Linear and Nonlinear Optical Absorptions of a Donor Impurity in Spherical Quantum Dots

An investigation of the optical properties of a hydrogenic donor in sphericalparabolic quantum dots has been performed by using the matrix diagonalization method. The optical absorption coefficient between the ground (L=0) and the first excited state (L=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 confinement strength. These results show the effects of the quantum size and the impurity on the optical absorption coefficient of a donor impurity quantum dot.     

ZnS/CdSe量子点中线性和三阶非线性光吸收系数研究

在有效质量近似下,利用量子力学的密度矩阵理论,采用无限深势阱模型,从理论上研究了ZnS/CdSe柱型核壳结构量子点中线性和三阶非线性光吸收系数。导出了柱型量子点中线性和三阶非线性光学吸收系数的解析表达式,分析了该系统在不同条件下线性和三阶非线性光吸收系数与入射光频率之间的关系。改变系统的参数,该系统的光吸收系数呈规律性变化。计算结果表明:弛豫时间τ、入射光强I和壳半径R2对系统的吸收系数α有很大的影响,从而为实验上研究核壳结构量子点的非线性光学效应提供了必要的理论依据。     

Linear and Nonlinear Optical Properties in Spherical Quantum Dots

We calculate the energy eigenvalues and the sate functions of one-electron Quantum Dot （QD） by using a combination of Quantum Genetic Algorithm （QGA） and Hartre-Fock-Roothaan （HFR） method. The linear and the third-order nonlinear optical absorption coefficients for the 1s-1p, 1p-1d, and 1d-1f transitions are examined as a function of the incident photon energy for three different values of the stoichiometric ratio. The results show that the stoichiometric ratio, impurity, relaxation time, and dot size have great influence on the optical absorption coefficients of QDs.     

Linear and Nonlinear Optical Absorptions of a Hydrogenic Donor in a Quantum Dot Under a Magnetic Field

The linear and nonlinear optical properties of a hydrogenic donor in a disc-like parabolic quantum dot in the presence of an external magnetic field are studied. The calculations were performed within the effective mass approximation, using the matrix diagonalization method and the compact density-matrix approach. The linear and nonlinear optical absorption coefficients between the ground （L =0） and the first excited state （L = 1） have been examined based on the computed energies and wave functions. We find that the linear, nonlinear third-order, and total optical absorption coefficients are strongly affected by the confinement strength of QDs, the external magnetic field, and the incident optical intensity.     

Linear and Nonlinear Optical Properties of Spherical Quantum Dots:Effects of Hydrogenic Impurity and Conduction Band Non-Parabolicity

Simultaneous effects of an on-center hydrogenic impurity and band edge non-parabolicity on intersubband optical absorption coefficients and refractive index changes of a typical GaAs/Al x Ga 1 x As spherical quantum dot are theoretically investigated,using the Luttinger-Kohn effective mass equation.So,electronic structure and optical properties of the system are studied by means of the matrix diagonalization technique and compact density matrix approach,respectively.Finally,effects of an impurity,band edge non-parabolicity,incident light intensity and the dot size on the linear,the third-order nonlinear and the total optical absorption coefficients and refractive index changes are investigated.Our results indicate that,the magnitudes of these optical quantities increase and their peaks shift to higher energies as the influences of the impurity and the band edge non-parabolicity are considered.Moreover,incident light intensity and the dot size have considerable effects on the optical absorption coefficients and refractive index changes.     

Calculation of Linear and Nonlinear Intersubband Optical Absorptions in Electric-Field-Biased Semiparabolic Quantum Wells

By using the compact density matrix approach and iterative procedure, a detailed procedure for the calculation of the linear and nonlinear intersubband optical absorption coefficients is given in the electric-field-biased semi-parabolic quantum wells (QWs). The simple analytical formulas for the linear and nonlinear optical absorption coefficients in the systems are also deduced. Numerical result on typical GaAs materials shows that, the linear and nonlinear optical absorption coefficients sensitively depend on the applied electric field and the confined potential frequency of the semiparabolic QW systems as well as the incident optics beam intensity.     

Effects of Electron-Phonon Interaction on Linear and Nonlinear Optical Absorption in Cylindrical Quantum Wires

The electron-phonon interaction influences on lineax and nonfineax optical absorption in cylindrical quantum wires （CQW） with an infinite confining potential axe investigated. The optical absorption coefficients are obtained by using the compact-density-matrix approach and iterative method, and the numerical results are presented for GaAs CQW. The results show that the electron-phonon interaction makes a distinct influence on optical absorption in CQW. The electron-phonon interaction on the wave functions of electron dominates the values of absorption coefficients and the correction of the electron-phonon effect on the energies of the electron makes the absorption peaks blue shift and become wider. Moreover, the electron-phonon interaction influence on optical absorption with an infinite confining potential is different from that with a finite confining potential.     

球形ZnS/CdSe/ZnS核壳壳量子点的三阶极化率参量相关特征

在有效质量近似下,利用量子力学的密度矩阵理论,采用无限深势阱模型导出了三层球型量子点的三阶非线性光学极化率(自聚焦)的解析表达式.通过数值计算,分析了ZnS/CdSe/ZnS球型核壳结构量子点的三阶极化率(自聚焦)与量子点尺寸和入射光频率之间的关系.结果显示,量子点尺寸增大时,自聚焦效应三阶极化率(自聚焦)的峰值高度增大,峰值位置红移.本文的讨论为实验研究和实际应用提供了理论依据,对于光电器件的研究和改进有参考价值.     

Four-Electron Systems in a Coupled Double-Layer Quantum Dots

Making use of the method of few-body physics, the energy spectrum of a four-electron system consisting in a vertically coupled double-layer quantum dot as a function of the strength ofa magnetic field is investigated. Discontinuous ground-state transitions induced by an external magnetic field are shown. We find that, in the strong coupling case, the ground-state transitions depend not only on the external magnetic field B but also on the distance d between double-layer quantum dots. However, in the case of weak coupling, the ground-state transitions occur in the new sequence of the values of the magic angular momentum. Hence, the interlayer separation d and electron-electron interaction strongly affect the ground state of the coupled quantum dots.     

柱形量子点中的三阶极化率

利用量子力学的密度矩阵理论,在有效质量近似下，采用有限深势阱模型，导出了柱形量子点的三阶非线性极化率的解析表达式.通过数值计算，分析了GaAs/AlβGa1－βAs柱形量子点的三阶非线性极化率与量子点尺寸、掺杂浓度、入射光的频率和偏振方向等参量的关系.结果表明，三阶极化率峰值位置与这些参量密切相关，并且对于确定频率和偏振方向的入射光以及材料的掺杂浓度，存在与之相匹配的量子点尺寸，使三阶极化率达到极大，比体材料的相应值高出2个数量级以上.     

量子点量子阱中的三阶非线性光学特性的研究

利用紧致密度矩阵近似方法,研究了一个特殊量子点量子阱中的三阶非线性光学特性(三次谐波产生),得到了量子点量子阱系统的三次谐波产生系数的解析表达式,而且考虑了量子点量子阱系统中的两种电子束缚态-壳层阱内与阱外两种束缚态。对CdS/HgS构成的典型的量子点量子阱进行了数值计算,得到了10^-15(m／v)^2量级的三次谐波产生系数,并且绘出了三次谐波产生系数作为量子点量子阱的尺寸和泵浦光子能量的函数曲线,最后对曲线的特征及其形成的原因进行了解析。     

Negative Trions Trapped by a Spherical Parabolic Quantum Dot

In this paper, a negatively charged exciton trapped by a spherical parabolic quantum dot has been investigated. The energy spectra of low-lying states are calculated by means of matrix diagonalization. The important feature of the low-lying states of the negatively charged excitons in a spherical quantum dot is obtained via an analysis of the energy spectra.     

Shallow Donor Impurity Ground State in a GaAs/AlAs Spherical Quantum Dot within an Electric Field

Using the configuration-integration methods （CI） [Phys. Rev. B 45 （1992） 19], we report the results of the Hydrogenie-impurity ground state in a GaAs/AIAs spherical quantum dot under an electric field. We discuss the variations of the binding energies of the Hydrogenic-impurity ground state as a function of the position of impurity D, the radius R of the quantum dot, and also as a function of electric field F. We find that the ground energy and binding energy of impurity placed anywhere depend strongly on the position of impurity. Also, electric field can largely change the Hydrogenic-impurity ground state only limiting to the big radius of quantum dot. And the differences in energy level and binding energy are observed from the center donor and off-center donor.     

Energy Spectra of Excitons Bound to a Neutral Acceptor in Quantum Dots

The energy spectra of the ground state for an exciton(X) trapped by a neutral acceptor (A⁰) in a quantum dot with aparabolic confinementhave been calculated as a function of the electron-to-hole mass ratio σ by using the hyperspherical coordinates.We find that the (A⁰,X) complex confined in aquantum dot has in general a larger binding energythan those in a two-dimensional quantum well and a three-dimensional bulksemiconductor, and the binding energy decreases with the increase of theelectron-to-hole mass ratio.     

Shallow Donor Impurity Ground State in a GaAs/AlAs Spherical Quantum Dot within an Electric Field

Using the configuration-integration methods {(CI)} [Phys. Rev.B 45 (1992) 19], we report the results of the Hydrogenic-impurity ground state in a GaAs/AlAs spherical quantum dot under an electric field. We discuss the variations of the binding energies of the Hydrogenic-impurity groundstate as a function of the position of impurity D, the radius R of the quantum dot, and also as a function of electric field F. We find that the ground energy and binding energy of impurity placed anywhere depend strongly on the position of impurity. Also, electric field can largely change theHydrogenic-impurity ground state only limiting to the big radius of quantum dot. And the differences in energy level and binding energyare observed from the center donor and off-center donor.     

Strong Electron-Phonon Interaction Effect in Quantum Dots

Strong-coupling polaron in a parabolic quantum dot is investigated by the Landau-Pekar variational treatment. The polaron binding energy and the average number of virtual phonons around the electron as a function of the effective confinement length of the quantum dot are obtained in Gaussian function approximation. It is shown that both the polaron binding energy and the average number of virtual phonons around the electron decrease with increasing the effective confinement length. The results indicate that the polaronic effects are more pronounced in quantum dots than those in two-dimensional and three-dimensional cases.     

Charged-Exciton Complexes in Quantum Dots

It is known experimentally that stable charged-exciton complexes can exist in low-dimensional semiconductor nanostructures. Much less is known about the properties of such charged-exciton complexes since three-body problems are very difficult to be solved, even numerically. Here we introduce the correlated hyperspherical harmonics as basis functions to solve the hyperangular equation for negatively and positively charged excitons (trions) in a harmonic quantum dot. 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 quantum dot. Based on symmetry analysis, the level crossover as the dot radius increases can be fully explained as the results of symmetry constraint.``     

量子盘中的三阶非线性光学极化率

利用密度矩阵方法讨论了量子盘中的三阶非线性光学极化率,导出了近共振条件下的三次揩波的解析表达式,并以GaAs量子盘为例进行了具体的计算。结果表明,GaAs量子盘中的三阶非线性光学极化率要比GaAs量子阱的大几十倍。而且,选取适当的参量,可在吸收系数相对较小的情况下获得较大的折射率。     

Charged-Exciton Complexes in Quantum Dots

It is known experimentally that stable charged-exciton complexes can exist in low-dimensional semiconductor nanostructures. Much less is known about the properties of such charged-exciton complexes since three-body problems are very difficult to be solved, even numerically. Here we introduce the correlated hyperspherical harmonics as basis functions to solve the hyperangular equation for negatively and positively charged excitons (trions) in a harmonic quantum dot. 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 quantum dot. Based on symmetry analysis, the level crossover as the dot radius increases can be fully explained as the results of symmetry constraint.     

Energy Spectra of Three-Electron Quantum Dots

By making use of the method of few-body physics, the energy spectra of three electrons confined by two-dimensional and three-dimensional quantum dots (QDs) are investigated. Using the present results, the size and shape effects of QDs on the spectra are revealed.