靶源位置对强场高次谐波相位匹配的影响

为提高强激光场与惰性气体靶作用产生的孤立阿秒激光脉冲的能量,给出了一种实现高次谐波过程中最佳谐波相位匹配的定量实验方法。研究了气体靶源与高斯型驱动激光场聚焦点相对空间位置对谐波相位匹配及谐波产率的影响,得出了其最佳相位匹配位置始终位于驱动激光场聚焦点后3~5 mm,而在聚焦点之前的位置区域,严重的高次谐波相位失配导致谐波产率非常低。同时,在最佳相位匹配条件下,高次谐波场与驱动场具有相类似的空间强度分布特性,该结果印证了目前通常采用的高次谐波场为高斯光束的假设。     

激光脉冲绝对相位对产生阿秒脉冲的影响

在用超短激光脉冲产生和测量阿秒脉冲的过程中，绝对相位成为一个非常重要的影响因素。讨论了绝对相位对阿秒脉冲的产生和激光振荡的关系的影响，发现高次谐波的产生过程非常稳定，阿秒脉冲和驱动它的激光振荡之间的稳定的相位关系并不会受绝对相位的随机性的破坏，从而为采用交叉相关技术直接测量由强场原子高次谐波过程产生的阿秒脉冲宽度提供了理论依据，正确解释了文献[13]上报道的实验结果。     

一维多原子分子离子与超短激光场作用产生的高次谐波

从经典方程出发，采用克兰克-尼科尔森中心差分法对一维多原子分子离子与一维超短强激光场相互作用产生的高次谐波进行了数值模拟计算，发现激光场的初始相位对高次谐波辐射谱有明显影响；通过进一步研究，给出了初步解释。从而，可以通过调节适当的激光初始相位和选取适当的系统来改善高次谐波的相位匹配，产生清晰的高次谐波辐射谱。     

极化门控制下中红外激光场驱动He原子产生的单个阿秒脉冲

由中红外激光场(波长为2128nm)驱动He原子,在极化门的控制下,通过强场近似方法(SFA)研究了He原子发射高次谐波的特点.研究表明,在这种组合场驱动下He原子可产生截止位置很高的高次谐波,并且在接近截止位置的平台区展现了超连续的特点,对该超连续部分的高次谐波进行叠加,可得到宽度为44.5as的单个超短脉冲.为了了解该超短阿秒脉冲的产生机理,我们对高次谐波谱的发射过程进行了时频分析,分析表明由于极化门的存在,有效地抑制了极化门以外的阿秒脉冲的发射,从而获得单个阿秒脉冲.     

极化门控制下中红外激光场驱动He原子产生的单个阿秒脉冲

由中红外激光场（波长为2128nm）驱动He原子,在极化门的控制下,通过强场近似方法(SFA)研究了He原子发射高次谐波的特点.研究表明,在这种组合场驱动下He原子可产生截止位置很高的高次谐波,并且在接近截止位置的平台区展现了超连续的特点,对该超连续部分的高次谐波进行叠加,可得到宽度为44.5as的单个超短脉冲．为了了解该超短阿秒脉冲的产生机理,我们对高次谐波谱的发射过程进行了时频分析,分析表明由于极化门的存在,有效地抑制了极化门以外的阿秒脉冲的发射,从而获得单个阿秒脉冲．     

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     

利用相位啁啾单色中红外激光场产生高次谐波

本文通过求解薛定谔方程,理论探索了高次谐波及孤立阿秒脉冲在相位调制的单色中红外激光场中的产生.研究结果表明,通过加入含时相位啁啾小量,能够有效的提高高次谐波的产生效率,获得超宽平台谐波谱及超短孤立阿秒脉冲.特别是在相位为0.3π的基础上,同时加入啁啾小量βt,在β=0.3时,可得到带宽为822 e V的超连续高次谐波平台.最后通过叠加第二平台高次谐波,可得到带宽仅为2.7 as的孤立阿秒脉冲.且脉冲强度比没有加入啁啾小量的情况下显著增强.     

7 fs超快强激光驱动Ar原子产生支持单个阿秒脉冲的高次谐波连续谱

采用脉冲宽度为7 fs,脉冲能量为0.4 mJ的超快强激光脉冲与气体盒子中Ar原子作用获得了高次谐波截止区连续谱,并发现当驱动激光稳定在不同的载波包络相位时,高次谐波的谱结构、谱调制深度和连续谱的带宽都有很大区别。在某些载波包络相位时获得了平滑的连续谱,调制深度小于17%,连续带宽达10 eV,从而支持时域上获得变换极限500 as的单个阿秒脉冲。     

利用脉宽10 fs偏振控制脉冲获得孤立阿秒脉冲

利用强场近似理论开展了具有较长脉宽的偏振控制脉冲与氦原子相互作用产生高次谐波和阿秒脉冲发射的理论研究.研究发现,当具有10 fs脉冲宽度的偏振控制脉冲被用作驱动脉冲时,只要恰当地调整两束反向旋转圆偏振脉冲峰值之间的时间延迟和强度比,即使不附加二次谐波脉冲,仍然可以得到效率较高且规则分布的高次谐波平台结构,傅里叶变换后得到了175 as的孤立短脉冲.该方案一方面通过调整两束脉冲峰值之间时间延迟突破了传统偏振控制方案中要求偏振门宽度为半个光学周期的限制,另一方面通过调整两束脉冲峰值之间的强度比避免了偏振门前端多个光学周期电场引起气体介质电离不利于谐波相位匹配的弊端.     

激光等离子体中由共振吸收诱发的高次谐波

用电容器模型计算了激光打靶过程中由共振吸收诱发的高效谐波，并在此基础上讨论了超短脉冲强激光的高次谐波作为一种短波长相干辐射源的可能性。     

采用附加共振高频场提高强场高次谐波的产生效率

提高高次谐波效率一直是强场物理研究中的一个重要课题,对此人们采取了很多方法,如采用双色场,实现相位匹配,附加一个强电场或强磁场等。本文在研究双色场高次谐波的过程中,发现原子的能级结构对于高次谐波的产生效率有极大的影响,据此提出利用附加共振高频场来大幅度提高高次谐波的产生效率。     

Optimization of multi-color laser waveform for high-order harmonic generation

With the development of laser technologies,multi-color light-field synthesis with complete amplitude and phase control would make it possible to generate arbitrary optical waveforms.A practical optimization algorithm is needed to generate such a waveform in order to control strong-field processes.We review some recent theoretical works of the optimization of amplitudes and phases of multi-color lasers to modify the single-atom high-order harmonic generation based on genetic algorithm.By choosing different fitness criteria,we demonstrate that:(i) harmonic yields can be enhanced by 10 to 100 times,(ii) harmonic cutoff energy can be substantially extended,(iii) specific harmonic orders can be selectively enhanced,and(iv) single attosecond pulses can be efficiently generated.The possibility of optimizing macroscopic conditions for the improved phase matching and low divergence of high harmonics is also discussed.The waveform control and optimization are expected to be new drivers for the next wave of breakthrough in the strong-field physics in the coming years.     

Characteristics of High Harmonics Generated in a Gas Filled Microcapillary

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Frequency dependence of quantum path interference in non-collinear high-order harmonic generation

High-order harmonic generation(HHG) driven by two non-collinear beams including a fundamental and its weak second harmonic is numerically studied. The interference of harmonics from adjacent electron quantum paths is found to be dependent on the relative delay of the driving pulse, and the dependences are different for different harmonic orders.This frequency dependence of the interference is attributed to the spatial frequency chirp in the HHG beam resulting from the harmonic dipole phase, which in turn provides a potential way to gain an insight into the generation of high-order harmonics. As an example, the intensity dependent dipole phase coefficient α is retrieved from the interference fringe.     

Probe of high-order harmonic generation in a hollow waveguide geometry using counterpropagating light

We use counterpropagating light to directly observe the coherent buildup of high harmonic generation in a hollow waveguide geometry. We measure, for the first time, coherence lengths for high photon energies that cannot be phase matched using conventional approaches. We also probe the transition through phase matching, the ionization level at which different harmonic orders are generated, and the change in the coherence length as the driving laser is depleted. These results directly prescribe the optimal structures or pulse trains required for implementing quasiphase matching.     

A nonperturbative quantum electrodynamic approach to the theory of laser induced high harmonic generation

Using nonperturbative quantum electrodynamics, we develop a scattering theory for high harmonic generation (HHG). A transition rate formula for HHG is obtained. Applying this formula, we calculate the spectra of high harmonics generated from different noble gases shined by strong laser light. We study the cutoff property of the spectra. The data show that the cutoff orders of high harmonics are greater than that predicted by the “3.17” cutoff law. As a numerical experiment, the data obtained from our repeated calculations support the newly derived theoretical expression of the cutoff law. The cutoff energy of high harmonics described by the new cutoff law, in terms of the ponderomotive energy U_p and the ionization potential energy I_p, is 3.34U_p+ 1.83I_p. The higher cutoff orders predicted by this theory are due to the absorption of the extra photons, which participate only the photon-mode up-conversion and do nothing in the photoionization process.     

耦合对相对论效应下的三次谐波的影响

研究了相对论效应引起的场量的非线性对谐波辐射源的影响；在入射光为长脉冲激光并计及各阶谐波间的耦合时，解析研究了三次谐波的振幅、频率、相位的变化及增长与饱和特性并计算了转化率，结果表明谐波间的耦合使幅值和转化率减少。     

Parametric phase conjugation for the second harmonic of a nonlinear ultrasonic beam

The effect of phase conjugation for the second harmonic of a focused ultrasonic beam was investigated experimentally and by numerical simulation. An ultrasonic pulse with the carrier frequency f=3 MHz was emitted into water and focused at a point between the source and the phase conjugating system. The phase conjugation for the second harmonic of the incident wave (2f=6 MHz) was performed in a magnetostrictive ceramic as a result of the parametric interaction of the incident wave with the pumping magnetic field (the pumping frequency was f_p=4f=12 MHz). The axial and focal distributions of sound pressure in the incident and conjugated beams were measured using a broadband PVDF membrane hydrophone. The corresponding calculations were performed by solving numerically the Khokhlov-Zabolotskaya-Kuznetsov (KZK) equation allowing for the nonlinearity, diffraction, and thermoviscous absorption. The results of measurements agreed well with the calculations and showed that the field of a conjugate wave adequately reproduces the field of the second harmonic of the incident wave. A certain advantage of focusing with the phase conjugation for the second harmonic was demonstrated in comparison with the operation at the doubled frequency of the incident wave. The results of this study can serve as a basis for the utilization of the phase conjugation of harmonics in ultrasonic tomography and nondestructive testing.     

Wave-front phase measurements of high-order harmonic beams by use of point-diffraction interferometry

We report the wave-front phase measurement of high-order harmonics employing point-diffraction interferometry. The high-order harmonics generated in a gas-filled hollow tube showed excellent spatial coherence over nearly the whole cross section of the harmonic beams. Using this coherent harmonic source in the extreme-ultraviolet-soft-x-ray region, we have demonstrated the operation of a point-diffraction interferometer and measured the wave-front phase of high-order harmonics.     

Molecular recollision interferometry in high harmonic generation

We use extreme-ultraviolet interferometry to measure the phase of high-order harmonic generation from transiently aligned CO(2) molecules. We unambiguously observe a reversal in phase of the high-order harmonic emission for higher harmonic orders with a sufficient degree of alignment. This results from molecular-scale quantum interferences between the molecular electronic wave function and the recolliding electron as it recombines with the molecule, and is consistent with a two-center model. Furthermore, using the combined harmonic intensity and phase information, we extract accurate information on the dispersion relation of the returning electron wave packet as a function of harmonic order. This analysis shows evidence of the effect of the molecular potential on the recolliding electron wave.