Light-induced ultrafast tunneling dynamics of a many-electron system: from weak to strong fields

Photoionization is one of the most possible processes that can happen when light interacts with anelectronic system. This seemingly simple phenomenon has expanded into a rich playground thanksto the rapid development of ultraintense and ultrashort light-source technology.In this doctoral dissertation, we investigate photoionization dynamics that can be measured ortriggered by advanced light sources. We focus on the theoretical understanding of (1) photoionizationprocesses assisted by ultrafast electron tunneling, i.e., short-lived shape-type resonance processes, and(2) properties that emerge as the size of the electronic system increases, with the heavy atom xenonserving as a benchmark system. To this end, many-electron Schrödinger equation is solved fromfirst principles using a combination of wave-packet propagation and non-Hermitian resonance-statetechniques. Two applications are presented in two distinct regimes of light-matter coupling.In the first application, we study the resonance dynamics of xenon induced by extreme ultraviolet(XUV) light in the perturbative regime of light-matter coupling. Here, the linear response of xenonis known to exhibit a plasma-like, collective feature, the giant dipole resonance (GDR). We find thatthe GDR is composed of two short-lived resonance states. Albeit hidden in XUV linear spectroscopy,this resonance substructure has been resolved lately using XUV nonlinear spectroscopy. In addition,by analyzing all types of resonance states and then their spectroscopic fingerprints, we explain thelimitation of linear-response studies and reveal the existence of collective multipole resonances. Ourwork offers a new way to understand collective electronic behavior in and beyond the linear regime.In the second application, we study the resonance dynamics of xenon induced by near infrared(NIR) light in the nonperturbative regime of light-matter coupling. Here, strong-field ionization (SFI),a process of both fundamental and technological importance, is examined inside each laser subcycle.In our experiment-theory collaboration, the SFI dynamics is followed by attosecond transient absorptionspectroscopy and is found to show an unexpected oscillatory character. With the aid of theory,we identify polarization as the origin of this oscillation. While the role of polarization had so far beenunnoticed in the strong-field regime, it comes into light due to the high polarizability of xenon and isexpected to influence the strongly driven dynamics of extended electronic systems