| In the last decades,laser technologies have been significantly advanced which have,in turn,promoted studies on the ultrafast dynamics of few-body quantum system,and exotic phenomena in atomic physics were reported every minute in literatures.Indeed,atomic physics has entered the new stage where both experimental and theoretical studies have the closest collaboration than ever.In this thesis,numerical and theoretical researches have been carried out on several unsolved problems that have appeared in the study of attosecond transient absorption spectroscopy in recent years.In the first part,the time-dependent buildup processes of Autler-Townes splitting in attosecond transient absorption spectra have been studied.Although such phenomenon have been observed in plenty of experiments,there still exists an open question of when the splitting is maximized.The answer to such question reviews how fast the system responds to the pump field.We work out,for the first time,a compact formula to quantify the buildup time of the Autler-Townes splitting,which is related to parameters associated to the laser-atom system,e.g.,the Rabi period,the laser cycle,and the pulse duration.The validity of the analytical expression is proved by comparison with the experimental data and the time-dependent Schrodinger equation(TDSE)results.Further studies show that the analytical formula is not only applicable to single excitation processes,but also to double excitation processes involving complex electron-electron interactions.Our findings are of potential in calibrating the zero time-delay in a transient absorption spectroscopy.In the second part,we further studied the sub-cycle dynamic of the light-induced state in the attosecond transient absorption spectrum,and gave analytical formula for the delay time when the light-induced state reached the maximum position(the strongest absorption moment).The results show that in the case of the blue detuned pump field,the response of matter to light is nearly instantaneous.It confirms the conjecture about this process for a long time.For the near-resonant and red-detuning cases,however,the present theory is different from the early scenario,i.e.,the buildup time is not divergent near resonance and it might even ratchet up reversely with the increasing of the negative detuning.In the third part,in the framework of linear response theory we reproduce the impulse response function of the system by attosecond transient absorption spectrum,and propose a general procedure of dipole moment reconstruction.Two examples of the self-reconstruction of extreme ultraviolet(XUV)-atom interaction(analytically)and the infrared(IR)-dressed helium system(numerically)are given as demonstration.A correlation coefficient is introduced to quantify the fidelity of the reconstruction,which reaches almost 100%over a wide range of laser parameters and thus indicates the robustness of our reconstruction method.Our theory provides an efficient method for understanding the recent experiment,explaining when and why the dipole moment can be well reconstructed solely from the absorption spectrum even without the exact knowledge of the attosecond probe pulse.In the last part of the work,the related research is extended from linearly polarized light to circularly polarized light.We present attosecond transient absorption spectroscopy of hydrogen atom in a circularly polarized two-color field.The effect of polarization on the absorption spectrum is preliminarily explored. |
