Dependence of photoemission efficiency on the pulsed laser cleaning of Tungsten photocathodes, part 1: Experimental

3+ :YAG laser with 5^th harmonic generator, generating 16 ps duration pulses at 213 nm, with energies up to 0.5 mJ. Experimental results concerning the action of laser pulses, as well as the effects of residual pressure on the cleanliness of the photocathodes surface are presented and discussed. Influence of laser pulses and residual pressure on the work function of the metal are also investigated. Received: 15 April 1996/Accepted: 5 November 1996     

Sound generation in the atmosphere upon exposure to high-power milli- and microsecond laser pulses

A review of works on generation of acoustic pulses in the atmosphere upon exposure to high-power laser pulses is presented in this paper. Characteristics of sound pulses in the atmosphere accompanying the propagation of high-power milli- and microsecond laser pulses, including the peak sound pressure level and the shape and spectrum of acoustic pulses, are discussed. The special features of acoustic pulses from individual liquid and solid aerosol particles and ensembles of monodisperse particles and polydisperse atmospheric aerosol are discussed. The characteristics of acoustic signals from long laser sparks and collective optical discharges are considered.     

超短超强激光与液体的相互作用研究

超短超强激光与液体靶相互作用时表现出许多有趣的特点，这明显区别于激光脉冲与固体或气体靶的相互作用情况．文章分别介绍了激光诱发等离子体所产生的高压冲击波、激光空泡、X射线、高能超热电子以及白光，对它们的产生机制及其各自的显著特征进行了综合描述．文章最后对超短超强激光脉冲与各种不同形态的液体靶相互作用的应用前景作了简单介绍。     

Selective ir laser chemistry of CDF3 in natural fluoroform

Selective decomposition of CDF₃ at natural abundance level (150ppm) in fluoroform has been achieved by infra-red multiple-photon excitation at moderate substrate pressure using 100 ns FWHM CO₂ laser pulses. Effects of energy fluence, number of laser pulses, buffer gas pressure and substrate pressure on decomposition yield and bulk selectivity are reported and discussed.     

Tunable picosecond pulses from a short-cavity dye laser under ultra-high pressure using diamond-anvil cell

It is demonstrated that broadly tunable picosecond pulses are generated from a dye laser of very short cavity utilizing a diamond-anvil cell, which operates at pressures up to 10 GPa. The pulses as short as 5 ps are obtained from the rhodamine 6G dye laser pumped by a frequency-doubled Q-switched Nd: YAG laser. The way of tuning is based on the pressure induced shift of the emission peak of the dye. The laser is tunable over 20 nm by changing the pressure of the cell within 4 GPa.     

Temperatures, pressures and stresses during laser shock processing

Mathematical models are developed to calculate the temperatures, pressures and stresses during laser shock processing for time-modulated (ramp-up, ramp-down and rectangular) laser pulses. Three different shock processing configurations are also considered: non-ablative exposure, ablative exposure and confined ablation with coating. The results for iron show that the plasma pressure reaches an average value of 9 GPa in direct ablation configuration and plays a dominant role for all three types of laser pulses. In the case of confined geometry, the plasma pressure reaches an average value of 20 GPa. All calculated pressures and stresses exceed the yield strength of the workpiece, indicating plastic deformation. It is also shown that pulses with short rise times yield higher plasma pressures.     

Dominance of radiation pressure in ion acceleration with linearly polarized pulses at intensities of 10(21) W cm(-2)

A novel regime is proposed where, by employing linearly polarized laser pulses at intensities 10(21) W cm(-2) (2 orders of magnitude lower than discussed in previous work [T. Esirkepov et al., Phys. Rev. Lett. 92, 175003 (2004)]), ions are dominantly accelerated from ultrathin foils by the radiation pressure and have monoenergetic spectra. In this regime, ions accelerated from the hole-boring process quickly catch up with the ions accelerated by target normal sheath acceleration, and they then join in a single bunch, undergoing a hybrid light-sail-target normal sheath acceleration. Under an appropriate coupling condition between foil thickness, laser intensity, and pulse duration, laser radiation pressure can be dominant in this hybrid acceleration. Two-dimensional particle-in-cell simulations show that 1.26 GeV quasimonoenergetic C(6+) beams are obtained by linearly polarized laser pulses at intensities of 10(21) W cm(-2).     

一种新型TEA CO2激光器的实验研究

设计了一种新型结构的激光器———双通道放电激励折叠腔TEA CO2激光器。该激光器通过提高脉冲能量输出水平来增加激光输出谱线数目和激光差分吸收雷达可探测的气体种类。它在增大激光器放电时间长度的同时保证了较小的激光器体积,更好地满足了激光差分吸收雷达光源体积小,重量轻,输出能量高的要求。得到激光器输出能量与气体工作气压的关系曲线,获得了激光器内工作气压与输出脉宽关系曲线,发现脉冲宽度大约在50~70ns之间变化,研究了气体压强对激光器输出脉冲峰值功率的影响。     

Zur Anregung und Emission im N2-Impulsgaslaser

A theoretical treatment of pulsed nitrogen laser operation is presented. The properties of laser pulses were investigated as functions of various parameters (electric circuit parameters, laser length, gas pressure). Based on this model some conditions are obtained which are necessary for effective laser pulse emission, and the known nitrogen laser devices are discussed.     

On the liquid explosive boiling on a substrate under action of subnanosecond laser pulse

The conditions and capability of acoustic recording of pressure pulses from explosive boiling of a transparent liquid on an absorbing substrate heated by nanosecond and subnanosecond laser pulses are analyzed.     

Effect of time profile of laser pulse on the excitation of sound signals in a liquid with a series of laser pulses

We have studied the sound generation with high repetition rate pulsed laser. We have solved the inhomogeneous wave equation for acoustic pressure in a liquid generated by a laser, using Green’s function formalism and convolution technique. To obtain the maximum pressure of the sound waves, we found the conditions on repetition rate and on period of laser pulse of various shapes. Our analysis shows that the sound generated in a liquid with a series of laser pulses is highly affected by the time profile of the pulses besides other parameters, namely laser beam diameter, laser beam optical wavelength, repetition rate and period of laser pulse. This effect is pronounced particularly in frequency domain. We found that the noise of higher harmonics in the generated sound can be greatly removed with the proper choice of the time profile of the laser pulses. It is found that the pressure is generated around the fundamental frequency for the half-sine and rectangular pulses, with the proper choice of repetition rate and period of pulse. The application of the present analysis for underwater communication is pointed out.     

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.     

双色光场驱动产生单个阿秒脉冲过程中的宏观效应

通过数值模拟三维传播方程研究了双色场产生高次谐波过程中宏观效应的影响,重点分析了阿秒脉冲的时域形状.在6 fs/800 nm和21.3 fs/400 nm组成的双色场中,通过调节双色场间的相对延时,双色场各焦点相对于气体盒子入口的间距和双色场束腰半径和气体压强等参数,可以在平台区得到超宽带连续谱.对该连续谱中420—540 eV的谐波直接进行滤波后可以得到时间同步性很好的单个阿秒脉冲.这种性质对实验上通过双色场方案产生超短的阿秒脉冲非常有利. 关键词： 传播效应 阿秒脉冲 时间同步性     

Enhancement of Ne-like Ar 46.9 nm laser output by mixing appropriate He ratio at low pressure

With pure Ar and gaseous mixtures of Ar-He, the laser spike of Ne-like Ar 46.9 nm pumped by capillary discharge is measured with XRD (X-ray diode) at low pressure. We observe the effects of the Ar pressure and the He ratio on the amplitude of laser pulse. Compared with the laser spike of pure Ar, a mixture of Ar and a small quantity of He enhances the laser output. The time of lasing onset and the duration of laser pulses are not affected by mixing He. In addition, a monochromator is used to measure the laser pulse at 46.9 nm. This is the first observation of laser output with a Ar-He mixture.     

Subnanosecond amplified spontaneous emission pulses by a nitrogen pumped dye laser

Pulses of 100 ps duration and peak power up to 100 kW are obtained with a dye laser pumped by an atmospheric pressure nitrogen laser of 0.5 ns duration. The shortening of the dye laser pulses is attributed to amplified spontaneous emission.     

超短脉冲激光照射下光栅Talbot效应的实验研究

理论上研究了超短脉冲激光照射下光栅的Talbot效应,得到了超短脉冲激光照射下光栅Talbot图像的性质.实验上采用严格的实验方法(频率分辨光学开关装置测量超短脉冲激光、反射式扩束系统对超短脉冲进行扩束)很好地验证了理论分析结果.理论分析和实验结果表明,超短脉冲激光照射下光栅的Talbot图像的对比度会显著下降,而且超短脉冲激光的脉宽越短,Talbot距离越大,Talbot图像的对比度会进一步下降.     

不同气氛下飞秒激光诱导硅表面微结构

利用钛宝石飞秒激光脉冲对单晶硅在SF6、空气和真空环境中进行了累积脉冲辐照,研究了硅表面微结构的演化。在SF6气氛中,在激光辐照的初始阶段,硅表面形成了1维的波纹结构,随着辐照脉冲数的增加,波纹结构演化成了2维凹凸结构。累积600个脉冲后,硅表面产生了准规则排列且具有大纵横比的锥形尖峰结构。该结构呈现高度相对较低、锥形尖端小球不明显的特征,分析认为主要与环境气压的大小有关。对比空气、SF6和真空中的微结构发现,尖峰的数密度依次减小;SF6中形成的尖峰高度最大,其次为真空,再次为空气。研究结果表明,真空、SF6和空气3种环境下微结构的形成及表面形貌主要由激光烧蚀、化学刻蚀和氧化决定。     

Nd-Glass laser with plasma mirror

A high power pulse Nd-glass laser system with plasma mirror is studied. Plasma is created on the surface of a solid target and the action is superradiantly triggered. Long trains of modelocked pulses are generated if the carbon or metal targets are used and Q-switched pulses are observed with dielectric targets like PVC. Tens of joules of light energy are extracted from the laser medium and absorbed by plasma in both regimes. Spatial and temporal structures of the laser beam are very reproducible, the laser action is insensitive to variations in air pressure inside the target chamber. Stimulated Brillouin back-scattering in the underdense plasma is discussed as the nonlinear mechanism governing the plasma reflectivity.     

Generation of far-infrared pulses by use of a passively mode-locked high-pressure CO2 laser

Summary Our present study demonstrates for the first time generation of frequency-tunable intense infrared radiation pulses in the nanosecond time regime by mode-locking a high pressure CO₂ laser using p-doped germanium as a saturable absorber. These pulses were transferred into the FIR region via stimulated Raman scattering of the CO₂ laser radiation in CH₃F resulting in subnanosecond FIR laser pulses.     

整形飞秒激光金属材料精细加工

实验通过二极管记录透射光信号随脉冲个数变化关系以及观测样品烧蚀形貌来研究不同实验条件对激光烧蚀的影响。使用的样品是厚度为50 m铝箔。实验中通过研究不同变量:激光焦点与样品的相对位置、激光的能量、背景气体压强以及脉冲形状对烧蚀加工过程和结果的影响,从而获得较好烧蚀效果的条件,达到控制烧蚀加工过程的目的。特别是通过使用不同形状的脉冲和具有一定规律的脉冲序列对样品进行烧蚀,发现某些形状的整形脉冲烧蚀结果明显优于变换极限脉冲。说明脉冲整形作为一种新的技术可以在激光精细钻孔领域得到更深入的研究和应用。