Resonant Perfect Absorption and Autoionization Dynamics Revealed by Attosecond Transient Absorption Spectroscopy

We introduce a general approach to manipulate and substantially enhance the resonant absorption property of a macroscopic 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. Numerical and analytical results on an ensemble of two-level systems demonstrate that the resonant absorption of light can be reduced or significantly enhanced by more than 5 orders of magnitude relative to that without laser manipulation, and resonant “perfect absorption” can be achieved at certain conditions. These results are further supported by large-scale calculations of the coupled time-dependent Schrödinger and Maxwell wave equations in helium. Experimentally, we report the transient-absorption measurement of sp_2,n± autoionization states in helium gas. The spectral signature of sp_2,4- shows up in the presence of a moderately intense visible pulse, which is otherwise suppressed due to its low dipole coupling to the ground state. The different temporal dynamics of these states are analyzed, and the roles of propagation effects in the line-shape manipulation are discussed.