Diploe-allowed optical absorption of an exciton in a spherical parabolic quantum dot

A investigation of the linear and nonlinear optical properties of an exciton in a spherical parabolic quantum dot has been performed by using the matrix diagonalization method. The optical absorption coefficients between the ground state (L=0,π=+1) and the first excited state (L=1,π=-1) have been examined based on the computed energies and wave functions. The results are presented as a function of the incident photon energy for the different values of the incident optical intensity and the confinement strength. We found the optical absorption coefficient is strongly affected by the incident optical intensity and the confinement strength.     

CdSe/HgSe/CdSe量子点量子阱(QDQW)晶粒的光学性质和电子结构

以CdSe纳米晶体为核,用胶体化学的方法,通过化学替代反应,获得了不同阱层或不同垒层的CdSeHgSeCdSe量子点量子阱(QDQW)晶体.紫外可见光吸收谱研究表明,通过调节QDQW中间HgSe阱层的厚度从0.9nm至0,可以调节QDQW颗粒的带隙从1.8变化至2.1eV,实现QDQW纳米晶体的剪裁.光致荧光(PL)谱研究显示,QDQW形成后,CdSeHgSe纳米颗粒表面态得到钝化,显现出发光强度加强的带边荧光峰.利用有效质量近似模型,对QDQW晶粒内部电子的1s—1s态进行了估算,估算结果总体趋势与实验数据相符 关键词： 量子点量子阱晶体 能带剪裁 加强的带边荧光峰     

Size-dependent optical properties and carriers dynamics in CdSe/ZnS quantum dots

Size-dependence of optical properties and energy relaxation in CdSe/ZnS quantum dots （QDs） were investigated by two-colour femtosecond （fs） pump-probe （400/800 nm） and picosecond time-resolved photoluminescence （ps TRPL） experiments. Pump-probe measurement results show that there are two components for the excited carriers relaxation, the fast one with a time constant of several ps arises from the Auger-type recombination, which shows almost particle sizeindependence. The slow relaxation component with a time constant of several decades of ns can be clearly determined with ps TRPL spectroscopy in which the slow relaxation process shows strong particle size-dependence. The decay time constants increase from 21 to 34 ns with the decrease of particle size from 3.2 to 2.1 nm. The room-temperature decay lifetime is due to the thermal mixing of bright and dark excitons, and the size-dependence of slow relaxation process can be explained very well in terms of simple three-level model.     

The electronic and optical properties of a triexciton in CdSe/ZnS core/shell quantum dot nanocrystals

In the study, we aim to investigate the electronic and optical properties of single excitons, biexcions and triexcitons in a CdSe/ZnS core/shell quantum dot nanocrystal. The electronic structure has been determined by solving of the Poisson–Schrödinger equations self-consistently. In calculations, the exchange-correlation effects between identical particles have been taken into account in the frame of the local density approximation. We have demonstrated that the optical properties of triexciton systems are remarkably different from the single and biexciton systems. Absorption peaks or transition energies of the triexciton system are well separated from those of single- and bi-exciton systems. We have observed that the core-radius dependent transition energy variations of triexcitons are higher when compared with single- and bi-excitonic systems. The transition energy shifts of double and triple excitons with respect to the single exciton have been calculated as a function of the core radius and we have shown that the energy shifts are inversely proportional with the radius. We have also investigated the radius-dependent changes in binding energies and lifetimes of the structures and the comparative results have been discussed in a detail manner.     

Size-dependent tunneling and optical spectroscopy of CdSe quantum rods

Photoluminescence excitation spectroscopy and scanning-tunneling spectroscopy are used to study the electronic states in CdSe quantum rods that manifest a transition from a zero-dimensional to a one-dimensional quantum-confined structure. Both optical and tunneling spectra show that the level structure depends primarily on the diameter of the rod and not its length. With increasing diameter, the band gap and the excited state level spacings shift to the red. The level structure was assigned using a multiband effective-mass model, showing a similar dependence on rod dimensions.     

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.     

Effect of capping layer of GaAlAs on the electronic and optical properties of GaAs spherical layer quantum dot

In the framework of effective-mass envelope function theory, we present a theoretical investigation of the electronic and optical properties for a system of nano layer consisting of spherical nano layer of GaAs (core) with capping layer of GaAlAs (shell). An exact analytical solution of the corresponding Schrödinger equation is obtained; in addition to that a transcendent equation for determination of electron energetic spectrum is derived as well. Dependences of the electron energy, threshold frequency and absorption coefficient versus the external radius, and the thickness of the capping layer are presented. The results showed that the electronic and the optical properties strongly depended on the thickness of the capping layer. We expect that such systems may be significant for practical application.     

Core-shell structure and quantum effect of CdSe/HgSe/CdSe quantum dot quantum well

Core-shell structure CdSe/HgSe/CdSe quantum dot quantum well (QDQW) nanocrystals were firstly synthesized and well controlled by colloidal chemical methods. High-resolution transmission electron microscope (HRTEM) photographs show that the CdSe core has a wurtzite crystal structure and the shell of HgSe has a hexagonal structure. The enhancement and blue shift of photoluminescence (PL) peak was observed which was attributed to the quantum confinement effect of carriers in the HgSe well.     

Nonlinear optical properties of a D system in a spherical quantum dot

The nonlinear optical properties of a D⁻ system confined in a spherical quantum dot represented by a Gaussian confining potential are studied. The great advantage of our methodology is that the model potential possesses the finite height and range. Calculations are carried out by using the method of numerical diagonalization of Hamiltonian matrix within the effective-mass approximation. We calculate the linear, third-order nonlinear and total optical absorption coefficients under the density matrix formalism. Numerical results for GaAs − Ga_1 − xAl_xAs QDs are presented. Our results show that the optical absorption coefficients in a spherical QD are much larger than their values for GaAs quantum wells. It is found that optical absorptions are strongly affected not only the confinement barrier height, dot radius, the electron-impurity interaction but also the position of the impurity.     

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 electronic structures and optical properties in CdSe/CdS Dot-in-rod colloidal nanostructures: Atomistic tight-binding theory

Using the atomistic tight-binding theory, in this study, the electronic structures and optical properties of CdSe/CdS dot-in-rod nanocrystals are modified by engineering the core and rod diameters. The ground electron states favor an s-like orbital, while the ground hole states are formed by light hole-like characteristics. The quantum confinement effect is attributed to the reduction in excitonic band gaps as the core and rod diameters increase. Changing the core and rod sizes produces excitonic band gaps across the visible light spectra. The highest oscillation strength, excitonic-binding energy, and dark-bright excitonic splitting are witnessed at a 3.0 nm CdSe core diameter. The oscillation strength, excitonic-binding energy, and dark-bright excitonic splitting decrease as the CdS rod diameters increase. This study highlights the importance of theoretically understanding and controlling the active materials’ structural parameters in optoelectronic technology.     

The correlation energies and nonlinear optical absorptions of an exciton in a disc-like quantum dot

Using exact diagonalization techniques, the low-lying states of an exciton, and the linear and nonlinear optical absorptions in a disc-like quantum dot are theoretically studied. The numerical results for the typical GaAs material show the so-called quantum size effect. Also, our study is restricted on the transition between the S state (L = 0) and the P state (L = 1). The optical absorption coefficients are greatly enhanced because of the induced size confinement. Meantime, we find that the total optical absorption coefficient is about two times bigger than that obtained by without considering exciton effects. Additionally, the optical absorption saturation intensity can be controlled by the incident optical intensity I.     

Exciton related optical absorption in a spherical quantum dot

An exciton in a spherical quantum dot is studied analytically within the effective mass approximation. A parabolic confinement under an electric field is considered. The linear and nonlinear optical absorption coefficients are calculated within the density matrix formalism. No assumptions are made about the strength of the confinement. It is shown how the competing mechanisms of the Coulomb interaction, the confinement and the applied static electric field affect the optical absorption.     

Near-field optical mapping of exciton wave functions in a GaAs quantum dot

Near-field photoluminescence imaging spectroscopy of naturally occurring GaAs quantum dots (QDs) is presented. We successfully mapped out center-of -mass wave functions of an exciton confined in a GaAs QD in real space due to the enhancement of spatial resolution up to 30 nm. As a consequence, we discovered that the spatial profile of the exciton emission, which reflects the shape of a monolayer-high island, differs from that of biexciton emission, due to different distributions of the polarization field for the exciton and biexciton recombinations. This novel technique can be extensively applied to wave function engineering in the design and the fabrication of quantum devices.     

Excited-state absorptions of an exciton confined in a quantum dot

The optical absorptions of an exciton with the higher excited states in a disc-like quantum dot are investigated. Calculations are made by using the method of numerical diagonalization of Hamiltonian matrix within the effective-mass approximation. With typical semiconducting GaAs based materials, the linear, third-order nonlinear, total optical absorption coefficients and refractive index changes have been calculated for the s–p, p–d, and d–f transitions. The results show that as the angular momentum quantum number of transitions increases, the absorption peaks shift towards lower energies and the absorption intensities increase.     

The nonlinear optical rectification of a confined exciton in a quantum dot

An exciton in a disc-like quantum dot (QD) with the parabolic confinement, under applied electric field, is studied within the framework of the effective-mass approximation. The nonlinear optical rectification between the ground and the first-excited states has been examined through the computed energies and wave functions in details for the excitons. The results show that the optical rectification susceptibility obtained in a disc-like QD reach the magnitude of 10⁻² m/V, which is 3-4 orders of magnitude higher than in one-dimensional QDs. It is found that the second-order nonlinear optical properties of exciton states in a QD are strongly affected by the confinement strength and the electric field.     

Ultrafast spectroscopy of CdS/CdSe quantum dots

Results from the nonresonance spectroscopy of CdS/CdSe quantum dots (composites of CdSe–CdS nanoparticles (core–shell)) are presented. The nonlinear optical properties of CdS/CdSe QDs in PMMA are studied with fs pulses at 1053 nm using the transient lens technique. QDs generate rapidly oscillating signals with amplitude rise times of around 200 fs and decay times of around 500 fs, while pure PMMA polymer only generates an oscillating signal whose envelope coincides with its autocorrelation function.     

Combined effects of hydrostatic pressure and temperature on nonlinear properties of an exciton in a spherical quantum dot under the applied electric field

Within the framework of the effective-mass approximation, we have calculated the combined effects of hydrostatic pressure, temperature and applied electric field on an exciton confined in a typical GaAs/Ga_0.7Al_0.3As quantum dot. Several inputs of the confinement potential, hydrostatic pressure, temperature, and applied electric field have been considered. Our findings suggest that (1) the effect of the confinement strength is dominant over the electric field effect, (2) the oscillator strength is an increasing function of the hydrostatic pressure, (3) the absorption coefficients and energy difference depend strongly on the hydrostatic pressure but weakly on the temperature, (4) the absorption coefficients with considering excitonic effects are stronger than those without considering excitonic effects and the absorption peak will move to the right side induced by the electron-hole interaction, (5) the applied electric field may effect either the size or the position of absorption peaks of excitons.