脉冲激光沉积法制备的ZnO薄膜的低阈值电抽运紫外随机激射

分别采用直流反应溅射法和脉冲激光沉积法在硅衬底上沉积ZnO薄膜, 用X射线衍射、扫描电镜、光致发光谱等手段对两种方法沉积的ZnO薄膜的结晶状态、 表面形貌和光致发光等进行了表征. 进一步对比研究了以上述两种方法制备的ZnO薄膜作为发光层的金属-绝缘体-半导体结构器件的电抽运紫外随机激射. 结果表明, 与以溅射法制备的ZnO薄膜作为发光层的器件相比, 以脉冲激光沉积法制备的ZnO薄膜为发光层的器件具有更低的紫外光随机激射阈值电流和更高的输出光功率. 这是由于脉冲激光沉积法制备的ZnO薄膜中的缺陷更少, 从而显著地减少了紫外光在光散射过程中的光损耗. 关键词： 随机激射 ZnO薄膜 脉冲激光沉积 溅射     

离散式任意充磁角度Halbach永磁电机解析模型研究

Halbach电机因其自身优势在新型船舶推进、海洋洋流发电等方面受到广泛关注. 本文在假设铁磁材料线性和定子内表面光滑的条件下, 通过将任意充磁角度Halbach阵列等效为两组90° Halbach(或180° Halbach)阵列的矢量合成, 提出了一种分析离散式任意充磁角度Halbach永磁电机气隙磁场的解析方法; 通过对电机中磁标量势的傅里叶级数进行计算, 推导出了最简单的90° Halbach永磁电机在极坐标系下的气隙磁密表达式, 并在此基础上, 给出了任意充磁角度Halbach电机永磁体磁化强度在一个极下的表达式, 进而得出任意充磁角度Halbach电机气隙磁密的分布, 并分析了气隙磁密与电机极对数、永磁体厚度和充磁角度间的关系. 最后通过有限元和试验结果验证了本文方法的正确性. 关键词： 离散式Halbach电机 任意充磁角度 矢量等效 解析模型     

Thinking machines

We propose that a sufficiently advanced civilisation may employ Cepheid variable stars as beacons to transmit all-call information throughout the galaxy and beyond. They might employ a pulsed neutrino beam to trigger the expansion of a Cepheid at an earlier than normal time, generating a binary signature of normal period or artificially shortened period. One can construct many scenarios wherein it would be desirable for such a civilisation of star ticklers to transmit data to anyone else within viewing range. The beauty of employing Cepheids is that these stars can be seen from afar (we monitor them out through the Virgo cluster), and any developing technological society would seem to be likely to closely observe them as distance markers. Records exist of Cepheids for well over 100 years. We propose that these (and other regularly variable types of stars) be searched for signs of phase modulation (in the regime of short pulse duration) and patterns, which could be indicative of intentional signalling.     

Photonic bandgap via nonlinear modulation assisted by spontaneously generated coherence

A four-level double-ladder atomic system with two upper states coupled to the excited state by a standing-wave trigger field is explored to generate photonic bandgap (PBG) structure. With the assistance of spontaneously generated coherence (SGC) from the two decay pathways, we can obtain single or double fully developed PBG when the trigger field is far away from resonance or resonant. While in the absence of SGC, the atomic medium becomes strong absorptive to the probe field, and therefore the resulting PBGs are severely malformed or even cannot be opened up. Numerical results show that the PBG structure is originated from the third-order cross Kerr nonlinear modulation between the probe and trigger fields. This mechanism differs from the recent schemes based on linear modulation.     

On the scatter ranges for the elastic moduli of random aggregates of general anisotropic crystals

A randomly inhomogeneous material may have macroscopic properties (elasticity, conductivity) scattered over some uncertainty intervals, despite the idealistic uniqueness assumption of homogenization theory. Based on minimum energy principles and certain statistical isotropy-symmetry hypotheses, our partly third-order bounds on the effective properties of random polycrystals are expected to estimate those scatter ranges. Explicit expressions are given and calculated for the elastic moduli of the random aggregates of some known monoclinic and triclinic crystals, which yield results in agreement with those calculated for higher-symmetry crystals: the moduli are determinable within an accuracy of two or three significant digits in most cases. It is shown, however, that with some real-world exotic crystals the bounds may fall far apart, and further theoretical and experimental studies on them deserve attention.     

Jamming and crystallization in athermal polymer packings

Dense packings of chains of hard spheres possess characteristic features that do not have a counterpart in corresponding packings of monomeric spheres especially near the maximally random jammed (MRJ) state. From the modelling perspective the additional requirement that spheres keep their connectivity while maximizing the occupied volume fraction imposes severe constraints on generation algorithms of dense chain configurations. The extremely sluggish dynamics imposed by the uncrossability of chains precludes the use of deterministic or stochastic dynamics to generate all but dilute polymer packings. As a viable alternative, especially tailored chain-connectivity-altering Monte Carlo (MC) algorithms have been developed that bypass this kinetic hindrance and have actually been able to produce packings of hard-sphere chains in a volume fraction range spanning from infinite dilution up to the MRJ state. Such very dense athermal polymer packings share a number of structural features with packings of monomeric hard spheres, but also display unique characteristics due to the constraints imposed by connectivity. We give an overview of the most relevant results of our recent modeling work on packings of freely-jointed chains of tangent hard spheres about the MRJ state, local structure, chain dimensions and their scaling with density, topological constraints in the form of entanglements and knots, contact network at jamming, and entropically driven crystallization.     

A Novel Approach to All-Optical Generation of Ultra-Wideband Signals

Abstract

The generation of ultra-wideband signals in the optical domain is highly desirable for ultra-wideband-over-fiber systems, which has recently become a topic of interest. In this article, a novel and simple approach to achieve all-optical generation of ultra-wideband signals is proposed, which is based on delaying and superimposing optical Gaussian pulses with opposite polarities. The proposed system is capable of generating both ultra-wideband monocycle and doublet pulses, and the polarity of the generated ultra-wideband monocycle pulses can be fast-switched to implement pulse polarity modulation with the required bit pattern. A model to describe the proposed system is developed, and the generation of ultra-wideband signals is demonstrated with simulations and a proof-of-concept experiment.     

The Design of Passive Optical Networking+Ethernet over Coaxial Cable Access Networking and Video-on-Demand Services Carrying

Abstract

Video on demand is a very attractive service used for entertainment, education, and other purposes. The design of passive optical networking+Ethernet over coaxial cable accessing and a home gateway system is proposed. The network integrates the passive optical networking and Ethernet over coaxial cable to provide high dedicated bandwidth for the metropolitan video-on-demand services. Using digital video broadcasting, IP television protocol, unicasting, and broadcasting mechanisms maximizes the system throughput. The home gateway finishes radio frequency signal receiving and provides three kinds of interfaces for high-definition video, voice, and data, which achieves triple-play and wire/wireless access synchronously.     

Overview of DWDM/ODC Project

Abstract

This article introduces the main achievements resulting from the DWDM/ODC project. The five areas of research activity within the DWDM/ODC project cover some of the main issues of design and development of dense wavelength division multiplexing systems for transparent optical networks. These issues are: performance assessment with arbitrary optical filtering; performance of signaling formats; dispersion compensation strategies for directly and externally modulated systems in presence of nonlinear transmission-induced degradation; and the impact of noise and crosstalk in the extent of transparent optical networks. All five areas of research activity have contributed significantly to a better understanding of the limitations present in dense wavelength division multiplexing systems.     

Solvable models of temporally correlated random walk on a lattice

We seek the conditional probability functionP(m,t) for the position of a particle executing a random walk on a lattice, governed by the distributionW(n, t) specifying the probability ofn jumps or steps occurring in timet. Uncorrelated diffusion occurs whenW is a Poisson distribution. The solutions corresponding to two different families of distributionsW are found and discussed. The Poissonian is a limiting case in each of these families. This permits a quantitative investigation of the effects, on the diffusion process, of varying degrees of temporal correlation in the step sequences. In the first part, the step sequences are regarded as realizations of an ongoing renewal process with a probability densityψ(t) for the time interval between successive jumps.W is constructed in terms ofψ using the continuous-time random walk approach. The theory is then specialized to the case whenψ belongs to the class of special Erlangian density functions. In the second part,W is taken to belong to the family of negative binomial distributions, ranging from the geometric (most correlated) to the Poissonian (uncorrelated). Various aspects such as the continuum limit, the master equation forP, the asymptotic behaviour ofP, etc., are discussed.