The Fluid Phenomena In the Crystallization Of the Protein Crystal

This paper reports that an optical diagnostic system consisting of Mach-Zehnder interferometer with a phase shift device and image processor has been used for study of the kinetics of protein crystal growing process. The crystallization process of protein crystal by vapour diffusion is investigated. The interference fringes are observed in real time. The present experiment demonstrates that the diffusion and the sedimentation influence the crystallization of protein crystal which grows in solution, and the concentration capillary convection associated with surface tension occurs at the vicinity of free surface of the protein mother liquor, and directly affects on the outcome of protein crystallization. So far the detailed analysis and the important role of the fluid phenomena in protein crystallization have been discussed a little in both space- and ground-based crystal growth experiments. It is also found that these fluid phenomena affect the outcome of protein crystallization, regular growth, and crystal quality. This may explain the fact that many results of space-based investigation do not show overall improvement.

1.58 mu m InGaAs quantum well laser on GaAs

We demonstrate the 1.58 mu m emission at room temperature from a metamorphic In0.6Ga0.4As quantum well laser grown on GaAs by molecular beam epitaxy. The large lattice mismatch was accommodated through growth of a linearly graded buffer layer to create a high quality virtual In0.32Ga0.68As substrate. Careful growth optimization ensured good optical and structural qualities. For a 1250x50 mu m(2) broad area laser, a minimum threshold current density of 490 A/cm(2) was achieved under pulsed operation. This result indicates that metamorphic InGaAs quantum wells can be an alternative approach for 1.55 mu m GaAs-based lasers. (C) 2007 American Institute of Physics.

基于双层ID空间的Peer-to-Peer文件系统设计与实现

提出并实现了一种建立在Peer-to-Peer 搜索策略上的自组织、自适应、高效和可靠的文件系统DISPFS(Double ID Space basedPeer-to-peer File System)。它在双层ID 空间中构造虚拟存储节点,不仅有效地取得了文件系统内的负载均衡、提高系统利用率,而且保证了动态环境中文件的可靠、快速获取。试验数据表明,DISPFS 在系统接近满负荷运行和文件插入/删除操作频繁的双重压力下依然保持优良的性能。

Dislocation Reduction in GaN on Sapphire by Epitaxial Lateral Overgrowth

High quality GaN is grown on GaN substrate with stripe pattern by metalorganic chemical vapor deposition by means of epitaxial lateral overgrowth. AFM,wet chemical etching, and TEM experiments show that with a two-step ELOG procedure, the propagation of defects under the mask is blocked, and the coherently grown GaN above the window also experiences a drastic reduction in defect density. In addition, a grain boundary is formed at the coalescence boundary of neighboring growth fronts. The extremely low density of threading dislocations within wing regions makes ELOG GaN a potential template for the fabrication of nitride-based lasers with improved performance.

VSR:多sink机会移动传感器网络的数据收集

机会移动传感器网络可应用在野生动物监控,或利用手持设备嵌入的传感器收集城市信息等场景,往往需要将数据从源节点传输到多个基站中的任一个.提出了一个基于虚拟空间的路由机制VSR(virtual space-based routing),采用"存储-携带-转发"的传输模式实现数据收集.每个传感器节点根据与多个sink节点的期望传输延迟映射成高维空间中的一个坐标点,消息传输对应于从源节点移动到空间原点的过程.细粒度的转发决策特性,使VSR自适应于网络的动态变化,具有很好的鲁棒性.此外,VSR机制具有很低的计算和存储开销,非常适合资源受限的传感器节点.两种不同随机特性场景下的模拟实验验证了VSR机制比ZebraNet的基于历史的转发机制和随机转发机制的性能更好.

Performance analysis of bit error rate for free space optical communication with tip-tilt compensation based on gamma-gamma distribution

In this paper, the gamma-gamma probability distribution is used to model turbulent channels. The bit error rate (BER) performance of free space optical (FSO) communication systems employing on-off keying (OOK) or subcarrier binary phase-shift keying (BPSK) modulation format is derived. A tip-tilt adaptive optics system is also incorporated with a FSO system using the above modulation formats. The tip-tilt compensation can alleviate effects of atmospheric turbulence and thereby improve the BER performance. The improvement is different for different turbulence strengths and modulation formats. In addition, the BER performance of communication systems employing subcarrier BPSK modulation is much better than that of compatible systems employing OOK modulation with or without tip-tilt compensation.

Fetntosecond pulse shaping with space-to-time conversion based on planar optics

We propose a novel structure of planar optical configuration for implementation of the space-to-time conversion for femtosecond pulse shaping. The previous apparatuses of femtosecond pulse shaping are 4f Fourier-transforming type system that is usually large, expensive, difficult to align. The planar integration of free-space optical systems on solid substrates is an optical module with the attractive advantages of compact, reliable and robust. This apparatus is analyzed in details and the design of the particular lens for femtosecond pulse shaping based on planar optics is presented. (c) 2006 Elsevier GmbH. All rights reserved.

Investigation on multiple-port microcylinder lasers based on coupled modes

Microcylinder resonators with multiple ports connected to waveguides are investigated by 2D finite-difference time-domain (FDTD) simulation for realizing microlasers with multiple outputs. For a 10 mu m radius microcylinder with a refractive index of 3.2 and three 2 mu m wide waveguides, confined mode at the wavelength of 1542.3 nm can have a mode Q factor of 6.7 x 10(4) and an output coupling efficiency of 0.76. AlGaInAs/InP microcylinder lasers with a radius of 10 mu m and a 2 mu m wide output waveguide are fabricated by planar processing techniques. Continuous-wave electrically injected operation is realized with a threshold current of 4 mA at room temperature, and the jumps of output power are observed accompanying a lasing mode transformation.

Low threshold lasing of GaN-based vertical cavity surface emitting lasers with an asymmetric coupled quantum well active region

We have fabricated and characterized GaN-based vertical cavity surface emitting lasers (VCSELs) with a unique active region structure, in which three sets of InGaN asymmetric coupled quantum wells are placed in a half-wavelength (0.5 lambda) length. Lasing action was achieved under optical pumping at room temperature with a threshold pumping energy density of about 6.5 mJ/cm(2). The laser emitted a blue light at 449.5 nm with a narrow linewidth below 0.1 nm and had a high spontaneous emission factor of about 3.0x10(-2). The results indicate that this active region structure is useful in reducing the process difficulties and improving the threshold characteristics of GaN-based VCSELs.

Optical microwave generation based on DFB lasers

A new device of two parallel distributed feedback ( DFB) laser integrated monolithically with Y-branch waveguide coupler was fabricated by means of quantum well intermixing. Optical microwave signal was generated in the Y-branch waveguide coupler through frequency beating of the two laser modes coming from two DFB lasers in parallel, which had a small difference in frequency. Continuous rapidly tunable optical microwave signals from 13 GHz to 42 GHz were realized by adjusting independently the driving currents injected into the two DFB lasers.

GaSb based midinfrared equilateral-triangle-resonator semiconductor lasers

Theoretical calculations of the mode characteristics of an equilateral-triangle resonator (ETR) with a 10 mu m cavity side length show that the fundamental mode, with longitudinal mode index of 25, has a wavelength of 2.185 mu m and a longitudinal mode separation of 100 nm. This mode has a quality factor (similar to 2x10(5)) that is much larger than the first (similar to 5x10(4)) and second (similar to 3x10(4)) order modes, indicating that single fundamental mode lasing should be accessible over a broad wavelength tuning range. An electrically injected ETR based on this design is fabricated from an InGaAsSb/AlGaAsSb/GaSb, graded-index separate-confinement heterostructure, laser diode wafer with a 2.1 mu m emission wavelength. This device achieved single mode, continuous wave operation at 77 K with a threshold current of 0.5 mA and a single mode wavelength tuning range of 3.25 nm, which is accomplished by varying the injection current from 0.5 to 6.0 mA. (C) 2008 American Vacuum Society.

Design of quantum cascade microcavity lasers based on Q factor versus etching depth

The choice of the etching depth for semiconductor microcavities is a compromise between a high Q factor and a difficult technique in a practical fabricating process. In this paper, the influences of the etching depth on mode Q factors for mid-infrared quantum cascade microcylinder and microsquare lasers around 4.8 and 7.8 mu m are simulated by three-dimensional (3D) finite-difference time-domain (FDTD) techniques. For the microcylinder and the microsquare resonators, the mode Q factors of the whispering-gallery modes (WGMs) increase exponentially and linearly with the increase in the etching depth, respectively Furthermore, the mode Q factors of some higher order transverse WGMs may be larger than that of the fundamental transverse WGM in 3D microsquares. Based on the field distribution of the vertical multilayer slab waveguide and the mode Q factors versus the etching depth, the necessary etching depth is chosen at the position where the field amplitude is 1% of the peak value of the slab waveguide. In addition, the influences of sidewall roughness on the mode Q factors are simulated for microsquare resonators by 2D FDTD simulation. (C) 2009 Optical Society of America