纤锌矿氮化镓基阶梯量子阱中界面声子的色散谱与散射率

本文理论分析了纤锌矿GaN-基阶梯量子阱中的电子-界面光学声子散射性质。阶梯量子阱中的解析的界面声子态及Frhlich电子-声子相互作用哈密顿被导出了。在考虑强内建电场效应及能带的非抛物性特性的情况下,阶梯量子阱结构精确解析的电子本征态也被给出了。以一个四层纤锌矿AlN-基阶梯量子阱为例进行了数值计算。结果发现,系统中存在四支界面光学声子模,这一观察明显不同于对称的GaN/AlN单量子阱与双量子阱的情况。这一差异被归结为阶梯量子结构的非对称性。GaN-基阶梯量子阱中的子带内散射率与子带间散射率比GaAs-基阶梯量子阱的结果大一个数量级,这被归因于GaN-基晶体大的电子-声子耦合常数。GaN-基阶梯量子阱的子带内散射率表现出与GaAs-基体系类似的结构参数依赖关系,但两类体系的子带间散射率对阶梯量子阱结构参数依赖则明显不同,这被归结为GaN-基阶梯量子阱结构中强的内建电场效应及带的非抛物性。结果还表明,高频界面声子模相对于低频界面声子模,对散射率的贡献更大。     

纤锌矿MgxZn1-xO/Mg0.3Zn0.7O抛物量子阱中极化子能级

采用改进的Lee-Low-Pines(LLP)中间耦合方法研究纤锌矿Mg x Zn1-x O/Mg0.3Zn0.7O抛物量子阱材料中的极化子能级,给出极化子基态能量、跃迁能量(第一激发态到基态)和不同支长波光学声子对电子态能级的贡献随量子阱宽度d的变化规律。理论计算中考虑了纤锌矿Mg x Zn1-x O/Mg0.3Zn0.7O抛物量子阱材料中声子模的各向异性和介电常数、声子(类LO和类TO)频率等随空间坐标Z变化(SD)效应对极化子能量的影响。结果表明,Mg x Zn1-x O/Mg0.3Zn0.7O抛物量子阱中电子与长波光学声子相互作用对极化子能级的移动很大,使得极化子能量明显降低。阱宽较小时,半空间长波光学声子对极化子能量的贡献较大,而定域长波光学声子的贡献较小;阱宽较大时,情况则正好相反。在d的变化范围内,电子与长波光学声子相互作用对极化子能级的移动(约67~79 meV)比Al x Ga1-x As/Al0.3Ga0.7As抛物量子阱中的相应值(约1.8~3.2 meV)大得多。因此,讨论ZnO基量子阱中电子态问题时要考虑电子与长波光学声子的相互作用。     

《发光学报》2008年分类目次

一、发光学的一般问题抛物量子点中弱耦合激子的光学声子平均数……………………………………………………………………………(1-1)声子色散对抛物量子点中极化子基态能量的影响……………………………………………………………………(1-5)纤锌矿GaN/AlxGa1-xN量子阱中极化子能     

纤锌矿GaN/AlxGa1-xN量子阱中极化子能量

采用LLP变分方法研究了纤锌矿GaN/Al_xGa_1-xN量子阱材料中极化子的能级,给出极化子基态能量、第一激发态能量和第一激发态到基态的跃迁能量与量子阱宽度和量子阱深度变化的函数关系。研究结果表明,极化子基态能量、第一激发态能量和跃迁能量随着阱宽L的增大而开始急剧减小,然后缓慢下降,最后接近于体材料GaN中的相应值。基态能量和第一激发态到基态的跃迁能量随着量子阱深度的增加而逐渐增加,窄阱时这一趋势更明显。纤锌矿氮化物量子阱中电子-声子相互作用对能量的贡献比较大,这一值(约40meV)远远大于闪锌矿(GaAs/Al_xGa_1-xAs)量子阱中相应的值(约3meV)。因此讨论GaN/Al_xGa_1-xN量子阱中电子态问题时应考虑电子-声子相互作用。     

纤锌矿氮化物量子阱中光学声子模的三元混晶效应

本文先比较了几种常用方法(修正的无规元素等位移模型、虚晶近似和简化相干势近似等)对纤锌矿三元混晶体声子频率的拟合结果,再选用与实验数据接近的拟合方法,结合介电连续和单轴晶体模型导出含纤锌矿三元混晶In_xGa_1－xN和Al_xGa_1－xN单量子阱各类光学声子模的色散关系,进一步分析了声子模随组分的变化. 结果表明,修正的无规元素等位移模型对单模性纤锌矿     

纤锌矿In_(0.19)Ga_(0.81)N/GaN量子阱中光学声子和内建电场对束缚极化子结合能的影响

用改进的Lee-Low-Pines变分方法研究纤锌矿In0.19Ga0.81N/GaN量子阱结构中束缚极化子能量和结合能等问题,给出基态结合能、不同支长波光学声子对能量和结合能的贡献随阱宽和杂质中心位置变化的数值结果.在数值计算中包括了该体系中声子频率的各向异性和内建电场对能量和结合能的影响、以及电子和杂质中心与长波光学声子的相互作用.研究结果表明,In0.19Ga0.81N/GaN量子阱材料中光学声子和内建电场对束缚极化子能量和结合能的贡献很大,它们都引起能量和结合能降低.结合能随着阱宽的增大而单调减小,窄阱中减小的速度快,而宽阱中减小的速度慢.不同支声子对能量和结合能的贡献随着阱宽的变化规律不同.没有内建电场时,窄阱中,定域声子贡献小于界面和半空间声子贡献,而宽阱中,定域声子贡献大于界面和半空间声子贡献.有内建电场时,定域声子贡献变小,而界面和半空间声子贡献变大,声子总贡献也有明显变化.在In0.19Ga0.81N/GaN量子阱中,光学声子对束缚极化子能量和结合能的贡献比GaAs/Al0.19Ga0.81As量子阱中的相应贡献(约3.2—1.8和1.6—0.3 meV)约大一个数量级.阱宽(d=8 nm)不变时,在In0.19Ga0.81N/GaN量子阱中结合能随着杂质中心位置Z0的变大而减小,并减小的速度变快.随着Z0的增大,界面和半空间光学声子对结合能的贡献缓慢减小,而定域光学声子的贡献缓慢增大.     

纤锌矿型量子级联激光器的界面与受限声子比较(英文)

利用传输矩阵方法对纤锌矿型量子级联激光器有源区的界面与受限声子进行了研究.计算结果显示:在纵光学频域有一组界面声子与二组受限声子存在,而且最低频率的两个界面声子在一定条件下能转变为受限模式.通过比较界面声子与受限声子的色散及声子势发现两者有很大不同,且计算电声散射速率可以发现由这两种声子引起的散射率是可比拟的.     

纤锌矿型量子级联激光器的界面与受限声子比较

利用传输矩阵方法对纤锌矿型量子级联激光器有源区的界面与受限声子进行了研究.计算结果显示:在纵光学频域有一组界面声子与二组受限声子存在,而且最低频率的两个界面声子在一定条件下能转变为受限模式.通过比较界面声子与受限声子的色散及声子势发现两者有很大不同,且计算电声散射速率可以发现由这两种声子引起的散射率是可比拟的.     

氮化物半导体耦合量子阱中非线性光整流特性:压电效应与自发极化效应

考虑了由于压电与自发极化引起的强内电场效应,基于密度矩阵与久期处理方法,理论考察了纤锌矿氮化物半导体耦合量子阱体系的非线性光整流特性。根据已经成功建立的耦合量子阱的内建电场模型,精确求解了体系的电子本征态。以典型的GaN/InxGa1-xN纤锌矿氮化物耦合量子阱为例进行了数值计算,结果发现共振光整流系数达到了10-6m/V的量级(体系的偶极矩阵元大小超过了2nm),这比同样尺寸的单氮化物量子阱的相应值高一个数量级。而且,计算还发现光整流系数对耦合量子阱的结构与掺杂组分呈现非单调的依赖关系,这一特性被归结为体系的强内建电场与量子尺寸效应对载流子受限特性的强烈竞争。计算结果还表明,通过选择小尺寸阱宽与垒宽的耦合量子阱,适当降低掺杂组分,可在氮化物耦合量子阱中获得较强的光整流效应。     

量子阱中极化子的声子平均数

采用有效质量近似下的变分法,考虑到电子同时与表面光学声子和体纵光学声子相互作用,研究了无限深量子阱中极化子的表面光学声子平均数,体纵光学声子平均数和光学声子平均数。讨论了电子与体纵光学声子耦合强度α,阱宽L和势垒材料Al_xGa_1-xAs中Al的含量x对上述光学声子平均数的影响。以GaAs/Al_xGa_1-xAs量子阱为例进行了数值计算。结果表明:量子阱中表面光学声子平均数随耦合强度α,阱宽L和Al含量x增大而增大。量子阱中体纵光学声子平均数随耦合强度α,阱宽L的增大而增大。光学声子平均数随耦合强度α,阱宽L和Al含量x的增大而增大。     

Guided-Wave Two-Dimensional Acousto-Optic Scanner Using Proton-Exchanged Lithium Niobate Waveguide

Two-dimensional scanning of a 0.6328 mum guided-light beam has been realized using noncolinear acousto-optic (AO) coplanar Bragg diffraction together with colinear AO guided-mode to substrate radiation-mode conversion in a Z-cut Xpropagation LiNbO3 proton-exchanged (PE) waveguide. The two surface acoustic (SAW) waves utilized are at the center frequencies of 500 and 200 MHz, propagating in the Y and X axes, respectively. Two-dimensional scanning of approximately 720 resolvable light beam spots, namely, 18 40 (horizontal vertical) scanning, has been demonstrated using a light beam of 1.0 mm aperture. The total number of resolvable beam spots can be greatly increased from 720 by simply utilizing SAW transducers of larger bandwidth and a light beam of greater aperture. It should also be possible to significantly increase the diffraction efficiency from 3 % by optimizing the parameters of the PE waveguide and the SAWs.     

Effect ofγ-radiation on the spectral luminescence characteristics of impure cadmium tungstate crystals

Results of a study of the effect of γ-radiation on the spectral luminescence properties of cadmium tungstate crystals doped with silver, bismuth, and molybdenum cations are presented. Spectral characteristics of the nondnnnoped crystals are briefly described. Absorption and photo and X-ray luminescence spectra of the crystals taken before and after exposure to γ-radiation (5.5·10⁴ Gy) are compared. It is found that the spectral characteristics of the crystals doped with silver, bismuth, and molybdenum cations do not change markedly after the exposure. The relation between the type of impurity-induced defects, individual characteristics of the impurity cations, and the character of the effect of γ-radiation on the spectral luminescence properties of impure crystals is analyzed (preliminarily). Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 1, pp. 55–60, January–February, 1997.     

Hybrid Wavelength and Code Division Multiple Access in Optical Networks

A review of schemes for multiple access in fiber optic networks shows that a hybrid of wavelength and code division multiple access (WCDMA) combines the best features of both. In particular, the hybrid scheme retains the large information carrying capacity of wavelength division multiple access (WDMA) and flexibility of code division multiple access (CDMA). In this paper WDMA, optical CDMA (OCDMA), and WCDMA networks are discussed. In OCDMA networks, concept of incoherent and coherent coding including inverse decoding and matched filter is introduced. The delay performance of both WDMA and WCDMA networks, under the simple suboptimum access protocols based on cyclic search, is computed. It has been shown quantitatively that tuning delay significantly affects the delay performance of both WDMA and WCDMA networks. Futhermore, delay performance of WCDMA networks is always better than the WDMA networks for the same tuning delay, load, and number of users.     

Femtosecond Transform-Limited Pulse Generation by Compensating for the Linear Chirp of SPM Spectra in Dispersion Shifted Fibers

A simple technique of pulse compression, based on the linear chirp compensation of self-phase modulation (SPM) spectra in dispersion shifted fibers, is demonstrated. The optimization procedure is carried out, for a short span of a single-mode fiber, using a parabolic law, which describes the behavior of the squared output pulse width versus the pump peak power in the case of Gaussian pulses. The experimental results give a minimum pulse duration of 233 fs, which is in good agreement with the model. Shorter and coherent pulses, down to 90 fs, have been obtained by inserting an interference filter at the optical output.     

Sorting of bending magnets for the SSRF booster

The Shanghai Synchrotron Radiation Facility(SSRF)booster ring,a full energy injector for the storage ring,is deigned to accelerate the electron beam energy from 150MeV to 3.5GeV that demands high extraction efficiency at the extraction energy with low beam loss rate when electrons are ramping.Closed orbit distortion(COD)caused by bending magnet field uniformity errors which affects the machine performance harmfully could be effectively reduced by bending magnet location sorting.Considering the affections of random errors in measurement,both ideal sorting and realistic sorting are studied based on measured bending magnet field uniformity errors and one reasonable combination of bending magnets which can reduce the horizontal COD by a factor of 5is given as the final installation sequence of the booster bending magnets in this paper.     

Measurement of nonlinear coefficient of optical fiber based on small chirped soliton transmission

We measure the waveform and phase curves of short optical pulses before and after transmission over different lengths of fibers by use of the pulse analyzer with the frequency-resolved optical gating (FROG),and numerically simulate pulse evolution under the experimental conditions.The nonlinear coefficient of the fiber is given by comparing the experimental results with the numerical ones.Difference between the experiment and numerical simulation is analyzed.     

Resonance scattering of canonical elastic shells in absorbing fluid medium

Resonance scattering of elastic spherical shell and cylindrical shell while the surrounding fluid medium has absorption is studied. The normal mode solution derived using exact elastic theory and the separation of variables is still applicable. However, the scattering form function has to be modified for the absorbing medium, otherwise the unreasonable result would be obtained. The backscattering form function in the absorbing medium is redefined, and the form function of elastic spherical and cylindrical shell with vacuum or solid matter filled is calculated in various absorption conditions. The results show that the absorption of surrounding fluid leads to notable attenuation of the coincidence resonances in the mid-frequency, but it has a little influence on the low-frequency resonance scattering induced by the filler inside the shell.     

Single-beam self-referenced phase-sensitive surface plasmon resonance sensor with high detection resolution

A versatile and low-cost single-beam self-referenced phase-sensitive surface plasmon resonance(SPR)sensing system with ultra-high resolution performance is presented.The system exhibits a root-mean-square phase fluctuation of ±0.0028.over a period of 45 min.i.e.a resolution of±5.2×10-9 refractive index units.The enhanced performance has been achieved through the incorporation of three design elements:a true single-beam configuration enabling complete self-referencing so that only the phase change associated with SPR gets detected,a differential measurement scheme to eliminate spurious signals not related to the sensor response,and the elimination of retardation drifts by incorporating temperature stabilization in the liquid crystal phase modulato .Our design should bring the detection sensitivity of non-labeling SPR biosensing closer to that achievable by conventional fluorescence-based techniques.     

Influences of SiO2 protective layers and annealing on the laser-induced damage threshold of Ta2O5 films

Ta2O5 films are prepared on BK7 substrates with conventional electron beam evaporation deposition.The effects of SiO2 protective layers and annealing on the laser-induced damage threshold (LIDT) of the films are investigated.The results show that SiO2 protective layers exert little influence on the electric field intensity(EFI)distribution,microstructure and microdefect density but increase the absorption slightly.Annealing iS effective on decreasing the microdefect density and the absorption of the films.Both SiO2 protective layers and annealing are beneficial to the damage resistance of the films and the latter is more effective to improve the LIDT.Moreover,the maximal LIDT of Ta2O5 films is achieved by the combination of SiO2 protective layers and annealing.