Role of blue-shift length in macroscopic properties of high-harmonic generation

IF 2.8 2区 物理与天体物理 Q2 PHYSICS, MULTIDISCIPLINARY
Margarita Khokhlova, Vasily Strelkov
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Abstract

The production of brighter coherent XUV radiation by intense laser pulses through the process of high-harmonic generation (HHG) is one of the central challenges in contemporary nonlinear optics. We study the generation and spatial propagation of high harmonics analytically and via ab initio simulations. We focus on the length scales defining the growth of the harmonic signal with propagation distance and show that the well-known coherence length limits HHG only for relatively low driving intensities. For higher intensities, the photoionisation of the medium, naturally accompanying HHG, leads to essentially transient phase matching and laser frequency blue shift. By systematically taking both of these factors into account, we demonstrate that the behaviour of the harmonic signal at higher intensities is defined by another length scale—the blue-shift length. In this generation regime the XUV intensity at a given frequency first grows quadratically and then saturates passing the blue-shift length, but the total harmonic efficiency continues growing linearly due to the linear increase of the harmonic line bandwidth. The changeover to this generation regime takes place for all harmonic orders roughly simultaneously. The rate of the efficiency growth is maximal if the atomic dispersion is compensated by photoelectrons near the centre of the laser pulse. Our theory offers a robust way to choose the generation conditions that optimise the growth of the harmonic signal with propagation.
蓝移长度在高次谐波生成宏观特性中的作用
强激光脉冲通过高次谐波发生(HHG)过程产生更明亮的相干 XUV 辐射是当代非线性光学的核心挑战之一。我们通过分析和ab initio模拟研究了高次谐波的产生和空间传播。我们重点研究了定义谐波信号随传播距离增长的长度尺度,结果表明,众所周知的相干长度仅在相对较低的驱动强度下限制了高次谐波的产生。对于更高的强度,介质的光离子化自然会伴随着 HHG,导致本质上的瞬时相位匹配和激光频率蓝移。通过系统地考虑这两个因素,我们证明谐波信号在较高强度下的行为是由另一个长度标度--蓝移长度--定义的。在这一生成机制中,给定频率下的 XUV 强度先是二次增长,然后在经过蓝移长度时达到饱和,但由于谐波线带宽的线性增长,总谐波效率继续线性增长。所有谐波阶次大致同时转换到这种产生机制。如果原子色散由激光脉冲中心附近的光电子补偿,则效率增长速度最大。我们的理论提供了一种稳健的方法来选择产生条件,从而优化谐波信号在传播过程中的增长。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
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来源期刊
New Journal of Physics
New Journal of Physics 物理-物理:综合
CiteScore
6.20
自引率
3.00%
发文量
504
审稿时长
3.1 months
期刊介绍: New Journal of Physics publishes across the whole of physics, encompassing pure, applied, theoretical and experimental research, as well as interdisciplinary topics where physics forms the central theme. All content is permanently free to read and the journal is funded by an article publication charge.
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