Förster energy transfer from a semiconductor quantum well to an organic material overlayer

We predict an efficient electronic energy transfer from an excited semiconductor quantum well to optically active organic molecules of the nearby medium (substrate and/or overlayer). The energy transfer mechanism is of the F?rster type and, at semiconductor-organic distances of about 50 ?, can easily be as fast as 10-100 ps, which is about an order of magnitude shorter than the effective exciton lifetime in an isolated quantum well. In such conditions, the Wannier-Mott exciton luminescence is quenched and the organic luminescence is efficiently turned on. We consider both free as well as localized quantum well excitons discussing the dependence of the energy transfer rate on temperature and localization length. A similar mechanism for the non-radiative energy transfer to the organic overlayer molecules from unbound electron-hole pairs excited in the 2D continuum is shown to be much less competitive with respect to other relaxation channels inside the inorganic quantum well (in particular, 2D exciton formation). Received 20 July 1998     

Laser-induced operations with charge qubits in a double-well nanostructure

We present the results of theoretical studies on operations with charge qubits in the system composed of two tunnel-coupled semiconductor quantum dots whose two lowest states (localized in different dots) define the logical qubit states while two excited states (delocalized between the dots) serve for the electron transfer from one dot to another under the influence of the laser pulse. It is shown that in the case of small energy separation between the excited levels, the optimal (from the viewpoint of minimal single-qubit operation time and maximum operation fidelity) strategy is to tune the laser frequency between the excited levels. The pulse parameters for implementation of the quantum NOT operation are determined. Analytical results obtained in the rotating-wave approximation are confirmed by rigorous numerical calculations.     

Inter-Layer Energy Transfer through Wetting-Layer States in Bi-layer InGaAs/GaAs Quantum-Dot Structures with Thick Barriers

The inter-layer energy transfer in a bi-layer InGaAs/GaAs quantum dot structure with a thick GaAs barrier is studied using temperature-dependent photoluminescence. The abnormal enhancement of the photoluminescence of the QDs in the layer with a larger amount of coverage considering the resonant Forster energy transfer between the at 110K is observed, which can be explained by wetting layer states at elevated temperatures.     

Full configuration interaction studies of phonon and photon transition rates in semiconductor quantum dots

Quantum chemical methods originally developed for studying atomic and molecular systems can be applied successfully to the study of few-body electron-hole systems in semiconductor nanostructures. A new computational approach is presented for studying the energetics and dynamics of interacting electrons and holes in a semiconductor quantum dot. The electron-hole system is described by a two-band effective mass Hamiltonian. The Hamiltonian is diagonalized in a configuration state function basis constructed as antisymmetric products of the electron one-particle functions and antisymmetric products of the hole one-particle functions. The symmetry adapted basis set used for the expansion of the one-particle functions consists of anisotropic Gaussian basis functions. The transition probability between electron-hole states consisting of different numbers of carrier pairs is calculated at the full configuration interaction level. The results show that the electron-hole correlation affects the radiative recombination rates significantly. A method for calculating the phonon relaxation rates between excited states and the ground state of quantum dots is described. The phonon relaxation calculations show that the relaxation rate is strongly dependent on the energy level spacings between the states.     

Nonequilibrium transport through a vertical quantum dot in the absence of spin-flip energy relaxation

We investigate nonequilibrium transport in the absence of spin-flip energy relaxation in a few-electron quantum dot artificial atom. Novel nonequilibrium tunneling processes involving high-spin states, which cannot be excited from the ground state because of spin blockade, and other processes involving more than two charge states are observed. These processes cannot be explained by orthodox Coulomb blockade theory. The absence of effective spin relaxation induces considerable fluctuation of the spin, charge, and total energy of the quantum dot. Although these features are revealed clearly by pulse excitation measurements, they are also observed in conventional dc current characteristics of quantum dots.     

Ultrafast carrier dynamics of resonantly excited InAs/GaAs self-assembled quantum dots

We study theoretically the time development of electronic relaxation in quantum dots. We consider the process of relaxation of the state with an electron prepared at the beginning of relaxation in the electronic ground state. We obtain a fast (in picoseconds) increase of electronic population in the excited state. Also, we consider the process of relaxation of an electron from an excited state in the dot. Here we obtain an incomplete depopulation of the electron from the excited state. We compare these results to experiments in which a fast decrease of luminescence is reported during the first period of relaxation after resonant excitation of the ground state. We estimate numerically the role of electron–LO–phonon (Fröhlich's coupling) mechanism in these processes. We show that this effect may be attributed to the influence of multiple scattering of quantum dot electrons on LO phonons. A single-electron two-energy-level quantum dot model is used to demonstrate this effect in an isolated semiconductor quantum dot.     

Steady State and Time Resolved Spectroscopic Study of CdSe and CdSe/ZnS QDs:FRET Approach

This article highlights some physical studies on the relaxation dynamics and Förster resonance energy transfer (FRET) of semiconductor quantum dots (QDs) to proximal dye molecule and the way these phenomena change with core to core-shell QD is discussed. Efforts to understand the optical and carrier relaxation dynamics of CdSe and CdSe/ZnS QDs are made by using absorption, steady-state fluorescence and time-resolved fluorescence (TCSPC) techniques. Steady-state as well as time-resolved fluorescence measurements were employed to evaluate the QD PL quenching induced by the proximal Rhodamine 101 dye molecule and to examine the influence of deep trap states on energy transfer efficiency. The FRET parameters such as spectral overlap, Förster distance, intermolecular distance for each donor-acceptor pair are determined and variation of these parameters from core to core-shell QD is discussed.     

Fast polariton relaxation in strongly coupled organic microcavities

We consider the regime of strong light-matter coupling in an organic microcavity, where large Rabi splitting can be achieved. As has been shown, the excitation spectrum of such a structure, besides coherent polaritonic states, contains a number of strongly spatially localized incoherent excited states. These states form the majority of the excited states of the microcavity and are supposed to play the decisive role in the relaxation dynamics of the excitations in the microcavity. We consider the non-radiative transition from an incoherent excited state into one of the coherent states of the lower polaritonic branch accompanied by emission of a high-energy intramolecular phonon. It is shown that this process may determine the lifetime of incoherent excited states in the microcavity. This observation may be important in the discussion of pump–probe experiments with short pulses. This process may also play an important role for the population of the lowest energy states in organic microcavities, and hence in the problem of condensation of cavity polaritons.     

Converting Wannier into Frenkel excitons in an inorganic/organic hybrid semiconductor nanostructure

Electronic coupling between Wannier and Frenkel excitons in an inorganic/organic semiconductor hybrid structure is experimentally observed. Time-resolved photoluminescence and excitation spectroscopy directly demonstrate that electronic excitation energy can be transferred with an efficiency of up to 50% from an inorganic ZnO quantum well to an organic [2,2-p-phenylenebis-(5-phenyloxazol), alpha-sexithiophene] overlayer. The coupling is mediated via dipole-dipole-interaction analog to the F?rster transfer in donor-acceptor systems.     

Förster Energy Transfer in Arrays of Epitaxial CdSe/ZnSe Quantum Dots Involving Bright and Dark Excitons

Using time-resolved photoluminescence (PL) spectroscopy, we establish the presence of the Förster energy transfer mechanism between two arrays of epitaxial CdSe/ZnSe quantum dots (QDs) of different sizes. The mechanism operates through dipole–dipole interaction between ground excitonic states of the smaller QDs and excited states of the larger QDs. The dependence of energy transfer efficiency on the width of barrier separating the QD insets is shown to be in line with the Förster mechanism. The temperature dependence of the PL decay times and PL intensity suggests the involvement of dark excitons in the energy transfer process.     

Dynamics of Förster Energy Transfer in Pyridium Salt-Doped Thin Films

Properties of photoluminescence and Förster energy transfer dynamics based on an organic pyridium salt trans-4-[p-(N-Hydroxyethyl-N-methylamino)Styryl]-N-methylpyridinium iodide (ASPI) and organic small molecule Alq3 in PMMA polymeric thin films are investigated by steady-state and time-resolved fluorescent spectra as well as theoretical calculation. The observation of reduced emission intensity and the fluorescent lifetime of Alq3 is demonstrated, while the ASPI emission gradually increases and is finally dominant in the PL spectra with increasing ASPI doping concentration. Such results show that there exists an efficient Förster energy transfer (FET) from Alq3 to ASPI due to the large spectral overlap between ASPI absorption and Alq3 emission. The difference between the theoretical FET efficiency and the experimental data is caused by the lower mobility of the Alq3 exciton in the MMA matrix.     

Selective spin coupling through a single exciton

We present a novel scheme for performing a conditional phase gate between two spin qubits in adjacent semiconductor quantum dots through delocalized single exciton states, formed through the interdot F?rster interaction. We consider two resonant quantum dots, each containing a single excess conduction band electron whose spin embodies the qubit. We demonstrate that both the two-qubit gate and arbitrary single-qubit rotations may be realized to a high fidelity with current semiconductor and laser technology.     

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     

Squeezed magnetobipolarons in two-dimensional quantum dot

In this Letter, a different method was given for calculating the energies of the magnetobipolarons confined in a parabolic QD (quantum dot). We introduced single-mode squeezed states transformation, which are based on the Lee-Low-Pines and Huybrechts (LLP-H) canonical transformations. This method can provide results not only for the ground state energy but also for the excited states energies. Moreover, it can be applied to the entire range of the electron-phonon coupling strength. Comparing with the results of the LLP-H transformations, we have obtained more accurate results for the ground state energy, excited states energies and binding energy of the bipolarons. It shows that the magnetic field and the quantum dot can facilitate the formation of the bipolarons when η is smaller than some value.     

双模自组织量子点光致发光的温度依赖性

采用稳态速率方程模型,对双模自组织量子点光致发光的温度依赖性进行了研究,模拟获得了不同温度下双模自组织量子点的光致发光光谱,并进一步研究了两组量子点分布的光致发光强度比的温度依赖性。研究表明:在低温下(<75K),两组量子点分布的发光强度比基本保持不变;随着温度的升高(75K相似文献   

非对称量子点中强耦合磁极化子激发态的性质

采用线性组合算符和幺正变换方法,研究非对称量子点中强耦合磁极化子的激发态性质。导出强耦合磁极化子的第一内部激发态能量、激发能量和从第一内部激发态到基态的跃迁谱线频率随量子点的横向和纵向有效受限长度、磁场的回旋频率和电子-声子耦合强度的变化关系。数值计算结果表明：第一内部激发态能量、激发能量和跃迁谱线频率随磁场的回旋频率和电子-声子耦合强度的增加而增大.随量子点的横向和纵向有效受限长度的减小而迅速增大,表现出奇特的量子尺寸效应。