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.     

三色激光控制量子路径生成短于30阿秒的孤立脉冲

采用多周期的800 nm钛宝石激光组合1600 nm中红外脉 冲辐照氦离子产生高次谐波发射功率谱. 相对于单色场情形, 谐波谱出现明显的双平台结构, 且在第二平台区出现了光滑的连续辐射谱, 其转化效率相对于第一平台低了约两个数量级. 通过附加脉宽为1 fs的27次谐波脉冲到双色激光场的特定时域, 可以控制电子电离在半个光学周期内迅速提升, 获得了由单一短量子路径贡献的连续辐射谱, 使得第二平台区谐波的转化效率相对于组合场情形增强4个数量级, 且连续谱的频谱范围从第二平台区扩展到第一平台区, 叠加该连续谱190次到285次谐波生成了脉宽为29 as的强、短孤立脉冲.     

Generation of the wavelength-tunable XUV pulse using the two-color and three-color infrared pulses

We present an efficient method to generate an ultrashort wavelength-tunable XUV pulse by using the harmonic selec- tive enhancement scheme. The results show that by properly controlling the delay times of a two-color field or a three-color field, selective enhancement of the harmonics with photon energies between 80 eV and 315 eV can be obtained. Fur- ther, a wavelength-tunable and bandwidth-controllable XUV radiation can be obtained by Fourier transformation of these enhanced harmonics.     

Attosecond pulse generation from two-electron harmonic emission spectrum

In this paper, we theoretically investigate the high-order harmonic generation and attosecond pulse generation when a two-electron He atom is exposed to the intense laser pulse. It shows that due to the two-electron double recombination mechanism, an extended plateau beyond the classical single-electron harmonic has been obtained on the two-electron harmonic spectrum. Further by using this two-electron harmonic extension scheme combined with the two-color field, two supercontinuum bandwidths with 200 e V have been obtained. As a result, a series of sub-60 as extreme ultraviolet(XUV)pulses have been directly generated.     

Enhancement and broadening of extreme-ultraviolet supercontinuum in a relative phase controlled two-color laser field

We experimentally demonstrate the generation of an extreme-ultraviolet (XUV) supercontinuum in argon with a two-color laser field consisting of an intense 7 fs pulse at 800 nm and a relatively weak 37 fs pulse at 400 nm. By controlling the relative time delay between the two laser pulses, we observe enhanced high-order harmonic generation as well as spectral broadening of the supercontinuum. A method to produce isolated attosecond pulses with variable width and intensity is proposed.     

利用远紫外光产生多重谐波截止能量延伸

数值研究了氦离子在红外激光场与远紫外激光场驱动下辐射高次谐波的过程.计算结果表明,当远紫外激光场加入后,电子在与母核回碰的过程中会再吸收远紫外光子,进而导致谐波截止能量以远紫外光子能量为间隔的延伸.随着远紫外激光场场强的增强,电子在回碰过程中会吸收更多的远紫外光子,因此多重谐波截止能量会进一步延伸.最后,随着红外激光场与远紫外激光场延迟时间的改变,多重谐波截止能量的延伸可以得到调控.     

Harmonic Extension and Enhancement Using a Two-Color Chirped Pulse and an Ultrashort Ultraviolet Pulse

A promising method to both extend the harmonic energy and enhance the harmonic intensity is presented when a He atom is exposed to the combination of a two-color chirped pulse and an ultrashort ultraviolet pulse. It is found that: (1) with the introduction of the two chirps, the harmonic energy has been remarkably extended; (2) by properly adding an ultrashort ultraviolet pulse to this synthesized two-color chirped pulse, the harmonic yield is enhanced by 3 orders of magnitude compared with the two-color chirped case; (3) the harmonic spectrum higher than the 96th order shows an ultrabroad supercontinuum with a 395 eV bandwidth, and the proper superposition over the harmonics directly produced a series of intense 40 as pulses which is 3 orders of magnitude higher than that in the two-color chirped case.     

传播效应对宽带极紫外连续谱的影响

宽带极紫外连续谱是获得阿秒脉冲的重要途径,在此过程中传播效应是影响宽带极紫外连续谱产生的重要因素,本文针对传播效应对宽带极紫外连续谱的影响,采用慢变波近似的一阶传播方程,研究了传播过程对双色场周期量级激光脉冲产生宽带连续谱及其对应的单个阿秒脉冲的影响.通过分析不同聚焦位置和介质长度的传播过程,发现介质位于焦点之后有利于产生连续谱,同时随着介质长度的变长,虽然单个阿秒脉冲峰值强度有所提高,但是产生的阿秒脉冲宽度也会增加.进一步的分析表明,在特定的双色场延迟优化下,传播效应不仅使得阿秒脉冲强度增强,还可以获得与单原子模型下具有同样脉宽的单个阿秒脉冲,而不会因为传播效应导致阿秒脉冲宽度变宽.     

利用超短的紫外光源来增强阿秒脉冲的强度

本文理论提出了一种利用超短紫外光源来增强阿秒脉冲强度的方法. 计算结果表明, 当适当的加入一束62nm或者125nm超短紫外光源到双色激光场时, 不仅高次谐波的截止能量被延伸了, 而且谐波的强度也比原双色场时有明显的增强. 尤其是当采用125nm光源时,谐波的强度被增强了2个数量级,并且形成了一个232eV的超长平台区. 最后通过叠加谐波次数从113次到170次可获得脉宽仅为43as的孤立阿秒脉冲, 其强度比原双色场情况下产生的阿秒脉冲增强了2个数量级.     

利用超短紫外光源增强阿秒脉冲的强度

提出了一种利用超短紫外光源来增强阿秒脉冲强度的方法. 计算结果表明,当适当的加入一束125 nm光源到双色正交激光场时,不仅高次谐波的强度比原双色场时增强了2个数量级,而且选择单一的量子路径对谐波发射起作用,进而形成了一个152 eV的超长平台区. 随后通过激光参数的优化,发现激光脉宽和偏振角对于谐波强度的增强不太敏感. 最后,通过叠加谐波可获得脉宽仅为38 as 的孤立阿秒脉冲,其强度比原双色场情况下产生的阿秒脉冲增强了2个数量级.     

利用超短的紫外光源来增强阿秒脉冲的强度

本文理论提出了一种利用超短紫外光源来增强阿秒脉冲强度的方法 .计算结果表明,当适当的加入一束62 nm或者125 nm超短紫外光源到双色激光场时,不仅高次谐波的截止能量被延伸了,而且谐波的强度也比原双色场时有明显的增强.尤其是当采用125 nm光源时,谐波的强度被增强了2个数量级,并且形成了一个232 e V的超长平台区.最后通过叠加谐波次数从113次到170次可获得脉宽仅为43 as的孤立阿秒脉冲,其强度比原双色场情况下产生的阿秒脉冲增强了2个数量级.     

Generation of high-order harmonic continuum with two-hump-structure infrared laser pulse

We experimentally investigate the high-order harmonic generation in argon gas cell driven by a multi-cycle broadband infrared laser pulse from a tunable optical-parametric-amplifier （OPA） source. The generation of high-order harmonic continuum with the cut-off photon energy up to 110 eV is observed by tuning the chirp of the 800-nm laser pulse which pumps OPA source. The generation of harmonic continuum is understood in terms of the two-hump structure of the OPA output spectrum and the optimal relative phase of the two humps. The demonstrated scheme is of importance for the generation of extreme ultraviolet （XUV） continuum at higher photon energy region.     

Generation of isolated attosecond pulses in bowtie-shaped nanostructure with three-color spatially inhomogeneous fields

We theoretically investigate high-order harmonic generation in a two-color multi-cycle inhomogeneous field combined with a 27 th harmonic pulse. By considering a bowtie-shaped gold nanostructure, the spatiotemporal profiles of enhanced plasmonic fields are obtained by solving the Maxwell equation using finite-domain time-difference method. Based on quantum-mechanical and classical models, the effect of 27 th harmonic pulse, temporal profile of enhanced plasmonic field and inhomogeneity on supercontinuum generation are analyzed and discussed. As a result, broadband supercontinuum can be generated from our approach with optimized gap size of nanostructure. Moreover, these results are not sensitively dependent on the relative phase in the two-color field.     

双色场驱动不对称分子气体产生平台区超连续光谱

研究了双色场驱动下不对称分子气体产生的高次谐波. 通过计算机数值模拟,发现当波长为800 nm,脉宽为10 fs的基频场和波长为1200 nm的控制场组成的多光周期双色场作用于不对称分子时,不对称分子的高次谐波在平台区的末端呈现出超连续结构. 超连续谱谱宽达80 eV,可以支持傅里叶极限脉宽为50 as的脉冲输出. 传播后,超连续谱变得更平滑,而且,直接选取超连续谱上的一段,可以得到脉宽约为90 as的单个脉冲输出. 关键词： 超连续谱 阿秒脉冲     

附加谐波脉冲生成强的39阿秒孤立脉冲

采用多周期800 nm激光组合它的27次谐波脉冲生成超短、宽频的孤立阿秒脉冲.研究表明,脉宽为1 fs的27次谐波脉冲可以有效地控制电子动力学过程.将其加入到单色激光场的特定时域,可以控制电离概率在半个光学周期内迅速提升,使得谐波的转化效率相对于单色场情形增强4个数量级,并实现单一的短量子路径选择,出现频宽为108 eV的超连续辐射谱,叠加该连续谱140次到210次谐波获得了脉宽为39as的强、短孤立脉冲.与文献[7]中采用5 fs/800 nm激光附加紫外阿秒脉冲的方法相比,该方案生成的连续谱频带增宽一 关键词： 组合激光脉冲 连续辐射谱 阿秒脉冲     

60 fs长脉宽双色场作用下孤立阿秒脉冲的产生

通过数值求解一维含时Schrödinger方程,研究了具有较长脉宽双色激光脉冲与氢原子相互作用产生的高次谐波和阿秒(as)脉冲.这里的双色激光脉冲由一束基频钛宝石主脉冲与另一束红外附加脉冲构成.研究发现,当合成脉冲的脉宽选为12 fs时, 选取合适的附加脉冲波长,合成电场的振幅在始末端时间段能被大幅抑制, 仅中间部分的电场对谐波谱平台区和截止位置起主要贡献. 通过分析合成脉冲电场始末端时间段抑制的机理,进一步扩展了合成脉冲脉宽到60 as, 并得到160 as的孤立短脉冲.这是迄今为止在孤立阿秒脉冲产生研究中所采用的最长脉宽. 该方案中的合成脉冲等效于单一5 fs短脉冲的作用, 却克服了5 fs脉冲低输出能量导致的阿秒脉冲能量低的困难.     

Coherent Control of High Harmonic Generation Driven by Metal Nanotip Photoemission

Steering ultrafast electron dynamics with well-controlled laser fields is very important for generation of intense supercontinuum radiation. It can be achieved through coherent control of the symmetry of the interaction between strong-field laser fields and a metal nanotip. We employ a scheme of two-color laser pulses combined with a weak static field to realize the control of a single quantum path to generate high harmonic generation from a single solid-state nanoemitter. Moreover, a smooth and ultrabroad supercontinuum in the extreme ultraviolet region is obtained, which can produce a single attosecond pulse. Our findings are beneficial for efficient generation of isolated sub-100 as XUV pulses from solid-state sources.     

三色圆偏振激光组合脉冲驱动氦原子产生椭圆偏振的阿秒脉冲

我们提出利用频率为ω,3ω和强度较弱的2ω组成的三色圆偏振激光组合脉冲驱动氦原子能够得到椭偏率较大的阿秒脉冲链的一种方法.通过强场近似方法,计算了氦原子在两色和三色圆偏振激光组合脉冲驱动下发射高次谐波谱及其合成阿秒脉冲链,比较了氦原子(初态为s态)在这两种情况下发射高次谐波谱的特点及其合成阿秒脉冲链椭偏率的大小,结果发现,在反旋的两色ω,3ω激光脉冲基础上加入了频率为2ω的第三色激光脉冲联合作用到氦原子上,所得到的阿秒脉冲链的椭偏率相对于双圆场情况下有所增加,通过调整ω,3ω激光的强度比,并且选择适当的第三色激光的强度,对初态为s态的原子,仍能够得到具有较大椭偏率的阿秒脉冲链.     

Efficient generation of third harmonic radiation in air filaments: A revisit

In this paper we revisit the third harmonic generation from a femtosecond laser filament in air and its significant enhancement (~ 100 times) with a intercepting pump pulse, which has been reported very recently. The enhanced third harmonic is studied as a function of the pulse polarization, time delay between the pump and signal pulses, laser pulse energy, etc. We provide an explanation for the enhancement of third harmonic based on a quenching of interference effects present near filamentation threshold. Simulations based on a two-color propagation code reproduce well the experimental observations and confirm our interpretation.     

Generation of an isolated sub-30 attosecond pulse in a two-color laser field and a static electric field

We theoretically investigate high-order harmonic generation(HHG) from a helium ion model in a two-color laser field,which is synthesized by a fundamental pulse and its second harmonic pulse.It is shown that a supercontinuum spectrum can be generated in the two-color field.However,the spectral intensity is very low,limiting the application of the generated attosecond(as) pulse.By adding a static electric field to the synthesized two-color field,not only is the ionization yield of electrons contributing to the harmonic emission remarkably increased,but also the quantum paths of the HHG can be significantly modulated.As a result,the extension and enhancement of the supercontinuum spectrum are achieved,producing an intense isolated 26-as pulse with a bandwidth of about 170.5 eV.In particular,we also analyse the influence of the laser parameters on the ultrabroad supercontinuum spectrum and isolated sub-30-as pulse generation.