超短脉冲激光与微小水滴相互作用中电子密度和光场的时空分布

为了研究超短激光脉冲和液滴相互作用过程中电子密度和光场的变化,基于非线性麦克斯韦方程组和电离速率方程,构建了激光等离子体非线性瞬态时域耦合模型,对飞秒激光脉冲击穿微米量级水滴时的电子密度和光场的时空分布进行了计算.结果显示水滴的击穿阈值最小可达2 TW/cm~2,为同等条件下无边界水介质击穿阈值的1/4.随着脉冲能量增强,水滴内自由电子密度峰值区域逆着激光入射方向移动,且入射光越强,水滴对光传播的屏蔽越明显.光束在水滴出射端外部汇聚,汇聚点的光功率密度可达入射光的5倍,且时域波形出现压缩和变形.另外,水滴对激光能量的吸收系数随光强增大而增大,并最终趋于饱和.     

球差对圆偏涡旋贝塞耳-高斯光束聚焦场的影响

利用理查德-沃耳夫矢量衍射积分,分析了初级球差对圆偏振涡旋贝塞耳-高斯光束聚焦光场的影响,获得不同球差系数下的光强分布.结果表明,当初级球差系数增加,会聚光点偏离系统焦平面,纵向光场发生改变;焦点光强逐渐减小,焦平面光场发生变化;光轴上光强峰值逐渐减小,峰值两侧的旁瓣变化趋势相反,光强由对称分布变成非对称分布;初级球差对贝塞耳-高斯光束聚焦光场的影响不依赖其偏振态;离焦无法完全补偿初级球差对聚焦光场的影响.     

光场纵向调控研究进展(特邀)

得益于特殊的空间结构,振幅、相位、偏振态面内调控的光场在传输及与物质相互作用过程中展现了诸多引人瞩目的新效应和新现象.随着空间结构光场的深入研究和广泛应用,沿着传输方向的光场调控也逐渐引起注意,由此发展出了一系列具有新颖传播特性,以及空间三维结构的调控光场.本文综述了针对传输方向的光场调控研究进展,包括光场强度沿纵向的...     

纳秒脉冲激光诱导水等离子体屏蔽的实验研究

基于激光阴影法测量原理,建立了一个记录纳秒脉冲激光诱导等离子体屏蔽现象的实验光路系统,探索了一种等离子体屏蔽图像的记录方法,对纳秒脉冲激光诱导水击穿形成等离子体的现象进行了研究.记录了在不同能量状态下出现的激光诱导等离子体屏蔽图像,发现了激光诱导水介质的等离子体屏蔽效应随作用光能量呈增强的趋势,首次观测到了在纳秒脉冲激光作用下液体中出现的线型击穿现象.本文研究结果可为水下激光加工研究、医疗以及激光在液体中的传播特性研究提供实验依据.     

纳秒脉冲激光诱导水等离子体屏蔽的实验研究

基于激光阴影法测量原理,建立了一个记录纳秒脉冲激光诱导等离子体屏蔽现象的实验光路系统,探索了一种等离子体屏蔽图像的记录方法,对纳秒脉冲激光诱导水击穿形成等离子体的现象进行了研究.记录了在不同能量状态下出现的激光诱导等离子体屏蔽图像,发现了激光诱导水介质的等离子体屏蔽效应随作用光能量呈增强的趋势,首次观测到了在纳秒脉冲激光作用下液体中出现的线型击穿现象.本文研究结果可为水下激光加工研究、医疗以及激光在液体中的传播特性研究提供实验依据.     

激光剑丝追未来

飞秒激光成丝现象已经开创了从纯物理到应用研究的许多新的科学研究领域,其中包括源于量子和经典物理的多个应用研究。飞秒光丝中的新现象,例如激光强度钳制效应、空气激光、粒子数捕获、分子排列、分子超激发态、光丝诱导降雪、光丝化学等等,开辟了令人振奋的、多学科交叉新领域。激光强度钳制效应是一个复杂的物理现象,结果可以形成一个非常稳定的相互作用状态。粒子数束缚、分子排列、转动及其周期性可被用于脉冲太赫兹辐射的远程探测。光丝中的空气激光现象看起来像是一种新的激发机制,例如,光丝中激光强度可以把氮气和二氧化碳分子激发到一种粒子数反转的状态。与此同时,这种光丝诱导激光现象(或粒子数反转)在空气中"污染分子"形成的分子碎片中也被观察到。利用单一激光,通过探测特征荧光来远程识别污染物、气体或者固体样品已经至少在原理上得到证实。光丝诱导沉积现象已经在云室中变成现实。最后讨论了光丝研究领域未来的挑战以及光丝化学为光丝研究提供的一个新的机遇。     

激光空泡在近自由液面运动特性的实验研究

采用激光技术聚焦击穿液体产生空泡,利用高速摄像系统开展激光空泡与自由液面的相互作用的实验研究。实验研究发现,空泡与自由液面之间的无量纲距离对水下空泡的脉动特性和自由液面的水冢现象存在影响。通过大量实验总结了无量纲距离与空泡半径、空泡脉动周期、自由液面水柱的最大高度和产生水冢时间的相互关系。实验表明无量纲距离越小,空泡脉动周期越短,自由面的水冢现象越明显。同时统计出了在不同无量纲距离范围内所出现的5种不同水冢现象的规律。研究内容为空泡与自由液面相互作用的理论提供了实验依据。     

激光中的烟圈

正在许多高强度激光脉冲周围发现一种呈圆环状的新的光学涡旋,但人们之前从未注意到。被称为时空光学涡旋(STOVs)的现象是一类新的光学现象。在一个典型的光学涡旋中,光波包围着一条穿过光束中心的暗线(或者空洞)环绕前行。光场对于光学涡旋的环绕与光场的振荡方向(偏振)无关,而是由位相决定。例如,光场     

基于飞秒激光等离子体丝诱导击穿光谱探测土壤重金属Cr元素含量

本文基于飞秒激光等离子体丝诱导击穿光谱对土壤重金属Cr元素含量进行了实验研究.利用荧光法对等离子体丝的长度进行测量,给出了在不同焦距聚焦透镜作用下土壤中Cr425.5 nm的谱线强度空间分布,实验给出了Cr元素的定标曲线.实验结果表明,土壤中Cr元素浓度分析测量的相对标准偏差小于5%,土壤中重金属Cr元素的检测极限为7.85 ppm.表明飞秒激光等离子体丝诱导击穿光谱技术在土壤重金属Cr元素含量的定量探测方面是完全可行的.     

飞秒平顶光束经微透镜阵列在熔融石英中的成丝及其超连续辐射

实验研究了平顶激光光束经微透镜阵列在熔融石英中成丝的演化以及超连续辐射的产生,并进一步与高斯光束的成丝和超连续辐射进行了对比研究.分别对这两种光束的多丝传输进行了横向和纵向成像.结果表明,使用平顶光束可以获得更为均匀的多丝分布,成丝的起点也更为一致;尤其重要的是,相对于高斯光束,平顶光束可以使用更高的入射激光脉冲能量而不会造成介质的损伤,从而可以获得更高脉冲能量和更高转换效率的超连续辐射.     

1kHz飞秒激光脉冲在空气中传输成丝的演化过程

利用高重复频率(１kHz)、低能量的飞秒激光脉冲研究激光脉冲在空气中传输成丝的演化过程.采用成像的方法观测飞秒激光脉冲在大气传输过程中光束截面上光强分布变化，以及大气等离子体通道对光强分布的反作用.实验结果表明，激光脉冲在传输过程中先形成双丝结构，再逐渐合并成单丝结构.实验结果与利用非线性薛定谔方程耦合多光子电离的数值模拟结果基本一致. 关键词： 飞秒激光脉冲 等离子体通道 成像     

Experimental tests of the new paradigm for laser filamentation in gases

Since their discovery in the mid-1990s, ultrafast laser filaments in gases have been described as products of a dynamic balance between Kerr self-focusing and defocusing by free electric charges that are generated via multiphoton ionization on the beam axis. This established paradigm has been recently challenged by a suggestion that the Kerr effect saturates and even changes sign at high intensity of light and that this sign reversal, not free-charge defocusing, is the dominant mechanism responsible for the extended propagation of laser filaments. We report qualitative tests of the new theory based on electrical and optical measurements of plasma density in femtosecond laser filaments. Our results consistently support the established paradigm.     

Controlling a bunch of multiple filaments by means of a beam diameter

We demonstrate a three orders of magnitude increase and stability in the backscattered fluorescence signal from nitrogen molecules by terawatt femtosecond laser pulse induced air filaments using a new method. The method is based on squeezing the initial beam diameter using a telescope. The effect of laser shot-to-shot fluctuations was included in numerical simulations by a random distribution of the initial intensity in both squeezed and non-squeezed beams. Statistical processing of the simulation results shows that the average diameter of plasma channels as well as the total amount of free electrons generated in a bunch of multiple filaments in air is larger in the squeezed beam. Shot-to-shot stability of the simulated plasma density increases in the squeezed beam. The change of this plasma density with propagation distance is in good qualitative agreement with the change of the range-corrected nitrogen fluorescence signal with distance. PACS 42.65.Jx; 42.60.Jf; 42.68.Ay; 42.68.Wt     

Performance comparison of self-focusing with 1053- and 351-nm laser pulses

Hole boring characteristics of laser beams are studied using two different laser wavelengths in preformed plasmas with overdense regions. We have shown that a whole beam self-focusing is created in plasma with a considerable density scale length using a 1 microm wavelength laser. The whole beam self-focusing of this type could be used for guiding the ultrahigh intense laser pulse to a highly compressed core for studying the feasibility of a fast ignitor. There is a clear difference in the hole-boring characteristics between two laser wavelengths at 1053 and 351 nm, both in the experiment and the simulation. Using the third-harmonic laser, a whole beam self-focusing is never created. The 351-nm laser beam broke up into filaments resulting in plasma jets observed in our interferogram.     

光折变晶体LiNbO_3:Fe中的特殊散射现象

用柱面透镜把经针孔滤波器扩束的激光束沿某一个方向聚焦成细丝状(或长条状),细丝状光束垂直入射到LiNbO_3:Fe晶体上,在远场位置观察散射光.我们发现光散射的方向与晶体的c轴方向不一致,不仅沿着晶体的c轴方向有散射光出现,沿光束的条形方向也出现很强的散射光.     

Interaction of light filaments in air

This paper analytically investigates the interaction of light filaments generated by a femtosecond laser beam in air. It obtains the Hamiltonian of a total laser field and interaction force between two filaments with different phase shifts and crossing angles. The property of the interaction force, which leads the attraction or repulsion of filaments, is basically dependent on the phase shift between filaments. The crossing angle between two filaments can only determine the magnitude of the interaction force, but does not change the property of the force.     

铷原子热蒸气中强非线性效应产生激光模式图样的实验研究

研究了具有高斯横向分布的连续激光束单次通过铷原子蒸气后,在近共振附近铷原子蒸气中,由强的非线性克尔效应导致激光分裂成细丝的现象,并且这些细丝的衍射图样在远场通过相干叠加,可以形成具有规则结构的斑图模式.实验上研究了输入光功率,铷泡温度和抽运激光频率相对于⁸⁵Rb原子D₂线的不同失谐位置等因素对斑图模式的影响.由于铷原子的超精细能级结构,在铷原子蒸气中同时存在与三阶非线性效应相关的四波混频现象,通过扫描探测光的频率同时观察到具有斯托克斯和反斯托克斯光子的拉曼增益现象. 关键词： 铷原子蒸气 克尔效应 自聚焦 斑图     

The comparison study of diagnostics of light filaments in air

Long plasma channels in air induced by femtosecond laser pulses are investigated using three different methods, including the cross-section imaging, resistivity measuring and acoustic diagnostics. These methods are based on different properties of the light filaments. A comparison of the three diagnostics shows that the imaging method is the most precise one in studying the filaments distribution and evolution, that the sonographic method is the most convenient approach to detecting long plasma channels by detecting the acoustic wave generation, and that the resistivity measurement can only be applied for giving a rough estimate. The diagnostics of filaments allow us to choose appropriate detecting methods and provide further insight into the dynamic evolution of the light filaments in air.     

Laser beam filamentation in fractal aggregates

We investigate filamentation of a cw laser beam in soft matter such as colloidal suspensions and fractal gels. The process, driven by electrostriction, is strongly affected by material properties, which are taken into account via the static structure factor, and have impact on the statistics of the light filaments.     

fs激光在靶背表面产生的质子束成丝

 介绍了利用3TW/60fs钛宝石超短超强激光与20μm铜薄膜靶相互作用的实验。实验观测到质子束的角分布随激光功率密度有所变化。在较高的功率密度（～1×10¹⁸ W/cm²）时，观测到环状的质子束分布，发散角较大。在较低的激光功率密度（～2×10¹⁷ W/cm²）时，质子束发散角减小，质子束出现成丝现象。质子束的角分布实际上反映了从靶前输运到靶背的超热电子电流横向分布。在输运过程中，由于Weibel不稳定性会使超热电子电流出现空心化并最后破裂成丝。