Abnormal Energy Dependence of Photoluminescence Decay Time in InGaN Epilayer

We employ photoluminescence (PL) and time-resolved PL to study exciton localization effect in InGaN epilayers.By measuring the exciton decay time as a function of the monitored emission energy at different temperatures,we have found unusual behaviour of the energy dependence in the PL decay process. At low temperature, the measured PL decay time increases with the emission energy. It decreases with the emission energy at 200K, and remains nearly constant at the intermediate temperature of 12OK. We have studied the dot size effect on the radiative recombination time by calculating the temperature dependence of the exciton recombination lifetime in quantum dots, and have found that the observed behaviour can be well correlated to the exciton localization in quantum dots. This suggestion is further supported by steady state PL results.     

Temperature Dependence of Photoluminescence from Single and Ensemble InAs/GaAs Quantum Dots

We investigate the temperature dependence of photoluminescence from single and ensemble InAs/GaAs quantum dots systematically. As temperature increases, the exciton emission peak for single quantum dot shows broadening and redshift. For ensemble quantum dots, however, the exciton emission peak shows narrowing and fast redshift. We use a simple steady-state rate equation model to simulate the experimental data of photoluminescence spectra. It is confirmed that carrier-phonon scattering gives the broadening of the exciton emission peak in single quantum dots while the effects of carrier thermal escape and retrapping play an important role in the narrowing and fast redshift of the exciton emission peak in ensemble quantum dots.     

Interferometric spectroscopy for excitons in InP single quantum dots

Dynamical dephasing processes of an exciton and a charged exciton in single InP quantum dots were studied by using interferometric spectroscopy. Interferometric spectroscopy enabled us to observe with high sensitivity the dephasing of exciton or other exciton complexes in single quantum dots. In order to observe the dephasing of the exciton or exciton complexes, emitted single-photons generated from single InP quantum dots were detected through the Michelson interferometer. The contrast of the interferometric signal due to the exciton and the charged exciton shows non-exponential decay under band-to-band excitation for the GaInP matrix. The band-to-band excitation generates carriers trapped in the matrix and the trapped carriers modulate the energy of the quantum dots because of the quantum-confined Stark effect. Therefore the non-exponential decays are caused by energy fluctuation due to the trap carriers in the long timescale.     

Exciton tunnelling in ZnCdSe quantum well/CdSe quantum dots

Exciton tunnelling through a ZnSe barrier layer of various thicknesses is investigated in a Zn_0.72Cd_0.28Se/CdSe coupled quantum well/quantum dots (QW/QDs) structure using photoluminescence (PL) spectra and near resonant pump-probe technique. Fast exciton tunnelling from quantum well to quantum dots is observed by transient differential transmission. The tunnelling time is 1.8, 4.4 and 39 ps for barrier thickness of 10, 15 and 20 nm, respectively.     

Tunneling and electric-field effects on electron-hole localization in artificial molecules

I theoretically investigate the Stark shift of the exciton goundstate in two vertically coupled quantum dots as a function of the interdot distance. The coupling is shown to enhance the tuneability of the linear optical properties, including energy and oscillator strength, as well as the exciton polarizability. The coupling regime that maximizes these properties results from the detailed balance between the effects of the single-particle tunneling, of the electric field and of the carrier-carrier interaction. I discuss the relevance of these results to the possible implementation of quantum-information processing based on semiconductor quantum dots: in particular, I suggest the identification of the qubits with the exciton levels in coupled- rather than single-dots.     

Mid-infrared luminescence from coupled quantum dots and wells

Coupled nanostructures have been developed in the InAs/InSb/GaSb materials system in order to extend the emission wavelength further into the infrared, beyond 2 μm. The samples studied consist of a single narrow InAs quantum well grown below a layer of InSb quantum dots in a GaSb matrix, in which the coupling has been altered by changing the thickness of a GaSb spacer layer. The overall transition energy of the combined dot–well system is generally reduced with respect to the dots and well only but the dependence on spacer thickness is more complex than that expected from a simple envelope function model.     

Charged magneto-exciton states in semiconductor quantum dots

By embedding a layer of self-assembled quantum dots into a field-effect structure, we are able to control the exciton charge in a single dot. We present the results of photoluminescence experiments as a function of both charge and magnetic field. The results demonstrate a hierarchy of energy scales determined by quantization, the direct Coulomb interaction, the electron–electron exchange interaction, and the electron–hole exchange interaction. For excitons up to the triply charged exciton, the behavior can be understood from a model assuming discrete levels within the quantum dot. For the triply charged exciton, this is no longer the case. In a magnetic field, we discover a coherent interaction with the continuum states, the Landau levels associated with the wetting layer.     

Effect of the Electron-LO-Phonon Coupling on an Exciton Quantum Dot

The influence of the electron-LO-phonon coupling on energy spectrum of the low-lying states ofan exciton inparabolic quantum dots is investigated as a function of dot size. Calculations are made by using the method of few-bodyphysics within the effective-mass approximation. A considerable decrease of the energy in the stronger confinement rangeis found for the low-lying states of an exciton in quantum dots, which results from the confinement of electron-phononcoupling.     

Electroluminescence of CdSe/CdS quantum dots and the transfer of the exciton excitation energy in an organic light-emitting diode

A light emitting diode has been developed on the basis of multilayer nanostructures in which CdSe/CdS semiconductor colloidal quantum dots serve as emitters. Their absorption, photo-, and electroluminescence spectra have been obtained. The strong influence of the size effect and the density of particles in the layer on the spectral and electrophysical characteristics of the diode has been demonstrated. It has been shown that the rates of the transfer of the exciton excitation energy from organic molecules to quantum dots increase strongly even at a small increase in the radius of the core (CdSe) of a particle and depend strongly on the thickness of the shell (CdS) of the particle. The optimal arrangement of the layer of quantum dots with respect to the p-n junction has been estimated from the experimental data. The results demonstrate that the spectral characteristics and rates of the electron processes in light-emitting devices based on quantum dots incorporated into an organic matrix can be efficiently controlled.     

Tuning the cross-gap transition energy of a quantum dot by uniaxial stress

We show that a piezoelectric actuator can be used to apply uniaxial stress to a layer of self-assembled quantum dots. The applied stress leads to a change of the quantum dot's ground state exciton energy by up to a few hundred μeV. This approach allows the possibility of an in situ and continuous tuning of the stress at temperatures down to 4 K and offers an alternative to tuning by temperature and Stark effect. We measure the relative change in the charging energy to the n-doped back contact by capacitance and the change in the exciton energy by photoluminescence. By tuning the uniaxial stress we are able to perform reflection spectroscopy on a single dot.     

Charge and energy transfer in double asymmetric quantum wells with quantum dots

The luminescence and luminescence excitation spectra of CdSe/ZnSe quantum dots are studied in a set of double quantum wells with the ZnSe barrier of width 14 nm, the same amount of a deposited CdSe layer forming a deep well and shallow wells with different depths. It is found that for a certain relation between the depths of shallow and deep wells in this set, conditions are realized under which the exciton channel in the luminescence excitation spectrum of a shallow well dominates in the region of kinetic exciton energies exceeding 10 longitudinal optical phonons above the bottom of the exciton band of the ZnSe barrier. A model is developed for the transfer of electrons, holes, and excitons between the electronic states of shallow and deep quantum wells separated by wide enough barriers. It is shown that the most probable process of electronic energy transfer between the states of shallow and deep quantum wells is indirect tunneling with the simultaneous excitation of a longitudinal optical phonon in the lattice. Because the probability of this process for single charge carriers considerably exceeds the exciton tunneling probability, a system of double quantum wells can be prepared in which, in the case of weak enough excitation, the states of quantum dots in shallow quantum wells will be mainly populated by excitons, which explains experimental results obtained.     

Investigation of Dynamical Parameters of Exciton Confined in CdS Spherical Quantum Dots with a B-Spline Technique

Exciton energies as a function of radii of quantum dots in the range of 5-35 A are calculated based on effective mass approximation model with the B-spline technique and compared with experimental and other theoretical data for the CdS dots. This method leads to accurate and fast convergent exciton energy, which are in good agreement with experimental data in the whole confinement regime. The effect of penetration of wave function from the inside to the outside of the dots and the effect of dielectric constants are taken into account. The magnitudes of dynamical parameters are discussed. It is found that the different materials surrounding the CdS quantum dot affect not only the potential energy and Coulomb interaction energy of the system, but also the effective masses. The comparison shows that the effective mass approximation model can describe very well the quantum size effects observed experimentally on the exciton ground state energy.     

Fine structure of the exciton states in InAs quantum dots

The fine structure of the exciton states in InAs quantum dots grown by the Stranski-Krastanov method with short growth interruptions has been studied by microphotoluminescence at cryogenic temperatures. It has been demonstrated that, with increasing quantum-dot size, the splitting of the exciton states increases steadily to ～10² μeV. It has been shown that, in the exciton energy range of 1.3–1.4 eV, the magnitude of this splitting is comparable to the natural width of the exciton lines. This result is important for the development of entangled photon pair emitters based on InAs quantum dots.     

The structure transition from vertical alignment to anti-alignment of InAs/InP quantum dot multilayers

We calculate the minimum Gibbs free energy of the InAs/InP quantum dot multilayer by combining the method of moving asymptotes and the finite element method. Based on the principle of the least energy, the transition between vertically aligned and anti-aligned quantum dot multilayers is studied. We investigate the influence of quantum dot base size and density on critical spacer thickness for the transition. The study results indicate that the critical thickness increases with the decrease in the density of quantum dots, while the base size of the quantum dot is linear to the critical thickness when the density is given.     

应变补偿层对量子点生长影响的理论研究

量子点的光学特性与量子点的大小均匀性、密度、内部应变以及隔离层的厚度等有密切关系.文中从理论角度定量研究了GaNXAs1-X应变补偿层对InAs/GaAs量子点生长质量的改善作用,分析了应变补偿层对隔离层厚度减小的作用.讨论了应变补偿层的补偿位置和补偿层N组分X对量子点生长时局部应变和体系应变的补偿作用.分析了应变补偿层对体系应变的减少作用,并计算了相邻层量子点的垂直对准概率.研究结果对实验中应变补偿的优化和高质量量子点阵列的生长实现提供了理论依据.     

All-optical switching between quantum dot nanoarrays

An all-optical switching device is proposed, based on a sandwich structure comprising two-dimensional square-lattice nanoarrays of donor and acceptor quantum dots. The system operates on Förster energy transfer between the dark states of the individual nanoparticles, normally precluded by selection rules. On application of an off-resonant laser beam, a nonlinear mechanism activates transfer between spatially correlated quantum dots across an optically passive spacer layer, signifying an active switching action with parallel processing capability. In this report, electrodynamic theory is employed to analyse the system and to evaluate its energy transfer fidelity. The results of model calculations are presented in graphical form.     

The negatively charged exciton in double-layer quantum dots

The hyperangular equation for charged semiconductor complexes in a double-layer harmonic quantum dot was solved numerically by using the correlated hyperspherical harmonics as basis functions. By using this method, we have calculated the energy spectra of the low-lying states of a charged exciton as a function of the radius of the quantum dot and the binding energy spectra of the ground state as a function of the radius of the quantum dot for a few values of the distance between the vertically coupled dots and the electron-to-hole mass ratio. Received 3 December 1999     

Magnetooptics and dynamics of a magnetic polaron in semimagnetic CdSe/ZnMnSe quantum dots

The magnetooptics and picosecond dynamics of the radiative recombination of excitons in self-assembled semimagnetic CdSe/ZnMnSe quantum dots is studied at low temperatures. The behavior of individual quantum dots in a magnetic field and with an increase in temperature is indicative of a strong exchange interaction of excitons and magnetic Mn ions giving rise to a quasi-zero-dimensional exciton magnetic polaron. When the exciton energy exceeds the intracenter transition energy in Mn, the energy is rapidly transferred from excitons to Mn ions (faster than 20 ps). In the opposite case, a substantial red shift (～15 meV) of the emission line maximum is observed during the exciton lifetime ～500 ps, presenting the dynamics of the formation of a magnetic polaron with a characteristic time of ～110 ps.     

Exciton relaxation in Ga1 − xInxAs/GaAs self-organized quantum dots

The exciton dynamics in Ga_1 − xIn_xAs/GaAs self-organized quantum dots grown on GaAs (111)B substrates are studied by the time-resolved photoluminescence (PL). We have found the intra-dot exciton relaxation by the reduction of the linewidth and peak energy and also by the energy-dependent PL rise time in the transient PL spectra. Compared with the energy relaxation in the reference quantum wells, we have confirmed that the exciton relaxation in three-dimensionally confined quantum dots is slower than in the quantum wells.     

Controlling the energy transfer between near-field optically coupled ZnO quantum dots

We performed time-resolved spectroscopy of ZnO quantum dots (QD), and observed exciton energy transfer and dissipation between QD via an optical near-field interaction. Two different sizes of ZnO QD with resonant energy levels were mixed to test the energy transfer and dissipation using time-resolved photoluminescence spectroscopy. The estimated energy transfer time was 144 ps. Furthermore, we demonstrated that the ratio of energy transfer between the resonant energy states could be controlled.