Time-energy properties of an attosecond extreme ultra-violet pulse

The time-energy properties of high-order harmonic generation (HHG) are calculated for a linearly polarized 7-fs laser pulse with different carrier-envelope phases (CEPs). The quantum trajectory paths that contribute to an as (1 as=10-18 s) pulse in HHG are identified. The laser-duration dependence and the CEP dependence of HHG energy property are investigated. The study shows that an as extreme ultra-violet (XUV) pulse can be selected from HHG spectrum near cut-off energy with a bandpass optical filter. The theoretical prediction of the pulse duration is proportional to bandwidth. Analysis suggests that a measured narrowband as XUV pulse may consist of instantaneous shorter pulses each dependent on laser pulse duration, intensity, and CEP. These information can be used as references for producing, selecting, improving and manipulating (timing) as pulses.     

Carrier-envelope phase measurement using plasmonic-field-enhanced high-order harmonic generation of H atom in few-cycle laser pulses

We investigate the plasmonic-field-enhanced high-order harmonic generation (HHG) of H atom driven by few-cycle laser pulses, by solving the time-dependent Schrödinger equation (TDSE). Compared with the homogeneous field, HHG spectra generated by inhomogeneous field exhibit two-plateau structure. We analyze the origin of the two plateaus by using the semiclassical trajectory method. Our results from both classical and TDSE simulations show that the cutoffs of the two plateaus are dramatically affected by the carrier-envelope phase (CEP) of laser pulse in the inhomogeneous field, even for a little longer pulse. Thus, we can determine the CEP of driving laser based on the cutoff position of HHG generated in the inhomogeneous field.     

High-order harmonics generation by few-cycle and multi-cycle femtosecond laser pulses

This paper investigates experimentally high-order harmonic generation(HHG) of neon gas with 5-fs and 25-fs driving laser pulses.It has been demonstrated that the cutoff energy of the harmonic extreme ultraviolet photons is extended to 131 eV and the HHG spectrum near the cutoff region becomes continuum as the driving laser pulse duration is 5 fs;whereas much lower cutoff photon energy and discrete harmonic spectrum near the cutoff region are presented as the laser pulse duration is 25 fs.The results can be explained by the fact that neutral atoms can be exposed to more intense laser field before they are depleted by ionization because of the extremely short rising time of the few-cycle pulse.The 5-fs driving laser pulse paves the way of generation of coherent x-ray in the water window and single attosecond pulse.     

电荷共振增强电离和离解电离在H~+_2谐波辐射中的贡献

理论研究了电荷共振增强电离和离解电离在H~+_2谐波辐射中的贡献.结果表明:在少周期激光场下,谐波辐射只由电荷共振增强电离贡献产生,谐波光谱呈规则的奇次谐波.在多周期激光场下,谐波辐射由电荷共振增强电离和离解电离共同贡献产生,但是电荷共振增强电离在谐波辐射中起主要作用.并且低阶谐波呈现偶次谐波.最后,通过分析含时核运动,电离几率以及谐波辐射时频分析图解释了少周期和多周期激光场驱动H~+_2辐射谐波的过程.     

利用43飞秒的强激光脉冲实现单个阿秒脉冲输出的新机理

研究了多个光周期的长脉冲(43 fs)激光与氖原子相互作用产生的高次谐波的辐射特性. 通过计算机数值模拟，发现当激光场的强度超过饱和光强时，原子将会在几个光周期内被迅速电离，使得原子的高次谐波谱在截止区呈现出平滑的多平台结构. 同时，在高频波段离子产生的高次谐波的影响将变得十分显著. 通过选取合适的子平台进行频率叠加，在消除了离子的高次谐波的影响后，可以获得单个的阿秒软X射线脉冲. 关键词： 高次谐波 阿秒脉冲     

载波-包络相位稳定的周期量级近红外超短脉冲激光系统

利用光参量放大过程输出的闲置光载波-包络相位(CEP)被动稳定的特点,搭建了三级光参量放大(OPA)系统,获得了CEP稳定的近红外高能量超短激光脉冲(1.4 mJ/40 fs/1 kHz @ 1.8 μm),其CEP抖动为516 mrad(rms).经空心光纤展宽光谱和块体材料补偿色散,激光脉宽最终可被压缩至小于两个光学周期(<12 fs),脉冲能量达到0.54 mJ.该系统为单个阿秒脉冲的产生和其他高次谐波实验提供了优质的光源. 关键词： 光参量放大 周期量级近红外激光脉冲 载波-包络相位稳定     

Single-pass high-harmonic generation at 20.8 MHz repetition rate

We report on single-pass high-harmonic generation (HHG) with amplified driving laser pulses at a repetition rate of 20.8?MHz. An Yb:YAG Innoslab amplifier system provides 35?fs pulses with 20?W average power at 1030?nm after external pulse compression. Following tight focusing into a xenon gas jet, we observe the generation of high-harmonic radiation of up to the seventeenth order. Our results show that state-of-the-art amplifier systems have become a promising alternative to cavity-assisted HHG for applications that require high repetition rates, such as frequency comb spectroscopy in the extreme UV.     

High-order harmonic generation from organic molecules in ultra-short pulses

We have studied high-order harmonic generation (HHG) from organic molecules irradiated with near-infrared high intensity laser pulses of 70 fs and 240 fs duration. The molecular systems studied were the aromatics benzene and naphthalene and the alkanes cyclopropane and cyclohexane (cyclic) and n-hexane (linear). Harmonic intensities were measured both as a function of laser intensity (in the range 5×10¹³-5×10¹⁵ W cm^-2) and as a function of ellipticity of the laser field polarisation. The results were compared with those from the xenon atom. For 70 fs pulses, harmonic generation from the organic systems was similar to that of xenon, revealing an atom-like behaviour for molecules when the laser pulse duration is shorter than the fragmentation timescale of the molecule. We note significant differences between molecules with respect to HHG efficiencies and the suppression of HHG in larger species. We discuss these differences in the context of the molecular properties, electronic structure and behaviour of ionisation and fragmentation that result in enhancement of field ionisation in larger systems. Study of the polarisation ellipticity dependence of HHG shows that the harmonic yield in molecules is less sensitive to the polarisation than for atoms (xenon). This is consistent with the expected behaviour given the larger recollision cross-section presented by the core in the molecular system compared to the atom. Our results suggest that study of HHG from molecules exposed to ultra-short pulses is potentially a powerful tool for understanding the electron dynamics of molecules exposed to an intense field. Received 14 September 2000 and Received in final form 6 December 2000     

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.     

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.     

高次谐波辐射发射特性研究

报告由不同脉冲宽度(半高宽,FWHM)和不同载波-包络相位(CEP,Φ)的激光产生的高次谐波辐射能量输出时间特性即发射特性的研究结果. 计算表明,由宽度为几个周期的激光产生的高次谐波辐射的截止能量明显低于由无限长脉冲宽度激光产生的截止能量ω_max=3.17U_p+I_p(其中ω_max为光子角频率,U_p和I_p分别为激光有质动力势和原子的电离能). 例如,由两周期(FWHM),Φ＝15°的激光产生的高次谐波辐射的截止能量为ω_max=2.90U_p+I_p,此时发射特性单脉冲(即分布单脉冲)具有最大的能量带宽0.86U_p. 脉冲中心位置的载波相位和时间宽度分别为0.94rad(弧度)和1.29rad. 而该激光脉冲在Φ＝-75°时能产生截止能量为ω_max=2.70U_p+I_p,最大能量带宽为0.70U_p的双分布脉冲,其中心位置分别为-0.58rad和2.43rad,宽度分别为1.22rad和1.33rad. 随着激光脉冲宽度的增加,分布单脉冲的能量带宽比时间宽度下降得更快. 对于一定宽度的激光脉冲,所产生的分布单脉冲的能量带宽和时间宽度的CEP依赖性显示出180°的周期结构. 利用这个有趣的特点,在实验上可以通过调节CEP来选择分布脉冲的能量参数,也可用来定位和控制阿秒脉冲的时间参数. 理论分析指出,只要选择合适的阿秒X射线能量带宽,CEP不稳定性对于光电子谱和测量结果的影响将大为降低,甚至在最大程度上消除这种影响. 这些研究结果不仅有助于在物理上深入了解高次谐波辐射的动力学过程,而且对于进一步在实验上优化和选择阿秒单脉冲和双脉冲具有重要的参考和指导意义. 关键词： 高次谐波产生 鞍点方法 谐波发射特性 分布脉冲     

Relativistic laser plasmas for novel radiation sources

Relativistic laser-plasma interaction results in new sources of short-pulsed x-ray radiation. Here we consider two options. The first one is betatron radiation of electrons accelerated in underdense plasmas and oscillating in transverse fields of the laser wake. This radiation is incoherent and broadband, the pulse duration is comparable with that of the driving laser. The second option is the high harmonic generation (HHG) from overdense plasma surfaces. This radiation is coherent. The relativistic high harmonics are phase locked and emerge in the form of (sub-)attosecond pulses. One- and three-dimensional regimes of relativistic HHG from overdense plasmas are considered.     

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.     

Origination of gamma-ray burst pulses associated with the Doppler effect of spherical fireballs or uniform jet

Ryde and Petrosian have pointed out that the rise phases of gamma-ray burst (GRB) pulses originate from the widths of the intrinsic pulses and their decay phases are determined by the curvature effect of the expanding fireball surface based on their simplified formula. In this paper we investigate in detail the issue based on the formula in Ref.[20], which is derived based on a model of highly symmetric expanding fireballs, where the Doppler effect is the key factor to be concerned about, and no terms are omitted in their derivation. Our analyses show that the decay phases of the observed pulses originate from the contributions from both the curvature effect of the expanding fireball and the two timescales of the local pulses, and the rise phases of the observed pulses only come from the two timescales of the local pulses. Associated with a local pulse with both rise and decay portions, the light curve of GRBs in the rise portion is expected to undergo a concave phase and then a convex one, whereas that in the decay portion is expected to evolve by an opposite process. And the ratio of the concave timescale to the convex one in the rise phase of the observed pulse linearly increases with the ratio of the rising timescale to the decay one of the local pulse ($r_{\rm rd}$), whereas the ratio of the convex timescale to the concave timescale in its decay phase linearly decreases with $r_{\rm rd}$. The two correlations are independent of the local pulse forms and the rest-frame radiation forms. But the different forms of local pulses and the different values of $r_{\rm rd}$ gives rise to the diversity of the light curve pulse shapes. We test a sample of 86 GRB pulses detected by the BATSE instrument on board the Compton Gamma Ray Observatory and find that the characteristics do exist in the light curve of GRBs.     

高次谐波辐射光子的能量-激光相位关系研究

原子在强激光电场中高次谐波辐射的理论与实验研究是当今科学前沿之一.利用量子力学理论和鞍点方法,细致地研究了高次谐波辐射光子的能量与激光相位的关系.对于时间宽度无限长激光,在一个激光周期内特定相位处产生的高阶辐射(X射线)有特定的能量.能量分布在7180°处成峰,有高斯形函数的对称形状.给出了这种分布的参数化公式.对于不同宽度的飞秒激光,能量分布的成峰位置、最大值和带宽等参数会发生变化.计算表明,三个振荡周期(半高宽)的飞秒激光,当载波-包络相位为175°和105°时,可以分别得到纯净的阿秒单脉冲和双脉 关键词： 超短脉冲激光 高次谐波产生 鞍点方法 能量相位关系     

共振条件下载波包络相位效应对阈上电离谱的影响

本文通过求解三维含时薛定谔方程, 从理论上研究了共振条件下氢原子的光电子能谱与角分布随着激光脉冲载波包络相位(CEP)的变化规律. 研究结果表明: 在共振位置附近, 光电子能谱的强度与CEP有强烈的依赖关系; 进一步对各个分波的分析, 可以确定共振电离初态的角量子数. 此外, 利用光电子的角分布信息, 可以对多周期激光脉冲的初始相位信息进行高精度探测. 关键词： 阈上电离 超短脉冲 载波包络相位     

载波相位对超短脉冲谐波谱的影响

数值计算了线偏振的超短激光脉冲(脉冲持续时间为两个光学周期)与一维模型原子相互作用产生的高次谐波发射功率谱. 研究表明,当载波相位发生变化时,超短脉冲谐波谱的截止频率也随之改变,而且在特定相位下,谐波谱出现了明显的双平台结构. 对此,采用半经典的“三步”模型给出了合理解释,并利用小波时频分析方法证实“三步”模型可以准确预言超短脉冲谐波谱的截止频率. 关键词： 高次谐波 超短脉冲 载波相位     

Multi-mJ carrier envelope phase stabilized few-cycle pulses generated by a tabletop laser system

A compact system for the generation of few-cycle multi-mJ Carrier Envelope Phase (CEP) stabilized pulses is presented. At the output 1.9?mJ, 5.7?fs pulses were achieved after hollow fiber compression (HFC) of 5?mJ, 25?fs circularly-polarized pulses from a Ti:sapphire multipass chirped pulse amplifier (CPA). Polarization control of the generated pulses was done using all reflective phase retarders which can be nearly arbitrarily scaled for increasing energies. The CEP noise from the amplifier system is shown to be 190?mrad rms over a period of more than 7?hours. The full system, i.e., oscillator, amplifier, CEP stabilization, and HFC is compact enough to fit on a standard optical table.     

Carrier envelope phase effect on the spatial distribution of high-order harmonic generation in asymmetric molecule

The spatial distribution in high-order harmonic generation(HHG) from the asymmetric diatomic molecule He H~(2+) is investigated by numerically solving the non-Born–Oppenheimer time-dependent Schr?dinger equation(TDSE). The spatial distribution of the HHG spectra shows that there is little contribution in HHG around the geometric center of two nuclei(z = 1.17 a.u.) and the equilibrium internuclear position of the H nucleus(z = 3.11 a.u.). We demonstrate the carrier envelope phase(CEP) effect on the spatial distribution of HHG in a few-cycle laser pulse. The HHG process is investigated by the time evolution of the electronic density distribution. The time–frequency analysis of HHG from two nuclei in HeH~(2+) is presented to further explain the underlying physical mechanism.     

High power all-fibered femtosecond master oscillator power amplifier at 1.56 μm

Direct amplification of output from chirped pulse oscillator (CPO) to 3.3?W of average power (pulse energy of 118?nJ in 20?ps pulse duration before compression) was achieved in a properly designed cladding pumped large mode area Er-doped fiber. Various configurations of CPO cavity with different FWHM of output spectrum and pulse duration were investigated. Fourier limit compression with 480?fs pulse duration and 32?kW peak power has been obtained for pulses with 14.8?nm FWHM spectrum. Subsequent nonlinear compression in a standard SMF-28 fiber yielded pulses as short as 145?fs.