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91.
B. Macke B. Ségard 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2003,23(1):125-141
This paper deals with the apparent superluminal propagation of electromagnetic pulses in a linear dispersive medium. One specifically
examines the possibility that the pulse leaving the medium may be nearly identical to the incident one (low distortion) and
in significant advance of it (strongly negative group-delays). Favourable conditions are obtained in a dilute medium where
the required anomalous dispersion originates in an ensemble of narrow absorption or gain lines. Analytical expressions of
the advancement of the pulse centre-of-gravity and of the lowest order distortion are established from the transfer-function
of the medium. The experiments already achieved with arrangements involving a single absorption-line or a gain-doublet are
analysed in detail and compared. The considerable difficulties to overcome in order to attain advancements comparable to the
pulse width without important distortion are pointed out. New and promising schemes involving a narrow dip in a gain profile
or absorption-doublets are proposed.
Received 4 July 2002 / Received in final form 8 November 2002 Published online 4 February 2003
RID="a"
ID="a"e-mail: bruno.macke@univ-lille1.fr
RID="b"
ID="b"Unité Mixte de Recherche de l'Université et du CNRS (UMR 8523) 相似文献
92.
在真空环境中,利用空间反射电磁波测量装置,开展微波等离子体喷流对垂直和水平极化电磁波衰减的实验研究,分析不同极化电磁波在等离子体中衰减的影响因素.实验结果表明:以氩气为工质,真空室中微波等离子体喷流对垂直和水平极化电磁波具有显著的吸收效应.发生器流量、功率以及实验真空度对垂直极化电磁波在等离子体中的衰减影响明显.真空度和发生器功率对水平极化电磁波没有显著影响.
关键词:
等离子体中的电磁波
等离子体基本性质
电磁波 相似文献
93.
K. Jin Z. C. Wang Q. Zhang 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2007,42(2):325-332
We study the absorption spectra of a degenerate V-type
atom, where a resonant driving field and a probe field drive
different branches of transitions and a dc field is applied to
drive the transition between two excited states. The effects of
vacuum induced coherence (VIC) on the absorption spectra are
investigated. It is demonstrated that in some special cases the
VIC can lead to the depression of absorption and narrow resonance.
The origin of these features are discussed. When the pump field
and the dc field have the same intensity, it is interesting to
find that the whole absorption spectrum comes mainly from the
absorptions induced by the interferences among different
transitions between dressed states. 相似文献
94.
An approximate analytical propagation formula of the elliptical Gaussian beams through a paraxial ABCD optical system with a circular aperture is obtained on the basis of the expansion of a hard-edged aperture into a finite sum of complex Gaussian functions. And the parameter characteristics of the truncated beams including the beam propagation factor and the kurtosis parameter are also studied in detail by using the second-order-moments method. 相似文献
95.
Based on the truncated second-order moments method on the cylindrical coordinate systems and the incomplete gamma function, an analytical expression of the generalized beam propagation factor (MG2 factor) of hard-edged circular apertured diffracted Bessel–Gaussian beams is derived and illustrated numerically. It is shown that the MG2 factor of hard-edged diffracted BGBs mainly depends on the truncation parameter δ and the beam parameters m and η. The results can be reduced to that for the non-truncated Bessel–Gaussian beams case and that for the truncated fundamental Gaussian beams case under certain conditions, respectively. The power fraction is also discussed analytically and numerically. 相似文献
96.
为提高纯相位衍射光学元件的设计效果,实现高衍射效率的三维光场衍射传播控制,在原有GS迭代算法的基础上提出了新的相位加权迭代优化设计算法。此算法的特点是,建立多衍射输出平面迭代加权算法模型,并通过反馈各个设计输出平面在迭代计算过程中的设计误差,引入一定的相位动态加权整调策略,以达到更加优化的设计效果。以此算法设计一个纯相位衍射光学元件,将输入的高斯光束在距离输入面300mm~400mm内的每个平面上变换为2×2等强度光束阵列。通过对比实验发现此方法在原有算法基础上能进一步改善算法的收敛效果,提高整体设计质量,实现更加优化的运算。 相似文献
97.
Integrated noise model (INM) is the most internationally used software to calculate noise levels near airports. Take off, landing or pass by operations can be modeled by INM, but it does not consider aircrafts taxiing, which, in some cases, can be important to accurately evaluate and reduce airports’ noise assessment.Aircraft taxiing noise emission can be predicted using other prediction tools based on standards that describe sound attenuation during propagation outdoors. But these tools require data inputs that are not known: directivity and sound power levels emitted by aircraft during taxiing.This paper describes methods used to calculate directivity indexes and sound power levels, based on field measurements made in Madrid-Barajas Airport (Spain). Obtained results can be used as inputs for general purpose outdoor sound prediction software, which will be able to evaluate noise at airports vicinity as industrial noise.Directivity and sound power levels have been estimated in octave and third octave band terms, for several aircraft families. 相似文献
98.
Jiangfeng Luan 《Optik》2011,122(2):168-172
The temporal intensity distributions of the transmitted light through the tissue with lesion structures are investigated using the polarized Monte Carlo technique. Simulation results show that time-resolved curves of transmitted light change obviously for different lesion structures. When the lesion thickness increases, the intensity of transmitted light decreases for co-polarization detection and increases for cross-polarization detection. With the increase of the lesion depth, the amplitude rises for both co- and cross-polarization detection. A method considering the dependence of the peak temporal distribution on the lesion position is thus proposed to locate lesions inside live tissues. 相似文献
99.
Chen Rong Kazuya Mitsuji Xue Songtao Yoshihiro Sugimura Masanori Izumi 《Acta Mechanica Solida Sinica》1999,12(2):144-154
The purpose of this paper is to study the dynamic behavior of soft ground including a porous layer by considering the porosity
change. In order to take the porosity change into account, the concept of the volume fraction, which has been proposed in
continuum mechanics, is introduced. The constitutive equations presented by Bowen are applied to the analysis of the porous
media. According to Bowen's theory, the porosity is considered as a variable called the volume fraction and has its own constitutive
equation. The constitutive equation of the volume fraction has thermoelastic equation coefficients and is determined by the
strains of the solid and the fluid. This means that the compressibilities of the solid and the fluid are considered. When
the special condition is assumed, Bowen's theory can contain Biots's theory, which has been applied in earthquake engineering.
The wave propagation in the ground including a porous layer, modeled by Bowen's theory, is studied and compared with that
of Biot's theory. One-dimensional attenuation and surface amplitude are calculated. The effect of the volume fraction is discussed
with respect to the compressibilities of the solid and the fluid. 相似文献
100.
Haydar Aygün Keith Attenborough Walter Lauriks Philip A. Rubini Christian M. Langton 《Applied Acoustics》2011,(7):458-463
Comparisons between predictions of a Biot-Allard model allowing for angle-dependent elasticity and angle-and-porosity dependent tortuosity and transmission data obtained at normal incidence on water-saturated replica bones are extended to oblique incidence. The model includes two parameters which are adjusted for best fit at normal incidence. Using the same parameter values, it is found that predictions of the variation of transmitted waveforms with angle through two types of bone replica are in reasonable agreement with data despite the fact that scattering is not included in the theory. 相似文献