红外激光载波包络相位对氦原子的极紫外光(XUV)吸收谱的量子调控研究

Quantum control of the XUV photoabsorption spectrum of helium atoms via the carrier-envelope-phase of an infrared laser pulse

本文通过数值求解双电子含时薛定谔方程,研究了利用红外(IR)超短超强激光的载波包络相位(CEP)对氦(He)原子的极紫外光(Extreme Ultra-Violet,XUV)吸收谱进行量子调控的可能性.当XUV作用到He原子上时,原子存在两个电离通道:无明显电子关联的直接电离和带强烈电子关联的间接电离(即通过双激发态自发电离).两个通道相互干涉可在XUV吸收谱中形成人们所熟知的Fano共振线型,并且谱线的形状由两个通道间的比例决定.通过引入另外一束IR激光,我们发现,原子的XUV吸收谱将发生明显改变,即伴随着超短脉冲CEP的改变而呈现出从Fano线型到Lorentz线型的周期性连续变化.上述结果表明,通过合理地控制超短脉冲的CEP可以有效地调控两个电离通道之间的量子干涉,进而为探测和操控原子中的极端超快电子关联提供可能.

In the present paper, we investigate the quantum control of the XUV photoabsorption spectrum of helium atoms via the carrier-envelope-phase （CEP） of an infrared （IR） laser pulse by numerically solving the time-dependent one-dimensional （1D） two-electron Schr？dinger equation. The advantage of the 1D model is that the associated time-dependent Schrodinger equation （TDSE） can be solved numerically with high precision as taking full account of the interaction between the electrons and without making any assumptions about the dominant physical mechanisms. In our study, a single attosecond XUV pulse with broad bandwidth is used to create a wave packet consisting of several doubly-excited states. Helium atoms subjected to the XUV pulse can be ionized through two different pathways： either direct ionization into the continuum or indirect ionization via the autoionization of doubly excited states. The interference of these two paths gives rise to the well-known Fano line shape in the photoabsorption spectrum, which is determined by the ratio and relative phases of the two paths. In the presence of an IR laser pulse, however, we find that the Fano line profiles are strongly modified, in good agreement with recent experimental observations [C. Ott et al., Science 340, 716 （2013）; C. Ott et al., Nature 516, 374 （2014）]. At certain time delays, we can observe symmetric Lorentz, inverted Fano profiles, and even negative absorption cross sections, indicating that the XUV light can be amplified during the interaction with atoms. We fit the absorption spectra with the Fano line profiles giving rise to the CEP-dependent Fano q parameters, which are modulated from extremely large positive value to extremely large negative value. Since the q parameter is proportional to the ratio between the dipole matrix of the indirect ionization path and the dipole matrix of the direct ionization path;these results indicate that the quantum interference between the two ionization paths can be e？ciently controlled by the CEP of an ultrashort laser pulse, thus offering another possibility （in addition to the laser intensity and the time delay between the XUV pulse and the IR laser） of manipulating the extreme ultrafast electronic motion in atoms. Our predictions can be experimentally verified easily with the present experimental technique.