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首页双色激光场中阈上电离谱的偏振角依赖性:经典与量子效应对比
本文主要探讨了"阈值以上电离光谱"(Above-Threshold Ionization, ATI)在双色激光场中的特性,特别是在激光场中两个光子能量都低于原子电离阈值时的依赖性。作者利用频域理论(Frequency-Domain Theory),深入研究了原子在双色激光场中的离子化过程。 当双色激光场中每个光子的能量不足以单独使原子电离,即两个光子的能量之和仍小于原子的电离阈值时,ATI光谱表现出显著的对激光偏振方向的敏感性。具体来说,ATI光谱的形状和强度会随着两个激光偏振方向之间的夹角的变化而变化,这种现象体现了量子干涉效应(Quantum Interference)。量子通道的贡献在这个低能量区域起着关键作用,它们之间强烈的相互作用导致了光谱的复杂角度依赖性。 然而,当其中一个激光的光子能量接近或超过原子的电离阈值时,情况发生了变化。在这种情况下,ATI光谱变得独立于激光偏振角,呈现出阶梯状结构,这是因为较高的频率使得量子通道间的干扰减弱,通道的贡献按顺序逐渐减小。这表明,随着能量的提升,经典力学机制与量子力学机制之间的平衡被打破,从而导致了光谱结构的简化。 这些研究结果对于理解物质与高强度激光场之间的相互作用具有重要意义,不仅揭示了经典和量子行为的界限,还为设计和控制高能激光实验提供了理论基础。研究者们可以通过调整激光偏振和频率来调控ATI过程,这对于原子物理、分子物理以及光电子学等领域有着广泛的应用潜力。 这篇论文通过对两色激光场中阈值以上电离光谱的深入分析,展示了量子效应如何影响激光与原子相互作用的结果,并为相关领域的理论研究和实验设计提供了宝贵的指导。
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Chin. Phys. B Vol. 24, No. 7 (2015) 073201
Dependence of above-threshold ionization spectrum on
polarization directions of two-color laser fields
*
Liu Min(刘 敏)
a)b)
, Guo Ying-Chun(郭迎春)
a)†
, and Wang Bing-Bing(王兵兵)
b)‡
a)
Department of Physics, East China Normal University, Shanghai 200241, China
b)
Laboratory of Optical Physics, Beijing National Laboratory for Condensed Matter Physics,
Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
(Received 23 December 2014; revised manuscript received 5 March 2015; published online 18 May 2015)
Using the frequency-domain theory, we investigate the above-threshold ionization (ATI) process of an atom in two-
color laser fields. When both photon energies of the two-color laser fields are much smaller than the atomic ionization
threshold, the ATI spectrum depends on the angle between the two lasers’ polarization directions. While when the photon
energy of one laser is comparable with or larger than the atomic ionization threshold, the ATI spectrum is independent of
the angle, and only several dips appear at certain angles. By analyzing the contributions of different quantum channels, we
find that, for the case that both frequencies of the two color lasers are low, the quantum interferences between the channels
are strong, and hence the spectrum changes with the angle between the two lasers’ polarization directions. While for the
case that the frequency of one of the two color lasers is high, the contributions of the channels to the ATI spectrum decrease
dramatically with increasing channel order, hence the interferences between the channels disappear, and the ATI spectrum
has a step-like structure, which is independent of the angle between the two lasers’ polarizations. These results can shed
light on the study of the corresponding relation between classical and quantum mechanisms of the matter–laser interaction
in high-frequency laser fields.
Keywords: above-threshold ionization, two-color laser fields, quantum interference
PACS: 32.80.Rm, 42.50.Hz, 42.65.Ky DOI: 10.1088/1674-1056/24/7/073201
1. Introduction
The interaction between an atom and an intense laser
pulse can cause lots of fundamental strong-field physics
phenomena, such as above-threshold ionization (ATI), high-
harmonic generation (HHG), and nonsequential double ion-
ization (NSDI). In the above-threshold ionization, atoms or
molecules absorb more photons than necessary for ioniza-
tion, which was firstly discovered by Agostini et al.
[1]
Since
the discovery of ATI, the ionization of an atom in two-color
laser pulses has attracted much attention for understanding
the stimulated absorption and emission of photons in the ATI
process.
[2–7]
Muller et al. observed a sideband structure in the
ATI spectrum, which may be due to the stimulated absorption
and emission of low-frequency photons.
[2]
Koval et al. inves-
tigated the ATI process of an atom in a two-color laser field of
the first and the third harmonics. They found that the cutoff of
the ATI spectrum depends on the relative phase between the
two color laser fields.
[7]
Furthermore, Paulus et al. also found
that the cutoff of the photoelectron spectrum strongly depends
on the relative phase of the two color laser fields with the fre-
quencies being the fundamental and the second harmonics.
[5]
Recently, with the development of high-frequency at-
tosecond pulse techniques via HHG and free-electron lasers,
ionization of atoms in IR+XUV two-color laser fields has be-
come a hot topic.
[8–18]
Kazansky et al. investigated the angle-
resolved spectra of photoelectrons produced by IR+XUV two-
color laser pulses. Radcliffe et al. observed a plateau-shape
envelope of the ATI spectrum with increasing intensity of the
IR laser field. Moreover, Zhang et al. found a step-like struc-
ture of the ATI spectrum in IR+XUV two-color laser fields. In
this paper, we investigate the dependence of the ATI spectrum
on the polarization directions of the two-color laser fields. We
find that, when the photon energies of both lasers are much
smaller than the atomic ionization threshold, the ionization
channel interference is very obvious, and hence the ioniza-
tion spectrum strongly depends on the angle between the two
lasers’ polarization directions. While when one laser’s pho-
ton energy approaches or is larger than the atomic ionization
threshold, the channel interference disappears, hence the ATI
spectrum is independent of the angle between the two lasers’
polarization directions, and several dips appear in the spectrum
at certain angles.
2. Theoretical method
The frequency-domain theory for the ATI process of
an atom in a two-color laser field has been presented in
detail.
[18–20]
Here we briefly summarize the basic idea of this
theory for completeness and modify the theory for the case in
*
Project supported by the National Natural Science Foundation of China (Grant Nos. 11474348 and 61275128).
†
Corresponding author. E-mail: ycguo@phy.ecnu.edu.cn
‡
Corresponding author. E-mail: wbb@aphy.iphy.ac.cn
© 2015 Chinese Physical Society and IOP Publishing Ltd http://iopscience.iop.org/cpb http://cpb.iphy.ac.cn
073201-1
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