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Journal of Physics Conference Series

Analytical theory of attosecond transient absorption spectroscopy of perturbatively dressed systems

Daria Kolbasova, Robin Santra

Published Jan 1, 2020• 0 citations
Publisher

Abstract

Synopsis The current work provides an elementary explanation of the signals one can obtain in attosecond transient absorption experiments. We perform a perturbative analysis of the transient absorption signal as a function of the time delay between the excitation pulse and the dressing pulse. We discuss an unusual kind of experiment, where the information is gained from the absorption spectrum of an attosecond XUV pump pulse, and the probe pulse gives a reference time, allowing to measure the time evolution of a system. We illustrate our concepts through numerical calculations for a model atom.

DOI
10.1088/1742-6596/1412/7/072007

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Synopsis The current work provides an elementary explanation of the signals one can obtain in attosecond transient absorption experiments. We perform a perturbative analysis of the transient absorption signal as a function of the time delay between the excitation pulse and the dressing pulse. We discuss an unusual kind of experiment, where the information is gained from the absorption spectrum of an attosecond XUV pump pulse, and the probe pulse gives a reference time, allowing to measure the time evolution of a system. We illustrate our concepts through numerical calculations for a model atom.
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 propose a general procedure for the reconstruction of time-dependent dipole moment via transient absorption spectrum, within the framework of linear response theory. The reconstruction method is demonstrated with two examples: the self-reconstruction of extreme-ultraviolet-light-atom interaction (analytically) and the infrared-dressed helium system (numerically). A correlation coefficient is introduced to quantify the fidelity of the reconstruction, which reaches almost 100% over a wide range of laser parameters and thus indicates the robustness of our reconstruction method. Our theory provides a solid basis for understanding the recent experiment [Phys. Rev. Lett. 121, 173005 (2018)], explaining when and why the dipole moment can be well reconstructed solely from the absorption spectrum even without exact knowledge of the attosecond probe pulse.
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.
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.
Abstract Attosecond transient absorption (ATA) has been developed as an all-optical technique for probing electron dynamics in matter. Here we present a scheme that can modify the laser-induced state and the corresponding ATA spectrum via excitation by a pair of XUV attosecond pulses and by a time-delayed mid-infrared (MIR) laser probe. Different from the scheme of the electronic excitation by a single XUV attosecond pulse, the application of a pair of XUV pulses provides extra degrees of freedom, such as the time delay and the intensity ratio between two XUV pulses, which make it possible to adjust the pump process, resulting in the modification of the ATA spectrum. We show that by varying the time delay between the two XUV pulses, the population of the dark state and the ATA spectrum of the laser-induced state have periodic modulations. We also demonstrate that the peak of the ATA spectrum of the laser-induced state appears at a fixed time delay between the XUV pair and the MIR laser when the intensity ratio is large, and it changes with the time delay when the intensity ratio is small, which can be related to either one of two peaks in the population of the dark state.
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.