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Journal of Physics B Atomic Molecular and Optical Physics

Resonance effects and quantum beats in attosecond transient absorption of helium

Michael Chini, Xiaowei Wang, Yan Cheng, Zenghu Chang

Published Jun 10, 2014• 54 citations
Publisher

Abstract

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.

DOI
10.1088/0953-4075/47/12/124009

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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.
We present a theoretical study of the delay-dependent Autler-Townes (AT) splitting in transient absorption spectroscopy of an isolated attosecond pulse in helium atoms subject to a delayed infrared (IR) pulse. We concentrate on cases in which the IR pulse is resonant with the helium $1s2p\text{\ensuremath{-}}1s2s$ transition and provide a time-domain perspective of the dynamics in the delay-dependent pump-probe system. We identify several interesting delay-dependent features in the transient absorption spectrum such as AT splitting, oscillation between absorption and emission at the resonant absorption frequency, and sub-IR-cycles oscillations. We then explain the origins of these features in the time domain in terms of a strongly driven two-level system, in the language of population transfer and coherent control.
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 theoretically investigate the attosecond transient absorption spectrum of helium atom in the presence of an infrared-dressed laser pulse upon scanning their relative delay, with the particular emphasis on the chirp effect of the attosecond pulse. By numerically solving the fully three-dimensional time-dependent Schrödinger equation, we identify the attoscecond chirp can induce the temporal shift of the absorption spectrogram along the delay axis. Additionally, it is found that the extent of the temporal shift is dependent on both the position of the absorption line and the infrared pulse wavelength, which is well confirmed and reproduced by a three-level model. Moreover, we demonstrate that the observed features can be quantitatively explained in terms of the indirect two-photon absorption processes through some virtual states. This effect might provide a way to measure the chirp of attosecond pulse in an all-optical way.
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.
We calculate the transient absorption of an isolated attosecond pulse (IAP) by helium atoms subject to a delayed infrared (IR) laser pulse. With the central frequency of the broad attosecond spectrum near the ionization threshold, the absorption spectrum is strongly modulated at the sub-IR-cycle level. Given that the absorption spectrum results from a time-integrated measurement, we investigate the extent to which the delay dependence of the absorption yields information about the attosecond dynamics of the atom-field energy exchange. We find two configurations in which this is possible. The first involves multiphoton transitions between bound states that result in interference between different excitation pathways. The second involves the modification of the bound state absorption lines by the IR field, which we find can result in a subcycle time dependence only when ionization limits the duration of the strong-field interaction.
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.