron interactions between several electrons in a quantum dot with a tapered constriction by monitoring the behavior of the position of the absolute charge density maximum,Z_max, of each occupied state under DC electric fields. States of this system are localized in, and can be identified with, either the left- or right-hand region, separated by the neck of the constriction. To demonstrate the effect, two cases with three electrons in the quantum dot were studied: (1) One electron is in the left-hand side region and the other two electrons are in the right-hand side region. They occupy the two lowest energy states of the quantum dot system. The movement of theZ_maxof the singly occupied state through the constriction does not show any unusual behavior except that it can be accelerated by a resonance process. (2) All three electrons are in the left-hand side region and occupy the two lowest energy states in that region. In this case, theZ_max’s of the two states move through the constriction in a competitive manner which would not be anticipated on the basis of either energy considerations or the results of case 1. Furthermore, and most significantly, we show that this unusual behavior depends completely upon electron–electron interactions: if they are not taken into account, it does not occur. We show also that this competitive process can occur in a ground-state configuration.     

Strain-induced quantum dot superlattice

Coupled double quantum dots and quantum dot superlattices are formed by utilizing the strain of an InP island on top of a near-surface multi-quantum-well structure. The number and composition of the quantum wells together with the thickness of the barrier separating the quantum wells are varied to investigate the coupling of the wave functions of the carriers confined in separate vertically stacked dots. Photoluminescence studies show that the reduction of the barrier thickness and the increase of the number of wells enhance the coupling, which is observed as red shift and narrowing of the quantum dot peak. The calculated shifts of the peak positions agree closely with the experimental values.     

Non-radiative exciton recombination through excitation energy transfer in quantum dot clusters

Excitation energy transfer (EET) processes in CdSe/CdZnS quantum dot (QD) clusters have been investigated in this study by measuring their time-resolved and spectrally resolved fluorescence intensities. The contributions of radiative and non-radiative exciton recombination through EET are evaluated, where the latter is expected to occur in a large class of QD ensembles because of the presence of nonluminescent QDs. It appears that the fluorescence decay in larger QDs serving as acceptor does not show an initial rise, in addition the lifetime of the acceptor QD is independent of the excitation wavelength, suggesting that an EET is followed mostly by non-radiative recombination.     

Inelastic tunneling in a double quantum dot coupled to a bosonic environment

Coupling a quantum system to a bosonic environment always give rise to inelastic processes, which reduce the coherency of the system. We measure energy-dependent rates for inelastic tunneling processes in a fully controllable two-level system of a double quantum dot. The emission and absorption rates are well reproduced by Einstein's coefficients, which relate to the spontaneous emission rate. The inelastic tunneling rate can be comparable to the elastic tunneling rate if the boson occupation number becomes large. In the specific semiconductor double dot, the energy dependence of the inelastic rate suggests that acoustic phonons are coupled to the double dot piezoelectrically.