锗硅量子阱中近带边光跃迁的理论和实验研究

本文研究了SiGe/Si量子阱中近带边光跃迁的产生机制,对由杂质无规分布引起的近带边光跃迁给出了一个物理模型。用此模型计算了光跃迁偶极矩,给出了跃迁偶极矩的上限。提出了未掺杂SiGe/Si量子阱中近带边光跃迁的一种跃迁机制,认为是Ge原子周围波函数畸变的集体行为。用MBE方法生长了掺杂SiGe/Si量子阱材料,在低温下观测到近带边光跃迁。     

Depending on the electric and magnetic field of the linear optical absorption and rectification coefficient in triple quantum well

In this study, the alteration of the potential profile, the energy levels, the dipole matrix element and the resonant peaks of the linear optical absorption (OA) and optical rectification (OR) coefficients in GaAs/GaAlAs triple quantum well (TQW) are calculated as dependent on the applied electric field and the magnetic field. The results show that the shape of confined potential profile, the energy levels and the dipole moment matrix elements are changed as dependent on the external fields. Also, the resonant peaks of the OA and OR coefficients depend on the applied external field effects. Therefore, I hope that these results will provide important improvement in semiconductor device applications, for suitable choice of electric and magnetic field values. It may particularly be useful in technological applications that the structure of TQW changes with the strength and direction of the external electric field.     

The quasibound state model for self-consistent characteristics of semiconductor intersubband devices

The quasibound state model (QBSM) for determining the self-consistent conduction band profile and space charge density of semiconductor intersubband devices is presented. This new method is based on the quasibound (QB) state resonances of quantum structures. For heterostructures, the traditional self-consistent energy continuum model (ECM) calculates space charge by integration over the entire energy continuum, weighted by Fermi–Dirac statistics. In the present approach, the continuum of energy states of the heterostructure is accurately represented by a small number of QB states, and the space charge calculations are performed only at these eigen-energies. This approach significantly reduces the computational burden associated with all self-consistent algorithms. Theoretical formulation of QBSM is compared with the traditional ECM approach. The bound (B) and QB eigenenergies of the structure are obtained by solving the single-band effective-mass Schrödinger equation using the argument principle method. The performance and the accuracy of the QBSM are evaluated for a double-barrier resonant structure and an asymmetric Fabry–Perot electron-wave interference filter. The self-consistent electron density and potential profiles calculated by the present method are shown to be in excellent agreement with the results obtained from the traditional ECM model. In addition to requiring less computational time, the present method is easily implemented and may be applied equally well to biased/unbiased, symmetric/asymmetric heterostructures.     

Semiconductor intersubband laser/detector performance optimization using a simulated annealing algorithm

A numerical method for global optimization of semiconductor intersubband laser/detector performance parameters is presented. The single-band effective-mass Schroedinger equation is solved by employing the argument principle method (APM) to extract both the bound (B) and quasibound (QB) eigen-energies of the quantum heterostructure. APM is combined with a simulated annealing (SA) algorithm to determine a set of device design parameters such as potential barrier height V_i, layer thickness d_i, applied biasV_Bias , for which the intersubband device performance is within a predetermined convergence criterion. The method presented incorporates the energy-dependent effective mass of electrons in nonparabolic conduction bands. The performance of the method is evaluated for the design of an asymmetric Fabry–Perot electron-wave interference filter (laser structure) and a dual-band quantum well infrared photodetector (QWIP). Results with and without nonparabolic effects are presented. In addition, results from the present method are compared to results obtained via the optimization technique based on super-symmetric quantum mechanics (SUSYQM) for the case of an optically-pumped quantum cascade (QC) laser. The present method is shown to improve the device performance beyond that obtained via SUSYQM optimization. Further, the present model can handle many optimization parameters and can incorporate fabrication constraints to achieve physically realizable devices.     

Calculation of vibrational matrix elements of the CO2 molecule in the formalism of polynomials of the quantum numbers

The vibrational energies and the matrix elements of the electric dipole moment function and the rotation function are calculated for an arbitrary linear polyatomic molecule in the first order of perturbation theory using the formalism of polynomials of quantum numbers. The formulas obtained are tested for the example of the CO₂ molecule. The experimental data available at present are briefly analyzed. It is shown that the calculated matrix elements are in good agreement with the corresponding experimental data even in the first order of the theory.     

用时间平移矩阵方法对真空中V型三能级原子非线性光学性质的理论研究

运用时间平移矩阵研究了真空中V型三能级原子在外场作用下的非线性光学性质,具体推导了三阶偶极跃迁矩阵元随时间演化规律.结果发现:真空与原子间的耦合是产生非线性量子相干的重要原因.     

Quasibound states in semiconductor quantum well structures

We present a study on quasibound states in multiple quantum well structures using a finite element model (FEM). The FEM is implemented for solving the effective mass Schrödinger equation in arbitrary layered semiconductor nanostructures with an arbitrary applied potential. The model also includes nonparabolicity effects by using an energy dependent effective mass, where the resulting nonlinear eigenvalue problem was solved using an iterative approach. We focus on quasibound/continuum states above the barrier potential and show that such states can be determined using cyclic boundary conditions. This new method enables the determination of both bound and quasibound states simultaneously, making it more efficient than other methods where different boundary conditions have to be used in extracting the relevant states. Furthermore, the new method lifted the problem of quasibound state divergence commonly seen with many other methods of calculation. Hence enabling accurate determination of dipole matrix elements involving both bound and quasibound states. Such calculations are vital in the design of intersubband optoelectronic devices and reveal the interesting properties of quasibound states above the potential barriers.     

锗硅合金半导体中无声子参与光跃迁机制研究

研究了Ｓｉ１－ｘＧｅｘ合金半导体中无声子参与光嗅迁的产机制，对由杂质无规分布引起的无声子参与光跃迁给出了一个物理模型。用此模型计算了光跃迁偶极矩，给出了跃迁偶极矩的上限。提出了未掺杂Ｓｉ１－ｘＧｅｘ合金半导体中无声子参与光跃迁的一种跃迁机制，认为是Ｇｅ原子周围波函数畸变的集体行为。     

The optical Airy transform and its application in generating and controlling the Airy beam

We propose an optical Airy transform in this paper, and obtain the analytical expressions for the Airy transform of fundamental Gaussian beams and finite energy Airy beams. The setup for performing the optical Airy transform is presented. The Airy transform for Gaussian beams and finite energy Airy beams are theoretically calculated and analyzed. Our results show that the Airy beam can be conveniently generated and controlled through the optical Airy transform of the Gaussian beam. The optical Airy transform also can be used to directly modulate the beam parameters of the incident Airy beam, and it can transform the incident Airy beam into the Gaussian beam.     

Radial matrix elements and dipole moment function for the ground state of CO

Radial matrix elements 〈vJ|x^k|v′J′〉 for k = 0–5, v = 0–12, |v′- v| = 0–4, and J up to 150 have been calculated for CO using accurate wavefunctions obtained from the numerical solution of the Schrödinger equation with a second-order RKR potential curve. These are used in conjunction with a model dipole moment function (a Padé approximant which has the correct united and separated atom limits and R^?4 long-range behavior) to analyze the experimental intensity data. For all the levels considered, we conclude that an adequate representation of the dipole moment function is provided by a five-term power series expansion. This simplifies the computation of dipole moment matrix elements, typical results of which are presented to illustrate the dependence on the rotational and vibrational quantum numbers.     

The calculation of reactions of the type (a,ab) using the plane-wave expansion method: (I). General formalism for the direct one-step process

A formalism for the evaluation of the transition matrix element for reactions of the type (a, ab) in the direct distorted-wave approximation is presented. The plane-wave expansion method is used to represent the optical model wave function for the entrance and exit channels. Because of the use of the plane-wave expansion method the completely off-the-energy-shell behavior is easily demonstrated when distorted waves are used. The use of this expansion method also shows how difficult it is to extract the momentum dependence of the bound particle. We also apply the plane-wave expansion method to evaluate the transition matrix element for the effective f-operator DWTA approach. We also discuss an approximation to the complete transition matrix element which seems equivalent to the DWTA but yet makes use of completely off-the-energy-shell matrix elements.     

Breakdown of the atomic picture for the interband dipole matrix element and charge oscillation under interband optical excitation

Numerical results for a one dimensional model are used to illustrate the nature of charge oscillation under interband optical excitation in a quantum well with a narrow band gap. The charge oscillation shows molecular rather than atomic character ie charge is transported over the entire quantum well and not just over atomic distances. The associated dipole moment, which corresponds to the interband dipole matrix element, for narrow gap semiconductor quantum well structures can be in the order of nanometres rather than Ångstroms.     

Electronic and Optical Properties of a Lens Shaped Quantum Dot under Magnetic Field：Second and Third-Harmonic Generation

In the present work, we have studied electronic and optical properties of a lens-shaped quantum dot under an external magnetic field. For this goal, we have calculated the energy levels and wave functions using the finite element method (FEM) for different values of magnetic field. We have also studied effect of magnetic field on second harmonic generation (SHG) and third-harmonic generation (THG) in the lens-shaped quantum dot. In this regard, we have obtained an analytic expression for the SHG and THG by a compact density matrix approach and an iterative procedure. According to the obtained results, it is found that the presence of the magnetic field affects the symmetry of the system. The SHG and THG are decreased with increasing the magnetic field. The magnetic field has a great influence on the energy levels, wave functions, the SHG and THG in a lens shaped quantum dot.     

An analytical study on absorptive lineshape of a driven N-type open four-level system: Quantum interference effects

An open four-level system of having two pairs of closely spaced levels (N-type configuration) is driven by a single electromagnetic field and tuned resonant with the average frequency of four dipole allowed transitions. Under the Doppler free condition and by using a semiclassical formulation of atom-field interaction for four dipole allowed transitions, we derive the optical Bloch equations for the said four-level system coupled to the driving field. In order to obtain the field induced polarization and hence the absorptive lineshapes, we use the usual perturbation method for getting the approximate analytical solution to the coupled optical Bloch equations for the density matrix elements. Through the off-diagonal complex density matrix elements, we introduce the field dependent phase angles arising out of the quantum interference between the levels participating in dipole allowed transitions. The difference between the field dependent and field independent phases are pointed out. In particular, we investigate the effects of Rabi frequencies and the field dependent phases on the absorptive lineshape. The analytical expressions for the effective linewidths, effective detunings and the induced polarization clearly indicate the role of quantum interference.     

Electronic and Optical Properties of a Lens Shaped Quantum Dot under Magnetic Field: Second and Third-Harmonic Generation

In the present work, we have studied electronic and optical properties of a lens-shaped quantum dot under an external magnetic field. For this goal, we have calculated the energy levels and wave functions using the finite element method(FEM) for different values of magnetic field. We have also studied effect of magnetic field on second harmonic generation(SHG) and third-harmonic generation(THG) in the lens-shaped quantum dot. In this regard, we have obtained an analytic expression for the SHG and THG by a compact density matrix approach and an iterative procedure. According to the obtained results, it is found that the presence of the magnetic field affects the symmetry of the system. The SHG and THG are decreased with increasing the magnetic field. The magnetic field has a great influence on the energy levels, wave functions, the SHG and THG in a lens shaped quantum dot.     

Airy传递矩阵法与偏压下多势垒结构的准束缚能级

使用Airy函数和传递矩阵方法精确计算了一维定态薛定谔方程，并推广到多势垒结构，求解出有/无偏压作用的2,3势垒结构的准束缚能级，进一步研究了有/无偏压作用的2,3势垒结构的准束缚能级与有效质量和外加电压的关系，并对结论的正确性进行了验证． 另外，文中 还指出了有些文章中关于Airy传递矩阵法与计算偏压下多势垒结构的准束缚能级的错误陈述 . 关键词： 准束缚能级 有/无偏压作用的多势垒结构 Airy函数 透射系数     

反射式GaN光电阴极表面势垒对量子效率衰减的影响

针对反射式GaN光电阴极长波段量子效率衰减较大, 短波段量子效率衰减较小的实验现象, 在考虑谷间散射的情况下, 利用玻尓兹曼分布和基于Airy函数的传递矩阵法, 计算了发射电子能量分布, 分析了表面势垒变化对量子效率衰减的影响, 理论与实验符合较好. 激活层有效偶极子数的减少使表面势垒宽度和高度增加, 引起长波光子激发产生的发射电子能量分布衰减较大, 短波光子激发产生的发射电子能量分布衰减较小, 这是量子效率在长波段衰减较大, 短波段衰减较小的根本原因.     

Fluorescence in a Quantum System with Violated Symmetry

The model of a single multilevel one-electron atom with violated symmetry such that its transition dipole-moment operator has constant diagonal matrix elements, among which not all are pairwise equal to each other, has been studied. It has been shown that the expression for the far electromagnetic field of such an atom does not contain any appreciable contributions from the diagonal matrix elements of the transition dipole moment in an explicit form; thus, these matrix elements have an effect on fluorescence via the time dependence of non-diagonal matrix elements due to quantum non-linear processes of higher orders. It has also been demonstrated that a two-level quantum system, whose transition dipole operator has constant unequal diagonal matrix elements, can continuously fluoresce under excitation with monochromatic laser radiation at a much lower frequency than the frequency of the exciting radiation. The possibility of the experimental detection and practical application of this effect are discussed.     

双波泵浦非对称量子阱的光学整流效应

为了实现基于光整流方式的室温下宽调谐高效率太赫兹源,设计了一种适于双波长CO2激光器共振子带跃迁泵浦的双阱嵌套形非对称量子阱结构,结构组分为Al0.5Ga0.5As/Ga As/Al0.2Ga0.8As,采用密度矩阵及迭代方法计算了其二阶非线性光整流系数χo(2)表达式,在导带为抛物线形和非抛物线形两种条件下对χo(2)进行对比研究。计算结果表明,其偶极跃迁矩阵元随量子阱总阱宽的增大而逐渐减小。当固定量子阱总阱宽及其中一束泵浦光波长不变时,χo(2)随着另一束泵浦光波长的增加,呈现出先增大后减小的变化趋势。当深阱为7 nm、总阱宽为23 nm、两束泵浦光相等为10.64μm时,χo(2)达到最大值5.925×10-6m/V;随着总阱宽的增大,χo(2)曲线呈现"红移"现象,其原因为量子限制效应导致了不同阱宽条件下的量子阱能级值差不同,从而造成满足泵浦光光子能量与能级差共振条件的变化。导带为抛物线形和非抛物线形两种条件下的χo(2)的最大值对应泵浦光波长基本相同,χo(2)数值上的差异主要由跃迁矩阵元的不同导致。     

硼玻璃中Dy3+及Sm3+的辐射跃迁几率和无辐射跃迁几率

硼玻璃中掺杂Nd3+,Er3+,Tm3+等小能隙的稀土离子,由于硼玻璃声子能量大,多声子无辐射几率大,影响其发光效率。但由于硼玻璃熔点低,加工容易,对于民用上常用的Sm3+,Dy3+,Eu3+,Tb3+等大能隙物质,多声子无辐射跃迁不是主要因素的情况下,发光效率将如何是本文要研究的问题。另外,辐射跃迁性质的研究对于能量传递机理的研究也是很有意义的。