1.School of Physics and Electronic Science, East China Normal University, Shanghai 200241, China 2.Beijing National Laboratory of Condensed Matter Physics, Laboratory of Optical Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China 3.University of Chinese Academy of Sciences, Beijing 100049, China
Fund Project:Project supported by the National Natural Science Foundation of China (Grant Nos. 12074418, 11774411).
Received Date:06 March 2021
Accepted Date:27 April 2021
Available Online:09 October 2021
Published Online:20 October 2021
Abstract:High order harmonic generation (HHG) is an important phenomenon when atoms or molecules interact with an intense laser field. It can be used to generate ultrashort laser source, and can also be used to investigate the atomic and molecular dynamics and obtain the electric structure information of molecules. All these require to understand in depth the mechanism of HHG. There are complicated interference effects in HHG spectra of molecules due to multiple re-collision atomic centers in the molecule. In this paper, spectra of aligned O2 molecule in linearly polarized laser field is investigated by using the Lewenstein' s model. The dependence of the spectrum on the angle θ between the nuclear axis of the molecule and the laser polarization direction is obtained. It is shown that the maximum yield of HHG occurs at θ of 45°, which is in consistence with the experimental result. In addition, it is found that there exists a minimum value in the HHG spectrum for any given value of θ. The harmonic order corresponding to the minimum increases with θ increasing. It is found that the minimum comes from the coherent superposition of contributions from two channels. One channel refers to that the ionized electron from one atomic center, subjected to the electric field of the laser, moves back to its parent atomic center and there it combines with the molecule and emits harmonics; while the other channel is that the ionized electron generated from one atomic center move back to the other atomic center to complete the combination and emission of harmonics. The angle θ-dependent phase difference between contributions from these two channels is calculated and the harmonic order corresponding to the minimum value is obtained. Finally, the reason why the yield of HHG is low for the case of the molecular axis parallel to the laser polarization direction is different from that for the case of the molecular axis perpendicular to the polarization direction. For the parallel case, the contributions to HHG from the two channels are both small so that the amplitude of their coherent superposition is small. While for the perpendicular case, the individual contribution from each channel is not small but their destructive interference leads to small yield in harmonicspectrum. Keywords:aligned molecule/ high order harmonic generation/ interference effect
其中, $ \boldsymbol{d}(\boldsymbol{p}) = \left\langle {\boldsymbol{p}|\boldsymbol{r}|{\phi _0}} \right\rangle $为跃迁矩, $ \boldsymbol{p} $为电子的正则动量, ${\phi _0}$为分子的基态波函数, $S(\boldsymbol{p},\; t, \;\tau )= $$ {\displaystyle \int _{t-\tau }^{t}{\rm{d}}t''\left\{\frac{{[\boldsymbol{p}-\boldsymbol{A}(t'')]}^{2}}{2}+{I}_{\text{p}}\right\}}$ 为经典作用量. 激光场的偏振方向在yz平面, 且与核轴的夹角为$\theta $, 核轴在z轴上, 如图1(a)所示. 用A1, A2表示两个氧核的位置. 图 1 (a)准直的O2分子与光电场示意图; (b)第一类通道; (c)第二类通道 Figure1. (a) Aligned O2 molecule and the polarizing electric field of laser; (b) the first type of path; (c) the second type of path.
3.O2分子的高次谐波谱计算了O2的高次谐波, 采用的连续的线偏振激光场波长为$800{\kern 1 pt} \, {\text{nm}}$, 强度为$5.18 \times {10^{14}}\, {{{\rm{W/c}}}}{{{{\rm{m}}}}^{{2}}}$. 垂直电离势为${I_{\text{p}}} = 12.72$ eV. O2分子的核间距为$2.2\, {\text{a}}{\text{.u}}{\text{.}}$ 计算结果如图3所示. 用偶极矩模方的对数来表达谐波的相对强度. 图3中的蓝色实线表达了激光场偏振方向与核轴成不同夹角$ \theta $时的高次谐波谱. 需要说明的是, 谐波谱仅有奇次谐波, 偶次谐波的强度为零. 由图3可知, 1)谐波强度在角度为45°时最大, 这与实验结果[16]一致. 2)谐波的截止频率不随角度变化. 也就是说, 不管光电场的偏振方向与核轴的夹角如何, 光传输方向上的谐波的截止频率都相同, 基本符合${I_{\text{p}}} + 3.17{U_{\text{p}}} = 71\omega $. 3)每个角度的谐波谱上都有一个凹陷, 即一个极小值, 对应图中蓝色箭头所示. 此极小值的位置(凹陷对应的谐波阶次)是角度的函数, 随角度的增大而增大. 定性地符合Lein等[10]由理论公式推导出的极小值条件$R\cos \theta = \lambda $, 其中$\lambda = 2{\text{π}}/k$为有效的电子德布罗意波长, ${{{k^2}}}/({{2 m}}) = q\omega$是辐射的高次谐波的光子的能量. 图 3 O2在激光偏振与核轴成不同夹角$\theta $下的高次谐波谱, 每幅图中的黑色划线为第一类通道的贡献 $\log |{X_1} + {X_2}{|^2}$; 红色点划线为第二类通道的贡献$\log |{X_3} + {X_4}{|^2}$; 蓝色实线为两类通道叠加的结果 $\log |{X_1} + {X_2} + {X_3} + {X_4}{|^2}$ Figure3. The HHG spectrum of O2 with different $\theta $ between the polarizing direction of laser electric field and the nuclear axis of O2: In each panel, black dash line is the contribution from the first path $\log |{X_1} + {X_2}{|^2}$, red dot line represent that from the second path$\log |{X_3} + {X_4}{|^2}$, and blue solid line represents the addition of the above two paths $\log |{X_1} + {X_2} + {X_3} + {X_4}{|^2}$.