VPN介绍及其安全性问题探讨

本文简要介绍了一下VPN的发展历史及其主要技术,阐述了现在比较常见的三种VPN技术,从大体上介绍了它们的主要技术及其优缺点.重点介绍了引起人们关注的VPN安全问题.     

An adaptive approach for handling two‐dimension influence maximization in social networks

To synthetically and dynamically make strategic choices in social networks, a novel adaptive approach to deal with two‐dimension influence maximization problem (TIMP) is proposed with game‐based diffusion model, which can achieve trade‐off between diffusion time and the number of active nodes. At first, TIMP model is synthetically formulated, and diffusion time and the number of active nodes are defined mathematically. In particular, budget efficiency is presented to describe TIMP in order that an appropriate trade‐off between diffusion time and the number of active nodes can be reached. Then, an adaptive heuristic (A‐Heuristic) scheme is proposed to dynamically determine the initial seed nodes (ie, the most influential nodes). Finally, experiments are performed, and their results verify the superior performance of the proposed scheme in terms of budget efficiency, running time, and the number of active nodes.     

Mie‐Resonant Membrane Huygens' Metasurfaces

All‐dielectric metasurfaces have become a new paradigm for flat optics as they allow flexible engineering of the electromagnetic space of propagating waves. Such metasurfaces are usually composed of individual subwavelength elements embedded into a host medium or placed on a substrate, which often diminishes the quality of the resonances. The substrate imposes limitations on the metasurface functionalities, especially for infrared and terahertz frequencies. Here a novel concept of membrane Huygens' metasurfaces is introduced. The metasurfaces feature an inverted design, and they consist of arrays of holes made in a thin membrane of high‐index dielectric material, with the response governed by the electric and magnetic Mie resonances excited within dielectric domains of the membrane. Highly efficient transmission combined with the 2π phase coverage in the freestanding membranes is demonstrated. Several functional metadevices for wavefront control are designed, including beam deflector, a lens, and an axicon. Such membrane metasurfaces provide novel opportunities for efficient large‐area metadevices, whose advanced functionality is defined by structuring rather than by chemical composition.     

Si上Si1-xGex:C 缓冲层的载流子分布

用化学气相淀积方法在Si(100)衬底上生长了Ge组分渐变的Si1-xGex:C合金缓冲层.研究表明,较高温度下生长的Si1-xGex:C缓冲层中Ge的平均含量较高,其晶体质量要优于较低温度下生长的外延薄膜.载流子浓度沿衬底至表面方向逐渐上升且Si1-xGex:C缓冲层总体呈p型导电,存在一局域n型导电区,本文对其导电分布特性进行了分析研究.     

NCl(α)/I全气相碘激光与氧碘化学激光能量提取的分析比较

利用连续流光腔动力学模型,对氧碘化学激光(COIL)和基于NCl(α)-I传能的全气相碘激光(AGIL)的能量提取进行了分析和公式推导,得到了连续流光腔的光子通量解析表达式以及氧碘化学激光光子通量沿流动方向的分布和输出功率的简化计算公式.AGIL的体系较为复杂,不能得到光子通量沿流动方向分布和输出功率的简化计算公式.从能量提取和体积重量效率角度,对氧碘化学激光和基于NCl(α)-I传能的全气相碘激光进行了对比评价.     

Simultaneous Activation of Short‐Wave Infrared (SWIR) Light and Paramagnetism by a Functionalized Shell for High Penetration and Spatial Resolution Theranostics

Nanoparticle emitting short‐wave infrared (SWIR) light has received increased attention in the molecular imaging field due to its deeper tissue penetration, fast imaging, high sensitivity, and resolution. The simultaneously activated SWIR excited directly by an 808 nm laser and T₁‐weighted magnetic resonance imaging (MRI) signal are found in one single‐shell nanoparticle NaErF₄@NaGdF₄ (Er@Gd), which is used as a dual‐modality imaging contrast agent in vivo to accurately determine the position of tumors. The conjugated cypate is then aggregated on the surface of Er@Gd@SiO₂‐Cy/bovine serum albumin. With the guidance of dual modality imaging, photothermal therapy is effectively used to ablate tumors in a mouse model. The design of single‐shell nanomaterial activation of SWIR imaging and MRI signals is expected to provide a new strategy for high penetration and spatial resolution cancer theranostics.     

家庭影院功放有关知识简介

1分类由于国际国内目前还无家庭影院的统一确切定义,所以暂且认为家庭影院功放就是通过对声音信号的处理,使人在家中即可以欣赏到接近环绕立体声影院效果的视听系统中的核心部件。它能将录制的声音信号解码成几个独立的声道,经过功放,形成一个环绕立体声,从而具有近似影院的视听效果。据此,家庭影院功放大致分类如下：（1）按结构可以分为合并式和组合式。合并式：解码器和功放合为一体。组合式：解码器和功放分为两部分。（2）按解码方式可以分为杜比定向逻辑、家用THX、杜比AC－3等。（3）按主要放大器件分：电子管（胆机）、集成…     

基于有理函数式镜头畸变模型的摄像机标定

摄像机成像模型是决定视觉测量精度的关键因素之一。随着视觉测量精度的要求越来越高,提高摄像机模型和标定的精度是解决该问题的主要手段。将一种新的有理函数式镜头畸变修正模型引入到摄像机的成像建模中,以更好地修正镜头畸变引起的误差,从而提高摄像机成像模型的精度;并提出一种两步求解以及分步迭代优化的方法精确求解成像模型中各参数,以解决有理函数式畸变模型参数无法直接求解的问题。实验结果表明,基于新畸变修正模型的摄像机成像模型比常规畸变修正模型的标定精度有显著的提高,并且新模型对大误差的控制更有效,从而可整体提高视觉测量的精度。     

高性能数字频率合成器的设计与实现

通过对直接数字频率合成技术的研究,采用单片机AT89S51控制DDS芯片AD9854设计出一种高性能直接数字频率合成器。该数字频率合成器采用并行通信的方式传输控制字,通过改变控制字来改变输出频率,得到所需频率的正弦波。软件上采用菜单式、全部键盘控制方式。用4×4矩阵键盘控制,进行功能选择以及设置频率、幅度和相位控制字。界面显示用带中文字库的液晶TS-12864显示,实现了良好的人机交互,系统操作使用方便。用单片机控制DDS数字芯片实现的数字频率合成器,有着比模拟频率合成器更好的抗干扰性、频率分辨率和频谱纯度,同时有着更小的体积。系统经测试得到所需频率的正弦波,数字频率合成器设计成功。     

聚光型光伏发电的太阳能定位和跟踪系统

在资源紧缺的今天,太阳能以它恒定的来源和高强的辐射能量,日渐成为资源利用的首要选择。聚光型光伏发电的基本原理是采用带有菲尼尔透镜的太阳能电池板,利用图像采集传感器,拍摄参照物的太阳影子长度,并以与垂直投影做出的比较测出太阳的偏转角度,通过高速控制芯片,根据对采集信息的分析,控制传动机构调节太阳能电池板的偏转角度,实现对太阳的定位和跟踪,从而实现太阳能的高效采集。