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Physical Review A

Light-induced states in attosecond transient absorption spectra of laser-dressed helium

Shaohao Chen, M. Justine Bell, Annelise R. Beck, Hiroki Mashiko, Mengxi Wu, Adrian Nikolaus Pfeiffer, Mette B. Gaarde, Daniel M. Neumark, Stephen R. Leone, Kenneth J. Schäfer

Published Dec 17, 2012• 145 citations
Publisher

Abstract

Laser-dressed absorption in atomic helium is studied, both theoretically and experimentally, by transient absorption spectroscopy using isolated 400-as pulses centered at 22 eV and 12-fs near-infrared (NIR) pulses with 780-nm central wavelength. Multiple features in the helium singly excited bound-state spectrum are observed only when the NIR and attosecond pulses are overlapped in time. Theoretical analysis indicates that these light-induced structures (LISs) are the intermediate states in resonant, second-order processes that transfer population to multiple dipole forbidden states. The use of broadband, coherent extreme ultraviolet (XUV) radiation allows observation of these LISs without specifically tuning to a two-photon resonance, as would be required with narrowband XUV light. The strength and position of the LISs depend strongly on the NIR intensity and the pump-probe delay.

DOI
10.1103/physreva.86.063408

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Laser-dressed absorption in atomic helium is studied, both theoretically and experimentally, by transient absorption spectroscopy using isolated 400-as pulses centered at 22 eV and 12-fs near-infrared (NIR) pulses with 780-nm central wavelength. Multiple features in the helium singly excited bound-state spectrum are observed only when the NIR and attosecond pulses are overlapped in time. Theoretical analysis indicates that these light-induced structures (LISs) are the intermediate states in resonant, second-order processes that transfer population to multiple dipole forbidden states. The use of broadband, coherent extreme ultraviolet (XUV) radiation allows observation of these LISs without specifically tuning to a two-photon resonance, as would be required with narrowband XUV light. The strength and position of the LISs depend strongly on the NIR intensity and the pump-probe delay.
Light-induced states in He atoms were characterized using attosecond transient absorption spectroscopy. A 400 as pulse covering the 20–24 eV spectral range serves as the probe pulse, and the effect of a few-cycle near infrared pulse (12 fs, 780 nm) on the absorption spectrum is measured as a function of time delay and near-infrared intensities varying from (5.0 ± 2) × 1010 to (1 ± 0.4) × 1013 W/cm2. Light-induced states resulting from near-infrared coupling of 1s2p to 1s2s, 1s3d, and 1s3s states are observed. Absorption features that likely result from coupling of 1s3p to 1s4s, 1s4d, 1s5s, and 1s5d states are also observed. The light-induced states with the smallest detunings (1s3d and 1s3s) from the dressing frequency may shift to higher frequencies as the dressing intensity is increased.
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.
In the present paper, we investigate the time-resolved transient absorption spectroscopy of doubly excited states of helium atoms by solving the time-dependent two-electron Schr\"odinger equation numerically based on a one-dimensional model. The helium atoms are subjected to an extreme ultraviolet (XUV) attosecond pulse and a time-delayed infrared (IR) few-cycle laser pulse. A superposition of doubly excited states populated by the XUV pulse is identified, which interferes with the direct ionization pathway leading to Fano resonance profiles in the photoabsorption spectrum. In the presence of an IR laser, however, the Fano line profiles are strongly modified: A shifting, splitting, and broadening of the original absorption lines is observed when the XUV attosecond pulse and infrared few-cycle laser pulse overlap in time, which is in good agreement with recent experimental results. At certain time delays, we observe symmetric Lorentz, inverted Fano profiles, and even negative absorption cross sections indicating that the XUV light can be amplified during the interaction with atoms. We further prove that the above pictures are general for different doubly excited states by suitably varying the frequency of the IR field to coherently couple the corresponding states.
In the present paper, the time-resolved transient absorption spectroscopy of helium atoms is investigated based on the three-level modeling. The helium atoms are subjected to an extreme ultraviolet (XUV) attosecond pulse and a time-delayed infrared (IR) few-cycle laser field. The odd excited state are populated from the ground state by the XUV pulse due to the dipole selection rule, and probed by the time-delayed IR laser. The time-resolved transient absorption spectroscopy based on the different coupling mechanism demonstrate some different features, the photoabsorption spectrum based on three-level model with rotating wave approximation (RWA) cannot repeat the fast oscillation and the sideband structure which have been observed in the previous experimental investigation. The dressing effect of IR laser pulse on the ground state can contribute new interference structures in the photoabsorption spectrum.
We employ an extreme ultraviolet (XUV) pulse to impulsively excite dipole polarization in atoms or molecules, which corresponds to coherently prepared superposition of excited states. A delayed near infrared (NIR) pulse then perturbs the fast evolving polarization, and the resultant absorbance change is monitored in dilute helium, dense helium, and sulfur hexafluoride (SF6) molecules. We observe and quantify the time-dependence of various transient phenomena in helium atoms,includinglaser-inducedphase(LIP),time-varying(AC)Starkshift,quantumpathinterference, and laser-induced continuum structure. In the case of dense helium targets, we discuss nonlinear macroscopic propagation effects pertaining to LIP and resonant pulse propagation, which accoun tfor the appearance of new spectral features in transient lineshapes. We then use tunable NIR photons to demonstrate the wavelength dependence of the transient laser induced effects. In the case of molecular polarization experiment in SF6, we show suppression of XUV photoabsorption corresponding to inter-valence transitions in the presence of a strong NIR field. In each case, the temporal evolution of transient absorption spectra allows us to observe and understand the transient laser induced modifications of the electronic structure of atoms and molecules.
We present theoretical simulations of the attosecond transient absorption of singly-excited states of helium atoms in the presence of a dressing near-infrared or infrared laser. In particular, we aim to address several unresolved questions in the transient absorption of helium and to resolve the remaining discrepancies between theory and experiment. We initially focus on the forklike structures in the Autler–Townes splitting of the 1s2p state and the effects of resonant coupling to the 1s2s and 1s3s states. We find that the delay-dependent features of the Autler–Townes doublet depend strongly on both the laser frequency detuning from resonance and on the laser pulse duration, and explain the lack of such structures in current experimental data. Next, we identify the interference mechanism which causes the half-cycle oscillations in the absorption spectrum below the excited state manifold. Finally, we observe for the first time the presence of quantum beating in the simulated transient absorption spectrogram, and discuss the conditions under which such wavepacket dynamics could be observed experimentally.