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

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 Schrödinger 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 efficiently 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.