首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   84篇
  免费   15篇
  国内免费   3篇
化学   14篇
晶体学   1篇
数学   2篇
物理学   85篇
  2022年   2篇
  2021年   4篇
  2020年   1篇
  2019年   5篇
  2018年   3篇
  2017年   4篇
  2016年   3篇
  2015年   2篇
  2014年   7篇
  2013年   4篇
  2012年   2篇
  2011年   2篇
  2010年   3篇
  2009年   1篇
  2008年   4篇
  2007年   5篇
  2006年   7篇
  2005年   2篇
  2004年   6篇
  2003年   2篇
  2002年   7篇
  2000年   1篇
  1999年   3篇
  1998年   4篇
  1997年   3篇
  1996年   2篇
  1993年   1篇
  1991年   4篇
  1990年   2篇
  1989年   1篇
  1988年   1篇
  1987年   1篇
  1985年   1篇
  1983年   1篇
  1981年   1篇
排序方式: 共有102条查询结果,搜索用时 15 毫秒
1.
本文基于衍射光学设计理论,提出了仅用一种简单结构实现波长和偏振态同时复用全息显示新方法.构建了不同入射条件下超表面微元的结构参数与透过相位之间的映射关系,建立了科学的评价函数,优化得到超表面每个像素点处最优的单一结构超表面微元尺寸.仿真结果表明,本文设计的超表面实现了波长为532 nm的x线偏振光和波长为633 nm的y线偏振光入射显示不同形状字符的功能.  相似文献   
2.
小麦白粉病和条锈病是我国两种最普遍、最具破坏性的小麦病害,且田间常常混合发生。由于病源和发病机理不同,有必要对这两种病害进行准确区分和识别,以采取不同的防治措施。基于ImSpector V10E高光谱成像系统采集的条锈菌侵染叶片、白粉菌侵染叶片和健康叶片(共计320个)在375~1 017 nm范围内的高光谱图像,利用高斯平滑等预处理方法得到三种小麦叶片的平均光谱曲线,发现小麦白粉病和条锈病的敏感波段均集中在550~680 nm的色素强吸收位置,且趋势基本一致。针对两种病害的响应波段交叉重叠的问题,通过主成分分析-载荷法(PCA)、连续投影算法(SPA)和竞争性自适应重加权算法(CARS)对小麦叶片的光谱信息进行有效降维,分别优选出3、6、30个敏感波段和特征波长;在此基础上,采用最小二乘-支持向量机和极限学习机两种分类算法分别基于全波段、PCA、SPA和CARS的优选波段,建立白粉病、条锈病和健康叶片的判别模型。结果表明,8种模型的准确识别率均在94.58%以上。其中,主成分分析-载荷法结合极限学习机模型最优,训练集与校正集的正确识别率分别为99.18%和100%,且结构简单,仅含有三个变量(占全波段的1.1%)。最后,通过对小麦白粉病、条锈病以及健康叶片的显微结构分析,发现病菌入侵叶片,破环细胞结构,导致叶绿素含量减少,光合作用效能降低,进而使得小麦在可见光波段光吸收程度减弱,反射率增大。可见,利用作物的高光谱图像信息能够准确地识别不同类型的小麦病害,为研发作物病害在线识别的多光谱系统提供重要的理论依据。  相似文献   
3.
高光谱图像和叶绿素含量的水稻纹枯病早期检测识别   总被引:1,自引:0,他引:1  
基于高光谱成像技术和化学计量方法,实现了对水稻纹枯病病害的早期检测识别。以幼苗时期的水稻植株为研究对象,对其进行纹枯病病菌侵染,获得染病植株,采集358~1 021 nm波段范围的高光谱图像,三次实验共240个样本,包括染病植株120个样本和健康植株120个样本。根据高光谱图像的光谱维,对染病水稻叶片和健康水稻叶片提取感兴趣区域(ROI),利用感兴趣区域的光谱数据,对其进行Savitzky-Golay(SG)平滑、Savitzky-Golay(SG)一阶求导、Savitzky-Golay(SG)二阶求导、变量标准化(SNV)和多元散射校正(MSC)预处理,建立线性判别分析(LDA)和支持向量机(SVM)分类模型,结果表明:采用SG二阶求导预处理后的线性判别分析(LDA)模型取得了较好的性能,正确识别率在建模集达98.3%,在预测集达95%;利用载荷系数法(x-loading weights, x-LW)对原始光谱和5种预处理的光谱数据进行特征波长提取,然后根据选取的特征波长建立线性判别分析(LDA)和支持向量机(SVM)分类模型,其中采用SG二阶求导预处理后提取的12个特征波长的线性判别分析(LDA)模型取得了较好的性能,其正确识别率在建模集达97.8%,在预测集达95%,而且基于载荷系数法建立的模型性能与全波段相当,可以通过载荷系数法减少数据量对水稻纹枯病病害进行识别;根据高光谱图像的图像维,研究了基于图像主成分分析、基于概率滤波和基于二阶概率滤波的图像特征提取方法,利用提取的特征变量建立反向传播神经网络(BPNN)和支持向量机(SVM)分类模型,其中基于图像主成分分析的反向传播神经网络(BPNN)模型取得了较好的性能,建模集准确识别率达90.6%,预测集的准确识别率达83.3%;根据高光谱图像光谱维和图像维的最优模型,特将叶绿素含量作为建模的另一个特征,分别与光谱特征、图像特征组合,建立反向传播神经网络(BPNN)和线性判别分析(LDA)模型,提出基于光谱特征加叶绿素含量、图像特征加叶绿素含量和光谱、图像特征加叶绿素含量三种组合方式,其中,光谱特征和图像特征分别与叶绿素组合的方式比之前单独的光谱和图像特征建模性能都有所提升,而且三种组合方式中光谱特征加叶绿素含量的反向传播神经网络(BPNN)建模方式取得本研究所有建模方式中较优的性能,其准确识别率在建模集达100%,在预测集达96.7%。以上研究表明,基于高光谱图像和叶绿素含量对水稻纹枯病病害进行早期识别是可行的,为水稻病害的早期识别提供了一种新方法。  相似文献   
4.
管桦  黄垚  李承斌  高克林 《物理学报》2018,67(16):164202-164202
近年来,冷原子技术和激光技术促进了高精度光频标的发展,有望在建立时间基准、推动基础研究和满足国家需求等方面发挥重要的作用.本文介绍了中国科学院武汉物理与数学研究所近年来在高准确度钙离子(~(40)Ca~+)光频标研究方面的进展:采用新的ULE腔系统,实现了729 nm钟跃迁激光器1—100 s的频率稳定度均优于2×10~(-15),通过对外场和环境效应的控制及克服,特别是囚禁离子运动效应的抑制,获得单个钙离子光频标的不确定度优于5.5×10~(-17);通过两台光频标的比对,测得20000 s的稳定度也进入10~(-17)量级;基于高精度钙离子光频标平台,进行了相关精密测量的工作,包括:基于全球定位系统的超高精度远程光频绝对值测量方案,第二次测量了钙离子的光频跃迁绝对值,该测量结果再次被国际时间频率咨询委员会采纳,更新了钙离子的频率推荐值;精确测量了钙离子的钟跃迁魔幻波长,由此提出新型的全光囚禁离子光频标的方法;精密测量了钙离子的亚稳态寿命等参数.以上工作推动了基于冷原子的精密测量工作.  相似文献   
5.
方亮  赵建林  甘雪涛  李鹏  张晓娟 《光子学报》2014,39(11):1921-1927
 通过数值模拟飞秒脉冲在具有双零色散波长的光子晶体光纤中的传输过程,详细分析了超连续谱的产生和控制机制.结果表明:中心波长处于反常色散区的泵浦脉冲在高阶非线性和高阶色散等作用的调制下,将演化为基孤子和正常色散区的两个色散波|该色散波进而经与之相位匹配的基孤子相干加强而使频谱展宽形成超连续谱,同时两个色散波上出现了干涉引起的振荡现象.进一步对比三种结构的光子晶体光纤中超连续谱的特点,定量分析了两色散波对超连续谱的限制作用,阐述了结构参量对超连续谱的影响.基于上述结论,结合对色散波的中心波长与光子晶体光纤的色散曲线、结构参量之间关系的分析,提出了设计光子晶体光纤的结构来控制超连续谱的方法.作为例证,通过优化光子晶体光纤结构理论上实现了频谱分量覆盖可见光区的平坦超连续谱.  相似文献   
6.
Abstract

This article presents an advanced optical model for simulation of alien wavelengths in multi-domain and multi-vendor dense wavelength-division multiplexing networks. The model aids optical network planners with a better understanding of the non-linear effects present in dense wavelength-division multiplexing systems and better utilization of alien wavelengths in future applications. The limiting physical effects for alien wavelengths are investigated in relation to power levels, channel spacing, and other factors. The simulation results are verified through experimental setup in live multi-domain dense wavelength-division multiplexing systems between two national research networks: SURFnet in Holland and NORDUnet in Denmark.  相似文献   
7.
ABSTRACT

Modified coupled-cluster (CC) methods such as linearized coupled-cluster doubles (LinCCD), approximate coupled pair (ACP D14), 2CC (from nCC family), parameterized CCSD (pCCSD) and distinguishable cluster (DCSD) can have their advantages over general CC methods. Though these methods include connected clusters of single and double excitations at most, distinguishable cluster, parameterized CC and approximate coupled pair methods, in particular, have been shown to produce quantitatively correct results in benchmark studies. To put these methods on a stronger foothold, it is essential to understand the rationale for their success: mimicking the effect of connected triple excitations. We exploit the relation between CC and many body perturbation theory (MBPT) in general, and between CCSD and MBPT(4)/MP4 in particular, to take a step towards bringing clarity to this persisting conundrum. Our aim here is to look for numerical signs of ‘addition by subtraction’ or ‘inclusion by deletion’ effect that is likely behind the success of these modified CCD or CCSD methods. We achieve this by revisiting well-studied examples of single and multiple bond dissociation and comparing the performance of these modified CCSD methods with higher-level CC methods. Though our results are qualitative in nature, we hope this would lead to more rigorous analysis in future studies.  相似文献   
8.
In this paper we study the excitation of photonic nanojets (PNJ) in 3D dielectric cuboids by surface plasmons at telecommunication wavelengths. The analysis is done using the effective refractive index approach. It is shown that the refractive index contrast between the regions with and without cuboid should be roughly less than 2 in order to generate jets at the output of the cuboid. The best performance at λ0 = 1550 nm is obtained when the height of the cuboid is 160 nm producing a jet just at the output interface with a subwavelength resolution of 0.68λ0 and a high intensity enhancement (×5) at the focus. The multi‐wavelength response is also studied demonstrating that it is possible to use the proposed structure at different wavelengths. Finally, the backscattering enhancement is numerically evaluated by inserting a metal particle within the PNJ region, demonstrating a maximum value of ~2.44 dB for a gold sphere of radius 0.1λ0.

  相似文献   

9.
We have developed a method to make real-time, continuous, noninvasive measurements of muscle oxygenation (Mox) from the surface of the skin. A key development was measurement in both the visible and near infrared (NIR) regions. Measurement of both oxygenated and deoxygenated myoglobin and hemoglobin resulted in a more accurate measurement of Mox than could be achieved with measurement of only the deoxygenated components, as in traditional near-infrared spectroscopy (NIRS). Using the second derivative with respect to wavelength reduced the effects of scattering on the spectra and also made oxygenated and deoxygenated forms more distinguishable from each other. Selecting spectral bands where oxygenated and deoxygenated forms absorb filtered out noise and spectral features unrelated to Mox. NIR and visible bands were scaled relative to each other in order to correct for errors introduced by normalization. Multivariate Curve Resolution (MCR) was used to estimate Mox from spectra within each data set collected from healthy subjects. A Locally Weighted Regression (LWR) model was built from calibration set spectra and associated Mox values from 20 subjects using 2562 spectra. LWR and Partial Least Squares (PLS) allow accurate measurement of Mox despite variations in skin pigment or fat layer thickness in different subjects. The method estimated Mox in five healthy subjects with an RMSE of 5.4%.  相似文献   
10.
In this study, we successfully studied water‐soluble extract from Radix isatidis. Optimized conditions of MAE were listed, the sample can be extracted completely in 10 minutes under microwave power of 400W and solid/liquid ratio of 1:80. Active compounds in water‐soluble extract from R. isatidis were identified with HPLC‐DAD/ESI‐MS, these compounds followed by cytidine, uridine, guanosine, (R,S)‐goitrin and adenosine. RODWs–HPLC as a new sensitive chromatography were also first proposed and investigated, we favoringly used this method for simultaneous determination of these active constitutents in water‐soluble R. isatidis extract. Chromatographic separation was performed on a Diamonsil C18 column (5 μm, 150 mm × 4.6 mm) with a mobile phase gradient consisting of methanol and water at a flow‐rate of 1.0 mL/min, detection wavelengths 240, 250, 260 and 270 nm, the retention times of the tested five compounds were about 4.2, 5.8, 11.1, 14.2 and 20.8 min respectively, the limits of detection were 15, 12, 20, 5.8 and 24 ng/mL for cytidine, uridine, guanosine, (R,S)‐goitrin and adenosine respectively, their linear ranges were between 0.045 and 350 μg/mL with correlation coefficient (R) of 0.9998‐0.9999. The relative standard deviations (RSDs) of intra‐day and inter‐day assays were 0.30‐2.36% and 0.86‐2.54% respectively. Extraction recoveries were 94.25‐106.21%. This novel analytical method was shown to be simple, low‐cost, sensitive and reliable for multiple components in complex or undeveloped materials via MAE, ESI‐MS and RODWs‐HPLC.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号