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Physical Review A

Direct manipulation of atomic excitation with intense extreme-ultraviolet laser fields

Yu He, Haohan Shi, Nan Xue, Alexander Magunia, Shaohua Sun, J. J. Ding, Bitao Hu, Zuoye Liu

Published Apr 20, 2022• 6 citations
Publisher

Abstract

The coherent excitation and manipulation of a two-level system with ultrashort intense extreme-ultraviolet laser fields is investigated theoretically, based on numerically solving the time-dependent Schr\"odinger equation. We are particularly interested in the dynamical phase excursion of the energy states over the course of the interaction and the resulting spectral modifications. Fitting the absorption line shapes with the Fano profile quantifies the asymmetry parameter and the corresponding dipole phase offset, capturing the phase difference of the state coefficients after the interaction. Nonperturbative analytical calculations using rectangular driving pulses are employed, yielding physical insights into the dependence of the dipole phase shift on the external field. The validity of the formulas is validated by comparing their predictions with numerical results, which proves to be robust against the variation of laser parameters. The present investigation of strong-field dressing effects complements recent attosecond transient absorption studies assuming weak excitation, and marks a ubiquitous phenomenon that should be generally considered for the interaction of matter with intense laser fields.

DOI
10.1103/physreva.105.043113

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The coherent excitation and manipulation of a two-level system with ultrashort intense extreme-ultraviolet laser fields is investigated theoretically, based on numerically solving the time-dependent Schr"odinger equation. We are particularly interested in the dynamical phase excursion of the energy states over the course of the interaction and the resulting spectral modifications. Fitting the absorption line shapes with the Fano profile quantifies the asymmetry parameter and the corresponding dipole phase offset, capturing the phase difference of the state coefficients after the interaction. Nonperturbative analytical calculations using rectangular driving pulses are employed, yielding physical insights into the dependence of the dipole phase shift on the external field. The validity of the formulas is validated by comparing their predictions with numerical results, which proves to be robust against the variation of laser parameters. The present investigation of strong-field dressing effects complements recent attosecond transient absorption studies assuming weak excitation, and marks a ubiquitous phenomenon that should be generally considered for the interaction of matter with intense laser fields.
We report on the experimental observation of a strong-field dressing of an autoionizing two-electron state in helium with intense extreme-ultraviolet laser pulses from a free-electron laser. The asymmetric Fano line shape of this transition is spectrally resolved, and we observe modifications of the resonance asymmetry structure for increasing free-electron-laser pulse energy on the order of few tens of Microjoules. A quantum-mechanical calculation of the time-dependent dipole response of this autoionizing state, driven by classical extreme-ultraviolet (XUV) electric fields, evidences strong-field-induced energy and phase shifts of the doubly excited state, which are extracted from the Fano line-shape asymmetry. The experimental results obtained at the Free-Electron Laser in Hamburg (FLASH) thus correspond to transient energy shifts on the order of a few meV, induced by strong XUV fields. These results open up a new way of performing nonperturbative XUV nonlinear optics for the light-matter interaction of resonant electronic transitions in atoms at short wavelengths.
We show that by combining strong-field dressing and resonant propagation of XUV pulses, the transition of absorption lines from their natural Lorentzian profiles through Fano and complex multipeak shapes all the way back to broadened near-Lorentzian profiles can be achieved, in the limit of optically thick samples. The final stage of this spectral modification can be understood in terms of a significant temporal stretching and delay of the resonant XUV pulse as it propagates through the dense gas, which alters the ultrafast absorption that is modified by the time-synchronized few-femtosecond laser pulse. We first demonstrate this concept in numerical calculations, both using a model system and through a fully coupled solution of the time-dependent Schrödinger equation and the Maxwell wave equation. The applicability and generality of the underlying mechanism is then illustrated in proof-of-principle attosecond transient absorption measurements in a helium gas with a widely varying atomic density. These results provide insights into the interaction of ultrashort laser pulses with dense media and its coherent control.
We report on numerical results revealing line-shape asymmetry changes of electronic transitions in atoms near-resonantly driven by intense extreme-ultraviolet (XUV) electric fields by monitoring their transient absorption spectrum after transmission through a moderately dense atomic medium. Our numerical model utilizes ultrashort broadband XUV laser pulses varied in their intensity (1014–1015 W/cm2) and detuning nearly out of resonance for a quantitative evaluation of the absorption line-shape asymmetry. It will be shown how transient energy shifts of the bound electronic states can be linked to these asymmetry changes in the case of an ultrashort XUV driving pulse temporally shorter than the lifetime of the resonant excitation, and how the asymmetry can be controlled by the near-resonant detuning of the XUV pulse. In the case of a two-level system, the numerical model is compared to an analytical calculation, which helps to uncover the underlying mechanism for the detuning- and intensity-induced line-shape modification and links it to the generalized Rabi frequency. To further apply the numerical model to recent experimental results of the near-resonant dressing of the 2s2p doubly excited state in helium by an ultrashort XUV free-electron laser pulse we extend the two-level model with an ionization continuum, thereby enabling the description of transmission-type (Fraunhofer-like) transient absorption of a strongly laser-coupled autoionizing state.
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(1P)2s2p({}^{1}P)2s2p(1P) autoionizing state, which is coupled to the 2s2(1S)2{s}^{2}{(}^{1}S)2s2(1S) autoionizing state by a femtosecond infrared laser with the intensity in the order of 1012{}^{12}12 W/cm2{}^{2}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 $\ensuremath{\delta}$-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.
We present a theoretical study of transient absorption and reshaping of extreme ultraviolet (XUV) pulses by helium atoms dressed with a moderately strong infrared (IR) laser field. We formulate the atomic response using both the frequency-dependent absorption cross section and a time-frequency approach based on the time-dependent dipole induced by the light fields. The latter approach can be used in cases when an ultrafast dressing pulse induces transient effects, and/or when the atom exchanges energy with multiple frequency components of the XUV field. We first characterize the dressed atom response by calculating the frequency-dependent absorption cross section for XUV energies between 20 and 24 eV for several dressing wavelengths between 400 and 2000 nm and intensities up to 1012{}^{12}12 W/cm2{}^{2}2. We find that for dressing wavelengths near 1600 nm, there is an Autler-Townes splitting of the $1s\ensuremath{\rightarrow}2p$ transition that can potentially lead to transparency for absorption of XUV light tuned to this transition. We study the effect of this XUV transparency in a macroscopic helium gas by incorporating the time-frequency approach into a solution of the coupled Maxwell-Schr\"odinger equations. We find rich temporal reshaping dynamics when a 61-fs XUV pulse resonant with the $1s\ensuremath{\rightarrow}2p$ transition propagates through a helium gas dressed by an 11-fs, 1600-nm laser pulse.
Line shapes observed in attosecond transient absorption spectra (ATAS) encode information on the phase of the optical dipole response. Understanding and controlling the phase evolution of interference fringes is among the fundamental goals of exploring coherent interaction. Here, we simulate the attosecond transient absorption in the near-infrared (NIR) dressing field by considering a three-level model in the adiabatic approximation. We find that the absorption line shape can be changed from Lorentzian to Fano to inverted Lorentzian and back again at two-photon dressed energy levels “ω ± 2ωι”, and line shapes evolve in half-cycle periodicity (T/2). Moreover, the direction of phase evolution of “ω ± 2ωι” is opposite, while the phases of “ω ± 2ωι” are roughly the same at certain delays with an interval of approximately a quarter-cycle periodicity (T/4). These results demonstrate that absorbance of two-photon dressed energy levels “ω ± 2ωι” can qualitatively map the AC Stark shift. These observations provide new insights into the control of spectral line shapes and open the way for the modulation of quantum optical interference in the laser-driven atom.