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Physical Review A

Ultimate capabilities for few-cycle pulse formation via resonant interaction of XUV radiation with IR-field-dressed atoms

T. R. Akhmedzhanov, M. Yu. Emelin, V. A. Antonov, Yevgeny V. Radeonychev, Mikhail Yu. Ryabikin, Olga A. Kocharovskaya

Published Feb 28, 2017• 6 citations
Publisher

Abstract

We perform an ab initio study of the ultimate capabilities and limits of applicability of the method for few-cycle pulse formation via the resonant interaction of extreme ultraviolet (XUV) radiation with atoms dressed by a moderately strong infrared (IR) laser field proposed in two earlier works [Y. V. Radeonychev et al., Phys. Rev. Lett. 105, 183902 (2010) and V. A. Polovinkin et al., Opt. Lett. 36, 2296 (2011)]. Taking into account all the multiphoton processes in the systems under consideration on the basis of numerical solution of the three-dimensional time-dependent Schr\"odinger equation (TDSE) in the single-active-electron approximation, we show the possibilities to produce 1.1-fs pulses from 124.6-nm XUV radiation via the linear Stark effect in atomic hydrogen, as well as 500-as pulses from 58.4-nm XUV radiation via excited-state ionization in helium. We derive a generalized analytical solution, which takes into account the interplay between sub-laser-cycle Stark effect and excited-state ionization and allows us to analyze the results of TDSE calculations. We found that the ultimate intensity of the IR field suitable for few-cycle pulse formation via the linear Stark effect or excited-state ionization is limited by the threshold for atomic ionization from the resonant excited state or the ground state, respectively. We show that the pulses with shorter duration can be produced in the medium of ions with higher values of the ionization potential.

DOI
10.1103/physreva.95.023845

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We perform an ab initio study of the ultimate capabilities and limits of applicability of the method for few-cycle pulse formation via the resonant interaction of extreme ultraviolet (XUV) radiation with atoms dressed by a moderately strong infrared (IR) laser field proposed in two earlier works [Y. V. Radeonychev et al., Phys. Rev. Lett. 105, 183902 (2010) and V. A. Polovinkin et al., Opt. Lett. 36, 2296 (2011)]. Taking into account all the multiphoton processes in the systems under consideration on the basis of numerical solution of the three-dimensional time-dependent Schr"odinger equation (TDSE) in the single-active-electron approximation, we show the possibilities to produce 1.1-fs pulses from 124.6-nm XUV radiation via the linear Stark effect in atomic hydrogen, as well as 500-as pulses from 58.4-nm XUV radiation via excited-state ionization in helium. We derive a generalized analytical solution, which takes into account the interplay between sub-laser-cycle Stark effect and excited-state ionization and allows us to analyze the results of TDSE calculations. We found that the ultimate intensity of the IR field suitable for few-cycle pulse formation via the linear Stark effect or excited-state ionization is limited by the threshold for atomic ionization from the resonant excited state or the ground state, respectively. We show that the pulses with shorter duration can be produced in the medium of ions with higher values of the ionization potential.
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The amplification of ultrashort extreme-ultraviolet (XUV) pulses in argon in high-order harmonic generation processes is studied by using the time-dependent Schr\"odinger equation in the spin-free one-active-electron and single-atom approximation. We consider a neutral argon atom initially in the valence 3p state and a sufficiently intense two-cycle driving infrared (IR) pulse for the atom to be mainly ionized after the first laser cycle. The correlated dynamics and transitions from the valence 3p to a virtual subvalence 3s state and in the ionized regions are examined by synchronizing a 1.5-fs XUV pulse to the IR pulse. The calculated single-atom gain spectrum (26--45 eV) agrees with recent experimental measurements. We discuss different channels that can be present in the gain process as a function of pulse parameters and through an analysis of the dynamics of the populations in terms of field-free eigenstates. When the XUV pulse is considered at the end of the driving IR field the amplification is due to contributions of stimulated recombination from excited Rydberg and low energy continuum states to the 3s and 3p states of argon. In regions where the IR field is intense, high energy and angular momentum states are populated and the medium can interact with the pulses through bound and continuum states involving parametric transitions, which is further confirmed by studying classical electron trajectories. We discuss how these parametric interactions might be suitable for amplification of photon energies far from the ionization threshold as observed in the experiments.
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We suggest a highly efficient method of ultrashort pulse formation from resonant XUV radiation due to sub-laser-cycle modulation of the excited state of non-hydrogen-like atoms by a nonionizing IR laser field. This modulation results in formation of the Raman-Stokes and anti-Stokes sidebands in coherently forward-scattered radiation, which, in turn, leads to formation of short pulses, when the phases of the sidebands are matched. This method is a generalization of a recently suggested technique [V. A. Antonov et al., Phys. Rev. A 88, 053849 (2013)] for a non-hydrogen-like medium. The possibility to form 2-fs XUV pulses in the gas of helium atoms and 990-as XUV pulses in the plasma of Li+ ions with efficiencies over 80% is shown.
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.