非平行偏振双色场驱动产生脉宽稳定的单个宽谱阿秒脉冲

提出一种直接得到脉宽稳定的单个宽谱阿秒脉冲的新方法.利用波长为800 nm脉宽为5 fs的超短脉冲叠加上偏振方向与主脉冲成π/3,脉宽同样为5 fs的二次谐波脉冲驱动氦原子,可以得到宽度达到50 eV的超连续谱.当二次谐波的强度大于10¹⁴ W/cm²时,超连续谱的位置以及谱宽几乎不会随着二次谐波脉冲的强度的改变而改变.对85—125 eV的超连续谱进行滤波可以直接得到100 as左右的单个阿秒脉冲,这个性质对于实验上获得单个宽谱阿秒脉冲而言是非常有利的. 关键词： 阿秒脉冲 超连续谱     

空间非均匀场下核间距对不对称分子谐波发射的影响

数值研究了空间非均匀场下核间距离对于不对称分子HeH2+发射高次谐波以及阿秒脉冲的影响。计算结果表明,通过适当调节HeH2+分子离子的核间距离以及空间非均匀参数,不仅谐波发射的截止能量得到了延伸,而且单一的短量子路径也被选择出来对谐波发射起作用。随后适当引入第二束控制激光场,谐波截止能量得到了进一步扩展,形成了两个带宽分别在431eV和372eV的连续平台区。 最后,通过适当的叠加谐波,可获得脉宽为31as-65as的一系列阿秒X射线光源。     

Generation of 88 as Isolated Attosecond Pulses with Double Optical Gating

Isolated attosecond pulses with a duration of 88 as are generated in the spectral range of 29-72 eV using double optical gating technique.The gate width is set to be shorter than half the optical cycle to avoid carrier envelop phase stabilization of the 4.2 fs driving laser pulses centered at 800 nm.The attosecond pulse duration is measured with the technique of frequency resolved optical gating for complete reconstruction of attosecond bursts.     

阿秒电离门调控宽带超连续谱的传播特性

通过数值模拟三维传播方程研究了双色场阿秒电离门技术调控产生高次谐波的传播特性. 发现波长800和400 nm的5 fs双色场电离门调控产生的谐波在传播0.5 mm后由于短轨道满足相位匹配条件,实现了短轨道的选择,在平台区得到了高效率、无调制的宽带超连续谱（谱宽达80 eV）. 模拟不同传播距离的结果表明该连续谱能够在介质中稳定存在. 对该连续谱中60—90 eV的谐波直接进行滤波可以得到高效率、脉冲宽度约为135 as的单脉冲. 这种性质对实验上实现利用阿秒电离门调控产生高效率宽谱单阿秒非常有利. 关键词： 传播效应 阿秒脉冲 超连续谱     

量子路径控制生成宽频软X射线连续辐射谱

数值研究了双色激光脉冲与一维氩原子相互作用的谐波发射功率谱.双色激光脉冲由800 nm和1600 nm的两束激光组成,脉冲持续时间为9.7 fs.计算结果表明,相对相位为π时,谐波截止频率从单色场情形的58 eV大幅度地展宽到316 eV,获得频宽为93 eV的软X射线连续辐射谱.此外,还采用短程的普薛耳－特勒势和长程的软化库仑势进行比对,数值结果表明该连续辐射谱的存在具有普适性.通过半经典的“三步”模型和时频分析小波变换方法对该连续辐射谱进行分析,发现其主要由单一的“短”量子路径生成,直接叠加频率从22 关键词： 红外双色激光脉冲 连续辐射谱 阿秒脉冲     

正交偏振双色激光场作用下生成的孤立阿秒脉冲

我们理论研究了正交偏振双色激光场作用下的高次谐波发射和孤立阿秒脉冲的产生.当y方向加一束中红外激光脉冲(12.5 fs/2000 nm),x方向加一束强度较弱的激光脉冲(12 fs/800 nm)时,我们得到从250 eV到350 eV的超连续谐波平台,在平台范围内叠加50 eV的谐波,可以得到一个脉宽约为97as的孤立阿秒脉冲. 通过时频分析,我们解释了高次谐波发射的物理机制.     

中红外组合激光场调控宽带超连续谱的产生

本文对原子在波长为2000nm、脉宽为12.5fs的中红外驱动脉冲和脉宽为1.3fs的紫外控制脉冲叠加形成的组合场中产生的高次谐波进行了研究.通过组合场驱动氦原子得到了谱宽为230eV的超连续谱.利用组合场产生的谐波比单独利用中红外脉冲产生的谐波的强度高了3个量级.对超连续谱进行滤波并调节组合场中两束激光的延迟时间,可以直接产生100as的单个脉冲.我们发现超连续谱的宽度和位置几乎不会因为驱动场和控制场强度的变化而改变,这种性质有利于从实验上获得单个宽谱阿秒脉冲.     

Attosecond time-resolved autoionization of argon

Autoionization of argon atoms was studied experimentally by transient absorption spectroscopy with isolated attosecond pulses. The peak position, intensity, linewidth, and shape of the 3s3p?np 1P Fano resonance series (26.6-29.2 eV) were modified by intense few-cycle near infrared laser pulses, while the delay between the attosecond pulse and the laser pulse was changed by a few femtoseconds. Numerical simulations revealed that the experimentally observed splitting of the 3s3p?4p 1P line is caused by the coupling between two short-lived highly excited states in the strong laser field.     

利用两束同色圆偏振激光场产生水窗区间阿秒脉冲

数值研究了氦原子在两束同色圆偏激光场下发射高次谐波以及阿秒脉冲的特点.计算结果表明,当适当调节两束激光场的波长,相位以及时间延迟时,不仅高次谐波谱的平台区域能得到很大的扩展,而且谐波的强度也有较大的增强,形成了一个500 e V的超长平台区.最后,通过适当的叠加谐波平台上的谐波,可获得一系列脉宽为35 as左右的水窗区间阿秒脉冲.     

利用两束同色圆偏振激光场产生水窗区间阿秒脉冲

数值研究了氦原子在两束同色圆偏激光场下发射高次谐波以及阿秒脉冲的特点.计算结果表明,当适当调节两束激光场的波长,相位以及时间延迟时,不仅高次谐波谱的平台区域能得到很大的扩展,而且谐波的强度也有较大的增强,形成了一个500eV的超长平台区.最后,通过适当的叠加谐波平台上的谐波,可获得一系列脉宽为35as左右的水窗区间阿秒脉冲.     

Mapping attosecond electron wave packet motion

Attosecond pulses are produced when an intense infrared laser pulse induces a dipole interaction between a sublaser cycle recollision electron wave packet and the remaining coherently related bound-state population. By solving the time-dependent Schr?dinger equation we show that, if the recollision electron is extracted from one or more electronic states that contribute to the bound-state wave packet, then the spectrum of the attosecond pulse is modulated depending on the relative motion of the continuum and bound wave packets. When the internal electron and recollision electron wave packet counterpropagate, the radiation intensity is lower. We show that we can fully characterize the attosecond bound-state wave packet dynamics. We demonstrate that electron motion from a two-level molecule with an energy difference of 14 eV, corresponding to a period of 290 asec, can be resolved.     

Characterization of attosecond XUV pulses from photoelectron spectra of atoms

A method to characterize attosecond extreme ultra violet (XUV) pulses from photoelectron spectra of atoms is presented. A pump pulse prepares a coherent superposition of two atomic bound states, from which photoionization takes place after variable time delays by the attosecond XUV pulse. Information on the spectral phase of the attosecond XUV pulse is extracted from the analysis of photoelectron spectra as a function of photoelectron energy and time delay. Together with information on the spectral intensity obtained from a separate optical measurement, a temporal shape of the attosecond XUV pulse can be precisely reconstructed. After the theoretical formulation of the problem, we present numerical examples for H atom and show that, depending on the choice of energy separation of two bound states, a different accuracy is reached to characterize attosecond XUV pulses.     

Single attosecond pulse generation with intense mid-infrared elliptically polarized laser pulses

We have studied theoretically high-harmonic-order and single attosecond pulse generation with elliptically polarized laser pulses at wavelengths ranging from the visible to the mid-infrared. Results of ab initio simulations of the time-dependent Schr?dinger equation show that the ellipticity dependence of the high-harmonic signal intensifies with increasing wavelength of the driving pulse and saturates in the mid-infrared. The isolation of single attosecond pulses using the polarization gating method in the mid-infrared is due to an effective suppression of side pulses as compared with an operation at Ti:sapphire wavelengths.     

Chirped attosecond photoelectron spectroscopy

We study analytically the photoionization of a coherent superposition of electronic states and show that chirped pulses can measure attosecond time scale electron dynamics just as effectively as transform-limited attosecond pulses of the same bandwidth. The chirped pulse with a frequency-dependent phase creates the interfering photoelectron amplitudes that measure the electron dynamics. We show that at a given pump-probe time delay the differential asymmetry oscillates as a function of photoelectron energy. Our results suggest that the important parameter for attosecond science is not the pulse duration, but the bandwidth of phased radiation.     

Waveform-controlled near-single-cycle milli-joule laser pulses generate sub-10 nm extreme ultraviolet continua

We demonstrate the generation of waveform-controlled laser pulses with 1?mJ pulse energy and a full-width-half-maximum duration of ～4 fs, therefore lasting less than two cycles of the electric field oscillating at their carrier frequency. The laser source is carrier-envelope-phase stabilized and used as the backbone of a kHz repetition rate source of high-harmonic continua with unprecedented flux at photon energies between 100 and 200?eV (corresponding to a wavelength range between 12-6?nm respectively). In combination we use these tools for the complete temporal characterization of the laser pulses via attosecond streaking spectroscopy.     

Generation of isolated attosecond pulses with a specific waveform two-color laser field

We theoretically propose a new method for generating intense isolated attosecond pulses during high-order harmonic generation (HHG) process by accurately controlling electron motion with a two-color laser field,which consists of an 800-nm,4-fs elliptically polarized laser field and a 1400-nm,～43-fs linearly polarized laser field.With this method,the supercontinua with a spectral width above 200 eV are obtained,which can support a ～15-as isolated pulse after phase compensation.Classical and quantum analyses explain the controlling effects well.In particular,when the pulse duration of the 800-nm laser field increases to 20-fs,sub-100-as isolated pulses can be obtained even without any phase compensation.     

High-order harmonics extension and isolated attosecond pulse generation in three-color field: Controlling factors

In the present Letter, we theoretically discuss the optimum conditions for generating ultrashort attosecond pulse in three-color field with a model He exposed to the intense 5 fs, 800 nm fundamental field and the two weak control fields of 25 fs, 400 nm and 25 fs, 1600 nm. Through investigating the controlling factors in HHG spectra generation via manipulating the laser parameters of the three fields, we demonstrate that properly increasing the pulse intensity of 800 nm and 1600 nm fields at the same time with zero phase effects is an effective way to generate short attosecond pulses. Finally, an isolated pulse of 7 as is predicted by Fourier transforming an ultrabroad XUV continuum of 393 eV with phase compensation.     

Single attosecond pulse generation in the multicycle-driver regime by adding a weak second-harmonic field

We present a method of producing single attosecond pulses by high-harmonic generation with multicycle driver laser pulses. This can be achieved by tailoring the driving pulse so that attosecond pulses are produced only every full cycle of the oscillating laser field rather than every half-cycle. It is shown by classical and quantum-mechanical model calculations that even a minor addition (1%) of phase-locked second-harmonic light to the 800 nm fundamental driver pulse for high-harmonic generation leads to a major (15%) difference in the maximum kinetic energies of the recombining electrons in adjacent half-cycles.     

阿秒脉冲测量的研究进展

对于发生在原子范围内的电子动力学过程的观测需要阿秒量级的时间分辨率.理论和实验研究都已证明,用周期量级超短脉冲直接泵浦的高次谐波过程可以产生阿秒脉冲序列甚至单个的阿秒脉冲,将阿秒脉冲用于测量超快动力学过程之前先要对阿秒脉冲本身性质做出描述,传统的自相关方法和互相关方法不能直接推广到阿秒量级超短脉冲的测量.文章详细介绍了近几年发展起来的阿秒脉冲测量方法,分析了它们的分辨极限和局限性.     

Attosecond angle-resolved photoelectron spectroscopy

We report experiments on the characterization of a train of attosecond pulses obtained by high-harmonic generation, using mixed-color (XUV+IR) atomic two-photon ionization and electron detection on a velocity map imaging detector. We demonstrate that the relative phase of the harmonics is encoded both in the photoelectron yield and the angular distribution as a function of XUV-IR time delay, thus making the technique suitable for the detection of single attosecond pulses. The timing of the attosecond pulse with respect to the field oscillation of the driving laser critically depends on the target gas used to generate the harmonics.