Ultrafast Electronic Dynamics In Macroscopic Gaseous Media

Since the inception of the laser,the production and applicaton of intense ultra-short light pulses have been pushing the frontiers of modern science and technology.Transient absorption spectroscopy based on attosecond and femtosecond laser pulses has turned out to be a powerful tool for probing and steering electronic dynamics.By virtue of the single-atom response,experimental and theoretical investigations have made great success in many fields and advanced our understanding of light-matter in-teraction.As an all-optical technique,the rise in target density could not only increase the signal-to-noise ratio of the transient-absorption data,but also induce a series of in-teresting physical phenomena,which requires going beyond the approximation of a thin medium and incorporating macroscopic propagation effects.In addition,the dynamical processes of complex systems often involve multielectrons,in which the considera-tion of electron correlation effects is necessary.Aiming for a better understanding and manipulation of ultrafast electronic dynamics in macroscopic media,we perform the following experimental and theoretical investigations:1.We measure the spectral signatures of the two single-electron transitions 5s2S₁/2→5p²P₁/2,3/2in Rb gas with different laser intensities and cell temperatures（atomic densities）by static and transient absorption spectroscopy,and observe the break-down of the weak-excitation and dilute-gas approximations.In static absorption inves-tigation,the resonant absorption profiles change from natural Lorentzian to Fano line shapes with the increase of pulse intensity.Further,as the atomic density increases,we observe broader Fano-like shapes and the appearance of novel substructures in absorp-tion lines.Boths effects observed in the experiment are well-described by analytical and numerical calculations,and the resulting spectral lines are revealed to be governed by the coexistence of self-modulation and propagation effects.Moreover,by employ-ing two near-infrared pulses with a variable interpulse delay to excite and couple the Rb atoms,their transient absorbance is measured from the dilute-gas limit into high-densities cases.Our results reveal that the absorption lines are reshaped with substruc-tures emerging therein with increasing atomic density.However,the spectral evolutions are rather distinctive for positive and negative time delays,pointing towards differ-ent mechanisms behind them.By numerically solving the coupled Liouville-Maxwell equations,we reproduce the intensity-and density-controlled line shapes,and validate that the oscillating signals for negative and positive time delays orginate from different coherence channels.These results not only pave the way toward studying strong-field dynamics in dense systems,but could also be applied in the shaping of high-frequency pulses.2.Motivated by the limitation of weak-field excitation and the pioneer experimen-tal demonstration of strong-field dressing of atomic double excitation,we theoretically investigate the coherent excitation and manipulation of a two-level system with ultra-short intense extreme-ultraviolet laser fields.By fitting the resonant absorption line shape with Fano profile,we obtain the asymmetric parameter q and the corresponding dipole phase offset.The latter quantifies the phase difference of the state coefficients after the interaction with the laser pulses.The dependence of this phase excursion on different driving pulse parameters（pulse intensity,central frequency,pulse duration,chirp）is further explored,which can be well-explained by a nonperturbative analyti-cal model using rectangular driving pulses.The present investigation of strong-field dressing effects marks a different way to control electronic dynamics at short wave-lengths,and is complementary to recent attosecond transient absorption studies treating extreme-ultraviolet pulse excitation perturbatively.3.Through attosecond transient absorption spectroscopy,we introduce and demon-strate a general approach to manipulate the resonant absorption property of a macro-scopic medium.By emptying the population of the excited state after its excitation,the polarization decay of the target system is temporally reshaped and confined.The tunable temporal gate between excitation and termination allows us to tailor the tail of the excitation pulse developed during propagation,which thus interferes controllably with the original pulse.Firstly,we employed this approach to the 2s2p autoioniza-tion state in helium,and simulated the formation and reshaping of a Fano resonance in macroscopic media.The numerical calculations in low pressure agree well with the previously reported experimental results.However,beyond the region where the single-atom approximation holds,we observed the emergence of spectral spikes and spectral substructures.By further analysis,the links between these features imprinted in spectral profiles and the formation of temporal characteristics in the excitation field are identified.Afterwards,the time-gating approach is applied to an ensemble of two-level systems.The numerical and analytical results demonstrate that even at moderate optical depths,the resonant absorption of light can be reduced or significantly enhanced by more than 5 orders of magnitude relative to that without laser manipulation.Further-more,the quasicomplete extinction of light at the resonant frequency,here referred to as resonant perfect absorption,can be achieved at certain conditions.For the special case of resonant excitation,it is revealed to be connected with the formation of zero-area pulses in the time domain.The presented concept is further supported by large-scale calculations of the coupled time-dependent Schr?dinger equation in the single active electron approximation and the Maxwell wave equation in helium.These results sug-gest that the absorbance of the system could be switched very drastically through an auxiliary pulse and optical depth,which marks possibilities of controlling attosecond dynamics.4.We report the attosecond transient-absorption measurement of sp2,n±doubly excited states in helium gas,and investigate the electron correlation effects.A textbook example governed by electron-electron correlation is autoionization,where the picture of single active electron breaks down.Due to the Coulomb interaction between the two electrons,there are two kinds of autoionization states in helium,sp2,n±,which possess distinct parameters such as photoabsorption cross sections and energy widths.Exper-imentally,the spectral signature of sp2,4-gets significantly enhanced in the presence of a moderately intense visible pulse,and exhibits different dynamical behavior com-pared with sp2,n+states.We further measure the results for different pulse intensities and gas pressures,and analyze the roles of propagation effects in the spectral line-shape manipulation.The understanding of these dynamics would enable a closer scutiny of electron-electron correlations.Finally,the conducted investigations in the present dissertation are summarized and the future explorations are proposed.