Influence of circular aperture on high-order harmonic generation

The influence of circular aperture on the intensity of high-order harmonic generation (HHG) with intense femtosecond laser pulse was studied both experimentally and theoretically. The intensity variety of HHG with the diameter of circular aperture was observed in pulsed Ar gas. The result was discussed and interpreted in terms of the theory of Hankel transform. It is found that using the Gaussian beam truncated by an aperture could enhance the conversion efficiency of HHG at certain conditions.     

靶源位置对强场高次谐波相位匹配的影响

为提高强激光场与惰性气体靶作用产生的孤立阿秒激光脉冲的能量,给出了一种实现高次谐波过程中最佳谐波相位匹配的定量实验方法。研究了气体靶源与高斯型驱动激光场聚焦点相对空间位置对谐波相位匹配及谐波产率的影响,得出了其最佳相位匹配位置始终位于驱动激光场聚焦点后3~5 mm,而在聚焦点之前的位置区域,严重的高次谐波相位失配导致谐波产率非常低。同时,在最佳相位匹配条件下,高次谐波场与驱动场具有相类似的空间强度分布特性,该结果印证了目前通常采用的高次谐波场为高斯光束的假设。     

Pulsed microwave-driven argon plasma jet with distinctive plume patterns resonantly excited by surface plasmon polaritons

Atmospheric lower-power pulsed microwave argon cold plasma jets are obtained by using coaxial transmission line resonators in ambient air.The plasma jet plumes are generated at the end of a metal wire placed in the middle of the dielectric tubes.The electromagnetic model analyses and simulation results suggest that the discharges are excited resonantly by the enhanced electric field of surface plasmon polaritons.Moreover,for conquering the defect of atmospheric argon filamentation discharges excited by 2.45-GHz of continued microwave,the distinctive patterns of the plasma jet plumes can be maintained by applying different gas flow rates of argon gas,frequencies of pulsed modulator,duty cycles of pulsed microwave,peak values of input microwave power,and even by using different materials of dielectric tubes.In addition,the emission spectrum,the plume temperature,and other plasma parameters are measured,which shows that the proposed pulsed microwave plasma jets can be adjusted for plasma biomedical applications.     

Effect of laser focus in two-color synthesized waveform on generation of soft x-ray high harmonics

Synthesis of multi-color laser pulses has been developed as a promising way to improve low conversion efficiency of high-order harmonic generation (HHG). Here we systematically study the effect of laser focus in a two-color waveform on generation of macroscopic HHG in soft x-rays. We find that the dependence of HHG yields on laser focus at low or high gas pressure is sensitive to the characteristics of single-atom harmonic response, in which "short"-or "long"-trajectory emissions can be selectively controlled by changing the waveform of two-color synthesized laser pulse. We uncover the phase-matching mechanism of HHG in the gas medium by examining the propagation of the two-color waveform and the evolution of time-frequency emissions of high-harmonic field. We further reveal that the nonlinear effects, such as geometric phase, atomic dispersion, and plasma defocusing, are responsible for modification of two-color waveform upon propagation. This work can be used to find better macroscopic conditions for generating soft x-ray HHG by employing two-color optimized waveforms.     

High field physics and extreme nonlinear optics

Despite the fact that high order harmonic generation (HHG) aims at serving as a table-top light source for imaging applications with extremely high spatial resolution, a general lack of accurate conversion efficiency measurements exists in the field. Here, we present such a measurement for a HHG setup with a semi-infinite gas cell. By combining measurements with a calibrated photodiode sensitive in the extreme ultra-violet (XUV) and spatially resolved spectral measurements of HHG spectra, we are able to determine conversion efficiencies of the HHG process as well as brilliance values for individual harmonics. The method is explained in detail and applicable to any target geometry.     

脉冲激光偏振方向对氧分子高次谐波的影响——基于含时密度泛函理论的模拟

采用含时密度泛函方法,结合赝势模型和电子交换相关作用的广义梯度近似,模拟了氧分子在超强飞秒激光脉冲作用下的高次谐波产生现象,并研究了激光脉冲偏振方向对氧分子高次谐波的影响.结果表明氧分子的高次谐波谱具有典型原子谐波谱的特征;当θ=45°(激光偏振方向与分子轴向夹角)时,谐波谱强度最大.这与文献报道的实验结果基本一致.     

Plasma effect on the phase matching of high harmonic generation

By optimizing the phase matching condition of high harmonic generation(HHG) from a supersonic neon gas jet,the enhanced HHG in the region of 60-70 eV has been selected.Three-dimensional numerical calculation shows that plasma plays a significant role in the phase matching process of HHG in a supersonic gas jet with short medium length.Due to plasma formation,the harmonic emission decays as the laser intensity reaches over 3.5 × 10 14 W/cm 2.The plasma induces the broadening and blue shift of the HHG spectra,which provides a method for fine-tuning the harmonic wavelength.     

脉冲激光偏振方向对氮分子高次谐波的影响－－基于含时密度泛函理论的模拟

采用含时密度泛函方法，结合赝势模型和电子交换相关作用的广义梯度近似，模拟了氮分子在超强飞秒激光脉冲作用下的高次谐波产生现象，并研究了激光脉冲偏振方向对氮分子高次谐波的影响.结果表明氮分子的高次谐波谱具有典型原子谐波谱的特征；谐波谱强度随着θ(激光偏振方向与分子轴向夹角)的增大而减小.这与J.Itatanl在Nature上报道的实验结果基本一致. 关键词： 含时密度泛函理论 激光偏振方向 高次谐波 氮分子     

Simulation of Processes on a Target Induced by a Jet System and Laser Radiation

The mathematical model and the results of computer simulation of the processes of compression, heating, and energy release in a target under the hybrid influence of a system of pulsed plasma jets and intense laser radiation in magnetic inertial thermonuclear fusion are presented. The hybrid effects of intense energy flows on a single-layer cylindrical target are compared in the experimental and simulation conditions. The main radiation magnetic and gas dynamic parameters of the compressed and heated target plasma are evaluated as well.     

Emission Spectra of Light Inert Gases Ne and Ar in the 3–20 nm Range under Pulsed Laser Excitation Using Various Gas Jets as Targets

Optics and Spectroscopy - The emission spectra of neon and argon gas jets in the range of 3–20 nm when excited by pulsed laser radiation have been studied. The Nd:YAG laser with λ = 1064...     

数值研究二维含时薛定谔方程

采用二维渐近边界条件,将任意极化激光与原子相互作用的二维含时Schr(o)dinger方程无穷空间初值问题转化为有界空间的初边值问题,近而将截断后的初边值问题离散成线性正则方程组,而后利用辛算法求解正则方程得到含时波函数.最后利用含时波函数求得高次谐波谱,证明二维渐近边界条件和辛算法是合理而有效的.     

Femtosecond enhancement cavity with kilowatt average power

Femtosecond enhancement cavity(fsEC) has been proved to be a powerful tool in a diverse range of applications.Here, we report the recent progresses in building an fsEC on kilowatt level average power, with the aim of realization of intracavity high harmonic generation(HHG) and extension of the wavelength of femtosecond optical frequency comb from infrared(IR) to extreme ultraviolet(XUV). Upon mode-matching optimization and cavity length locking, an intracavity average power of 6.08 kW is reached and the corresponding buildup is 225. After introducing noble gas of Xe into the focus region, clear sign of plasma has been observed. The generated HHG is also coupled out by a sapphire plate placed at Brewster's angle for the fundamental laser. Our work paves the way for the realization of an XUV comb.     

Hydrodynamics of pulsed supersonic underexpanded jets: Spatiotemporal characteristics

The gasdynamic parameters of nonsteady expansion of He, Ar, N₂, and SiH₄ from a sonic nozzle into a space with reduced background gas pressure were experimentally studied for moderate values of n (10³–10⁶) and the Reynolds number (Re_L∼10⁰–10²). The jet set times necessary for the formation of pulsed jets of a given finite duration are determined. The results are generalized in terms of dimensionless similarity parameters. The laws of motion of the leading and trailing fronts in pulsed jets of various gases are established. The leading front of a pulsed jet propagates at a velocity significantly smaller than the limiting steady value. The jet expansion dynamics is determined by the ratio of the momentum of the expanding gas to that of the background gas displaced from the flow region. The length of the steady flow region in a pulsed jet monotonically decreases downstream from the source and drops with increasing background gas pressure because of the loss of jet particles in the trailing rarefaction wave; this length increases with the initial momentum because the background gas is more intensively displaced from the flow region.     

Utilization of ultrasound to enhance high-speed water jet effects

Continuous high-speed water jets are presently used in many industrial applications such as cutting of various materials, cleaning and removal of surface layers. However, there is still a need for further research to enhance the performance of pure water jets. An obvious method is to generate water jets at ultra-high pressures (currently up to 700 MPa). An alternate approach is to eliminate the need for such high pressures by pulsing of the jet. This follows from the fact that the impact pressure on a target generated by a slug of water is considerably higher than the stagnation pressure of a corresponding continuous jet. Ultrasonically forced modulation of a continuous stream of water represents the most promising method of pulsed jet generation because of its simplicity and practicality. A pulsed jet is generated by modulating a continuous stream of water by ultrasonic waves. A velocity transformer connected to a piezoelectric transducer is located axially inside a nozzle to induce longitudinal pulsations in the water. An extensive laboratory research program is in progress to understand the basic principles of the process and to optimize the nozzle design for several applications. The results reported in this paper show that the performance of such a pulsed jet is far superior to that of a continuous jet operating at the same parameters. Experimental results obtained with the ultrasonic vibration of a tip situated inside the nozzle indicate that using this technique one can achieve performance of the jet even order of magnitude higher in comparison to continuous jet at the same hydraulic parameters. Performance of ultrasonically modulated jets in cutting of various materials was tested in laboratory conditions. In this paper, results of measurement of dynamic pressure in the nozzle and force effects of modulated jets are presented together with results obtained in cutting of various materials using ultrasonically modulated water jets. The results are compared with those obtained with continuous jets at the same operating parameters. Potential of forced modulation of the jet in applications of cleaning, paint and coating removal from surfaces and concrete cutting in the process of repair of concrete structures is mentioned.     

Supersonic flow characteristics in laser grooving

The gas flow structure in laser grooving has been evaluated by numerical simulation and experiments in this study. A nozzle designed with a specific arrangement of compound jets for laser grooving has been tested. For this nozzle configuration the compressible flow structure of a shock wave induced by a supersonic side jet has been predicted in a numerical simulation using FLUENT, a computational fluid dynamics code and visualized in the experiment where the wall pressure on the grooving zone was evaluated and measured for jets impinging on the substrate at varying attacking angles of the side jet. The results obtained numerically and experimentally were comparable. In addition, the relationships between the material removal efficiency and the flow structure have also been established. It transpired that the attacking angle made a significant and dramatic improvement on the flow structure and grooving appearance and that a large wall pressure with a clear grooving profile can be obtained for attacking angles between 40° and 50° in the present test setup.     

Laser phase control of high-order harmonic generation at large internuclear distance: the H+ -H2+ system

Exact (Born-Oppenheimer) 3-D numerical solutions of the time-dependent Schr?dinger equation are obtained for the one electron linear H+-H2+ atom-molecule system at large internuclear distance R in interaction with two-cycles intense (I>10(14) W cm(-2)) 800 nm laser pulses. High-order harmonic generation (HHG) spectra are obtained with an energy cutoff larger than the atomic maximum of I(p)+3U(p), where I(p) is the ionization potential and U(p) is the ponderomotive energy. At large R, this extended cutoff is shown to be related to the nature of electron transfer, whose direction is shown to depend critically on the carrier-envelope phase (CEP) of the ultrashort pulse. Constructive and destructive interferences in the HHG spectrum resulting from coherent superpositions of electronic states in the H+-H2+ system are interpreted in terms of multiple electron trajectories extracted from a time profile analysis.     

红外激光场中共振结构原子对极紫外光脉冲的压缩效应

通过数值求解一维原子的含时薛定谔方程, 研究了具有共振结构的原子在双色场(红外激光(IR)+极紫外光(XUV)) 驱动下发射高次谐波的特征. 研究结果表明, 具有共振结构的原子所发射的高次谐波与无共振结构原子(简称为一般原子)发射的高次谐波有明显不同, 共振结构的原子除了在某一能量附近(原子的共振能量+电离能)高次谐波的强度有很大提高外, 它还对XUV光的响应较一般原子表现得更为敏感, 即使XUV光的强度较弱, 也能够明显提高XUV光脉冲中心频率附近的谐波强度, 更重要的是通过调节双色场的时间延迟, 能使输入的XUV光的脉宽得到明显的压缩, 通过时间-频率分析给出了发生这种现象的原因. 由此提出了通过滤波-连续反馈的方式可使XUV光的脉冲从200 as压缩至120 as左右.     

基于固体薄片超连续飞秒光源驱动的高次谐波产生实验

本文报道了采用基于熔石英薄片超连续的少周期飞秒光源驱动高次谐波产生的实验研究.实验中通过将重复频率1kHz的飞秒钛宝石激光放大器所输出的能量0.8mJ、脉宽30fs的脉冲聚焦到7片0.1mm厚的熔融石英片中,得到了覆盖带宽大于倍频程的展宽光谱.利用啁啾镜补偿色散后,经瞬态光栅频率分辨光学开关法测得脉宽为6.3fs,对应约2.3个光学周期.利用压缩后的激光脉冲聚焦作用于惰性气体,并通过调节尖劈插入量改变脉宽,分别测得了分立以及连续的高次谐波截止区信号,结果与6.3fs的脉冲宽度相符合.     

A shadowed off-axis production of Ge nanoparticles in Ar gas atmosphere by pulsed laser deposition

In this work, we report on the production of Ge nanoparticles (NPs) in an inert Ar gas atmosphere by pulsed laser deposition (PLD) at room temperature (RT). The direct deposition of energetic particles/droplets resulting from the ablation process of the target material has been avoided by using an original and customized off-axis shadow mask (shadowed off-axis) deposition set-up where the NPs deposition on the substrate takes place by means of scattering between the NPs formed in the vapor phase and the background Ar atoms. It is found that the Ar gas pressure parameter has a relevant role in the crystallization process, with better crystallinity obtained as the background Ar pressure is raised for the given experimental conditions.     

Enhanced X-ray emission in the 1-keV range from a laser-irradiated gas puff target produced using the double-nozzle setup

X-ray emission from a double-stream gas puff target irradiated with a nanosecond Nd:glass laser pulse was studied for the first time. The target was formed by pulsed injection of a high-density gas into a gas cloud by using the double-nozzle setup. This new concept allows a high-density gaseous target to be formed at a relatively large distance from the nozzle output. Enhanced X-ray emission in the 1-keV energy range and a smaller source size were observed as compared to the ordinary gas puff target created by pulsed injection of gas into vacuum. This new approach should be useful in the development of a laser-produced X-ray source for various applications. Received: 20 October 1999 / Published online: 27 January 2000