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.     

利用三色组合脉冲激光获得孤立阿秒脉冲发射

孤立阿秒脉冲因可以跟踪和控制原子及分子内电子的运动过程而备受关注.本文从理论上开展了氦原子在3束飞秒脉冲激光组合场辐照下产生的高次谐波和阿秒脉冲辐射的研究.组合激光场由16 fs/1600 nm,15 fs/1100 nm和5.3 fs/800 nm的钛宝石脉冲构成.与前两束脉冲合成的双色场产生谐波谱相比,附加钛宝石脉冲的三色场产生的高次谐波发射谱呈现出高转换效率及宽带超连续特性,超连续谱范围覆盖从230—690次谐波,傅里叶变换后实现了128 as高强度孤立短脉冲的产生.该结果归因于合成的三色场呈现出高功率及少周期的中红外飞秒脉冲激光特性,可以有效控制原子电离以及复合发生在中红外飞秒脉冲的一个有效光学周期内.     

Controls for the generation of high-order harmonics and attosecond pulses by an infrared laser field combined with a low-frequency pulse

We investigate high-order harmonic generations by controlling various quantum paths of harmonics in an infrared laser field which combines a low-frequency pulse.Both classical theory and the quantum wavelet transform method are used to understand the physics of harmonics.By adjusting the carrier envelope phase of the fundamental field,the intensities of harmonic spectra increase and the harmonics in the plateau become regular.Attosecond pulses each with a duration of 58 as are obtained directly by compressing the harmonics,and with phase compensation an isolated attosecond pulse less than 30 as can be generated.     

Controls for the generations of high-order harmonics and attosecond pulses by infrared laser field combined with a low-frequency pulse

We investigate high-order harmonic generations by controlling various quantum paths of harmonics in an infrared laser field which combines a low-frequency pulse. Both classical theory and quantum wavelet transform method are used to understand the physics of harmonics. By adjusting the carrier envelope phase of the fundamental field, the intensities of harmonic spectra increase and the harmonics in the plateau become regular. Attosecond pulses each with a duration of 58 as are obtained directly by compressing the harmonics, and with phase compensation an isolated attosecond pulse less than 30 as can be generated.     

基于半解析自洽理论研究相对论激光脉冲驱动下阿秒X射线源的产生

发展了一种描述相对论激光脉冲与稠密等离子体相互作用产生阿秒X射线源的半解析自洽理论.该理论模型不仅可以获得等离子体界面的振荡轨迹、振荡面电场和磁场等物理参数,而且能够精确计算出激光脉冲驱动下阿秒X射线源的频谱,结果与粒子模拟程序一致.理论计算结果表明阿秒X射线源的辐射特性与等离子体界面随时演化过程相关,在周期量级激光场驱动下等离子体界面振荡振幅呈现中心不对称,通过改变激光场的载波包络相位实现对等离子体界面振荡的控制,获得准单阿秒X射线源.     

Chirped polarization-gating technique for isolated attosecond pulse generation

We propose a scheme to generate broadband high harmonic supercontinuum and single attosecond pulse emission in time domain by employing a chirped polarization-gating laser field. It is shown that this scheme can significantly extend the high harmonic cutoff to higher energy region, and the produced isolated attosecond pulse is shorter than 100 attoseconds after proper dispersion compensation, when a 7 fs driving laser pulse is used.     

采用较长脉宽双色场获得孤立阿秒脉冲（第15届全国原子与分子物理会议）

通过数值求解一维含时薛定谔方程,本文研究了具有较长脉宽双色激光脉冲与氢原子模型相互作用产生的谐波辐射谱。研究发现,当采用12个光学周期的特定组合脉冲时,孤立阿秒脉冲可以实现。这里特定组合脉冲由两束类似频率脉冲( )构成,附加脉冲的作用恰是调整基频光电场的形状。不同于入射基频光,整形后的基频光在时间始末端电场振幅被抑制,只有中间部分电场对谐波谱有贡献,它的作用等同于单一6飞秒超短脉冲的效果。从而扩展获得孤立阿秒脉冲所需脉宽从6飞秒到12飞秒。     

采用较长脉宽双色场获得孤立阿秒脉冲

通过数值求解一维含时薛定谔方程,本文研究了具有较长脉宽双色激光脉冲与氢原子模型相互作用产生的谐波辐射谱.研究发现,当采用12个光学周期的特定组合脉冲时,孤立阿秒脉冲可以实现.这里特定组合脉冲由两束类似频率脉冲(ω,ω+δω)构成,附加脉冲的作用恰是调整基频光电场的形状.不同于入射基频光,整形后的基频光在时间始末端电场振幅被抑制,只有中间部分电场对谐波谱有贡献,它的作用等同于单一6飞秒超短脉冲的效果.从而扩展获得孤立阿秒脉冲所需脉宽从6飞秒到12飞秒.     

静电场对高次谐波时频特性的影响

通过数值求解含时薛定谔方程，发现在静电场的作用下，对原子高次谐波谱截止区附近的谐波进行叠加可以得到脉冲间隔为一个光周期的阿秒脉冲链. 利用这种性质，用12fs波长为800nm的激光附加强度为激光强度0.25%的静电场脉冲驱动He原子可以得到脉宽为220as的单个脉冲. 当静电场与激光场幅度之比达到0.39时，原子谐波谱的截止区可以被拓宽到I_p+9.1U_p，同时，次数高于I_p+0.7U_p的谐波的相位几乎是锁定的，因此可以得到谱宽大于40eV且相位锁定的谐波. 对这部分谐波进行叠加就可以得到脉宽小于100as的阿秒脉冲链.     

Ionization phase-match gating for wavelength-tunable isolated attosecond pulse generation

High-order harmonic emission can be confined to the leading edge of an 800 nm driver laser pulse under moderately intense focusing conditions (7×10¹⁴ W/cm²) (Pfeifer et al. in Opt. Express 15:17120, 2007). Here, the experimentally observed curtailment of harmonic production on the leading edge of the driver pulse is shown to be controlled by an ionization-induced phase-matching condition. The transient plasma density inherent to the process of high-harmonic generation terminates the harmonic emission by an ultrafast loss of phase matching on the leading edge of the laser pulse. The analysis is supported by a reconstruction of the in situ intensity envelope of the driver pulse with attosecond temporal resolution, performed by measurements of the carrier-envelope phase dependence of individual half-cycle harmonic cutoffs. The method opens the way to wavelength-tunable isolated attosecond pulse generation.     

利用偏振门方案和静电场组合控制阿秒脉冲的产生

本文理论研究了5 fs,800 nm的左旋圆偏振和右旋圆偏振激光组合的偏振门方案叠加静电场作用下氦原子的高次谐波发射及阿秒脉冲的产生.研究发现,加入的静电场可以控制量子轨道,进而抑制量子干涉效应.当静电场系数α为0.1时,高次谐波谱出现双平台结构且比较平滑,通过小波分析,发现对第二平台有贡献的主要是一个短轨道,通过叠加第二平台区域50 eV(160 eV～210 eV)的连续谐波,我们得到了80 as的阿秒脉冲.     

Attosecond pulse trains generated using two color laser fields

We investigate the spectral and temporal structure of high harmonic emission from argon exposed to an infrared laser field and its second harmonic. For a wide range of generating conditions, trains of attosecond pulses with only one pulse per infrared cycle are generated. The synchronization necessary for producing such trains ensures that they have a stable pulse-to-pulse carrier envelope phase, unlike trains generated from one color fields, which have two pulses per cycle and a pi phase shift between consecutive pulses. Our experiment extends the generation of phase stabilized few cycle pulses to the extreme ultraviolet regime.     

Three-dimensional numerical investigations of the laser-beam interactions in an undulator

Laser-beam interaction in an undulator is commonly suggested in the development of free electron laser(FEL)schemes. In this paper, a three-dimensional algorithm is developed to assist in laser-beam interaction simulation in an undulator, which is built on the basis of the fundamentals of electrodynamics, i.e.the electron's behavior is determined by the magnetic field and the laser electric field in the time domain. On the basis of the algorithm, the detuning effect in a laser heater, the carrier envelope phase effect of a few-cycle laser in attosecond X-ray FEL schemes and output wavelength tuning in a high gain harmonic generation FEL are numerically discussed.     

Spatial control of recollision wave packets with attosecond precision

We propose orthogonally polarized two-color laser pulses to steer tunneling electrons with attosecond precision around the ion core. We numerically demonstrate that the angles of birth and recollision, the recollision energy, and the temporal structure of the recolliding wave packet can be controlled without stabilization of the carrier-envelope phase of the laser, and that the wave packet's properties can be described by classical relations for a point charge. This establishes unique mapping between parameters of the laser field and attributes of the recolliding wave packet. The method is capable of probing ionic wave packet dynamics with attosecond resolution from an adjustable direction and might be used as an alternative to aligning molecules. Shaping the properties of the recollision wave packet by controlling the laser field may also provide new routes for improvement of attosecond pulse generation via high harmonic radiation.     

Isolated sub-30-attosecond pulse generation using a multicycle two-color chirped laser and a static electric field

We present a theoretical investigation of high-order harmonic generation in a chirped two-color laser field, which is synthesized by a 10-fs/800-nm fundamental chirped pulse and a 10-fs/1760-nm subharmonic pulse. It is shown that a supercontinuum can be produced using the multicycle two-color chirped field. However, the supercontinuum reveals a strong modulation structure, which is not good for the generation of an isolated attosecond pulse. By adding a static electric field to the multicycle two-color chirped field, not only the harmonic cutoff is extended remarkably, but also the quantum paths of the high-order harmonic generation （HHG） are modified significantly. As a result, both the extension of the supercontinuum and the selection of a single quantum path are achieved, producing an isolated 23-as pulse with a bandwidth of about 170.6 eV. Furthermore, the influences of the laser intensities on the supercontinuum and isolated attosecond pulse generation are investigated.     

Conclusive evidence of an attosecond pulse train observed with the mode-resolved autocorrelation technique

We report on the direct observation of an attosecond pulse train with a mode-resolved autocorrelation technique. The chirp among the three harmonic fields is specified by analyzing two-photon above-threshold ionization spectra of electrons, resulting in a pulse duration that should be shorter than 450 as, which is, to our knowledge, the first determination of the chirp in the attosecond pulse train with an autocorrelation technique. These results will open the way to full characterization of an attosecond pulse train with its envelope.     

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.     

Generation of Near-Circularly Polarized Attosecond Pulse with Tunable Helicity by Unidirectionally Rotating Laser Field

A method to produce circularly polarized attosecond pulses with tunable helicity from CO molecule by using an unidirectionally rotating laser field is proposed. It is found that broadband harmonic supercontinuum with circular polarization can be generated from the oriented CO molecule. This enables the production sub-100 attosecond isolated pulse with the ellipticity as high as 0.9 at the macroscopic level. Moreover, the helicity of the generated high-order harmonics and the attosecond pulse can be tuned by adjusting the orientation of the CO molecule. This method will be beneficial for the studies on chiral-specific dynamics and magnetic circular dichroism on an attosecond time scale.     

Self‐Referenced Measurement of Light Waves

The wave nature of light has been known for more than 100 years. However, it is still very difficult to directly sample an optical field transient on a time scale below the oscillation period. It has been naturally believed that the field oscillation can be detected only by using a reference pulse that has a shorter duration than the period of the oscillation. In this Letter, an experimental demonstration of a self‐referenced light wave measurement is shown. Frequency‐resolved optical gating capable of carrier‐envelope phase determination was used to measure the complete electric field evolution of a 21‐fs few‐cycle pulse without any reference pulses. A second harmonic generation frequency‐resolved optical gating spectrogram and an interferogram between second harmonic and self‐diffraction signals were simultaneously recorded. The phase‐controlled light waves of few‐cycle pulses were clearly observed by using this method.