基于非绝热项讨论的隧穿延迟时间调控【增强内容出版】
1 引言
强场作用下电子的隧穿是原子分子超快动力学中最基础的现象,对隧穿过程进行深入的讨论有利于理解更多高阶非线性超快动力学过程[1-5]。根据Keldysh理论[6],当绝热参数
然而,当绝热参数
理论研究表明,隧穿电子初始动量能有效表征隧穿过程的非绝热性[21],同时,非绝热初始横向动量可近似表示为瞬时有效的Keldysh参数
本研究发现少周期OTC激光场作用下,瞬时有效Keldysh参数对电离时移方向的预测会失效。通过建立基于强场近似理论(SFA)[25-26]-虚时间方法[27]的非绝热隧穿模型,计算了各时刻最概然电离轨迹的初始动量
2 理论方法
2.1 非绝热模型
将SFA与虚时间方法结合,隧穿电离过程可以看作电子从时间
式中:
2.2 单色线偏振激光场
对于单色线偏振激光:
由
2.3 圆偏振激光场
而对于圆偏振激光:
在圆偏振激光脉冲中,由于瞬时场频率始终为定值
2.4 双周期OTC激光场
对于更加复杂的OTC激光场作用下的隧穿电离,由于瞬时有效场频率不断随时间变化,因此其非绝热特性时而与单色线偏振激光相似,时而与圆偏振激光相似。同时为了凸显隧穿耗能
式中:
将
3 结果与讨论
研究了氢原子在双周期含包络OTC激光场作用下的非绝热隧穿电离现象。越来越多的实验和数值模拟结果证明,OTC激光脉冲可以对亚周期的电子动力学行为进行有效的调控[32-34]。
图 1. 激光场波形及瞬时有效Keldysh参数。(a)双周期OTC激光电场波形;(b)瞬时有效Keldysh参数
Fig. 1. Laser field waveform and instantaneous effective Keldysh parameter. (a) Electric field waveform of OTC laser field; (b) instantaneous effective Keldysh parameter
在Keldysh准静态极限条件下,绝热电离轨迹一般有两种通道:吸收多个光子能量的垂直通道(竖直向上点划线)和以恒定能量隧穿势垒的水平通道(水平向右点划线),如
图 2. 结合势与两种隧穿轨迹
Fig. 2. Combined potential barrier and two kinds of tunneling trajectories
3.1 Keldysh参数对隧穿延迟时间预测的失效
为了定性地描述隧穿能量和初始动量与瞬时电离概率之间的关系,揭示OTC激光脉冲发生隧穿延迟现象的原因,通过SFA-虚时间方法计算了
图 3. 非绝热隧穿电离动量谱分析。(a)SFA-虚时间方法得到的隧穿电离电子动量分布;(b)隧穿延迟时间
Fig. 3. PMD analysis of non-adiabatic tunneling ionization. (a) PMD obtained by SFA with imaginary-time method; (b) tunneling delay time
根据隧穿电离模型,在绝热条件下激光场负矢势与最概然动量分布应完美重合,但在
为了确定隧穿延迟时间的大小,分别计算半周期内电场强度随时间的变化关系以及PMD中最概然电离率与电离时间的函数关系。如
根据
然而在计算中出现了与上述预测相反的数值模拟结果,电离时移出现在了瞬时有效Keldysh参数变大的方向上,即
为了更深层次地剖析影响瞬时电离概率的因素,解释与瞬时有效Keldysh参数预测相反的电离时移现象,分别计算了
由于初始动量
3.2 双色激光场强比对隧穿延迟时间的调控
图 5. 不同场强比作用下的PMD。(a) ;(b) ;(c)
Fig. 5. PMD with different field intensity ratios. (a) ; (b) ; (c)
接下来详细讨论OTC激光场强比
图 6. 场强比 时隧穿延迟时间与非绝热特性分析。(a)隧穿延迟时间;(b)非绝热特性分析
Fig. 6. Analysis of tunneling delay time and non-adiabatic characteristics at . (a) Tunneling delay time; (b) analysis of non-adiabatic tunneling ionization characteristics
经过上述的讨论得出,OTC激光场强比的变化会影响隧穿电离中的非绝热性,因此,统计了隧穿延迟时间随OTC激光场强比变化的曲线,如
图 7. 改变场强比对隧穿延迟时间的调控
Fig. 7. Regulation of tunneling delay time by changing the field intensity ratio
3.3 双色激光相位差对隧穿延迟时间的调控
调整OTC激光场强比可以改变PMD的结构并影响电离延迟时间的大小,是由于初始动量
图 8. 相位差为 时非绝热隧穿电离PMD分析。(a)PMD;(b)隧穿延迟时间
Fig. 8. PMD analysis of non-adiabatic tunneling ionization at phase difference. (a) PMD; (b) tunneling delay time
根据隧穿延迟时间的定义,两个电离时移中只有最大电场强度与最大电离率之间的时间间隔
图 9. 改变相位差对隧穿延迟时间的调控
Fig. 9. Regulation of tunneling delay time by changing the phase difference
4 结论
综上所述,少周期含包络OTC激光脉冲作用下非绝热隧穿现象成因较为复杂,Keldysh参数已无法准确描述各电离时刻隧穿过程的非绝热特性。因此,分别讨论了电离电子的初始动量和隧穿耗能两个非绝热项对瞬时电离概率的显著影响,确立了后者在瞬时电离概率影响因素中的主导地位。在此基础上,通过改变OTC激光场强比以及相位差的方式,控制了非绝热项随电离时间的变化曲线,从而改变各电离时刻隧穿过程的非绝热性,最终实现对隧穿延迟时间的有效调控。
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Article Outline
周涛, 许梦瑶, 张赛, 许伯强, 崔森. 基于非绝热项讨论的隧穿延迟时间调控[J]. 激光与光电子学进展, 2024, 61(5): 0532001. Tao Zhou, Mengyao Xu, Sai Zhang, Boqiang Xu, Sen Cui. Regulation of Nonadiabaticity-Induced Tunneling Delay Time[J]. Laser & Optoelectronics Progress, 2024, 61(5): 0532001.