2000 Optical Society of America

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北航考博辅导班：2019北京航空航天大学光学考博难度解析及经验分享根据教育部学位与研究生教育发展中心最新公布的第四轮学科评估结果可知，全国共有40所开设光学专业的大学参与了2017-2018光学专业大学排名，其中排名第一的是南京大学，排名第二的是哈尔滨工业大学，排名第三的是中国科学技术大学。

作为北京航空航天大学实施国家“211工程”和“985工程”的重点学科，北京航空航天大学的光学一级学科在历次全国学科评估中均名列第十一。

下面是启道考博整理的关于北京航空航天大学光学考博相关内容一、专业介绍光学专业是研究光(电磁波)的行为和性质，以及光和物质相互作用的物理学科。

传统的光学只研究可见光，现代光学已扩展到对全波段电磁波的研究。

如今常说的光学是广义的，是研究从微波、红外线、可见光、紫外线直到 X射线的宽广波段范围内的，关于电磁辐射的发生、传播、接收和显示，以及跟物质相互作用的科学。

光学是物理学的一个重要组成部分，也是与其他应用技术紧密相关的学科。

本专业培养具有坚实的光学理论基础和基本实验技能，具有较强的创新能力;了解本领域的发展现状和研究动态，熟悉光学发展的国际前沿动态;能从事科研、教学或承担专门技术工作，具有较强的综合能力、语言表达能力及写作能力的高级人才。

北京航空航天大学物理科学与核能工程学院博士招生专业专业代码及名称：1、070207 光学考试科目详细内容，请咨询招生学院。

二、综合考核1.我院根据申请人的申请材料，进行初步审核。

根据初步审核结果，择优确定进入考核的候选人；2.考核采取面试方式进行差额复试，对学生的学科背景、专业素质、操作技能、外语水平、思维能力、创新能力以及申请人分析、解决问题以及进行创新的综合能力等进行考察；复试成绩组成：特别注意：复试满分为400分，单科成绩低于60分的考生将不予录取。

四、申请材料1）通过中国研究生招生信息网（网址）完成网上报名（报名系统开放时间：2018年11月2日9：00至2018年11月15日9：00），打印系统生成的报名信息表，并按表中规定完整填写后签字盖章。

Long lifetime plasma channel in air generated by multiple femtosecond laser pulses and anexternal electrical fieldJiabin Zhu, Zhonggang Ji, Yunpei Deng, Jiansheng Liu, Ruxin Li, and Zhizhan Xu State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Fine Mechanics (SIOM), ChineseAcademy of Sciences, Shanghai 201800, Chinajiabinzhu@Abstract: The lifetime of a plasma channel produced by self-guidingintense femtosecond laser pulses in air is largely prolonged by adding a highvoltage electrical field in the plasma and by introducing a series offemtosecond laser pulses. An optimal lifetime value is realized throughadjusting the delay among these laser pulses. The lifetime of a plasmachannel is greatly enhanced to 350 ns by using four sequential intense100fs(FWHM) laser pulses with an external electrical field of about350kV/m, which proves the feasibility of prolonging the lifetime of plasmaby adding an external electrical field and employing multiple laser pulses.© 2006 Optical Society of AmericaOCIS codes: (320.7120) ultrafast phenomena; (350.5400) plasmasReferences and links1. A. Braun, G. Korn, X. Liu, D. Du, J. Squier, and G. Mourou, “Self-channeling of high-peak-powerfemtosecond laser pulses in air,” Opt. Lett. 20, 73-75 (1995).2. E. T. J. Nibbering, P. F. Curley, G. Grillon, B. S. Prade, M. A. Franco, F. Salin, and A. Mysyrowicz,“Conical emission from self-guided femtosecond pulses in air,” Opt. Lett. 21, 62-64 (1996).3.Miguel Rodriguez, Riad Bourayou, Guillaume Méjean, Jérôme Kasparian, Jin Yu, Estelle Salmon,Alexander Scholz, Bringfried Stecklum, Jochen Eislöffel, Uwe Laux, Artie P. Hatzes, RolandSauerbrey, Ludger Wöste, and Jean-Pierre Wolf.“Kilometer-range nonlinear propagation offemtosecond laser pulses,” Phy. Rev. E 69, 036607 (2004).4.S. A. Hosseini, Q. Luo, B. Ferland, W. Liu, N. Akozbek, G. Roy, S.L. Chin, “Effective length of filamentsmeasurement using backscattered fluorescence from nitrogen molecules,” Appl. Phys. B 77, 697-702(2003).5.R. Ackermann, K. Stelmaszcyk, P. Rohwetter, G. Mejean, E. Salmon, J. Yu, J. Kasparian, G. Mechain,V.Bergmann, S. Schaper, B. Weise, T. Kumm, K.Rethmeier, W. Kalkner, L. Wöste, and J. P. Wolf,“Triggering and guiding of megavolt discharges by laser-induced filaments under rain conditions,”Appl.Phys. Lett. 85, 5781-5783 (2004).6. F. Vidal, D. Comtois, C.-Y. Chien, A. Desparois, B. La Fontaine, T. W. Johnston, J.-C. Kieffer, H. P.Mercure, and F. A. Rizk, “Modeling the triggering of streamers in air by ultrashort laser pulses,” IEEETrans. Plasma Sci. 28, 418–433 (2000).7.J. Kasparian, M. Rodriguez, G. Méjean, J. Yu, E. Salmon, H. Wille, R. Bourayou, S. Frey, Y.-B. André,A. Mysyrowicz, R. Sauerbrey, J.-P. Wolf, and L. Wöste, “White-Light Filaments for AtmosphericAnalysis,” Science 301, 61-64 (2003).8.H. Yang, J. Zhang, W. Yu, Y. J. Li, and Z. Y. Wei,“Long plasma channels generated by femtosecondlaser pulses,” Phys. Rev. E 65, 016406(2001).9.X. Lu, Xi Ting Ting, Li Ying-Jun, and Zhang Jie, “Lifetime of the plasma channel produced by ultra-shortand ultra-high power laser pulse in the air,” Acta Physica Sinica 53, 3404-3408 (2004).10.Hui Yang, Jie Zhang, Yingjun Li, Jun Zhang, Yutong Li, Zhenglin Chen, Hao Teng, Zhiyi Wei, andZhengming Sheng, “Characteristics of self-guided laser plasma channels generated by femtosecond laserpulses in air,” Phys. Rev. E 66, 016406(2002).11.X .M .Zhao, Jean-Claude Diels, Cai Yi Wang, and Juan M. Elizondo, “Femtosecond Ultraviolet LaserPulse Induced Lightning Discharges in Gases,” IEEE J. Quantum Electron. 31. 599-612(1995).12.M.A. Biondi, “Recombination,” in Principles of Laser Plasmas, G. Bekefi, ed. pp.125-157 (New York,Wiley, 1976)#68045 - $15.00 USD Received 17 February 2006; revised 9 May 2006; accepted 10 May 2006 (C) 2006 OSA29 May 2006 / Vol. 14, No. 11 / OPTICS EXPRESS 491513. Quanli Dong, Fei Yan, Jie Zhang, Zhan Jin, Hui Yang, Zuoqiang Hao, Zhenglin Chen, Yutong Li, ZhiyiWei, and Zhengming Sheng, “The measurement and analysis of the prolonged lifetime of the plasmachannel formed by short pulse laser in air,” Acta Physica Sinica 54, 3247-3250 (2005).14. Jiansheng Liu, Zuoliang Duan, Zhinan Zeng, Xinhua Xie, Yunpei Deng, Ruxin Li, and Zhizhan Xu,“Time-resolved investigation of low-density plasma channels produced by a kilohertz femtosecond laser inair,” Phys. Rev. E 72, 026412 (2005).The generation of light filaments in air has attracted broad interest [1-4] due to their applications for lightning protection [5-6] and atmospheric remote sensing [7]. The filaments remain stable over tens of meters or more, which is much longer than the beam’s Rayleigh distance [1-3]. This self-guiding effect has been attributed to a dynamic balance between beam self-focusing (owing to the optical Kerr effect) and defocusing (owing to medium ionization). A high degree of ionization as well as a long lifetime of light filaments is preferred in practical application. Recent research on the lifetime of light filaments reported that the lifetime of a light filament could be enhanced by bringing in a second long-pulse laser after a femtosecond laser pulse mainly due to the optical detachment effect [8-10]. The electron density owing to the optical detachment effect maintains itself at about 12313310~10cm cm −− [9]. We hope to further increase the degree of ionization during the total lifetime of a plasma channel.In our experiment, we applied a high voltage electrical field in the plasma channel induced by a femtosecond laser pulse in air. Results show that the lifetime of the plasma channel had been prolonged and also the degree of ionization increased. The lifetime of the plasma channel reaches about 60 ns with a field of about 350kV/m. We investigated the variation of the lifetime of the plasma channel with the increase in electric field. In addition, we brought in a second femtosecond laser pulse and found that the lifetime of the filament can reach 200 ns with a delay of 60 ns between the first and second pulse. Finally, the lifetime of plasma channel was enhanced to 350 ns by using four sequential laser pulses, which proves the feasibility of prolonging the lifetime of plasma by employing multiple laser pulses.The experiments were performed with a 10-Hz chirped-pulse amplification Ti-sapphire laser system. A plasma channel was produced by a 2-mJ, 100-fs chirped laser pulse at 790 nm with a focusing lens of f=50 cm. An electrical field which can be adjusted in a range of 0-350kV/m was applied along the plasma channel. The experimental arrangement is shown in Fig. 1. The configuration of the electrodes here for the high voltage is sharp-point. The distance between two electrodes is about 3 cm. The variation of the electrical signals in the channel indicates the decay of electron density. And Electron decay rate is directly related to the length of plasma’s life. Therefore, we measure the lifetime of the plasma channel by detecting voltage from probe c in the channel.Fig. 1. Experimental setup; Electrodes a, b, and probe c are set close to the path of the plasmachannel induced by femtosecond laser pulse.#68045 - $15.00 USD Received 17 February 2006; revised 9 May 2006; accepted 10 May 2006(C) 2006 OSA 29 May 2006 / Vol. 14, No. 11 / OPTICS EXPRESS 4916We have measured the electrical signals when the fields are 0, 250, and 350kV/m respectively. Meanwhile, through a longitudinal diffraction detection method [14], the initial electron density was estimated at about 17310cm −and the diameter of the plasma channel was about 100m μ. The visible length of the plasma channel was over 4 cm.As shown in Fig. 2(a), the decay time of the electrical signal (defined as the duration lasting from the maximum value to 5% of the maximum value), increased by about 3 folds when the electrical field increased to 350 kV/m (dash-dotted line c). As we expected, the variation of the electrical signals in the channel showed that the lifetime of the plasma channel was prolonged when the electrical field increased. On the other hand, the solid line in Fig. 2(b), resulting from a theoretical model, which will be discussed later based on Eq. (1)-(3), depicts the evolution of electron density in the absence of an electrical field. We calculated that within 20 ns the electron density would be expected to fall to 31410−cm . Here, the initial electron density in our calculation was of the same order magnitude as the measurement in our experiment (17310cm −). Therefore, we expected that within the same 20 ns the electron density in the plasma would remain above 31410−cm . We regard this level as an indication of the lifetime of a plasma channel. In Fig. 2(a), compared to line a, line b and c indicate increased lifetimes of 40 and 60 ns respectively. Our experiment results show that an electrical field added in the plasma channel can affect the characteristics of the plasma and prolong the lifetime of the plasma channel.#68045 - $15.00 USD Received 17 February 2006; revised 9 May 2006; accepted 10 May 2006(C) 2006 OSA 29 May 2006 / Vol. 14, No. 11 / OPTICS EXPRESS 4917Fig. 2. (a) Measured electrical signals (solid line a, dashed line b, and dash-dotted line ccorrespond to electrical fields of 0V/m, 250kV/m, 350kV/m respectively); (b) Theoreticalcalculation with initial condition that 173210e n cm −=×.In order to further extend the lifetime of the plasma channel, we added a second femtosecond laser pulse with the external electrical field still in place. The delay between the two laser pulses was adjusted and the corresponding lifetime of the plasma channel is measured as shown in Fig. 3 and Fig. 4. As we can see in Fig. 3, the lifetime is prolonged to about 150 ns when the delay between two pulses is 40 ns. With a delay of 60 ns, the lifetime increases to 200 ns. As shown in Fig. 4, further increase in delay (100 ns) no longer leads to further extension of the lifetime. This is because the distance between the two laser pulses is so long that the interaction between them is less pronounced than in situations with shorter delay time.#68045 - $15.00 USD Received 17 February 2006; revised 9 May 2006; accepted 10 May 2006(C) 2006 OSA 29 May 2006 / Vol. 14, No. 11 / OPTICS EXPRESS 4918A multi-pulse scheme is employed here to reach a longer lifetime. In our experiment, we added three more laser pulses to the original laser pulse with a delay between two consecutive pulses at about 70 ns. This was done to obtain an optimal effect on the lifetime. These multiple laser pulses were generated by passing a main laser pulse through beam splitters and setting long-range fixed delays. The electrical field remained at about 350kV/m. The energy of the original pulse was 0.4 mJ and those of the later three laser pulses are all about 0.1 mJ ±0.1 mJ due to long-range propagation. The measured electrical signal is shown in Fig. 5 with a total lifetime of about 350 ns. As we can see, the signal caused by subsequent pulses is not as intense as in the double-pulse experiments conducted. This is due to the relatively low energy of later pulses. According to our double-pulse experimental results, we can expect that with relatively high energy of each later pulse at about 0.4 mJ, the lifetime of the plasma channel can be increased longer than what we acquired in Fig. 5. Therefore, we can conclude that a multi-pulse scheme with an electrical field added is efficacious for the extension of the lifetime of the plasma channel.-0.010.000.010.020.030.040.050.06e l e c t r i c a l s i g n a l (a .u .)t(ns)Fig. 3. Electrical signals in double-pulse scheme. The energies of two pulses with the delay of20 ns are 0.5 mJ and 0.4 mJ respectively. The energies of two pulses with the delay of 40 ns arealso 0.5 mJ and 0.4 mJ respectively. #68045 - $15.00 USD Received 17 February 2006; revised 9 May 2006; accepted 10 May 2006(C) 2006 OSA 29 May 2006 / Vol. 14, No. 11 / OPTICS EXPRESS 4919Fig. 4. Electrical signals in double-pulse scheme. The energies of two pulses with the delay of60 ns are 0.5 mJ respectively. The energies of two pulses with the delay of 100 ns are 0.3 mJrespectively.Fig. 5. Electrical signal in four-pulse scheme. The energy of the first pulse is 0.4 mJ, and theenergies of later pulses are all about 0.1 mJ. The delay between two contiguous pulses is 70 ns.The main mechanisms involved in the decay process of the plasma channel in a highelectrical field include the photo-ionization, impact ionization, dissociative attachments of electrons to oxygen molecules, charged particle recombination, detachments of electrons byion-ion collision, and electron diffusion. Among these effects, the attachment of electrons to oxygen molecules is detrimental to the lifetime of the plasma channel. The effect of#68045 - $15.00 USD Received 17 February 2006; revised 9 May 2006; accepted 10 May 2006 (C) 2006 OSA29 May 2006 / Vol. 14, No. 11 / OPTICS EXPRESS 4920detachments of electrons caused by ion-ion collision is relatively weak compared with the others and thus is omitted in our analysis. And the electron diffusion is a slow process, on the time scale of tens of s μ[11]. And electron generation and plasma formation are on the time scale of ns to s μ. At this time scale, effects from electron diffusion can be neglected. Therefore, we can estimate the lifetime of the plasma channel following the equation of continuity as follows [10，11] p e ep e e e n n n n tn βηα−−=∂∂ (1) p n np p e ep e pn n n n n tn ββα−−=∂∂ (2) p n np e n n n n tn βη−=∂∂ (3) where e n , p n , n n are electron density, positive ion density, and negative ion density in air respectively. α is the impact ionization coefficient. ηis the attachment rate. Initial conditions for theoretical analysis is that 173210e n cm −=×, 173210p n cm −=×, 0n n =.Through our simulation, αand ηin different electric fields did not exert a noticeable effect on the lifetime of a plasma channel. Therefore, we expect that ep βand np βmay play a role in extending the lifetime when an external electrical field is added.Without considering the effect of external electric field, a general expression of electron-ion recombination coefficient ep βas a function of electron temperature Te is [11, 12]:3120.39123110.702212(/) 2.03510,()(/) 1.13810,()0.790.21ep m s Te e N m s Te e O βββββ−−−+−−−+=×−=×−=+ (4)We take np ep ββ= in our calculation since the ion-ion recombination coefficient np β is of the same order of magnitude as the electron-ion recombination coefficient ep β.The theoretical simulation of the lifetime of the plasma channel is shown in Fig. 6. As line a, b and c shown, the lifetime of the plasma channel is prolonged from 20 ns to 60 ns as the dissociative recombination coefficient ep βand np β decrease.Potential energy curves play a role in dissociative recombination. In a favorable potential curve crossing case, a sharper falloff in this coefficient than 0.39Te −and 0.70Te −will occur with increasing incident electron energy [12]. When the external electrical field is added along the plasma channel, the incident energy of electrons will be increased. Meanwhile, Te can be assumed to thermalize at the same ambient air temperature as the gas molecules [11]. Because potential energy curves will change due to the external electrical field, we expect that a favorable potential curve crossing may exist in this case. And this can lead to a quicker falloff in ep βand np β, and corresponding extension in the lifetime as electron energy increases, as we can see from the comparison of line a, b and c shown in Fig. 6.#68045 - $15.00 USDReceived 17 February 2006; revised 9 May 2006; accepted 10 May 2006(C) 2006 OSA 29 May 2006 / Vol. 14, No. 11 / OPTICS EXPRESS 4921Fig. 6. Theoretical simulation with 417.410s α−=× and 712.510s η−=× [11]; Solid line a,dashed line b and dash-dotted line c correspond to different dissociative recombinationrates 1332.210/m s −×, 1330.810/m s −× and 1330.310/m s −× respectively.Similarly, in double-pulse and multi-pulse case, the dissociative recombination rate can decline more intensively than the case without an external electrical field and this will thus lead to an extension of the lifetime of the plasma channel. Moreover, the addition of the second and later pulses will again cause a large number of electrons due to photo-ionization [13]. With these extra electrons, the lifetime of the plasma channel will further extend.As a conclusion, characteristics of the lifetime of the plasma channel are investigated by adding an external electrical field and also extra laser pulses. The lifetime increases by 3 folds when the external electrical field is about 350kV/m in our experiment. We expect that a favorable crossing case may exist when an external electrical field is in place, and this can lead to a corresponding growth in the lifetime of the plasma channel. In addition, the lifetime of plasma channel is greatly enhanced to 350 ns by using four sequential intense 100fs (FWHM) laser pulses with the external electrical field (350kV/m). Therefore, we conclude that a multi-pulse scheme with an external electrical field added is feasible for greatly prolonging the lifetime of a plasma channel. This research is supported by a Major Basic Research project of the Shanghai Commission of Science and Technology, the Chinese Academy of Sciences, the Chinese Ministry of Science and Technology, and the Natural Science Foundation of China. #68045 - $15.00 USD Received 17 February 2006; revised 9 May 2006; accepted 10 May 2006(C) 2006 OSA 29 May 2006 / Vol. 14, No. 11 / OPTICS EXPRESS 4922。

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