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Physical Review Letters

XUV Rectification Effect in the IR-Dressed Medium

A. V. Flegel, M. V. Frolov

Published Dec 11, 2023• 1 citation
Publisher

Abstract

We show that the quasistatic dipole moment can be induced by a short extreme ultraviolet (XUV) pulse (XUV rectification effect) in atomic gas medium subjected to an intense infrared (IR) field (IR-dressed atoms). The general theory of the XUV rectification effect for a single IR-dressed atom is presented, which explicitly relates IR-modified polarizability of an atomic system in the XUV range with the induced quasistatic dipole moment. We illustrate general properties of the XUV rectification effect in an atomic system within the analytical zero-range potential model by presenting the dependence on the IR-field intensity and the time delay between XUV and IR pulses.

DOI
10.1103/physrevlett.131.243202

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We show that the quasistatic dipole moment can be induced by a short extreme ultraviolet (XUV) pulse (XUV rectification effect) in atomic gas medium subjected to an intense infrared (IR) field (IR-dressed atoms). The general theory of the XUV rectification effect for a single IR-dressed atom is presented, which explicitly relates IR-modified polarizability of an atomic system in the XUV range with the induced quasistatic dipole moment. We illustrate general properties of the XUV rectification effect in an atomic system within the analytical zero-range potential model by presenting the dependence on the IR-field intensity and the time delay between XUV and IR pulses.
The nonlinear response of an atomic system interacting with short extreme ultraviolet (XUV) and infrared (IR) pulses is considered within the adiabatic approach (for an IR field) and the perturbation theory (for an XUV pulse). The second-order perturbation theory in the XUV field is developed for the IR-dressed atom, and the laser-induced generation of a pulse with a doubled carrier XUV frequency is analyzed. We study the yield of harmonics in the frequency band of the generated doubled frequency XUV pulse as a function of the time delay between initial XUV and IR pulses. Our analytical and numerical results show that this dependence carries the time dependence of the intense IR pulse and can be utilized to retrieve the waveform of the IR pulse.
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.
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.
We investigate the absorption line shapes of laser-dressed atoms beyond the single-atom response, by using extreme ultraviolet (XUV) attosecond pulse trains to probe an optically thick helium target under the influence of a strong infrared (IR) field. We study the interplay between the IR-induced phase shift of the microscopic time-dependent dipole moment and the resonant-propagation-induced reshaping of the macroscopic XUV pulse. Our experimental and theoretical results show that as the optical depth increases, this interplay leads initially to a broadening of the IR-modified line shape, and subsequently, to the appearance of new, narrow features in the absorption line.
We present a theoretical study of the transient absorption of an ultrafast, extreme ultraviolet (XUV) pulse in the presence of an infrared (IR) control pulse in a macroscopic three-level helium gas. We concentrate on cases in which the IR pulse is shorter than the XUV pulse and induces transient transparency and amplification of the XUV light via coherent control. We give a time-dependent picture of the absorption process through the time dependence of the populations and relative coherences of the three states. We show that the XUV absorption can be dynamically changed into emission via phase control of the XUV-resonant 2p2p2p state through IR-induced coupling between the 2p2p2p and the 2s2s2s states. This can lead to compression and amplification of the XUV pulse when propagating through a laser-dressed macroscopic medium. We discuss the influence of atomic decoherence times on the XUV reshaping and show that fast decoherence means that the laser dressing acts more like a passive transmission window and less like a dynamical redistribution of energy.
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.