Efficient parametric uncertainty analysis within the hybrid Finite Element/Statistical Energy Analysis method

This paper is concerned with the development of efficient algorithms for propagating parametric uncertainty within the context of the hybrid Finite Element/Statistical Energy Analysis (FE/SEA) approach to the analysis of complex vibro-acoustic systems. This approach models the system as a combination of SEA subsystems and FE components; it is assumed that the FE components have fully deterministic properties, while the SEA subsystems have a high degree of randomness. The method has been recently generalised by allowing the FE components to possess parametric uncertainty, leading to two ensembles of uncertainty: a non-parametric one (SEA subsystems) and a parametric one (FE components). The SEA subsystems ensemble is dealt with analytically, while the effect of the additional FE components ensemble can be dealt with by Monte Carlo Simulations. However, this approach can be computationally intensive when applied to complex engineering systems having many uncertain parameters. Two different strategies are proposed: (i) the combination of the hybrid FE/SEA method with the First Order Reliability Method which allows the probability of the non-parametric ensemble average of a response variable exceeding a barrier to be calculated and (ii) the combination of the hybrid FE/SEA method with Laplace's method which allows the evaluation of the probability of a response variable exceeding a limit value. The proposed approaches are illustrated using two built-up plate systems with uncertain properties and the results are validated against direct integration, Monte Carlo simulations of the FE and of the hybrid FE/SEA models. © 2013 Elsevier Ltd.

Second-order rate region of constant-composition codes for the multiple-access channel

This paper presents an achievable second-order rate region for the discrete memoryless multiple-access channel. The result is obtained using a random-coding ensemble in which each user's codebook contains codewords of a fixed composition. It is shown that this ensemble performs at least as well as i.i.d. random coding in terms of second-order asymptotics, and an example is given where a strict improvement is observed. © 2013 IEEE.

Distinct two dimensional lateral ordering of self-assembled quantum dots

We report a quantum dot (QD) ensemble structure in which the in-plane arrangements of the dots are in a hexagonal way while the dots are also vertically aligned. Such a distinct lateral ordering of QDs is achieved on a planar GaAs(l 0 0) rather than on a prepatterned substrate by strain-mediated multilayer vertical stacking of the QDs. The analysis indicates that the strain energy of the lateral island-island interaction is minimum for arrangement of the hexagonal ordering. The ordered dots demonstrate strong photoluminescence (PL) emission at room temperature (RT) and the full width at half maximum of PL peak at RT is only 50 meV. (C) 2007 Elsevier B.V. All rights reserved.

Dynamics of quantum dissipation systems interacting with fermion and boson grand canonical bath ensembles: Hierarchical equations of motion approach

A hierarchical equations of motion formalism for a quantum dissipation system in a grand canonical bath ensemble surrounding is constructed on the basis of the calculus-on-path-integral algorithm, together with the parametrization of arbitrary non-Markovian bath that satisfies fluctuation-dissipation theorem. The influence functionals for both the fermion or boson bath interaction are found to be of the same path integral expression as the canonical bath, assuming they all satisfy the Gaussian statistics. However, the equation of motion formalism is different due to the fluctuation-dissipation theories that are distinct and used explicitly. The implications of the present work to quantum transport through molecular wires and electron transfer in complex molecular systems are discussed. (c) 2007 American Institute of Physics.

Structure and photoluminescence of InGaAs quantum dots formed on an InAlAs wetting layer

We have developed a new self-assembled quantum dot system where InGaAs dots are formed on an InAlAs wetting layer and embedded in the GaAs matrix. The structure is realized by special sample designation and demonstrated by low-temperature photoluminescence measurements. In contrast to the traditional InAs/GaAs quantum dots dominated by the ensemble effect, the temperature dependence of the photoluminescence of such a quantum dot structure behaves as decoupled quantum dots. This can be attributed to the enhanced potential confinement for the dots provided by a higher-energy barrier in the wetting layer.

Thermal redistribution of photocarriers between bimodal quantum dots

We study the photoluminescence (PL) properties of InAs/GaAs self-assembled quantum dots (QDs) by varying excitation power and temperature. Excitation power-dependent PL shows that there exists bimodal size distribution in the QD ensemble. Thermal carrier redistribution between the two branches of dots is observed and investigated in terms of the temperature dependence of their relative PL intensity. Based on a model in which carrier transfer between dots is facilitated by the wetting layer, the experimental results are well explained. (C) 2001 American Institute of Physics.

Unusual temperature-dependent optical properties of self-organized InAs/GaAs quantum dots at high excitation power

The temperature-dependent photoluminescence (PL) properties of InAs/GaAs self-organized quantum dots (QDs) have been investigated at high excitation power. The fast redshift of the ground-state and the first excited-state PL energy with increasing temperature was observed. The temperature-dependent linewidth of the QD ground state with high carrier density is different from that with low carrier density. Furthermore, we observed an increasing PL intensity of the first excited state of QDs with respect to that of the ground state and demonstrate a local equilibrium distribution of carriers between the ground state and the first excited state for the QD ensemble at high temperature (T > 80 K). These results provide evidence for the slowdown of carrier relaxation from the first excited state to the ground state in InAs/GaAs quantum dots.

Self-assembled quantum dots, wires and quantum-dot lasers

Molecular beam epitaxy-grown self-assembled In(Ga)As/GaAs and InAs/InAlAs/InP quantum dots (QDs) and quantum wires (QWRs) have been studied. By adjusting growth conditions, surprising alignment. preferential elongation, and pronounced sequential coalescence of dots and wires under specific condition are realized. The lateral ordering of QDs and the vertical anti-correlation of QWRs are theoretically discussed. Room-temperature (RT) continuous-wave (CW) lasing at the wavelength of 960 nm with output power of 3.6 W from both uncoated facets is achieved fi-om vertical coupled InAs/GaAs QDs ensemble. The RT threshold current density is 218 A/cm(2). A RT CW output power of 0.6 W/facet ensures at least 3570 h lasing (only drops 0.83 dB). (C) 2001 Elsevier Science B.V, All rights reserved.

Temperature dependence of electron redistribution in modulation-doped InAs/GaAs quantum dots

In this work we report the photoluminescence (PL) and interband absorption study of Si-modulation-doped multilayer InAs/GaAs quantum dots grown by molecular beam epitaxy (MBE) on (100) oriented GaAs substrates. Low-temperature PL shows a distinctive double-peak feature. Power-dependent PL and transmission electron microscopy (TEM) confirm that they stem from the ground states emission of islands of bimodal size distribution. Temperature-dependent PL study indicates that the family of small dots is ensemble effect dominated while the family of large dots is likely to be dominated by the intrinsic property of single quantum dots (QDs). The temperature-dependent PL and interband absorption measurements are discussed in terms of thermalized redistribution of the carriers among groups of QDs of different sizes in the ensemble. (C) 2000 Elsevier Science B.V. All rights reserved.

Self-assembled InAs/GaAs quantum dots and quantum dot laser

Systematic study of molecular beam epitaxy-grown self-assembled In(Ga)As/GaAs, In-AlAs/AlGaAs/GaAs, and InAs/InAlAs/InP quantum dots (QDs) is demonstrated. By adjusting growth conditions, surprising alignment, preferential elongation, and pronounced sequential coalescence of dots under the specific condition are realized. Room-temperature (RT) continuous-wave (CW) lasing at the wavelength of 960 nm with output power of 1 W is achieved from vertical coupled InAs/GaAs QDs ensemble. The RT threshold current density is 218 A/cm(2). An RT CW output power of 0.53 W ensures at least 3 000 h lasing (only drops 0.83 db). This is one of the best results ever reported.

Excitation transfer in vertically self-organized pairs of unequal-sized InAs/GaAs quantum dots

The excitation transfer processes in vertically self organized pairs of unequal-sized quantum dots (QD's), which are created in InAs/GaAs bilayers with different InAs deposition amounts in the first and second layers, have been investigated experimentally by photoluminescence technique. The distance between the two dot layers is varied from 3 to 12 nm. The optical properties of the formed pairs of unequal-sized QD's with clearly discernible ground-state transition energy depend on the spacer thickness. When the spacer layer of GaAs is thin enough, only one photoluminescence peak related to the large QD ensemble has been observed as a result of strong electronic coupling in the InAs QD pairs. The results provide evidence for nonresonant energy transfer from the smaller QDs in the second layer to the larger QD's in the first layer in such an asymmetric QD pair.

Photoluminescence study of InAs/GaAs self-organized quantum dots with bimodal size distribution

We have investigated the temperature dependence of the photoluminescence (PL) spectrum of self-organized InAs/GaAs quantum dots. A distinctive double-peak feature of the PL spectra from quantum dots has been observed, and a bimodal distribution of dot sizes has also been confirmed by scanning tunneling microscopy image for uncapped sample. The power-dependent PL study demonstrates that the distinctive PL emission peaks are associated with the ground-state emission of islands in different size branches. The temperature-dependent PL study shows that the PL quenching temperature for different dot families is different. Due to lacking of the couple between quantum dots, an unusual temperature dependence of the linewidth and peak energy of the dot ensemble photoluminescence has not been observed. In addition, we have tuned the emission wavelength of InAs QDs to 1.3 mu m at room temperature.

Influence of interdot electronic coupling on photoluminescence spectra of self-assembled InAs/GaAs quantum dots

The influence of interdot electronic coupling on photoluminescence (PL) spectra of self-assembled InAs/GaAs quantum dots (QDs) has been systematically investigated combining with the measurement of transmission electron microscopy. The experimentally observed fast red-shift of PL energy and an anomalous reduction of the linewidth with increasing temperature indicate that the QD ensemble can be regarded as a coupled system. The study of multilayer vertically coupled QD structures shows that a red-shift of PL peak energy and a reduction of PL linewidth are expected as the number of QD layers is increased. On the other hand, two layer QDs with different sizes have been grown according to the mechanism of a vertically correlated arrangement. However, only one PL peak related to the large QD ensemble has been observed due to the strong coupling in InAs pairs. A new possible mechanism to reduce the PL linewidth of QD ensemble is also discussed.

优化MISTY型结构的伪随机性

该文对4轮MISTY和3轮双重MISTY两种结构进行了优化。在保持其安全性不变的情况下，把4轮MISTY结构中第1轮的伪随机置换，用一个XOR-泛置换代替，第2，第3轮采用相同的伪随机置换，3轮结构中第1轮的伪随机置换用XOR-泛置换代替，其它轮相同。伪随机置换的数量分别由原来的4个变为2个，3个变为1个，从而缩短了运行时间，节省了密钥量，大大降低了结构的实现成本。

6H-SiC高场输运特性的多粒子蒙特卡罗研究

采用非抛物性能带模型，对6H-SiC高场电子输运特性进行了多粒子蒙特卡罗(Ensemble Monte Carlo)研究．研究表明：温度为296 K时，电子横向漂移速度在电场为2．0×104 V／cm处偏离线性区，5．O×10~5V／cm处达到饱和．由EMC方法得到的电子横向饱和漂移速度为1．95×10~7cm／s，纵向为6．0×10~6cm／s，各向异性较为显著．当电场小于1．0×10~6 V／cm时，碰撞电离效应对高场电子漂移速度影响较小．另一方面，高场下电子平均能量的各向异性非常明显．电场大于2．O×10~5V／cm时，极化光学声子散射对电子横向能量驰豫时间影响较大．当电场一定时，c轴方向的电子碰撞电离率随着温度的上升而增大．对非稳态高场输运特性的分析表明：阶跃电场强度为1．0×10~6 V／cm时，电子横向瞬态速度峰值接近3．O×10~7cm／s，反应时间仅为百分之几皮秒量级．