Time-resolved current response of a nanosecond laser pulse illuminated STM tip

Time-resolved dependence of the transient current through a ns laser pulse illuminated scanning tunneling microscope (STM) tip/sample gap in tunneling mode and out of tunneling range is presented. A self-designed fast STM-preamplifier (bandwidth 35 MHz) allows one to resolve the fine structure of the transient signal as well as the observation of some effects that are undetectable by using conventional low-band preamplifiers. The dependence of the threshold laser pulse intensity, which corresponds to the beginning of electron emission from tip (in non-tunneling mode), as a function of the tip/sample distance was investigated. At tip/sample distances from tunnel contact up to approximately 1 μm a linear dependence is found. This behavior is in good agreement with the theory for field enhancement in a STM tip/sample system. In tunneling mode a ns (fast component) as well as a μs (slow component) current response was found as a result of the laser pulse illumination. These data suggest the tip bending to be an important factor in clarifying the thermal/mechanical mechanism of laser-assisted surface nanomodification. Received: 4 May 1998 / Accepted: 29 January 1999 / Published online: 28 April 1999     

基于脉冲CO2激光锡等离子体光刻光源的极紫外辐射光谱特性研究

研究了不同条件下脉冲放电CO₂激光烧蚀平板锡靶产生的等离子体极紫外辐射特性, 设计并建立了一套掠入射极紫外平焦场光栅光谱仪, 结合X射线CCD探测了光源在6.5～16.8 nm波段的时间积分辐射光谱,得到了极紫外光谱随激光脉宽, 入射脉冲能量及背景气压的变化规律。实验结果发现：入射激光脉冲能量在30～600 mJ变化时,极紫外辐射光谱的强度随辐照激光脉冲能量的增加而增加, 但并不是线性关系, 具有饱和效应, 且产生极紫外辐射的脉冲能量阈值约为30 mJ,当激光脉冲能量为425 mJ时具有最高的转换效率,此时中心波长13.5 nm处2%带宽内的转换效率约为1.2%。激光脉冲半高全宽在50～120 ns范围内变化时, 极紫外辐射光谱的峰值位置均位于13.5 nm,光谱形状几乎没有什么变化, 但是脉宽从120 ns变到52 ns后,由于激光功率密度的提高,极紫外辐射强度也随之增强了约1.6倍。极紫外光谱的强度随背景气压的增大而迅速下降, 当腔内空气气压为200 Pa时, 极紫外辐射光子几乎被全部吸收,而当缓冲氦气气压为7×104 Pa时,仍能够探测到微弱的极紫外辐射信号,计算表明100 Pa的空气对13.5 nm极紫外光的吸收系数为3.0 m-1,而100 Pa的He气的吸收系数为0.96 m-1。     

基于双程放大的毛细管放电69.8 nm激光增益特性

建立了类氖氩C线69.8 nm激光的双程放大实验后反射腔结构,利用45 cm长毛细管作为放电负载得到了其双程放大输出.在相同初始实验条件下,分别测量了单程放大输出与双程放大输出的激光脉冲光强、脉冲宽度以及激光束散角.通过对比单程与双程输出实验结果,利用双程放大激光光强的计算公式,分析得到了增益持续时间大于4 ns,以及增益在毛细管径向上的分布特点.以上结果为建立谐振腔进行毛细管放电类氖氩激光的多程放大实验奠定了基础.     

Coherent probe-pump-based Brillouin sensor for centimeter-crack detection

We provide a theoretical explanation for a coherent probe-pump-based Brillouin sensor system that achieves centimeter spatial resolution with high-frequency resolution. It was recently discovered that, when a combination of cw and pulsed light (the probe beam) interacts with a cw laser (the pump beam), centimeter spatial resolution with high-frequency resolution can be achieved even though the probe-pulse duration is 1.5 ns [Opt. Lett. 29, 1485 (2004)]. Our study reveals that the coherent portion inside the pulse length of these two interactions caused by the same phase is responsible for this behavior. It allows us to detect 1.5-cm outer-layer cracks on an optical ground-wire cable.     

THz Radiation Generation via the Interaction of Ultra‐short Laser Pulses with the Molecular Hydrogen Plasma

In this paper, a two dimensional Particle In Cell‐Monte Carlo Collision simulation scheme is used to examine the THz generation via the interaction of high intensity ultra‐short laser pulses with an underdense molecular hydrogen plasma slab. The influences of plasma density, laser pulse duration and its intensity on the induced plasma current density and the subsequent effects on the generated THz signal characteristics are studied. It is observed that the induced current density in the plasma medium and THz spectral intensity are increased at the higher laser pulse intensities, laser pulse durations and plasma densities. Moreover, the generated THz electric field amplitude is reduced at the higher laser pulse durations. A wider frequency range for the generated THz signal is shown at the lower laser pulse durations and higher plasma densities. Additionally, it is found that the induced current density in hydrogen plasma medium is the dominant factor influencing the generation of THz pulse radiation. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Ionization focusing of a short intense laser pulse and generation of wake plasma waves

The propagation of a short intense laser pulse is studied in a gas taking into account the ionization of gas atoms by the high-frequency electromagnetic field of the pulse. The conditions are found under which the ionization structures produced by the laser pulse cause the pulse focusing accompanied by a substantial increase in its intensity. It is shown that the leading edge of the pulse is subjected to ionization refraction at the ionization front, the temporal profile of the pulse becoming steeper. This results in the efficient generation of a wake wave at the ionization front, which is amplified during the development of self-modulation instability. The amplitude of the wake plasma wave achieves a substantial value already at small paths of the pulse in matter (smaller than the diffraction length of the pulse).     

Long pulse ultraviolet laser emissions in an aerodynamically cooled electron-beam excited Ne-NF3 mixture

Long pulse laser action has been achieved at 3378.28 and 3481.96 Å in an electron beam excited supersonic flow. These laser emissions, attributed to NeII transitions, have been obtained when a mixture of Ne and NF₃ at a pressure of 0.2 atm. and aerodynamically cooled to 80 K was excited by a medium intensity electron beam. Laser pulse duration, about 350 ns fwhm for both lines, was only limited by the electron beam pulse length. The second of these laser lines had not been previously reported.     

Background gas collisional effects on expanding fs and ns laser ablation plumes

The collisional effects of a background gas on expanding ultrafast and short pulse laser ablation plumes were investigated by varying background pressure from vacuum to atmospheric pressure levels. For producing Cu ablation plumes, either 40 fs, 800 nm pulses from a Ti: Sapphire laser or 6 ns, 1,064 nm pulses from a Nd:YAG laser were used. The role of background pressure on plume hydrodynamics, spectral emission features, absolute line intensities, signal to background ratios and ablation craters was studied. Though the signal intensities were found to be maximum near to atmospheric pressure levels, the optimum signal to background ratios are observed ~20–50 Torr for both ns and fs laser ablation plumes. The differences in laser–target and laser–plasma couplings between ns and fs lasers were found to be more engraved in the crater morphologies and plasma hydrodynamic expansion features.     

Quenching effects on pulsed photoacoustic signals in NO2–air samples

A theoretical expression for the pulsed photoacoustic signal amplitude from NO₂–air mixtures is deduced based on a two-level system and the inhomogeneous wave equation, in the limit where the V-T relaxation time is longer than the laser pulse width (low buffer pressure). In this time limit, the photoacoustic signal from NO₂, at constant pressure, after excitation by pulses from a Nd:YAG laser at 532 nm, is measured at different buffer pressures. The relaxation rates are obtained by measuring the quenching of visible fluorescence induced by the same laser. The experimental dependence of the photoacoustic signal amplitude on air pressure shows a very good agreement with the model, where the measured relaxation rates are included.     

铁基非晶磁环的高频响应和ns级脉冲响应特性

为了获取高频和快脉冲条件下铁基非晶磁芯的重要特性参数,对三种国产铁基非晶磁环的高频响应和快脉冲响应特性进行了实验研究.通过1∶1变压器测试回路发现,在1～20 MHz等振幅正弦源信号作用下,磁芯响应信号幅度损失率从0陡增至50％～60％,当源信号频率继续增大时,响应信号幅度损失率维持在50％～60％.对磁芯快前沿脉冲响...     

全光纤多种子激光脉冲产生系统

报道一种可精确同步输出宽带啁啾脉冲、纳秒级整形脉冲以及窄带光参量啁啾放大抽运脉冲的全光纤多种子激光脉冲产生系统.采用掺镱光纤锁模激光器和掺镱单纵模光纤振荡器作为系统的光源,将掺镱光纤锁模激光器输出的超短脉冲分束啁啾展宽至0.9ns后,一路进行放大产生10μJ量级的宽带啁啾脉冲为高能拍瓦激光器系统提供种子光脉冲,另一路通过1.2nm滤波产生140ps的基元脉冲,通过光纤堆积器产生可编程整形的2.3ns脉冲,再经过放大达到10μJ量级,提供任意整形的压缩脉冲.同时,将部分掺镱光纤锁模激光器输出的超短脉冲进行光电转换并锁相后,产生与锁模超短脉冲高精度同步的电脉冲用于触发幅度调制器,将掺镱单纵模光纤振荡器输出的连续光削波放大,产生光参量啁啾放大抽运脉冲.该系统能够根据物理实验的需要,非常灵活地在输出各种脉冲之间做出选择.     

Simple electro-optic technique to generate temporally flat-top laser pulses

A simple technique is presented to generate temporally flat-top shaped laser pulses using electro-optic modulator (Pockels cell). It involves splitting of input laser pulse into two halves of equal intensity and then stacking together with appropriate optical delay to get a temporally flat-top laser pulse. It also allows generation of other pulse shapes by varying the relative intensity, delay, and phase between two halves of the input laser pulse. Temporally flat-top laser pulses of duration ~ 9 ns have been generated using ~ 7 ns duration incident laser pulses from a flash lamp pumped Q-switched Nd:glass laser oscillator. The rise and fall-time of the shaped pulse is limited by speed of electro-optic switch (Pockels cell), which is ~ 2 ns in the present case.     

激光诱导Ti等离子体电子温度的实验研究

室温,常压下,利用Nd∶YAG脉冲激光器产生的波长为1 064 nm, 脉宽12 ns,能量分别180, 230和280 mJ的脉冲激光冲击Ti靶,使用中阶梯光栅光谱仪检测了三种激光能量下对应的光谱。调节延时器DG645的延迟时间,检测了延迟0～500 ns时间范围内Ti等离子体对应激光能量下的发射光谱,分析光谱,可以得到了九条不同的的TiⅠ 和TiⅡ等离子体谱线,证明在该实验条件下,Ti靶能够充分吸收能量电离且离子谱线具有不同的演化速率,利用Saha-Boltzmann法计算并分析Ti等离子体电子温度,实验结果表明：相同的延迟时间,激光能量越大,谱线相对强度越大,电子温度越高,谱线相对强度的变化量随激光能量的变化量增大而增大;在延时0～150 ns内,三种激光能量下的等离子体电子温度和谱线的相对强度都随延迟时间的增加而快速下降,其中280 mJ激光能量下的等离子体电子温度和谱线强度下降速率较快;在150～250 ns范围内,电子温度和谱线强度均随延迟时间的增加有一个缓慢的上升,180 mJ激光能量下的等离子体电子温度和谱线强度的上升速率较快。250～500 ns范围内,三种激光能量下的电子温度和谱线强度均随延迟时间的增加而缓慢下降。     

Detection of water surface capillary wave by analysis of turning-point local signal data using a laser interferometer

A laser interferometry technique is developed to detect water surface capillary waves caused by an impinging acoustic pressure field. The frequency and amplitude of the water surface capillary waves can be estimated from the local signal data at some special points of the phase modulated interference signal, which is called the turning points. Demodulation principles are proposed to explain this method. Experiments are conducted under conditions of different intensity and different frequency driving acoustic signals. The results show the local signal data analysis can effectively estimate the amplitude and frequency of water surface capillary waves.     

1653 nm光参量放大器及在甲烷遥测方面的应用

基于掺氧化镁的周期性极化铌酸锂（MgO:PPLN）晶体,研究了连续波种子光注入结构的光参量放大器(OPA)。采用重复频率为10 kHz、最大平均输出功率为6 W的1064 nm高频脉冲激光作为抽运源,晶体的极化周期为31.02 m,在119 ℃工作温度下,将连续分布式反馈激光器（DFB）作为种子注入,在1652.3 nm处得到最大平均功率为125 mW的OPA信号光输出,量子转换效率为7.47％。实验通过改变注入种子光波长,实现了信号光nm级范围的调谐输出。信号光通过10 m外甲烷气池,观察到其光谱强度明显下降,初步验证了信号光在甲烷遥测领域的可行性。与用种子光直接检测相比,信号光强度更高,检测距离及灵敏度均有较大优势。     

Simulation of pulsed-laser-induced explosive boiling

Bulk evaporation process in absorbing condensed matter irradiated with laser pulses was studied using the one-dimensional thermal model with additional interfaces between different phases. Within this approach, it was shown that the repeated explosive boiling mode can be achieved using nanosecond laser pulses, if the nucleation time is shorter than 0.1 ns. This mode can be observed if the surface pressure is lower than the critical pressure P _c of the liquid-vapor phase transition. The dependences of these processes on the laser pulse intensity and duration, as well as on evaporation kinetics were studied. Explosive boiling and spallation of a transparent liquid film on a pulse-heated absorbing target, as well as the photoacoustic signal in the target before the explosion, were considered.     

基于光斑跟踪器的脉冲峰值保持电路的实现

针对激光光斑跟踪器接收信号处理系统中常规A/D采样电路对脉冲电压峰值采集的技术要求,设计了一种脉冲峰值保持电路。通过对该峰值保持电路的各项指标进行理论分析,采用宽带宽跨导放大器MAX436设计该峰值保持电路,并对该电路进行了仿真分析与实验研究,得到满足参数指标设计要求的实验结果:对于脉宽50 ns的激光脉冲信号,在输入信号幅度大于100 mV时,峰值保持电路的响应速度2 ns,下垂速率6.0 mV/s,保持精度1.1%。     

High repetition rate spiker-sustainer XeCl laser

An X-ray preionized spiker-sustainer XeCl-excimer laser operating at high repetition frequency (1000 Hz) and 220 W of average laser power is demonstrated. The pulse energy and the pulse duration (160 ns) are virtually independent of the repetition frequency. Comparison with aC-L-C circuit shows superior properties of spiker-sustainer excitation for high-frequency operation. Furthermore, a variation of the overshoot mode is proposed.     

Photoacoustic responce of the loaded surface of absorbing liquid to laser pulse with modulated intensity

The experimental results on the effect of erbium laser pulses (300–400 ns long, 2.94 μm wavelength) on the water surface loaded by a transparent sapphire plate 9.6 mm thick are presented. The laser pulse intensity was modulated with a period of 8.2 ns. The behavior of the phase of the modulated part of the photoacoustic response under given experimental conditions differs significantly from the case of irradiation of a free liquid surface [1, 2].     

激光脉冲能量对激光诱导Al等离子体辐射特征的影响

使用Nd：YAG激光器烧蚀金属Al靶获得等离子体,利用光谱时-空分辨技 术,在52 mJ～145 mL/pulse激光脉冲能量范围内,关于激光脉冲能量对激光诱导等离子体 辐射特征的影响进行了研究。使用的气体是Ar气,压强为10 kPa。结果发现,激光脉冲能量 升高,引起特征谱线强度增加,连续谱强度也增加;但能量过高,会击穿周围气体,产生气 体微等离子体。此时,特征谱和连续谱几乎不再增强;最大特征辐射强度在145 mJ、10 kPa 、靶前0.1 mm处、延时180 ns获得;同一条件下获得最强背景连续谱,而信号-背景差是在 145 mJ、10 kPa、靶前1.0 mm处、延时450 ns达到最大值。基于Al等离子体不同激光脉冲能 量下的时间-空间分辨谱,对结果进行了简单的讨论。并分别确定了获得最大特征辐射和信 号-背景差的条件。