全文获取类型
收费全文 | 2773篇 |
免费 | 262篇 |
国内免费 | 58篇 |
专业分类
化学 | 279篇 |
晶体学 | 9篇 |
力学 | 125篇 |
综合类 | 4篇 |
数学 | 40篇 |
物理学 | 2636篇 |
出版年
2024年 | 1篇 |
2023年 | 12篇 |
2022年 | 20篇 |
2021年 | 24篇 |
2020年 | 35篇 |
2019年 | 16篇 |
2018年 | 31篇 |
2017年 | 64篇 |
2016年 | 110篇 |
2015年 | 61篇 |
2014年 | 250篇 |
2013年 | 168篇 |
2012年 | 112篇 |
2011年 | 215篇 |
2010年 | 170篇 |
2009年 | 180篇 |
2008年 | 216篇 |
2007年 | 232篇 |
2006年 | 166篇 |
2005年 | 120篇 |
2004年 | 143篇 |
2003年 | 99篇 |
2002年 | 84篇 |
2001年 | 106篇 |
2000年 | 85篇 |
1999年 | 79篇 |
1998年 | 72篇 |
1997年 | 49篇 |
1996年 | 30篇 |
1995年 | 25篇 |
1994年 | 17篇 |
1993年 | 12篇 |
1992年 | 14篇 |
1991年 | 13篇 |
1990年 | 8篇 |
1989年 | 8篇 |
1988年 | 13篇 |
1987年 | 11篇 |
1986年 | 1篇 |
1985年 | 2篇 |
1984年 | 6篇 |
1982年 | 1篇 |
1981年 | 6篇 |
1980年 | 2篇 |
1978年 | 1篇 |
1977年 | 2篇 |
1973年 | 1篇 |
排序方式: 共有3093条查询结果,搜索用时 31 毫秒
171.
用Matlab编程的方法对长周期光纤光栅的透射光谱谱线特性进行了研究,数值模拟了长周期光纤光栅周期对光谱谱线的影响.研究结论为长周期光纤光栅的设计提供了理论依据. 相似文献
172.
173.
Muhammad Taher Abuelma’atti 《Applied Acoustics》2011,72(5):305-309
In this paper, closed-form mathematical expressions are obtained for the amplitudes of the harmonic and intermodulation products of the fiber optic Mach–Zehnder microphone excited by a multitone acoustical perturbation. The results show the strong dependence of the harmonic and intermodulation performance on the amplitude of the acoustical perturbation tones and the phase shift produced by the DC bias voltage applied to the reference fiber of the Mach–Zehnder interferometer. The results also show that depending on the phase shift produced by the DC bias voltage, either the third-order or the second-order intermodulation will be dominant. Compared to previously published results for other types of microphones, it appears that the phase shift produced by the DC bias voltage plays an important role in deciding the order of the dominant intermodulation product. 相似文献
174.
Energy bands formed by rotation–vibrational states of molecules in the presence of symmetry and their qualitative modifications under variation of some control parameters are studied within the semi-quantum model. Rotational variables are treated as classical whereas a finite set of vibrational states is considered as quantum. In the two-state approximation the system is described in terms of a fiber bundle with the base space being a two-dimensional sphere, the classical phase space for rotational variables. Generically this rank 2 complex vector bundle can be decomposed into two complex line bundles characterized by a topological invariant, the first Chern class. A general method of explicit calculation of Chern classes and of their possible modifications under variation of control parameters in the presence of symmetry is suggested. The construction of iso-Chern diagrams which split the space of control parameters into connected domains with fixed Chern numbers is suggested. A detailed analysis of the rovibrational model Hamiltonian for a D3 invariant molecule possessing two vibrational states transforming according to the two-dimensional irreducible representation is done to illustrate non-trivial restrictions imposed by symmetry on possible values of Chern classes. 相似文献
175.
A full-duplex radio-over-fiber (RoF) link with a novel scheme to generate 60 GHz mm-waves from a 5 GHz RF signal source is investigated. In the RoF downlink, the required frequency of the RF oscillator is reduced greatly. Since the optical carrier is not modulated by downstream data, part of it is reused to carry upstream data and the upstream data is transmitted to the central station using optical single-sideband modulation. In this way, a single wavelength is used for both downstream and upstream transmissions. Based on this scheme, a full-duplex RoF link is built and its transmission performance is analyzed. Theoretical analysis and numerical simulation show that the downstream signal cannot only eliminate code form distortion caused by time shift of the code edges, but also reduce the influence of the fading effect as the 60 GHz DSB optical mm-wave signal is transmitted along the fiber, and the upstream signal is immune to both fading effect and time shift of the code edges. 相似文献
176.
A. Harfouche 《Optics Communications》2011,284(22):5348-5352
It has been demonstrated in a previous work (Opt. Commun. 233 (2004) 39-43), for a monochromatic laser beam, that imperfect collimation leads to systematic errors in the determination of the spectral bandwidth of a Gaussian beam. In this work, we demonstrate that such transverse effects could also make wrong the measurement of the coherence length of a non-monochromatic Gaussian beam. In addition, we have demonstrated that this systematic error is increased when the laser beam under study is made up of a mixing of spatial modes. 相似文献
177.
A polarization beam splitter based on a self-collimation Mach-Zehnder interferometer (SMZI) in a hole-type silicon photonic crystal was proposed and numerically demonstrated. Utilizing polarization dependence of the transmission spectra of the SMZI and polarization peak matching (PPM) method, the SMZI can work as a polarization beam splitter (PBS) by selecting appropriate path length difference in the structure. Because of its intrinsic operating principle, the PBS possesses high polarization extinction ratios (PERs). As its dimensions are only several operating wavelengths, the PBS may have practical applications in photonic integrated circuits. 相似文献
178.
We derive analytical expressions for the single mode quantum field state at the individual output ports of a beam splitter when a single-photon Fock state and a coherent state are incident on the input ports. The output states turn out to be a statistical mixture between a displaced Fock state and a coherent state. Consequently we are able to find an analytical expression for the corresponding Wigner function. Because of the generality of our calculations the obtained results are valid for all passive and lossless optical four port devices. We show further how the results can be adapted to the case of the Mach-Zehnder interferometer. In addition we consider the case for which the single-photon Fock state is replaced with a general input state: a coherent input state displaces each general quantum state at the output port of a beam splitter with the displacement parameter being the amplitude of the coherent state. 相似文献
179.
Photonic generation of microwave signals using dual-wavelength single-longitudinal-mode fiber lasers
Photonic generation of microwave signals is demonstrated with a dual-wavelength single-longitudinal-mode erbium-doped fiber ring laser. Microwave signal with a frequency of 21.07 GHz is obtained by aligning the reflection band of a uniform fiber Bragg grating to the two transmission peaks within a fiber Bragg grating-based Fabry-Perot filter. Stable dual-wavelength single-longitudinal-mode operation is guaranteed by means of the combined filtering effect of a saturable absorber and the Fabry-Perot filter. This approach provides an effective solution to the photonic generation of high-frequency microwave signals. 相似文献
180.
This paper presents the modeling and characterization of an optical fiber grating for maximum reflectivity. Grating length and change in refractive index are the critical parameters in contributing to the performance of fiber Bragg grating. The wavelength chosen for analysis is from the third window to minimize the attenuation. The reflection spectra, bandwidth and side lobes strength were analyzed with different lengths and change in refractive index. The simulations are based on solving coupled mode equations by transfer matrix method that describes the interaction of guided modes. 相似文献