Quasi-bound states determination using a perturbed wavenumbersmethod in a large quantum box

A perturbed wavenumbers method (PWM) is presented that is capable of determining the quasi-bound-state eigenenergies and their lifetimes for quantum heterostructures having arbitrary potential profiles. The numerical method presented solves the single-band effective-mass Schrodinger equation without using complex energies. It is applicable to quantum structures that are symmetric, asymmetric, unbiased, or biased. For multiple quantum heterostructures, extensive comparisons of this numerical method with other currently used techniques are included. In addition, a modified density of states formulation is presented and applied to these example cases     

Gain spectra of quantum wires with inhomogeneous broadening

The effects of fabricational variations on the gain spectra of quantum wires are calculated within the limits of first-order perturbation theory. Gain spectra and density of states for 50-Å-radius and 150-Å-radius cylindrical quantum wires are calculated and plotted for several different fabrication tolerances. The wave functions for a finite, cylindrical potential are calculated and a quasicritical radius, below which the carriers are weakly confined by the potential, is established. This sets a lower limit on quantum wire size. Upper limits on the size of quantum wells, quantum wires, and quantum boxes are also discussed. The threshold current and differential gain of quantum-wire lasers and quantum wire array lasers are calculated. These calculations indicate a possible reduction in threshold current of one to two orders of magnitude as compared to the best quantum-well lasers     

A computational Fourier series solution of the BenDaniel-DukeHamiltonian for arbitrary shaped quantum wells

A new technique for solving the BenDaniel-Duke Hamiltonian using a Fourier series method is discussed. This method Fourier transforms the effective mass and potential profiles to calculate the eigenenergies and probability densities in transform space. Numerical solutions of the eigenenergies of a rectangular quantum well are compared to the finite difference, finite element, and transfer matrix methods. The eigenenergies of the envelope functions are computed and compared to the exact case made under a constant effective mass approximation for an asymmetric triangular and parabolic shaped quantum well. The necessity of using a variable effective mass in the BenDaniel-Duke Hamiltonian is shown by a comparison of the eigenenergies in the constant and variable effective mass cases. The Fourier series method is then used to analyze the effects of compositional gradients and electric fields on the eigenenergies and envelope functions for asymmetric coupled asymmetric triangular quantum wells     

Optical effects based on intersub-band-transitions in quantum wells

Unipolar devices based on intersub-band-transition have been developed in the last decade [Appl. Phys. Lett. 65 (1994) 2901]. However, this technique will not be directed applicable for the longer wavelengths corresponding to Tetrahertz frequencies. In this work we analyze a THz device based on the intersub-band-transitions of an asymmetric double quantum well. We first study the intersub-band optical absorption in superlattice made of asymmetric double quantum wells tailored as a three level system. By applying an external electric field we obtain the Wannier-Stark ladder and can tune the transition energies between the sub-bands to reach the THz absorption between the two excited sub-bands. Although the dipole moments are big, the relative THz absorption to the other frequencies is small. However, reported lifetimes for this system encourage the possibility of getting population inversion, which is the main condition to design a laser. With this goal, it is presented a detailed study of the geometric design of the asymmetric double quantum well by performing an accurate calculation of the energies and wave functions, the dipole moments and the electron-LO-phonon interaction form factors, which are important ingredients of the scattering rates results. Furthermore, we analyze the role of electron-LO-phonon scattering in THz devices.     

Numerical simulation of interaction processes in the electron-positron matter by the methods of the classical and quantum theories

Numerical simulation of interaction processes in the electron-positron matter (EPM) is numerically simulated in the framework of the specific research area, gamma-electronics, which is concerned with the problem of creation and long existence of an EPM with extremely high energy, which is released in the process of delayed annihilation. Interaction processes in the EPM are studied by the methods of the classical large-particle model and a quantum model using macroscopic wave functions of electrons and positrons. In contrast to the point kinematic approach used in quantum electrodynamics, large particles are considered as deformed bunches of charge.     

Negative differential resistance in a resonant quantum wirestructure

It is shown that the transmission characteristics of a semiconductor split-gate device with a double constriction exhibit resonant behavior similar to that in resonant-tunneling diodes (RTDs). The double constriction is modeled with cascaded quantum waveguides of different widths and analyzed by applying a mode-matching technique to the wave function at the discontinuities. At low temperature, the calculated current-voltage characteristics exhibit a region of negative differential resistance. Current-voltage characteristics are presented for different constriction widths and temperatures exhibiting peak-to-valley ratios of up to 4:1 which persist up to temperatures around 10 K     

二维H型四终端量子点阵列的量子输运

利用一个基于紧束缚模型的散射区格点上能量波动的二终端系统的透射和反射系数的一般计算方法,然后提出了基于实空间格点的H型四终端量子点阵列的系统,将其转化成二终端量子点阵列模型去探讨,通过利用实空间格林函数,计算了H型四终端量子点阵列的透射率,研究其量子输运问题。     

A Perturbation Method for the Analysis of Wave Propagation in Inhomogeneous Dielectric Waveguides with Perturbed Media

This paper presents a perturbation method for determining the modes and the propagation constants of TE and TM waves in inhomogeneous dielectric waveguides whose index distributions depart from well-known profiles; e.g., a parabolic profile for which exact solutions can be obtained. Applying the variable-transformation technique to the wave equations, the wave-equation problem is transformed into the related-equation problem. The approximate solutions of the wave equations are obtained solving the related equation. The method is applied to the analysis of lower order mode propagation in a near-parabolic-index medium. The first-order field functions and the second-order propagation constants are given.     

Nano-optical probing of exciton wave-functions confined in a GaAs quantum dot

We have enhanced the performance of near-field scanning optical microscopy (NSOM) in terms of the spatial resolution and the sensitivity in signal detection. A careful preparation of an aperture-NSOM probe provides us with a spatial resolution as high as 30 nm in fluorescence imaging spectroscopy. We have applied this technique to map out the center-of-mass wave functions of an exciton confined in a GaAs quantum dot (a monolayer-high island formed in a quantum well). The spatial profile of the exciton emission, which reflects the shape of the island, differs from that of biexciton emission, due to different distributions of the polarization field for the exciton and biexciton recombinations. A theoretical calculation of the spatial distribution of the polarization field quantitatively reproduced the experimental result. Furthermore, mapping of an excited state wave-function with a node structure is also demonstrated. The novel technique can be extensively applied to wave-function engineering in the design and fabrication of quantum devices.     

外加电场对GaN/AlxGa1-xN双量子阱中性施主束缚能的影响

在有效质量包络函数理论下,利用变分法计算了未加电场以及加入电场后 双量子阱中施主杂质各种情况下的束缚能,讨论了双量子阱中间势垒高度、施主杂质位置对杂质束缚能的影响。给出了加入电场后施主位置不同时的束缚能和波函数,以及量子阱宽度不同时的束缚能,并且计算了未加电场和加入电场后中间势垒高度变化以及宽度不同时的束缚能。当双量子阱中间垒宽一定时,束缚能随着阱宽的变化会出现一个峰值。在阱宽一定时,随着中间垒宽度的增加,束缚能逐渐减小,并在垒宽增加到一定宽度时双量子阱情况与单量子阱情况相似,束缚能不再明显变化。计算结果对设计和研究 量子阱发光和探测器件有一定的参考价值。     

基于量子隐写术的计算安全比特承诺协议

在量子比特承诺协议中,目前流行的方案没有很好地解决信道噪声的影响,实用性不强。根据量子隐写术对信息的隐藏性,提出一种新的量子比特承诺协议。提出了利用量子信道噪声结合遮盖比特隐藏敏感信息,同时采用量子纠错码的方法克服信道噪声,有效地抵抗了第三方窃听攻击和噪声对信息的影响和破坏。通过理论分析与仿真证明该协议的绑定性和完善隐蔽性;理论证明了方案的有效性,为量子密码协议的推广应用提供了理论基础。     

用分数维空间方法研究量子阱中激子效应对三次谐波产生的影响

运用分数维空间方法理论研究了GaAs/AlGaAs无限深和有限深方形量子阱中激子效应对三次谐波产生的影响。利用分数维空间模型获得波函数和束缚能级为空间维度的函数,而空间维度数是阱宽的函数。无限深方阱的维度数随着阱宽的减小从三维极限过渡到二维;而在有限深阱中,当维度数达到一个极值后,维度数随阱宽的减小而增大。采用密度矩阵和迭代法导出三次谐波的表达式。数值结果表明,考虑激子效应的三次谐波系数比只考虑电子状态的系数增大40%左右,并且三次谐波系数大小依赖于激子的受限程度。结果还表明在弛豫率较小情况下可以获得较大的三次谐波的系数。     

Quantum confined Stark effect in semiconductor quantum wellsincluding valence band mixing and Coulomb effects

A complete model for evaluating the electro-optic response of a semiconductor quantum well structure is presented. Heavy and light hole mixing in the valence band is included by using a variational technique to determine the basis states. The excitonic effects, which are clearly evident at room temperature in quantum well structures, arise from Coulomb interaction between the charged particles; the corresponding many body problem is treated in the framework of the second quantization approach, by writing the total Hamiltonian of the interacting electron-hole plasma. The electro-optic responses, for both TE (transverse electric) and TM (transverse magnetic) polarized light, are computed by summing up the contributions of all the different transitions; they show a good agreement with experimental results. Some examples of computed electro-optic spectra for GaAs/AlGaAs compounds are presented     

Analysis of stripe multilayer waveguides with effective index andfinite element methods

An algorithm for a fast and accurate analysis of stripe multiple quantum well (MQW) waveguides is presented. The technique is based on the combination of effective index and finite element methods. The stripe MQW structure is replaced by an equivalent planar waveguide, using the effective index method to derive the (refractive index) profile of the planar guide from the original problem. The propagation constants and field intensity profiles of E_mn^X and E_mn^Y modes for the equivalent structure are then calculated with the one-dimensional finite element method. The results obtained with this technique are presented for both symmetric and asymmetric stripe MQW structures with an arbitrary number of wells. Propagation characteristics obtained for a six-well MQW waveguide are in good agreement with those evaluated (for comparison) using full two-dimensional finite element analysis     

Application and optimization of PML ABC for the 3-D wave equation in the time domain

A three-dimensional algorithm with the perfectly matched layer (PML) absorbing boundary condition (ABC) for the scalar wave equation in the time domain is presented for general inhomogeneous lossy or loss-free problems. The proposed PML ABC is applicable to practical finite difference schemes treating the time-domain wave equation, such as the time-domain wave-potential (TDWP) technique and the time-domain scalar wave equation approaches to the analysis of optical structures. The time-domain wave equation for lossy media is expressed in terms of stretched coordinate variables. The algorithm is tested for homogeneous and inhomogeneous media. We demonstrate applications to open (radiation) problems and to port terminations in high-frequency circuit problems. New PML conductivity profiles are developed for use with the second order wave equation, which offer lower reflections in a wider frequency band in comparison with the commonly used (in finite-difference time-domain (FDTD) algorithms) profiles. The effect of the termination walls on the overall PML performance is studied and the best choices are singled out.     

基于QKD量子密钥扩展的方法研究

目前量子密钥分发所面临的最主要问题是如何实现远距离、高码率量子密钥传输.为提高量子保密通信系统量子密钥生成的速率,提出一种反射和位移相结合的技术对已有的码率较低的量子密钥进行扩展处理,提高量子密钥库的容量.实验结果表明,采用反射技术和位移技术不但能有效地扩展量子密钥随机序列,同时可以保持良好的随机性,是一种扩展量子密钥库的有效方法.     

纳米量子点能级以及能隙计算和修正

用一个简单模型分析了量子点产生能级的原因,推导出能级与量子点尺寸的表达式。根据量子点表面势垒为有限值的特点以及表面存在衰减波,提出量子点内电子波函数不完全满足驻波条件的观点,在量子点的能级表达式中引入一个修正系数。结合量子点表面衰减波的特征,给出量子点修正系数的计算方法。通过理论计算表明：量子点的能级以及能级之间的能量差（间隙）不但与量子点的尺寸有关,而且与其边界条件有关。特别是对于非光滑的表面,其表面势垒对量子点的能级与能隙影响非常大。     

Exact synthesis of three-qubit quantum circuits from non-binary quantum gates

Because of recent nano-technological advances, nano-structured systems have become highly ordered, making it quantum computing schemas possible. We propose an approach to optimally synthesise quantum circuits from non-permutative quantum gates such as controlled-square-root-of-not (i.e., controlled-V). Our approach reduces the synthesis problem to multiple-valued optimisation and uses group theory. We devise a novel technique that transforms the quantum logic synthesis problem from a multi-valued constrained optimisation problem to a permutable representation. The transformation enables us to use group theory to exploit the symmetric properties of the synthesis problem. Assuming a cost of one for each two-qubit gate, we found all reversible circuits with quantum costs of 4, 5, 6, etc., and give another algorithm to realise these reversible circuits with quantum gates. The approach can be used for both binary permutative deterministic circuits and probabilistic circuits such as controlled random-number generators and hidden Markov models.     

Manifestation of size-related quantum oscillations of the radiative exciton recombination time in the photoluminescence of silicon nanostructures

The spectra of steady-and nonsteady-state photoluminescence in silicon nanostructures are calculated using the envelope-function method. The distinguishing feature of this calculation is that it takes into account an additional size-related quantum effect that manifests itself in the nonmonotonic (oscillatory) dependence of the time of radiative pseudodirect (phononless) exciton transitions on the nanocrystallite size. This is due to the oscillating overlap integral of the electron wave functions of the X valleys in the conduction band and the hole wave functions of the Γ valley in the valence band. The mesoscopic effect, which manifests itself in an appreciable broadening of the energy spectrum of small nanocrystals because of the fact that the fluctuations of both the structure of heteroboundaries and the atomic arrangement of nanocrystals become significant, is also taken into account. The potential application of this approach in interpreting experimental photoluminescence spectra in low-dimensional silicon is analyzed.