Determination of the temporal structure of femtosecond laser pulses by means of laser-induced air plasma

A new approach is presented to reveal the temporal structure of femtosecond laser pulses by recording the correspond- ing time-resolved shadowgraphs of the laser-induced air plasma. It is shown that the temporal structures of femtosecond laser pulses, normally not observable by the ordinary intensity autocorrelator, can be detected through intuitively analyz- ing the ultrafast evolution process of the air plasma induced by the femtosecond laser pulses under examination. With this method, existence of pre- and post-pulses has been clearly unveiled within the time window of 4-150 fs in reference with the main 50-fs laser pulses output from a commercial 1-kHz femtosecond laser amplifier. The unique advantage of the proposed method is that it can directly provide valuable information about the pulse temporal structures＇ effect on the laser-induced ionization or material ablation.

Propagation of Intense Femtosecond Laser Pulses in Deuterated Methane and Deuterium Cluster Jets

Propagation of intense femtosecond laser pulses （60 fs, 800 nm, 120 m J, 6 × 10^17 W/cm^2 in vacuum） in supersonic （CD4）N and （D2）N cluster jets at different backing pressures was studied. Pump-probe interferometry is employed to investigate the propagation of laser beams in dense cluster jets by examining the electron density distribution of plasma chan- nels. It was found that propagation effects, including ionization-induced defocusing and laser attenuation of incident pulses, are very different in the （CD4）N and （D2）N cluster jets. Different ionization states of CD4 and D2 molecules were observed by analyzing the transverse electron density profiles of the plasma channels and should be considered as a major reason for the differences in the propagation effects. Numerical simulations of the nonlinear propagation of femtosecond laser pulses in （CD4）N and （D2）N cluster jets were performed, and the results indicated a good reproduction of the experimental data.

Competition between multiphoton/tunnel ionization and filamentation induced by powerful femtosecond laser pulses in air

In this work we present experiments by focusing 42 femtosecond laser pulses in air using three differentfocal length lenses: f=100, 30 and 5 cm. For the longest focal length, only the filament, which is aweak plasma column,is observed. When the shorter focal length lens is used, a high density plasma isgenerated near the geometrical focus and coexists with a weak plasma channel of the filamemt. Under thetightest focusing condition, filamentation is prevented and only a strong plasma volume appears at tehgeometrical focus.

Characteristics of filamentation in ZK7 glass by negatively chirped femtosecond laser pulses

The filamentation characteristics of femtosecond laser pulses in ZK7 glass are in- vestigated experimentally as a function of initial negative chirps. It is found that the filament threshold power grows rapidly and the filament length extends over a long distance with increasing initial temporal chirps. The measurement of supercon- tinuum reveals that the plasma generation process within filamentation becomes weaker as the initial negative chirp increases, leading to a self-guiding long light channel dominated by Kerr nonlinearity. The interference of transverse rings in multifilamentation of the chirped laser pulses is observed as well. Analyses and discussions give an interpretation of this chirp-induced ionization-free filamenta- tion. These results indicate that initial chirps will play a crucial role in the filament formation of ultrashort laser pulses in transparent media.

The dependence on optical energy of terahertz emission from air plasma induced by two-color femtosecond laser-pulses

The generation of terahertz （THz） emission from air plasma induced by two-color femtosecond laser pulses is studied on the basis of a transient photocurrent model. While the gas is ionized by the two-color femtosecond laser-pulses com- posed of the fundamental and its second harmonic, a non-vanishing directional photoelectron current emerges, radiating a THz electromagnetic pulse. The gas ionization processes at three different laser-pulse energies are simulated, and the corresponding THz waveforms and spectra are plotted. The results demonstrate that, by keeping the laser-pulse width and the relative phase between two pulses invariant when the laser energy is at a moderate value, the emitted THz fields are significantly enhanced with a near-linear dependence on the optical energy.

Spatiotemporal control of femtosecond laser filament-triggered discharge and its application in diagnosing gas flow fields

Precise control of the discharge in space and time is of great significance for better applications of discharge plasma.Here,we used a femtosecond laser filament to trigger and guide a highvoltage DC pulse discharge to achieve spatiotemporal control of the discharge plasma.In space,the discharge plasma is distributed strictly along the channel generated by the femtosecond laser filament.The breakdown voltage threshold is reduced,and the discharge length is extended.In time,the electrical parameters such as the electrode voltage and the electrode gap affect discharge delay time and jitter.By optimizing the parameters,we can achieve sub-nanosecond jitter of the discharge.Based on the spatiotemporal control of the discharge,we applied filamenttriggered discharge for one-dimensional composition measurements of the gas flow field.Besides,the technique shows great potential in studying the spatiotemporal evolution of discharge plasma.

Enhancement of third harmonic generation in air filamentation using obstacles

The intensity of third harmonic emission in air filamentation disturbed by copper fibers and alcohol droplets has been investigated experimentally. Enhancement of the third harmonic emission up to more than one order of magnitude has been observed. The physical mechanism of third harmonic enhancement is attributed to suppression of the destructive interference by comparison of the experimental results and it is closely related to the type, size, and relative position of the obstacles.

飞秒脉冲激光冲击强化中等离子体压力时空演化规律

为研究飞秒脉冲激光冲击强化中等离子体压力时空演化规律,利用考虑电子态密度(DOS)效应的模型计算了电子热容和电声耦合系数随电子温度的演化规律,并与采用QEOS(quotidian equation of state)模型计算结果进行了对比;提出DOS飞秒脉冲激光冲击强化模型,计算得到电子温度、晶格温度、等离子体羽位置时间演化规律和等离子体压力时空演化规律,并与QEOS飞秒脉冲激光冲击强化模型结果进行了对比。结果表明:DOS飞秒脉冲激光冲击强化模型计算得到的等离子体羽位置随时间的演化规律与实验结果吻合程度更好;增加激光能量或功率密度、考虑电子DOS效应会增加电子、晶格温度和等离子体压力。

飞秒光丝中等离子体密度时间演化特征

采用能够较为清晰、完整描述强飞秒激光等离子体通道内带电粒子产生过程及其演化的物理模型,进一步研究了飞秒光丝中等离子体密度的时间演化特征。计算结果表明:对于不同时间线型的脉冲,在等离子体通道形成过程中,氧气分子的电离贡献率及氮气分子的贡献率明显不同,不同线型的脉冲对高效维持高密度等离子体的寿命具有较大的影响。有效控制成丝脉冲线型能够达到对等离子体通道的高效利用。长脉冲、短波长虽能够获得较高密度等离子体通道,但其存活寿命却完全受限于通道的后期演化。

飞秒激光成丝诱导Cu等离子体的温度和电子密度

研究了飞秒激光成丝诱导铜击穿光谱,利用光发射光谱对产生的铜等离子体光谱强度沿着丝长度进行了测量,获得了在不同样品与聚焦透镜间距离的Cu(I)的强度分布.结果显示,由于强度钳箍效应成丝诱导的光谱在较大的透镜样品间距离范围内有较强的辐射强度.另外,利用玻尔兹曼图和斯塔克展宽计算了整个成丝繁衍距离中Cu等离子体温度和电子密度.

飞秒超强激光在空气中光丝现象的研究

综述了飞秒超强激光脉冲在大气中传输机理的研究进展和现状 ,阐述了光丝现象的形成原理及其传输机制 ,并对其实验方法及其在实际中的应用做了详细分析 。

飞秒激光等离子体通道传导能量特性的研究进展

基于强飞秒激光诱导等离子体通道的基本性质,分析了等离子体通道在传导电能和射频电磁能时的一些基本问题。综述了飞秒激光等离子体通道传输电能和电磁能方面的新概念和新技术,重点描述了该领域近几年所取得的一些理论及应用的最新成果。最后,展望了飞秒激光等离子体通道在传导能量技术领域中的发展趋势。

野外环境下太瓦飞秒激光等离子体通道特性研究

对太瓦(TW)飞秒激光在自然大气中传输时产生的超长等离子体通道的物理性质进行了研究。试验结果证实2 TW飞秒激光在大气中自由传输时实现了2km长的等离子体通道,长距离传输后通道内的等离子体电子密度约为10^11 cm^-3,仍然保持良好的导电性。高压放电试验也证实了有等离子体通道存在,可以将放电电压降低30%,说明激光诱导高压放电的有效性。本次试验研究表明长度为公里量级、长寿命大气等离子体通道的实现是可行的,将为激光引雷、大气监测、人工干预气候等实用化应用扫清技术上的障碍。

单个飞秒激光脉冲在玻璃内部产生多次微爆的研究

在入射能量不同的情况下,用近红外飞秒激光脉冲在重钡火石玻璃(ZBaF15)内部产生了色心和微爆,在熔融石英的内部产生了多次微爆现象.实验表明:入射能量较高时,色心的中间会产生微爆;在松聚焦条件下,一个飞秒激光脉冲在透明介质内会激发多次微爆.基于飞秒激光脉冲在透明介质内的自聚焦效应和自由电子等离子体的自散焦效应,理论分析了多次微爆产生的原因;也讨论了飞秒激光脉冲诱导玻璃折射率改变的机理.

时空分布的超短激光脉冲烧蚀电介质材料时等离子体的产生

若考虑聚焦平面内固定区域 ,脉冲空间分布可简化为一维问题 在此基础上 ,从脉冲的传输过程出发 ,本文分析了时空分布的超短激光脉冲与电介质材料相互作用时等离子体形成模式 在材料内部累积的能量与脉冲宽度、与空间相关的损伤区域。

各阶克尔非线性折射率对强激光大气传输的影响

用数值方法研究了各阶克尔非线性折射系数对强激光传输的影响,结果显示第7阶克尔非线性系数和第9阶克尔非线性系数协同影响激光传输,当第7阶克尔效应起作用时,忽略第9阶克尔效应的作用会导致光强随传输距离震荡剧烈,传输非常不稳定。要得到稳定传输需要第7阶克尔非线性系数和第9阶克尔非线性系数同时起作用或同时不起作用。在自聚焦阶段第3阶非线性折射率对折射率改变起主要作用,在稳定光丝传输阶段第7阶克尔非线性系数,第9阶克尔非线性系数起主要作用。第5阶克尔系数在整个传输过程中对折射率的改变一直比较小。

面向在轨遥感应用的飞秒激光成丝调制延长方法研究

飞秒激光在大气中的远距离传输对于实时快速的遥感探测具有重要的应用价值。文章通过在聚焦镜前增加圆孔石英板,提出了一种延长激光光丝产生等离子体通道的新方法,结果显示飞秒激光等离子体通道延长了一倍;同时,研究了不同激光脉冲能量下,圆孔石英板的直径和厚度对形成的等离子体通道的影响,发现石英板中心圆孔的直径对形成光丝长度具有显著影响。基于延长的飞秒激光光丝,探测了氦气中充入空气量对混合气体光谱的影响,发现光谱信息对氦气的浓度变化非常敏感。

强飞秒激光大气传输成丝特性研究

基于对强飞秒激光大气传输的非线性薛定谔方程的变分法,模拟研究飞秒激光大气传输成丝特性。研究了在是否考虑多光子吸收损耗的情况下,激光成丝状态的演化。研究表明:多光子吸收损耗会破坏自聚焦与等离子体散焦之间的动态平衡,进而引起成丝的不稳定性;在不同的入射功率下,成丝的起点位置会发生移动;在实验室条件下,p_(in)≥3p_(cr)时才可能形成多次聚焦成丝传输。继续增加激光入射功率时,光束因强烈聚焦发生崩塌而成多丝结构。

利用等离子体通道实现空间太阳能无线能量传输

空间太阳能电站是高效利用太阳能的有效途径,受到了国际的广泛关注。空间无线能量传输技术是空间太阳能电站的关键技术之一。文章首先分析了微波与激光无线能量传输技术的问题,根据空间太阳能传输的具体需求进一步提出基于等离子体通道的无线能量传输技术,并对基于此项技术的空间太阳能传输系统进行了论证,完善并发展了空间太阳能传输模型。基于等离子体通道的无线能量传输技术为多个领域的能量补给提供了新颖的技术途径,是具有广阔应用前景的战略选择。

不同能量下环形高斯强激光光束在空气中的非线性传输

以线性传输模型为对比研究了不同初始能量的环形光束在空气中的非线性传输。结果显示,在传输初始阶段,非线性克尔效应减弱了线性空间啁啾导致的聚焦作用。线性聚焦使能量向光轴方向流动导致强光强核心,在光轴附近生成类高斯脉冲结构,增强了非线性自聚焦效应,导致光强急剧增加形成光丝。环形光束的初始能量的大小能够影响自聚焦焦距、光丝长度和光强通量。自聚焦焦距随着初始脉冲能量的增加而减小,但自聚焦焦距与初始功率的开方不成反比(这点与高斯光束不同)。光丝长度随着初始脉冲能量的增加而增加。轴上光强通量随初始脉冲能量的增加而增加。