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Influence of cutting off position of plasma filament formed by two-color femtosecond laser on terahertz generation
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Femtosecond laser filamentation is a method of generating terahertz, which has wide application in terahertz subwavelength resolution imaging. In this paper, the plasma filament formed by femtosecond laser focusing was terminated with an alumina ceramics at different positions and the influence of the cutting off position of the plasma filament on the terahertz wave was studied. The results showed that the terahertz amplitude increases as the position approaches the end of the filament gradually. The stability of amplitude and peak frequency of the terahertz generated by the filament formed by two-color femtosecond laser via a lens with a longer focal length is lower than that through a lens with a shorter focal length, especially the terahertz amplitude at the end of the filament. The study will be helpful for future researchers in the field of THz sub-wavelength imaging utilizing femtosecond laser filament. 相似文献
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《中国光学快报(英文版)》2021,(8)
The spatial distribution of the forward-propagating amplified spontaneous emission(ASE) of nitrogen molecular ions during femtosecond laser filamentation in air is studied via numerical simulations. The results suggest that the divergence angle and signal intensity are extremely sensitive to the external focal length. Concurrently, we show that the optical Kerr effect plays a significant role in concentrating the directivity of ASE signals, particularly in cases of loose focusing. Furthermore,the simulations demonstrate that ASE signals are enhanced for a tight focus, although the corresponding filament length is shorter. The main physical mechanism underlying this process is the competition between the plasma defocusing and optical Kerr effects. The result is important for filamentation-based light detection and ranging applied to remote sensing. 相似文献
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The focusing properties of partially coherent vortex wave fields are studied. Expressions are derived for the intensity distribution and the degree of coherence near the geometrical focus. It is found that the size of coherence vortex dark core in the focal region depends on the topological charges and normalized coherence lengths. It is found that the desired vortex dark core near the geometrical focus can be generated by choosing appropriate values of parameters. The degree of coherence possesses a pair of phase singularities regions in the geometrical focus neighbourhood. 相似文献
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Yu. E. Geints A. A. Zemlyanov N. A. Izyumov A. A. Ionin S. I. Kudryashov L. V. Seleznev D. V. Sinitsyn E. S. Sunchugasheva 《Journal of Experimental and Theoretical Physics》2013,116(2):197-205
We report on the results of laboratory experiments of filamentation of sharply focused gigawatt femtosecond laser radiation passed through various aperture diaphragms in air. For the multiple filamentation regime, the dependences of the length and spatial structure of the filamentation region on the initial beam profile are established. It is found that light beam profiling by a diaphragm leads in some cases to a displacement of the filamentation region and to repeated self-focusing of radiation behind the linear focal waist. In the beam of the same power in the absence of a diaphragm and in the regime of the formation of a single filament, this effect terminates in front of the geometrical focus. The experimental results are illustrated by numerical simulation data. 相似文献
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从理论上研究部分相干光被一环形透镜聚焦,在焦点附近的轴上点的光强分布。研究结果表明,当部分相干光被一环形透镜聚焦时,最大聚焦光强不在几何焦点,而是位于透镜与几何焦点之间,出现焦移现象。并且,焦移量不仅依赖于透镜外半径的菲涅耳数,还依赖于部分相干光的空间相干度和中心拦截比。透镜的菲涅耳数越小,焦移越大;部分相干光的空间相干度越低,焦移越大。当菲涅耳数一定时,环形透镜的中心拦截比越大,焦移越大;当空间相干度很小时,情况就变得比较复杂。 相似文献
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W. Liu O. Kosareva I.S. Golubtsov A. Iwasaki A. Becker V.P. Kandidov S.L. Chin 《Applied physics. B, Lasers and optics》2003,76(3):215-229
The competition between femtosecond laser pulse induced optical breakdown and femtosecond laser pulse filamentation in condensed
matter is studied both experimentally and numerically using water as an example. The coexistence of filamentation and breakdown
is observed under tight focusing conditions. The development of the filamentation process from the creation of a single filament
to the formation of many filaments at higher pulse energy is characterized systematically. In addition, strong deflection
and modulation of the supercontinuum is observed. They manifest themselves at the beginning of the filamentation process,
near the highly disordered plasma created by optical breakdown at the geometrical focus.
Received: 9 July 2002 / Revised version: 15 November 2002 / Published online: 19 March 2003
RID="*"
ID="*"Corresponding author. Fax: +1-418/6562-623, E-mail: wliu@phy.ulaval.ca 相似文献
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The true focal spot of a focusing system may be defined as either the location of the maximum on-axis intensity or the location of the minimum beam diameter that contains a certain fraction of the power. Diffraction causes this true focus to be shifted from the ideal geometrical optics prediction, the effect being greatest at largeF-numbers. Specifically, a theoretical and experimental study is made of the largeF-number focusing properties of a thin lens in contact with a uniformly illuminated aperture. The results are clarified by a comparison to the focal spot shift for a Gaussian beam. 相似文献
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利用“泵浦-探测”非共线双等离子体通道方法实现三次谐波增强。泵浦光与探测光间的耦合作用能有效改善探测光光丝中的饱和效应,克服光强钳制对谐波强度增加的限制。实验中将两束能量分别为4.4和10.2 mJ、中心波长均为810 nm、脉宽均为60 fs的超短脉冲在空气中非共线聚焦,能各自形成光丝且产生微弱三次谐波。当强光在时域上超前于弱光时,强光会预先成丝形成等离子体通道,对后续弱光产生调制,使探测光产生的三次谐波强度明显增强。实验发现270 nm谐波能量增加的显著区域内频谱宽度出现振荡变化,当两光以27.3 mrad小角度相交于几何焦点前约15 mm,且探测光滞后约55 fs时,获得的能量增长倍率达到近70倍,对应谱宽约为5 nm。 相似文献
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根据几何光学中的光线方程,推导出了折射率跃变界面处光折射的代数方程,依据该方程讨论了球面界面的"焦点"、"焦距"问题并用计算机进行了模拟.该方法对学生理解光的传播行为及提高几何光学的教学效果有帮助. 相似文献
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为了获得多种类型的波长量级聚焦光斑,研究了一种新型涡旋光束,高次方涡旋光束经过大数值孔径透镜的聚焦。基于矢量德拜积分公式,理论上研究了线偏振的高次方涡旋光束经过大数值孔径透镜的聚焦特性。研究了涡旋光束的拓扑荷数和幂次方数对聚焦平面光强和电场x分量的相位分布的影响。研究结果表明,通过控制涡旋光束的拓扑荷数和幂次方数可以产生不同类型的聚焦光强分布,例如尺寸约为2个波长大小的实心和空心型聚焦光斑。此外,与普通的涡旋光束聚焦不同,高次方涡旋光束聚焦后的奇点并不在焦点处。这些特殊的聚焦光斑有望在微粒的操控等领域中得到应用。 相似文献
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部分相干修正贝塞尔高斯光束通过球差透镜的聚焦特性 总被引:2,自引:2,他引:0
由部分相干光的传输理论出发,对部分相干修正贝塞尔高斯光束通过球差透镜聚焦后轴上光强分布进行了研究.数值计算表明:当光谱相干度较小时,正、负球差对应的最佳聚焦点位于无球差时对应的最佳聚焦点的两侧;当光谱相干度较大时,正、负球差对应的最佳聚焦点将位于无球差时对应的最佳聚焦点的左侧.研究进一步表明,最佳聚焦点随着透镜菲涅耳数的增大向几何焦点方向靠近,并趋于一个定值.当光束菲涅耳数为1时,无球差时对应的最佳聚焦点逐渐趋于0.908,该值与光谱相干度无关,当透镜具有球差时,该值与光谱相干度有关. 相似文献
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A. F. Glova A. Yu. Lysikov D. D. Malyuta S. S. Nelyubin P. I. Peretyatko Yu. F. Ryzhkov 《Physics of Atomic Nuclei》2016,79(14):1663-1670
An experimental installation with a laser plasmatron based on a continuous wave CO2 laser with a radiation power of up to 3.5 kW has been created. The plasmatron design makes it possible to bring out the plasma jet into atmospheric air both along and across the laser beam direction. The spatial temperature distributions on the metal substrate surface heated by the plasma jet are measured. The threshold power for optical discharge maintenance as a function of the gas flow rate and the focal length of the focusing lens are obtained for an Ar and Ar/CH4/H2 gas mixture under atmospheric pressure; the radiation spectrum of the discharge plasma is measured. A one-dimensional model of the discharge for estimation of its geometrical parameters in a convergent laser beam with consideration of radiation refraction on the discharge is given. 相似文献
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为了提高长焦距变焦镜头的温度稳定性,分析了温度对光学系统的影响。在CODE V里对长焦距变焦镜头光学系统进行温度仿真分析,发现温度对光学系统的像质影响很大。采用无热化技术成熟的主动调焦式补偿方式对光学系统的温度效应进行补偿,发现采取补偿后像质有很大改善,但在相机的最大空间频率处的MTF还是较低,且变焦系统各焦距时的最佳像面并不能齐焦。采用机械被动式补偿和主动调焦式补偿混合的方式进行温度补偿,使得变焦系统在要求的全温度范围内有良好的像质,短焦时MTF大于0.5,长焦时的MTF大于0.25,并且焦距也得到补偿,各焦距时的最佳像面齐焦并与CCD的像面位置重合。 相似文献
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An analysis is presented of the possibilites of focusing the microwave energy by means of a magnetoactive plasma. The basic parameters of a plasma lens are calculated, and the way a magnetoactive plasma may be used in the making of lenses with a variable focal length is pointed out. 相似文献
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Weiwei Zhang Jing Liu Guangcai Chang Zhan Shi Ming Li Yuqi Ren Xiaowei Zhang Futing Yi Peng Liu Weifan Sheng 《Journal of synchrotron radiation》2016,23(5):1091-1096
A new prism‐array lens for high‐energy X‐ray focusing has been constructed using an array of different prisms obtained from different parabolic structures by removal of passive parts of material leading to a multiple of 2π phase variation. Under the thin‐lens approximation the phase changes caused by this lens for a plane wave are exactly the same as those caused by a parabolic lens without any additional corrections when they have the same focal length, which will provide good focusing; at the same time, the total transmission and effective aperture of this lens are both larger than those of a compound kinoform lens with the same focal length, geometrical aperture and feature size. This geometry can have a large aperture that is not limited by the feature size of the lens. Prototype nickel lenses with an aperture of 1.77 mm and focal length of 3 m were fabricated by LIGA technology, and were tested using CCD camera and knife‐edge scan method at the X‐ray Imaging and Biomedical Application Beamline BL13W1 at Shanghai Synchrotron Radiation Facility, and provided a focal width of 7.7 µm and a photon flux gain of 14 at an X‐ray energy of 50 keV. 相似文献
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Multiple foci and a long filament observed with focused femtosecond pulse propagation in fused silica 总被引:2,自引:0,他引:2
Multiple foci and a long filament are observed when we focus a femtosecond laser pulse into a fused-silica sample. The dependences of the intensity distribution of the plasma luminescence on the pulse energy and the numerical aperture (NA) of the focusing objective are investigated. Multiple foci are observed when NA of =0.65 . A long filament tail is formed instead of multiple foci when the NA is 0.85. A physical image of femtoseond pulse propagation is given by a model based on the nonlinear Schr?dinger equation. 相似文献
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对尺读望远镜的结构与原理进行分析研究,给出在不同情况下望远镜调焦手轮的具体调节方法,避免学生在实验时盲目地调节调焦手轮。 相似文献