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.     

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.     

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.     

空间非均匀啁啾双色场驱动下氦离子的高次谐波以及孤立阿秒脉冲的产生

本文理论上研究了初态为基态与第一激发态等权叠加的一维氦离子在空间非均匀啁啾双色场驱动下氦离子的高次谐波发射及孤立阿秒脉冲的产生. 研究表明, 一维氦离子在空间非均匀啁啾双色场驱动下发射的高次谐波相对于均匀场情况截止位置得到明显扩展, 得到了光滑的超连续谱,并应用半经典三步模型解释了高次谐波发射的物理机理. 通过小波变换的方法对连续谱进行了时频分析, 并且与电子的经典运动轨迹进行了对比分析, 结果显示在空间非均匀场中长量子轨道消失, 短量子轨道加强. 讨论了空间非均匀啁啾双色场中时间延迟对谐波和孤立阿秒脉冲产生的影响, 发现适当调整时间延迟值可以得到较大延展的光滑的超连续谐波谱, 本方案中时间延迟为t₀=1.6πup/ω₁时得到了最大延展, 通过对谐波中600次到680次(80次)谐波合成得到32 as的孤立脉冲.     

Stochastic resonance in a time-delayed asymmetric bistable system with mixed periodic signal

This paper studies the phenomenon of stochastic resonance in an asymmetric bistable system with time-delayed feedback and mixed periodic signal by using the theory of signal-to-noise ratio in the adiabatic limit. A general approximate Fokker--Planck equation and the expression of the signal-to-noise ratio are derived through the small time delay approximation at both fundamental harmonics and mixed harmonics. The effects of the additive noise intensity $Q$, multiplicative noise intensity $D$, static asymmetry $r$ and delay time $\tau$ on the signal-to-noise ratio are discussed. It is found that the higher mixed harmonics and the static asymmetry $r$ can restrain stochastic resonance, and the delay time $\tau $ can enhance stochastic resonance. Moreover, the longer the delay time $\tau $ is, the larger the additive noise intensity $Q$ and the multiplicative noise intensity $D$ are, when the stochastic resonance appears.     

Anharmonic resonances with recursive delay feedback

We consider application of time-delayed feedback with infinite recursion for control of anharmonic (nonlinear) oscillators subject to noise. In contrast to the case of a single delay feedback, recursive delay feedback exhibits resonances between feedback and nonlinear harmonics, leading to a resonantly strong or weak oscillation coherence even for a small anharmonicity. Remarkably, these small-anharmonicity induced resonances can be stronger than the harmonic ones. Analytical results are confirmed numerically for van der Pol and van der Pol-Duffing oscillators.     

Bispectrum and reconstruction of modulation signal extracted from ship-radiated noise

I.IntroductionRecentyearshigh-ordcrstatisticshavebeguntofindwideapP1icabilityindiversefie1dssuchassonar,radar,plasmaphysics,biomedicine,seismicdataprocessing,harmonicretrieva1,adaptivefi1teringandarrayprocessingl'-'l=Thcsestatisticsnotonlyrevea1amp1itudeinforma-honaboutaprocess,butalsorevca1phaseinformation.Insigna1processing,thesearethreemotivationsbehindtheuseofhighcr-ordcrspectra:1.TosupprcssGaussiannoiseofunknownspectra1charactcristicsindctcction,paramcterestimation,andsignalreconstructi…     

Yield enhancement of elliptical high harmonics driven by bicircular laser pulses

We theoretically investigate the yield enhancement of elliptical high harmonics in the interaction of molecules with bicircular laser pulses by solving the time-dependent Schrödinger equation. It is shown that by adjusting the relative intensity ratio of the two bicircular field components in specific ranges the yield of the molecular high harmonics for the plateau and cutoff regions can be respectively enhanced. To analyze this enhancement phenomenon, we calculate the weights of the electron classical trajectories. Additionally, we also study the ellipticity distribution of harmonics for different intensity ratios. We find that these enhanced harmonics are elliptically polarized, which we mainly attribute to the recombination dipole moment of the major weighted trajectories. These enhanced elliptical extreme ultraviolet and soft x-ray radiations may serve as essential tools for exploring the ultrafast dynamics in magnetic materials and chiral media.     

利用空间非均匀场偏振门方案产生超短的X射线光源

理论提出了一种利用空间非均匀场偏振门方案延伸高次谐波截止能量以及获得超短X射线光源的方法。计算结果表明, 当适当调节两束圆偏振激光场的延迟时间以及空间非均匀参数时,不仅高次谐波谱的平台区域能得到很大的扩展,并且谐波谱上的干涉也明显减小,形成了一个由单一量子路径贡献而成的310eV的超长平台区。谐波相位分析显示,平台区谐波相位呈线性分布。最后,通过适当的叠加谐波平台上的谐波, 可获得一系列相位稳定的脉宽在50as以下的超短X射线光源。     

利用空间非均匀场偏振门方案产生超短的X射线光源

理论提出了一种利用空间非均匀场偏振门方案延伸高次谐波截止能量以及获得超短X射线光源的方法。计算结果表明, 当适当调节两束圆偏振激光场的延迟时间以及空间非均匀参数时,不仅高次谐波谱的平台区域能得到很大的扩展,并且谐波谱上的干涉也明显减小,形成了一个由单一量子路径贡献而成的310eV的超长平台区。谐波相位分析显示,平台区谐波相位呈线性分布。最后,通过适当的叠加谐波平台上的谐波, 可获得一系列相位稳定的脉宽在50as以下的超短X射线光源。     

原子在两色组合激光场中产生的单个阿秒脉冲

利用分裂算符法求解含时薛定谔方程,对一维氦原子处于红外场（IR）与紫外场（UV）组合的两色激光场中产生的高次谐波进行研究,分析了由高次谐波产生的阿秒脉冲的特征. 发现在组合场中可以通过增加红外场的强度,来缩短阿秒脉冲的宽度,通过调节组合场中两束激光的时间延迟,提高一个电子轨道对谐波的贡献,而抑制另一个电子轨道的贡献,从而得到低于100 as的单个脉冲. 关键词： 强激光场 组合场 高次谐波 阿秒脉冲     

任意偏振双色相干场高次谐波

系统地研究了任意偏振双色相干场高次谐波，结果表明单个圆偏振光不能产生高次谐波，在适当的条件下，某些谐波可以部分或完全消失，而某些谐波可以得到加强，产生更高次谐波高转换效率的条件是双色线偏振场的结合。     

激光脉宽对谐波频移和振幅强度的影响

数值研究了激光脉宽对H_2~+和T_2~+谐波辐射的影响.计算结果表明:(i)对于谐波频移现象:在少周期激光场下,H_2~+和T_2~+谐波辐射呈现红移.随着激光脉宽增大,H_2~+谐波辐射呈现蓝移; T_2~+谐波辐射红移减弱.(ii)对于谐波振幅强度:H_2~+和T_2~+谐波辐射强度会随着激光脉宽增大而增强.但是,在少周期激光场下,H_2~+谐波截至能量附近的强度要大于T_2~+.在多周期激光场下,T_2~+谐波截至能量附近的强度要大于H_2~+.     

Spontaneously generated walking X-shaped light bullets

In quadratic nonlinear media with normal dispersion and nonvanishing group velocity mismatch between fundamental wave and second-harmonic pulses, the wave packets of two harmonics can be locked together in propagation in the form of walking X-shaped light bullets. The output wave packets are developed into X-shaped light bullets with significant group delay time due to mutual dragging and significant shifting in spatiotemporal spectrum due to delayed nonlinear phase shift. We also show that spontaneously generated phase-front tilting can balance the dragging force induced by group velocity mismatch and lead to zero-velocity walking X-shaped light bullets.     

Nonadiabatic effects in the molecular high-order harmonic generation from two-center molecular ions

The nonadiabatic effects in the molecular high-order harmonic generation (MHHG) from H₂⁺ and its isotope have been theoretically investigated beyond the Born-Oppenheimer approximations. It is found that (i) due to the asymmetric contributions of the harmonic emission on the rising and the falling parts of the laser field, the frequency red-shifts in the MHHG can be found. As the initial vibrational state or the pulse intensity increases, the red-shifts of the harmonics are decreased and even the blue-shifts of the harmonics can be found in the higher pulse intensity with the higher vibrational state. With the increase of the pulse duration, the frequency red-shifts and the blue-shifts in the MHHG are decreased and enhanced, respectively. The shifts of the harmonics from the heavy nuclei are decreased compared with those from the light nuclei. (ii) Due to the coupling of the electron-nuclear dynamics, the non-odd harmonics can be produced at the larger internuclear distance. With the increase of the initial vibrational state or the pulse duration, the generations of the non-odd harmonics can be enhanced. As the pulse intensity increases, the intensities of the non-odd harmonics are first increased and then decreased at the much larger pulse intensity. Due to the slower nuclear motion, the generations of the non-odd harmonics are decreased as the nuclear mass increases. (iii) With the increase of the alignment angle of the molecule ions, the nonadiabatic effects in MHHG (including the shifts of the harmonics and the generations of the non-odd harmonics) are decreased.     

Enhancing and unmasking the harmonics of a complex tone

Alternately eliminating and reintroducing a particular harmonic of a complex tone can cause that harmonic to stand out as a pure tone-separately audible from the rest of the complex-tone background. In the psychoacoustical literature the effect is known as "enhancement." Pitch matching experiments presented in this article show that although harmonics above the 10th are not spectrally resolved, harmonics up to at least the 20th can be enhanced. Therefore, resolution is not required for enhancement. Further, during those experimental intervals in which a harmonic is eliminated, excitation pattern models suggest that listeners should be able to hear out a neighboring harmonic-separately audible from the background. The latter effect has been called "unmasking." In the present article we provide the first experimental evidence for unmasking. Harmonics of 200 Hz, with harmonic numbers between about 5 and 16, are readily unmasked. Their pitches are usually matched by sine tones with frequencies that are not exactly those of the unmasked harmonics but are shifted in a direction away from the frequency of the pulsed harmonic. Phase relationships among the harmonics that produce temporally compact cochlear excitation lead to reduced enhancement but greater unmasking.     

Harmonic generation by atoms in circularly polarized laser fields: far-off and near resonance regimes

The generation of harmonics by atoms interacting with two laser fields having coplanar circular polarizations and an integral frequency ratio is addressed through ab initio numerical simulations. A detailed characterization of a few specific harmonics is given. In particular, the two different cases where the total energy absorbed through photons is far off or close to the energy gap between different atomic states are investigated. It is found that the conversion efficiency in the harmonic generation is strongly dependent on the inner atomic structure and in certain specific cases it can be significantly enhanced within a small frequency range. PACS 42.50.Hz; 42.65.Ky     

Attosecond electron wave-packet interference observed by transient absorption

We perform attosecond time-resolved transient absorption spectroscopy around the first ionization threshold of helium and observe rapid oscillations of the absorption of the individual harmonics as a function of time delay with respect to a superimposed, moderately strong infrared laser field. The phase relation between the absorption modulation of individual harmonics gives direct evidence for the interference of transiently bound electronic wave packets as the mechanism behind the absorption modulation.     

HeH2+分子多通道谐波发射的理论研究

利用非波恩奥本海默近似模型,理论研究了强激光场下HeH2+分子发射高次谐波的特点。计算结果表明,HeH2+分子的谐波截止能量要大于经典预言值。理论分析表明,延伸的谐波截止能量是由于激发态激光诱导电子跃迁所产生的多通道电子电离-回碰过程所导致的。随后利用双色空间非均匀激光场机制,谐波截止能量进一步延伸,形成了1个由单一量子路径贡献而成的482eV的超长平台区。该辐射能量相当于单色激光场场强被增强了3倍的结果。并且高振动激发态会产生强度更高的谐波光谱。最后,通过适当的叠加谐波平台上的谐波, 可获得一系列脉宽在30as的超短X射线光源。