How to realize an ultrafast electron diffraction experiment with a terahertz pump: A theoretical study

To integrate a terahertz pump into an ultrafast electron diffraction (UED) experiment has attracted much attention due to its potential to initiate and detect the structural dynamics both directly. However, the deflection of the electron probe by the electromagnetic field of the terahertz pump alters the incident angle of the electron probe on the sample, impeding it from recording structural information afterwards. In this article, we studied this issue by a theoretical simulation of the terahertz-induced deflection effect on the electron probe, and came up with several possible schemes to reduce such effect. As a result, a terahertz-pump-electron-probe UED experiment with a temporal resolution comparable to the terahertz period is realized. We also found that MeV UED was more suitable for such terahertz pump experiment.     

超强激光超热电子激发的Kα X射线发射研究

建立了无色散型X射线谱仪. 利用SILEX-I激光装置的超强激光辐照固体物质,分别在靶前、后定量测量了Cu和Mo物质在不同激光功率密度时的X射线谱和Kα光子产额,推导了不同激光强度时的Kα X射线光子转换效率. 实验发现,打靶激光能量越高,靶后出射的Kα产额越高,100μm Mo靶可获得10^-5量级转换效率. 关键词： X射线发射 激光-物质相互作用 Kα谱仪')" href="#">Kα谱仪     

Properties of long light filaments in natural environment

The multiple filamentation of terawatt femtosecond(fs) laser pulses is experimentally studied in a natural environment.A more than 30-m long plasma filament with a millimeter diameter is formed by the collimated fs laser pulse freely propagating in an open atmosphere. This study provides the first quantitative experimental data about the electron density of a long range light filament in the atmosphere. The electron density of such a filament is quantitatively detected by using an electric method, showing that it is at the 10~(11)-cm~(-3) level.     

基于可变类FCM算法的多光谱遥感影像分割

为了自动确定多光谱遥感影像中地物目标类别数,该文提出一种基于可变类模糊C均值(Fuzzy C-Means, FCM)的多光谱遥感影像分割方法。首先定义像素与聚类的非相似性测度并据此构建目标函数,而后通过求解目标函数得到最优模糊隶属度和聚类中心。其次,研究模糊因子与影像地物目标类别数的关系,并通过定义划分熵(Partition Entropy, PE)指数优选模糊因子,选择PE指数值稳定收敛后所对应的最小模糊因子值为最优模糊因子,根据模糊因子与类别数的关系得到最优类别数,从而实现了影像的可变类分割。最后,利用提出算法分别对合成和真实多光谱遥感影像进行分割实验,实验结果表明,提出算法不仅能自动确定影像的最优类别数,还能获得较好的分割结果,为实现自动确定遥感影像中地物目标类别数提供新方法。     

基于改进噪声方差估计的小波阈值语音增强

提出了一种估计语音噪声方差的改进方法,利用语音信号小波系数尺度间的相关性,去除了语音信号细节小波系数对噪声方差估计的干扰。仿真结果表明,方法能准确估计语音信号的噪声方差,尤其当语音噪声较小时,方法比传统的Donoho法更准确。     

MoS2和MoTe2同质结和异质结中的表面电势排列

过渡金属硫族化合物（TMD）薄层仅改变几何形状（如层厚）就可调节带隙、电子亲和势和费米能级，使器件设计更灵活。但因缺乏费米能级排列信息，TMD同质/异质结器件常因未知的能带弯曲而偏离预期。利用扫描开尔文探针显微镜（SKPM）表征了TMD同质/异质结，结果显示，MoS₂和MoTe₂同质结的费米能级随层厚增加向本征费米能级移动（背景掺杂浓度降低），而MoTe₂/MoS₂异质结中探测到宽耗尽区和强光响应，同时给出表面污染（分子尺度）对单层TMD表面电势的影响。上述发现将在器件设计中帮助精准堆叠范德华（vdW）层。     

基于虚拟游标效应的夹层多模光纤马赫-曾德尔干涉仪高灵敏温度传感器

提出一种基于夹层多模光纤马赫-曾德尔干涉仪（MZI）与虚拟游标效应结合的高灵敏度温度传感器，进行了理论分析和实验验证。该光纤MZI是通过在两段1 mm长的阶跃型多模光纤之间拼接一段20 mm的渐变多模光纤制成，夹层多模光纤MZI作为游标效应的传感干涉仪，利用信号处理对该光纤MZI的干涉光谱进行变频得到参考干涉仪光谱，通过叠加传感干涉仪和参考干涉仪的波形实现虚拟游标光谱。实验结果表明，在40～100℃温度范围内，该传感系统的温度灵敏度为3.884 nm/℃，与单个夹层多模光纤MZI相比，灵敏度提高了37.346倍。与传统的光纤游标传感器相比，所提出的虚拟游标夹层多模光纤温度传感器具有灵敏度极高、尺寸紧凑、制作简单、成本低、结果更可靠等优点。