全文获取类型
收费全文 | 11901篇 |
免费 | 1699篇 |
国内免费 | 1396篇 |
专业分类
化学 | 8091篇 |
晶体学 | 147篇 |
力学 | 859篇 |
综合类 | 95篇 |
数学 | 1426篇 |
物理学 | 4378篇 |
出版年
2024年 | 18篇 |
2023年 | 169篇 |
2022年 | 188篇 |
2021年 | 291篇 |
2020年 | 393篇 |
2019年 | 346篇 |
2018年 | 308篇 |
2017年 | 295篇 |
2016年 | 454篇 |
2015年 | 431篇 |
2014年 | 567篇 |
2013年 | 809篇 |
2012年 | 887篇 |
2011年 | 968篇 |
2010年 | 654篇 |
2009年 | 712篇 |
2008年 | 752篇 |
2007年 | 724篇 |
2006年 | 683篇 |
2005年 | 627篇 |
2004年 | 493篇 |
2003年 | 392篇 |
2002年 | 347篇 |
2001年 | 327篇 |
2000年 | 287篇 |
1999年 | 317篇 |
1998年 | 327篇 |
1997年 | 266篇 |
1996年 | 265篇 |
1995年 | 224篇 |
1994年 | 244篇 |
1993年 | 161篇 |
1992年 | 149篇 |
1991年 | 130篇 |
1990年 | 138篇 |
1989年 | 84篇 |
1988年 | 78篇 |
1987年 | 86篇 |
1986年 | 82篇 |
1985年 | 71篇 |
1984年 | 48篇 |
1983年 | 33篇 |
1982年 | 35篇 |
1981年 | 26篇 |
1980年 | 20篇 |
1979年 | 15篇 |
1978年 | 14篇 |
1977年 | 8篇 |
1976年 | 8篇 |
1973年 | 10篇 |
排序方式: 共有10000条查询结果,搜索用时 109 毫秒
961.
Polarization-independent self-collimating bends and beam splitters in photonic crystals 总被引:1,自引:0,他引:1
Xiaopeng Shen Kui Han Xianqing Yang Yifeng Shen Haipeng Li Gang Tang Zhitian Guo 《中国光学快报(英文版)》2007,5(11)
Polarization independent bends and beam splitters for transverse electric (TE) and transverse magnetic (TM) polarizations have been demonstrated in two-dimensional (2D) photonic crystals (PhCs). In virtuel of equi-frequency contour analysis and finite-difference time-domain calculations, self-collimation behaviors for TE- and TM-polarizations are achieved at the same frequency. Simulation results show a 90-degree bend with 90% efficiency and beam splitters with about 96% total efficiency for both TE- and TMpolarizations, where the light is self-guided by the self-collimation effect. Such bends and beam splitters are expected to play important roles in optical devices where polarization insensitivity is needed. 相似文献
962.
Localized fields in the defect mode of one-dimensional photonic crystals with active impurity are studied with the help of the theory of spontaneous emission from two-level atoms embedded in photonic crystals.Numerical simulations demonstrate that the enhancement of stimulated radiation, as well as the phenomena of transmissivity larger than unity and the abnormality of group velocity close to the edges of photonic band gap, are related to the negative imaginary part of the complex effective refractive index of doped layers. This means that the complex effective refractive index has a negative imaginary part, and that the impurity state with very high quality factor and great state density will occur in the photonic forbidden band if active impurity is introduced into the defect layer properly. Therefore, the spontaneous emission can be enhanced, the amplitude of stimulated emission will be very large and it occurs most probably close to the edges of photonic band gap with the fundamental reason, the group velocity close to the edges of band gap is very small or abnormal. 相似文献
963.
单偏振控制器环形腔光纤激光器实验研究 总被引:1,自引:0,他引:1
理论分析了非线性偏振旋转环形腔作为类饱和可吸收体获得脉冲的物理机理。在光纤环形腔结构中,采用单个偏振控制器实现了非线性偏振旋转锁模,直接获得了脉冲宽度为131 fs的超短脉冲输出。实验中,采用性能稳定的976 nm半导体二极管激光器作为抽运源,使用高掺杂浓度的Er3 光纤为增益介质,通过调节偏振控制器,获得了光谱谱宽(3 dB带宽处)为23.5 nm的稳定锁模脉冲输出。脉冲中心波长为1535.9 nm,平均功率为5.91 mW,脉冲重复率为11.20 MHz。 相似文献
964.
Qiuhua Nie Tiefeng Xu Xiang Shen Xudong Zhang 《Journal of Physics and Chemistry of Solids》2007,68(4):477-481
A series of Er3+-doped Bi2O3-B2O3-SiO2-Na2O glasses with different hydroxyl groups were prepared and the interaction between the Er3+ ions and OH groups was investigated. Infrared spectra were measured in order to calculate the exact content of OH groups in samples. The observed increase of the fluorescence lifetime with the oxygen bubbling time has been related to the reduction in the OH content concentration evidenced by infrared (IR) absorption spectra, which confirmed that the OH groups were dominant quenching centers of excited Er3+ and a cause of concentration quenching of 1.5 μm band emission. Various nonradiative decay rates from 4I13/2 of Er3+ with the change of OH content were determined from the fluorescence lifetimes and radiative decay rates, which were calculated on the basis of Judd-Ofelt theory. 相似文献
965.
966.
Yb3+ singly doped and Yb3+/Al3+ co-doped high silica glass samples are prepared,and their luminescent properties are investigated.Al3+ is considered as a beneficial activator for Yb3+-doped glass.However,the experimental result shows that the addition of Al3+ results in the transformation from Yb3+ to Yb2+ and the improvement in the photon luminescence of Yb2+.These characteristics indicate the role of Al3+ in Yb3+-doped glass. 相似文献
967.
A new cyclic heptapeptide containing γ-aminobutyric acid in the ring, namely unguisin E (1), was isolated from the fermentation culture of Aspergillus sp. AF119. The structure was elucidated by spectroscopic analyses, including 1D and 2D NMR experiments, and HR ESI-Q-TOF
mass spectrometry, and by comparison with those reported. Compound 1 was evaluated for its antimicrobial activities by the paper diffusion method. 相似文献
968.
969.
970.
Hua Zhu Xiaoping Xu Wei Cui Yuanqing Zhang Huaping Mo Yu‐Mei Shen 《Journal of polymer science. Part A, Polymer chemistry》2011,49(8):1745-1752
This article describes the synthesis and characterization of hydroxyl tailored, molecularly defined biodegradable cooligomers capable of chelating rhenium for potential radiopharmaceuticals. New insights were gained during the synthesis of lactide based on dimethylaminopyridine‐catalyzed transesterification. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011 相似文献