Long lifetime plasma channel in air generated by multiple femtosecond laser pulses and an external electrical field

The lifetime of a plasma channel produced by self-guiding intense femtosecond laser pulses in air is largely prolonged by adding a high voltage electrical field in the plasma and by introducing a series of femtosecond laser pulses. An optimal lifetime value is realized through adjusting the delay among these laser pulses. The lifetime of a plasma channel is greatly enhanced to 350 ns by using four sequential intense 100fs( FWHM) laser pulses with an external electrical field of about 350kV/m, which proves the feasibility of prolonging the lifetime of plasma by adding an external electrical field and employing multiple laser pulses. (c) 2006 Optical Society of America.

Alignment of nanoparticles formed on the surface of 6H-SiC crystals irradiated by two collinear femtosecond laser beams

Nanoripples with periods of 150 and 80 nm are formed on the surface of 6H-SiC crystals irradiated by the p-polarized 800 nm and the s-polarized 400 nm femtosecond lasers, respectively. When both of the two collinear laser beams focus simultaneously on the sample surface, nanoparticles are formed on the whole ablation area, and they array in parallel lines. We propose and confirm that the second harmonics in the sample surface excited by the incident lasers plays an important role in the formation of nanostructures.

Mechanisms of femtosecond laser-induced breakdown and damage in MgO

Single-shot laser damage threshold of MgO for 40-986 fs, 800 nm laser pulses is reported. The pump-probe measurements with femtosecond pulses were carried out to investigate the time-resolved electronic excitation processes. A theoretical model including conduction band electrons (CBE) production and laser energy deposition was applied to discuss the roles of multiphoton ionization (MPI) and avalanche ionization in femtosecond laser-induced dielectric breakdown. The results indicate that avalanche ionization plays the dominant role in the femtosecond laser-induced breakdown in MgO near the damage threshold. (c) 2005 Elsevier B.V. All rights reserved.

超短脉冲照射下氟化锂的烧蚀机理及其超快动力学研究

研究了超短脉冲激光照射下LiF晶体的破坏机理及其超快动力学过程，利用扫描电镜和原子力显微镜等测试手段，观测了飞秒激光照射下LiF晶体的烧蚀形貌。利用烧蚀面积与激光脉冲能量的对数关系确定了LiF晶体的破坏阈值，并利用非线性玻璃棒展宽脉宽，得到了800nm激光作用下LiF破坏阈值对激光脉宽（50～1000fs）的依赖关系；利用抽运一探针超快探测平台，探测了LiF烧蚀过程中反射率的变化。采用雪崩击穿模型，并根据晶体材料反射率与材料的介电常量的依赖关系，通过数值计算，模拟了材料烧蚀阈值与脉宽的依赖关系及材料激发过

飞秒双脉冲激光在硅和石英中的光学破坏

The optical breakdown thresholds (OBTs) of typical dielectric and semiconductor materials are measured using double 40-fs laser pulses. By measuring the OBTs with different laser energy and different time delays between the two pulses, we found that the total energy of breakdown decrease for silica and increase for silicon with the increase of the first pulse energy. (C) 2005 Optical Society of America.

飞秒激光作用下全向高反膜破坏的激发过程

设计和制备了全向高反膜SiO2／TiO2，研究了它在不同脉冲宽度、不同脉冲能量的飞秒激光作用下的破坏阈值和烧蚀深度．利用发展的抽运．探针方法，研究了抽运脉冲作用下材料中导带电子的超快激发和能量沉积过程，建立并求解了飞秒激光激发材料和材料的激发对抽运光自身反作用的耦合动力学模型．模型较好地揭示了材料破坏的激发过程．

相对论飞秒激光脉冲在真空中对预加速电子的加速

通过求解电子运动的相对论方程，发现预加速电子在超强超短激光脉冲的作用下可以获得很高的能量增益．飞秒激光脉冲的上升沿在焦点附近的区域有效加速电子后，电子和光脉冲一起传播一段距离（远大于瑞利长度）后，激光强度变得很弱，从而使脉冲下降沿对电子的减速作用可以忽略不计，因此电子只经历加速过程而没有被减速，当电子和光脉冲分离时，电子获得了很高的能量增益．当光强为10^19W／cm^2，电子的初始能量为MeV量级时，电子的能量增益可以达到0．1GeV．进一步讨论了电子的能量增益与电子的初始条件与激光脉冲的参数之间的关系

飞秒激光脉冲在正色散固体材料中的自压缩

实验研究了正色散固体介质中的激光脉冲自压缩现象，证明了无需任何外加色散补偿情况下，固体透明介质中的自聚焦传输过程可使高功率飞秒激光脉冲实现时域脉冲压缩，并详细研究了输出脉冲的时域和频域特性随入射脉冲强度的演化规律．实验结果表明脉冲自压缩量随入射脉冲强度的增加呈递增趋势，然而当入射光强增大到足以引起超连续谱及锥形辐射产生时，脉冲时域形状会发生分裂．此外还发现发散光束入射情况下同样可以观察到脉冲自压缩现象．

高强度飞秒激光脉冲在空气中的自压缩

利用高重复频率(1kHz)、吉瓦级飞秒激光脉冲实验验证了高强度飞秒脉冲在空气中的自压缩现象，研究了入射脉冲在不同初始啁啾情况下经空气中聚焦成丝后，时域及频域特性随入射脉冲能量的变化规律．实验结果表明，在无需后继色散补偿情况下，高强度飞秒脉冲仅通过在空气中的非线性传输过程就可以实现脉冲压缩；在入射脉冲为负啁啾情况下，实验观察到脉冲光谱及时域宽度同时得到压缩，并可获得比激光源所能提供的更短的近双曲正割型变换限脉冲．

不同偏振飞秒激光经块状材料传输后的脉宽压缩

实验研究了线偏振和圆偏振状态下的飞秒强激光脉冲在块状材料中的传输过程。不同偏振的激光脉冲在传输过程中得到了不同程度的光谱展宽，经色散补偿后，脉冲时域宽度均得到了压缩。详细分析了压缩脉冲的脉宽以及啁啾情况与入射激光脉冲能量之间的关系，比较了飞秒激光在线偏振及圆偏振情况下的不同压缩效果。在线偏振入射光情况下得到了最短21fs的压缩脉冲宽度，在圆偏振情况下得到的最短脉冲宽度为22fs。实验结果表明，这种光谱展宽与色散补偿方式对圆偏振光同样适用，而且圆偏振的入射激光将更有利于对更高能量的脉冲进行压缩。在色散补偿量

飞秒激光对氟化镁烧蚀机理研究

用扫描电镜（SEM）研究了氟化镁在800nm超短脉冲激光作用下的单枪表面烧蚀形貌．根据烧蚀斑面积与激光脉冲能量间的对数关系，测得烧蚀阈值与激光脉宽的关系曲线（55—750fs）．计算了导带电子的双光子吸收，改进了多速率方程模型．很好地解释了实验结果．

飞秒激光作用下透明介质材料的反射率演化

利用800nm抽运和400nm探针技术，测量了CaF2和MgO的时间分辨反射率，研究了材料的电子激发和弛豫超快动力学过程。采用耦合动力学模型，探讨了飞秒激光对透明介质材料的激发，以及材料的激发对抽运激光在材料中的传输、分布和反射特性的影响。根据这个理论模型计算了时间分辨反射率的演化，计算结果和实验结果相吻合。研究表明，多光子电离（MPI）和碰撞电离（II）在介质材料的导带电子激发中起着重要的作用。

负啁啾高强度飞秒脉冲在正常色散材料中传输特性研究

利用块状正常色散材料作非线性介质，实验研究了负啁啾强激光飞秒脉冲在正常色散材料的传输过程中，脉冲波形及频谱随入射脉冲能量的变化特性．实验发现，负啁啾脉冲经过材料后激光光谱在被整形和压缩的同时，脉冲宽度也被压缩．实验获得的压缩脉冲宽度短于相等光谱宽度下的双曲正割转换极限脉冲宽度。数值模拟结果显示，在实验条件下模拟结果具有同样的变化特性．

Neutron source from thin foil confined by two laser pulses

Neutron production from a thin deuterium-tritium (D-T) foil irradiated by two intense femtosecond laser pulses from opposite sides with zero phase difference is studied analytically and numerically. For the interaction of a laser pulse of amplitude a = 7, focal area 100 mu m(2) and areal density 4.4 x 10(18) cm(-2) with a D-T plasma foil, about 1.17 x 10(21) neutron s(-1) can be obtained, much more than from other methods. The profiles of the ion and electron densities are also calculated.

利用块状介质进行飞秒强激光脉冲的腔外压缩

高强度飞秒激光脉冲的腔外压缩是获得高次谐波阿秒脉冲驱动源的必要手段。实验研究了超强超短飞秒激光脉冲在经过块状介质后的光谱展宽和色散补偿压缩现象。单脉冲能量0．26mJ，脉宽50fs的激光脉冲经透镜在空气中聚焦后再入射到块状材料上，出射脉冲光谱被展宽到接近40nm。由于在块状材料中的自聚焦效应，出射光束质量变好并保持较小的空间啁啾。利用熔融石英棱镜对补偿带有正色散的出射脉冲，最后得到〉0．1mJ，19fs的压缩脉冲。利用SPIDER装置测量了出射脉冲的脉宽和光谱相位。整个系统的能量效率大约为35％，压缩后的