Complete characterization of molecular dynamics in ultrashort laser fields

Reaction Microscope-based, complete, and time-resolved Coulomb explosion imaging of vibrating and dissociating D(2)(+) molecules with femtosecond time-resolution allowed us to perform an internuclear distance (R-)dependent Fourier analysis of the corresponding wave packets. Calculations demonstrate that the obtained two-dimensional R-dependent frequency spectra enable the complete characterization of the wave packet dynamics and directly visualize the field-modified molecular potential curves in intense, ultrashort laser pulses.     

电子在超短激光脉冲修正场中的动力学特性研究

给出了一种精确描述超短、紧聚焦激光脉冲的新方法，其思路是根据两个无量纲小量ε=1/(ω0t0)和s=1/(k0w0)（其中ω0=ck0为中心振荡频率，t0为脉冲延迟时间，w0为激光束腰半径）进行展开来计算脉冲的高阶修正场.在激光束近轴近似表达式的基础上，给出了高斯脉冲一阶修正场的解析表达式，并研究发现其振幅和相位相对于零阶修正场（即长脉冲近似）的修正量都在ε的量级甚至更小.另外对电子在超短高斯脉冲一阶修正场中的动力学特性研究发现：对于ω0t0>20的情况，零阶修正场可以正确地描述电子被光场加速的特性；当ω0t0<20时，则需要采用高阶修正场. 关键词： 超短激光脉冲 激光加速     

紫外超短脉冲激光放大过程中脉宽压缩的方案设计

采用放电泵浦KrF准分子激光放大器放大波长为248.4nm的紫外超短脉冲激光。对于能量为0.7mJ、脉宽为550fs的输入脉冲,在光束直径保持10mm不变的条件下,能量放大到15mJ,脉宽展宽到1200fs。为了压缩输出脉冲宽度,分析了群速度色散和自相位调制效应对脉宽展宽的影响。利用棱镜对,采用4种不同的实验方案对脉冲引入负的线性频率啁啾,以补偿KrF准分子激光放大器CaF2窗镜中的群速度色散和自相位调制对脉冲引入的正的线性频率啁啾。结果表明:在放大器之前放置棱镜对的方式可以在保持输出脉冲能量为15mJ的同时,在棱镜对间距为110cm的条件下,将输出脉冲宽度压缩到370fs,输出波长为248.4nm、带宽为0.4nm。     

紫外超短脉冲激光放大过程中脉宽压缩的方案设计

采用放电泵浦KrF准分子激光放大器放大波长为248.4 nm的紫外超短脉冲激光。对于能量为0.7 mJ、脉宽为550 fs的输入脉冲,在光束直径保持10 mm不变的条件下,能量放大到15 mJ,脉宽展宽到1 200 fs。为了压缩输出脉冲宽度,分析了群速度色散和自相位调制效应对脉宽展宽的影响。利用棱镜对,采用4种不同的实验方案对脉冲引入负的线性频率啁啾,以补偿KrF准分子激光放大器CaF2窗镜中的群速度色散和自相位调制对脉冲引入的正的线性频率啁啾。结果表明：在放大器之前放置棱镜对的方式可以在保持输出脉冲能量为15 mJ的同时,在棱镜对间距为110 cm的条件下,将输出脉冲宽度压缩到370 fs,输出波长为248.4 nm、带宽为0.4 nm。     

Formation of ultrashort electron pulses on scattering of an electron beam by a standing laser wave of ultrashort duration

The scattering of a well collimated electron beam by a strong standing laser wave of ultrashort duration, giving rise to a great number of scattered photons, is considered. This type of scattering is found to cause an electron beam to divide effectively into two parts. Ultrashort laser pulses are shown to be capable of forming ultrashort electron bunches whose length is governed by the laser beam diameter.     

Vortices in ultrashort laser pulses

The propagation of optical vortices nested in broadband femtosecond laser beams was studied both numerically and experimentally. Based on the nonlinear Schrödinger equation, the dynamics of different multiple-vortex configurations with varying topological charge were modelled in self-focussing and self-defocussing Kerr media. We find a similar behavior in both cases regarding the vortex–vortex interaction. However, the collapsing background beam alters the propagation for a positive nonlinearity. Regimes of regular and possibly stable multiple filamentation were identified this way. Experiments include measurements on pairs of filaments generated in a vortex beam on an astigmatic Gaussian background with argon gas as the nonlinear medium. Spectral broadening of these filaments leads to a supercontinuum which spans from the visible range into the infrared. Recompression yields <19 fs pulses. Further optimization may lead to much better recompression.     

Peculiarities of the ionization of many-electron systems and generation of attosecond pulses in ultrashort laser fields

The ionization of a model two-electron atom in the field of a strong ultrashort laser pulse is studied by numerical integration of the nonstationary Schrödinger equation describing the dynamics of a quantum system in the field of an electromagnetic wave. Pecularities of the two-electron ionization are analyzed for pulses whose duration amounts to one to two periods of oscillation of the electric field of the wave at different frequencies of the incident radiation. For extremely short pulses, the double ionization is found to be suppressed. This effect is caused by the finiteness of the interelectron energy exchange time during the laser action. Peculiarities of the generation of high-order harmonics and single XUV attosecond pulses upon ionization of atoms by laser pulses, whose duration is within one to two optical cycles, are investigated.     

Electronic and nuclear responses of fixed-in-space H2S to ultrashort intense laser fields

The Coulomb explosion dynamics of H2S, H2S3+-->H+ +S+ + H+, in ultrashort intense laser fields (12 fs, approximately 2 x 10(14) W/cm2) is studied by the coincidence momentum imaging of the three fragment ions. Different electronic and nuclear responses are identified depending on the direction of laser polarization epsilon in the molecular frame. The dependence can be interpreted in terms of the electronic and bonding characters of charge transfer states of H2S coupled to the electronic ground state.     

Electron impact ionization of atomic clusters in ultraintense laser fields

In this paper we report on inner ionization of Xe_n clusters (n = 55- 2171) in ultraintense Gaussian laser fields (peak intensity I = 10¹⁵- 10²⁰ Wcm^-2, pulse width τ= 25 fs, frequency 0.35 fs^-1). The cluster inner ionization process is induced by the barrier suppression ionization (BSI) mechanism and by electron impact ionization (EII), which occurs sequentially with the BSI. We address electron impact ionization of clusters, which pertains to inelastic reactive processes of the high-energy (100 eV–1 keV per electron) nanoplasma. We utilized experimental data for the energy dependence of the electron impact ionization cross-sections of Xe^j+ (j = 1-10) ions, which were fit by an empirical three-parameter Lotz-type equation, to explore EII in clusters by molecular dynamics simulations. Information was obtained on the yields and time-resolved dynamics of the EII levels (i.e., number n_imp of electrons per cluster atom) in the Xe_n clusters and their dependence on the laser intensity and cluster size. The relative long-time (t = 90 fs) yields for EII, n_imp/n_ii (where n_ii is the total inner ionization yield) are rather low and increase with decreasing the laser intensity. In the intensity range I = 10¹⁵-10¹⁶ Wcm^-2, n_imp/n_ii = 0.21 for n = 2171 and n_imp/n_ii = 0.09-0.14 for n = 459, while for I = 10¹⁸-10²⁰ Wcm^-2, n_imp/n_ii = 0.01-0.05. The difference Δn_imp between the EII yield at long time and at the termination of the laser pulse reflects on ionization dynamics by the nanoplasma when the laser pulse is switched off. For Xe₂₁₇₁ in the lower intensity domain, Δ n_imp = 0.9 at I = 10¹⁵ Wcm^-2 and Δn_imp = 0.4 at 10¹⁶ Wcm^-2, reflecting on EII by the persistent nanoplasma under “laser free” conditions, while in the higher intensity domain of I = 10¹⁷ - 10¹⁸ Wcm^-2, Δn_imp is negligibly small due to the depletion of the transient nanoplasma.     

Influence of ultrashort laser pulse duration on the X-ray emission spectrum of plasma produced in cluster target

Line emission spectrum of a laser plasma produced in an argon cluster jet target was measured on the n ¹ P1?1¹ S ₀ (n=5–9) transitions of the helium-like Ar XVII ion for a pulse duration varying from 45 fs to 1.1 ps and a constant fluence of ～10⁵ J/cm². The independent modeling of the relative intensities of the transitions from the n=5,..., 10 levels, as well as of the 2¹ P ₁? 1² S ₀ and 2³ P ₁?1² S ₀ lines and dielectronic satellites indicates that the electron temperature is anomalously low and that the electron density in emitting plasma increases with shortening the laser pulse. The excitation from the ground state by a small fraction of hot electrons is expected to be the main channel of populating the Ar XVII levels.     

Measurement of ultrashort laser pulses

A proposal for measuring the pulse shape of ultrashort laser pulses is described. The proposed method consists of a Gamo triple intensity-interferometer to measure the triple-correlation function of the light pulses. A subsequent signal processing algorithm is used to reconstruct the true shape of the light pulses out of their triple-correlation function. The suggested method is quite insensitive to noise, as shown by means of computer simulations.     

Raman-Nath approximation for diffraction of atom in the laser field of counterpropagating waves: Instantaneous switching of interaction

Kapitsa-Dirac scattering problem is solved in Raman-Nath approximation without restrictions on the value of resonance detuning. Formula for the scattering amplitude is obtained in the form of a definite integral. It is shown that in case of Gauss-type superposition initial states the spectrum of scattering has new regularities more needed in atomic optics.     

Microablation with ultrashort laser pulses

This paper reports the micromachining results of different materials (Al, Si, InP and fused silica) using a Ti : sapphire laser at the wavelength of 800 and 267 nm with variable pulse lengths in the range from 100 fs to 10 ps. The hole arrays with a diameter up to a few μm through microdrilling are presented. We discussed how an effective suppression of the thermal diffusion inside the ablated materials and an effective microablation could be realized. If the laser fluence is taken only slightly above the threshold, a hole array can be drilled with diameters even smaller than the wavelength of the laser. Some examples are presented in the paper.     

Second harmonic beam analysis, a sensitive technique to determine the duration of single ultrashort laser pulses

The second harmonic beam generated in a noncollinear arrangement allows the observation of the autocorrelation functio: A compact optical beam splitter and imaging system eliminates alignment problems. Single pulses of 1 ps duration and approximately 10⁻⁸ joule energy are readily observed using an optical multichannel analyser.     

超短激光脉冲

概述了超短激光脉冲技术的发展进程、研究进展及其在液态分子研究方面、超快现象等领域的应用。     

Simulation of ultrashort double-pulse laser ablation

In this paper, we study the mechanisms of femtosecond double-pulse laser ablation of metals. It was previously shown experimentally that the crater depth monotonically drops when the delay between two successive pulses increases. For delays longer than the time of electron-ion relaxation the crater depth can be even smaller than that produced by a single pulse. The results of the performed hydrodynamic simulation show that the ablation can be suppressed due to the formation of the second shock wave. The modeling results of the double-pulse ablation obtained for different delays correlate with the experimental findings.     

周期量级超短激光脉冲作用下导带电子的光吸收与碰撞电离

激光照射下光学材料的损伤过程中，导带电子的加热和碰撞电离是非常重要的过程，影响着导带电子的产生、晶格能量的沉积和破坏.分析了Drude模型的局限性，从经典力学出发求解了周期量级激光场中导带电子的运动方程，计算了导带电子的光吸收和碰撞电离，分析了激光强度、载波相位等对碰撞电离的影响. 关键词： 周期量级超短激光脉冲 导带电子 碰撞电离     

Ionization of two-electron atom in ultrashort laser pulse

The method of numerical integration of the classical equations of motion was used to study interaction of a model two-electron atom with ultrashort laser pulses. Mechanisms and specific features of the ionization process were analyzed in a broad range of laser-field parameters.     

超短脉冲激光场中间二氯苯的激发态动力学

利用飞秒时间分辨的飞行时间质谱技术研究了间二氯苯的激发态动力学.间二氯苯分子吸收一个200 nm或者267 nm的光子被抽运到激发态,随后再吸收多个800 nm的光子被电离.实验获得了电离产生的离子质谱信号及其随抽运探测激光延迟时间的变化曲线.在200 nm时,分子被抽运到激发态(π,π*),可观察到三个相互竞争的解离通道的寿命:内转换到排斥态(n,σ*)或者(π,σ*)并发生快速解离,其寿命约(0.15±0.01)ps;内转换到基态的高振动态,能量在基态"热"振动态间弛豫的寿命约为(4.94±0.08)ps;系间窜越到相邻的三重态从而发生预解离过程,其寿命约为(110.09±4.33)ps.在267 nm时,分子被抽运到第一激发态的低振动态,可观察到一个长寿命(约(1.06±0.05)ns)的系间窜越过程.除此之外,在碎片离子信号中还观察到了激发态与基态的高振动态之间的内转换过程.