Absorption and emission of single attosecond light pulses in an autoionizing gaseous medium dressed by a time-delayed control field

Abstract

An extreme ultraviolet (EUV) single attosecond pulse passing through a laser-dressed dense gas is studied theoretically. The weak EUV pulse pumps the helium gas from the ground state to the 2s2p( 1 P ) autoionizing state, which is coupled to the 2s 2 ( 1 S) autoionizing state by a femtosecond infrared laser with the intensity in the order of 10 12 W/cm 2 . The simulation shows how the transient absorption and emission of the EUV are modified by the coupling laser. A simple analytical expression for the atomic response derived for δ-function pulses reveals the strong modification of the Fano lineshape in the spectra, where these features are quite universal and remain valid for realistic pulse conditions. We further account for the propagation of pulses in the medium and show that the EUV signal at the atomic resonance can be enhanced in the gaseous medium by more than 50% for specifically adjusted laser parameters, and that this enhancement persists as the EUV propagates in the gaseous medium. Our result demonstrates the high-level control of nonlinear optical effects that are achievable with attosecond pulses.

Probing dipole-forbidden autoionizing states by isolated attosecond pulses

“…This is in contrast to the common assumption in previous studies to neglect C-C transitions when applying long pulses [35][36][37][38] and ultrashort pulses [11,18,19]. By fitting the model to the TAE-TDSE result, the C-C dipole matrix elements D 1s p,1s s and D 1s p,1s s are determined to be about 1 a.u./eV.…”

“…(18) and (19) without the α(t) terms, and they are then added back as iterative corrections. The coupling between the continua through α(t) greatly reduce the number of coupled equations in the numerical calculation from 2N 2 to 2N , where N is the number of energy steps in the range in concern.…”

“…(13), (14), (18), and (19), and with a given set of atomic and field parameters, the total wave function in the form of Eq. (1) is uniquely determined.…”

“…Most recently, the Fano line shapes modulated by time-delayed intense fields in transient absorption spectroscopy have been measured and analyzed [8,9], which has also brought to light the astounding potential to shape and to control the attosecond pulses [13]. In these studies and the related theoretical works, the dynamics of the Fano wave packet were mostly implemented as attosecond streaking [14,15] or resonant coupling [16][17][18][19] of the Fano states, if not carried out by ab initio calculations [20][21][22], which were numerically expensive and less intuitive. While the attention has been put on the directly measurable resonances under the dressing field, the properties of the dipole-forbidden resonances (DFRs) in the coupling were largely overlooked.…”

“…Then a multilevel model with parametrized atomic structure is developed and applied to the case, where the comparison to the virtual experiment optimizes the atomic parameters of the DFRs. In our previous model consisting of coupled AISs [19], the transitions between the AISs were reduced to the transitions involving only the bound statesbound-bound (B-B) transitions-by ignoring the second-order (two-electron) transitions. This naive picture successfully predicted the inversion of Fano line shapes under intense dressing fields, but the effects on the DFRs were missing.…”