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.     

Direct observation of laser filamentation in high-order harmonic generation

We investigate the spatial evolution of a laser pulse used to generate high-order harmonics (orders ranging from 45 to 91) in a semi-infinite helium-filled gas cell. The 5 mJ, 30 fs laser pulses experience elongated focusing with two distinct waists when focused with f/125 optics in 80 Torr of helium. Extended phase matching for the generation of harmonics occurs in the region between the double foci of the laser, where the laser beam changes from diverging to converging.     

Control of high-order harmonic generation with two-colour laser field

We numerically investigate the high-order harmonic generation with two-colour optical field,taking into considera-tion the propagation effects.Some harmonics can be dramatically enhanced at a certain delay between the fundamental pulse and its second harmonics.Choice of the enhanced harmonics can be realised by changing the time delay between the two laser pulses.     

高次谐波辐射光子的能量-激光相位关系研究

原子在强激光电场中高次谐波辐射的理论与实验研究是当今科学前沿之一.利用量子力学理论和鞍点方法,细致地研究了高次谐波辐射光子的能量与激光相位的关系.对于时间宽度无限长激光,在一个激光周期内特定相位处产生的高阶辐射(X射线)有特定的能量.能量分布在7180°处成峰,有高斯形函数的对称形状.给出了这种分布的参数化公式.对于不同宽度的飞秒激光,能量分布的成峰位置、最大值和带宽等参数会发生变化.计算表明,三个振荡周期(半高宽)的飞秒激光,当载波-包络相位为175°和105°时,可以分别得到纯净的阿秒单脉冲和双脉 关键词： 超短脉冲激光 高次谐波产生 鞍点方法 能量相位关系     

Efficient high-order harmonic generation in a two-color laser field

We report efficient high-order harmonic generation achieved using a two-color laser field. By inserting a second-harmonic generation crystal in the path of a converging laser pulse, the overall efficiency for high-harmonic generation was enhanced by more than an order of magnitude. In addition, even-order harmonics were also generated. The 2(2n+1)th -order harmonics were very strong and, in particular, the 38th harmonic (21.6 nm), obtained with a single laser shot of 2 mJ, saturated the X-ray charge-coupled device (CCD) of an extreme-ultraviolet spectrometer even with an Al filter of 1.5 m thickness installed in front of the CCD. PACS 42.65.Ky; 32.80.-t; 42.65.Re     

The study of high-order harmonic generation using ellipticity-modulated laser pulse

The high-order harmonic generation using ellipticity-modulated laser field is investigated experimentally and theoretically. The laser field with time-varying ellipticity is obtained using two quarter wave plates. The harmonic spectral intensity peaks appear periodic for the localization induced by the ellipticity modulation of the laser pulse. We calculate the single-atom response to the driving laser field using the Lewenstein model and compare the calculated and measured harmonic spectra generated with ellipticity-modulated driving laser pulses. The theoretical simulation and the experiment results agree well with each other.     

Heterodyne mixing of laser fields for temporal gating of high-order harmonic generation

The concept of heterodyne mixing of laser fields is theoretically applied to the process of high-harmonic generation to enhance and modulate the kinetic energy of the active electron on subcycle time scales. A very small amount of intensity in the heterodyne field creates a significant modification of the electron kinetic energy, due to its amplification by the strong fundamental field in the kinetic-energy term, in which the heterodyne mixing occurs. Quantum calculations are carried out to verify the predictions of the classical results, demonstrating very good qualitative and quantitative agreement. Applications of the heterodyne-mixing concept are the extension of the harmonic cutoff to higher photon energies and the temporal gating of attosecond pulse production.     

Atomic high-order harmonic generation from a circularly polarized laser pulse

We demonstrate theoretically that the high-order harmonic of an atom can be generated by a circularly polarized laser pulse.The harmonic spectrum shows a clear cutoff with an energy Ip + 2Up.In particular,the high-order harmonic generation comes from the multiple recombination of the ionized electron with non-zero initial velocity.These results are verified by the classical model theory and the time-frequency analysis of a harmonic spectrum.     

激光驱动晶体发射高次谐波的特性研究

通过数值模拟激光驱动下电子在周期性势阱中的动力学行为, 研究了晶体在激光场中发射高次谐波的特性. 研究发现在一定的激光波长和光强驱动下, 晶体发射的谐波谱会呈现出双平台结构, 经分析后得知第一个平台主要来自于最低导带与价带间的电流(电子-空穴对复合), 第二个平台主要来源于较高导带与价带间的电流(电子-空穴对复合), 且两个平台的截止位置处的能量都与激光场的振幅呈线性关系. 在少周期激光驱动下, 晶体谐波第二平台的截止位置与激光的载波相位呈单调变化, 由此我们提出可以利用晶体谐波第二平台的截止位置来确定少周期激光的载波相位. 进一步研究发现, 在啁啾激光驱动下, 晶体发射谐波谱的第二平台有较大变化, 第二平台的发射效率会随啁啾参数而改变, 能够通过改变啁啾激光场来提高晶体谐波第二平台的发射效率.     

高次谐波辐射发射特性研究

报告由不同脉冲宽度(半高宽,FWHM)和不同载波-包络相位(CEP,Φ)的激光产生的高次谐波辐射能量输出时间特性即发射特性的研究结果. 计算表明,由宽度为几个周期的激光产生的高次谐波辐射的截止能量明显低于由无限长脉冲宽度激光产生的截止能量ω_max=3.17U_p+I_p(其中ω_max为光子角频率,U_p和I_p分别为激光有质动力势和原子的电离能). 例如,由两周期(FWHM),Φ＝15°的激光产生的高次谐波辐射的截止能量为ω_max=2.90U_p+I_p,此时发射特性单脉冲(即分布单脉冲)具有最大的能量带宽0.86U_p. 脉冲中心位置的载波相位和时间宽度分别为0.94rad(弧度)和1.29rad. 而该激光脉冲在Φ＝-75°时能产生截止能量为ω_max=2.70U_p+I_p,最大能量带宽为0.70U_p的双分布脉冲,其中心位置分别为-0.58rad和2.43rad,宽度分别为1.22rad和1.33rad. 随着激光脉冲宽度的增加,分布单脉冲的能量带宽比时间宽度下降得更快. 对于一定宽度的激光脉冲,所产生的分布单脉冲的能量带宽和时间宽度的CEP依赖性显示出180°的周期结构. 利用这个有趣的特点,在实验上可以通过调节CEP来选择分布脉冲的能量参数,也可用来定位和控制阿秒脉冲的时间参数. 理论分析指出,只要选择合适的阿秒X射线能量带宽,CEP不稳定性对于光电子谱和测量结果的影响将大为降低,甚至在最大程度上消除这种影响. 这些研究结果不仅有助于在物理上深入了解高次谐波辐射的动力学过程,而且对于进一步在实验上优化和选择阿秒单脉冲和双脉冲具有重要的参考和指导意义. 关键词： 高次谐波产生 鞍点方法 谐波发射特性 分布脉冲     

Dramatic enhancement of high-order harmonic generation

We present a dramatic enhancement [Phys. Rev. Lett. 91, 043002 (2003)] of high-order harmonic generation by simultaneous irradiation of booster harmonics. A key feature of our experiment is the use of mixed gases (Xe and He) with different ionization energies. The harmonics from Xe atoms act as a booster to increase the harmonic yield from He by a factor of 4 x 10(3). The dominance of the dramatic enhancement effect is supported by simulation with the time-dependent Schr?dinger equation as well as the observed spatial characteristic of the generated harmonics and dependence on medium conditions.     

双色激光场中高次谐波转化效率的提高

采用渐近边界条件和辛算法数值求解双色激光场与模型P?schl-Teller势相互作用的含时Schr?dinger方程，计算了电离概率、平均位移、高次谐波以及跃迁概率；数值结果显示添加6倍频光的双色激光场能使高次谐波转化效率明显提高.分析了双色激光场极大地提高高次谐波转化效率的机理，并且通过对产生特定级次谐波的跃迁概率的计算，给出高次谐波转化效率提高的具体数值. 关键词： 渐近边界条件 辛算法 双色激光场 高次谐波     

超短脉冲谐波辐射过程中的量子路径分析

本文数值计算了脉宽为半个光学周期的超短脉冲在不同载波包络相位下的谐波波谱,通过比较谐波波谱截止频率与电子通过主要路径可获得的最大动能的差异,研究了超短脉冲谐波辐射过程中的量子路径。结果显示,超短脉冲谐波辐射过程中电子获得动能的量子路径偏离主要量子路径,而且该偏离效应受载波包络相位的影响。     

超短脉冲谐波辐射过程中的量子路径分析

本文数值计算了脉宽为半个光学周期的超短脉冲在不同载波包络相位下的谐波波谱,通过比较谐波波谱截止频率与电子通过主要路径可获得的最大动能的差异,研究了超短脉冲谐波辐射过程中的量子路径。结果显示,超短脉冲谐波辐射过程中电子获得动能的量子路径偏离主要量子路径,而且该偏离效应受载波包络相位的影响。     

Carrier-envelope phase effects of over-saturated few-cycle laser pulses on high-order harmonic generation

We investigate theoretically the high-order harmonic generation driven by few-cycle laser pulses with intensities above the saturated intensity using the Lewenstein model. It is found that the carrier-envelope phase of the laser pulse has great influences on the coherence of the high-order harmonic spectrum in the plateau region when driven by a relatively weaker driver field in the oversaturated regime. While for a much higher intensity the coherence of the high-order harmonic spectrum is almost independent on the carrier-envelope phase.     

5fs驱动激光脉冲的高次谐波选择性优化

研究了在紧聚焦实验条件下, 采用5 fs激光脉冲与氩气相互作用产生的高次谐波特性. 通过优化系统色散、气体靶气压与位置等参数, 观察到在60–73 eV波段范围内高次谐波光谱接近一个量级的增强. 进一步通过对单原子模型和传播方程的数值求解及模拟相位匹配实验参数下高次谐波的产生过程, 发现相位匹配在所观察到的实验现象中起着关键作用, 得到了理论上与实验规律一致的结果. 关键词： 极紫外激光 高次谐波 超快光谱     

Influence of laser contrast on high-order harmonic generation from solid-density plasma surfaces

The influence of laser temporal contrast on high-order harmonic generation from intense laser interactions with solid-density plasma surfaces is experimentally studied. A switchable plasma mirror system is set up to improve the contrast by two orders of magnitude at 10 ps prior to the main peak. By using the plasma mirror and tuning the prepulse, the dependence of high-order harmonic generation on laser contrast is investigated. Harmonics up to the 21 st order via the mechanism of coherent wake emission are observed only when the targets are irradiated by high contrast laser pulses by applying the plasma mirror.     

超短脉冲高次谐波谱的截止频率分析

本文数值计算了超短脉冲(脉冲持续时间为两个光学周期)的高次谐波发射功率谱.研究结果表明,超短脉冲谐波谱的截止频率不再符合IP 3.17UP的截止规则(IP为原子电离势,UP为有质动力能).对此,本文采用"三步"模型给出了合理解释,并用小波变换验证了结果的正确性.     

Bohmian mechanics to high-order harmonic generation

This paper introduces Bohmian mechanics (BM) into the intense laser-atom physics to study high-order harmonic generation. In BM, the trajectories of atomic electron in an intense laser field can be obtained with the Bohm--Newton equation. The power spectrum with the trajectory of an atomic electron is calculated, which is found to be irregular. Next, the power spectrum associated with an atom ensemble from BM is considered, where the power spectrum becomes regular and consistent with that from quantum mechanics. Finally, the reason of the generation of the irregular spectrum is discussed.