高次谐波产生过程中测定电子隧穿电离时刻的研究

The tunneling ionization of atoms driven by strong laser fields has significant importance for deeply understanding the basic mechanism of strong field-matter interaction. Resolvation of the tunneling ionization times on attosecond time scale is the key for the investigation of tunneling ionization. On one hand, it is possible to resolve the tunneling ionization time on attosecond time scale due to the sensibility of the phase to the tunneling ionization time. And the Complete Reconstruction of Attosecond Bursts technique allows the measurement of the phase of harmonic radiations. On the other hand, the latest studies on transient absorption spectra show that, there is subcycle-oscillating energy structure of the atom dressed by laser fields, i.e. the laser-Coulomb-coupled potential barrier felt by the electron oscillates along with the laser field. As a result, the coupled potential barrier can not be treated as quasi-static, and its change on attosecond time scale has to be taken into account during the resolvation of tunneling ionization times. In this project, we will theoretically investigate the transient absorption spectrum observed by pulses with finite width, and its relationship with the atomic transient energy structure; discuss the change of tunneling ionization process induced by the sub-cycle oscillation of coupled potential barrier , providing the approach for accurately resolving tunneling ionization times by the use of the dynamical phases of high-order harmonics. According to the research of this project, we will try to provide theoretical basis for the experimental observation of tunneling ionization dynamics.

电子隧穿电离过程的研究对深入理解强激光场与物质相互作用的基本物理机制具有重要意义。精确测定电子的隧穿电离时刻是观测整个隧穿过程的关键。一方面，高次谐波辐射相位对隧穿电离时刻的敏感性，为在阿秒尺度上精确测定电子的隧穿电离时刻提供了思路，阿秒脉冲重构技术则为精确测量谐波辐射相位信息提供了可能。另一方面，瞬时吸收光谱的最新研究表明，激光场中的缀饰态原子能级结构存在亚光学周期振荡，即电子所感受到的激光—库仑场耦合势垒是随激光场振荡的。因此，在推算隧穿电离时刻时不能将耦合势垒看作准静态的，而必须考虑其在阿秒时间尺度上的瞬时变化。基于以上两点，本课题拟研究探测光脉冲对瞬时吸收光谱的影响，探索其与原子瞬态能级结构间的关系；研究激光—库仑耦合势场的亚周期振荡对隧穿电离过程的影响，探讨利用高次谐波辐射动力学相位精确测定电子隧穿电离时刻的理论方案，力求为实验观测隧穿电离过程提供必要的理论支持。

探索和理解强激光场与原子相互作用过程，是物理学研究的基本问题之一，对于理解原子中电子如何与库伦场及驱动光场相互作用机制，特别是在典型电子运动时间尺度——即阿秒尺度——上的动力学过程（隧穿过程等），有重要的意义。本项目主要研究了强激光场驱动下的原子谐波辐射光谱及瞬时吸收谱相位信息。研究发现：1）谐波辐射光谱不仅含有电子波包的强度信息，还含有波包的相位信息。并指出，通过分析光谱相位信息，可以获得原子缀饰态能级结构的瞬时信息。2）光谱相位信息中包含有动力学相位及几何相位两部分，并分别依赖于电子隧穿电离时刻。此结果对于推算确定电子隧穿电离时刻具有重要的理论意义。3）利用双超短脉冲吸收谱可以实现几何相位的观测，并指出，在特定外场参数条件下可以实现对几何相位的精确操控，这对实现量子信息的转换和传递具有重要的应用意义。