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摘要

利用广义伪谱方法精确数值求解了氢原子在强激光场中的三维含时薛定谔方程, 获得了强激光中氢原子的含时波函数, 利用时间依赖的偶极矩的傅里叶变换得到了高次谐波谱, 研究了氢原子在强激光场中发射低于电离阈值的谐波谱对激光强度的依赖性. 研究发现, 激光强度在低于电离阈值的谐波产生的通道选择的过程中扮演着重要角色, 主要有两种量子通道对阈下谐波的产生有贡献, 即广义的短轨道和长轨道, 其中长轨道对激光场强度比较敏感. 结合小波时频变换、经典轨道分析、以及强度依赖的量子通道选择分析, 本文阐明了其背后的物理机制.

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Abstract

High-order-harmonic generation (HHG) is a fundamental atomic and molecular process in strong laser fields and plays a crucial role in the development of ultrafast science and technology. The essential features in HHG, such as the above-threshold harmonic plateau and cutoffs, can be well understood by the semiclassical three-step model. The HHG cutoff occurs approximately at the energy $3.17{U_{\rm p}} + {I_{\rm p}}$ , where ${I_{\rm p}}$ is the atomic ionization potential, and ${U_{\rm p}}$ is the ponderomotive potential. In the past, most studies focused on the HHG above the ionization threshold, and the general pattern of the HHG spectrum can be qualitatively explained by means of the strong field approximation (SFA) and the quantum treatment of three-dimensional time-dependent Schr?dinger equation (TDSE). However, the SFA results in inadequate description for the process in the harmonic generation below the ionization threshold since it neglects the Coulomb potential and the detailed electronic structure of atoms. Recently, as a promising method to produce vacuum-ultraviolet frequency comb, the HHG in the near- and below-ionization threshold has been increased considerably. However, the dynamical origin of in these lower harmonics is less understood and largely unexplored. Here we perform an ab initio quantum study of the near- and below-threshold harmonic generation of hydrogen atom by means of the time-dependent generalized pseudospectral method. We study the intensity dependence of the harmonic spectra below the ionization threshold of hydrogen atom in the intense laser field. The high-order harmonic spectra are calculated by the Fourier transform of the atom induced dipole moment in the laser field. The below-threshold harmonic spectra yield is scaled as a function of the laser peak intensity. We find that the spectra yield in below-threshold harmonic generation (BTHG) dependents on the light intensity in the multiphoton ionization regime. And the laser intensity plays an important role in the channel selection process for BTHG. There are mainly two kinds of quantum channels to be responsible for the BTHG. Namely, the generalized short trajectories and the long trajectories, in which the long trajectories are more sensitive to the laser field intensity. Combining with wavelet time-frequency transform, semiclassical trajectories simulation, and quantum channel analysis associated with the laser intensity, the dynamical origin of the BTHG is uncovered.

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作者及机构信息

通信作者:李鹏程,lipc@nwnu.edu.cn

基金项目:国家级-从头算研究强激光场中的分子低能高次谐波动力学过程(11674268)

Authors and contacts

Corresponding author:Li Peng-Cheng,lipc@nwnu.edu.cn

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施引文献

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