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Physical Review A

Second-harmonic generation of a short XUV pulse interacting with an IR-laser-dressed atom

Alexander A. Romanov, Alexander A. Silaev, Н. В. Введенский, Ia. V. Breev, A. V. Flegel, M. V. Frolov

Published Dec 5, 2022• 8 citations
Publisher

Abstract

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.

DOI
10.1103/physreva.106.063101

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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 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.
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.
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.
We demonstrate Rabi oscillations in nonlinear ionization of argon by an intense femtosecond extreme ultraviolet (XUV) laser field produced by high-harmonic generation. We monitor the formation of Ar2+\mathrm{A}{\mathrm{r}}^{2+}Ar2+ as a function of the time delay between the XUV pulse and an additional near-infrared (NIR) femtosecond laser pulse, and show that the population of an Ar+∗\mathrm{A}{\mathrm{r}}^{+*}Ar+∗ intermediate resonance exhibits strong modulations both due to an NIR laser-induced Stark shift and XUV-induced Rabi cycling between the ground state of Ar+\mathrm{A}{\mathrm{r}}^{+}Ar+ and the Ar+∗\mathrm{A}{\mathrm{r}}^{+*}Ar+∗ excited state. Our experiment represents a direct experimental observation of a Rabi-cycling process in the XUV regime.
We present a theoretical study of transient absorption spectra of laser-dressed hydrogen atoms, based on numerical solutions of the time-dependent Schr\"odinger equation. The timing of absorption is controlled by the delay between an extreme ultra violet (XUV) pulse and an infrared (IR) laser field. The XUV pulse is isolated and several hundred attoseconds in duration, which acts as a pump to drive the ground-state electron to excited ppp states. The subsequent interaction with the IR field produces dressed states, which manifest as sidebands between the 1s-npnpnp absorption spectra separated by one IR-photon energy. We demonstrate that the population of dressed states is maximized when the timing of the XUV pulse coincides with the zero crossing of the IR field, and that their energies can be manipulated in a subcycle time scale by adding a chirp to the IR field. An alternative perspective to the problem is to think of the XUV pulse as a probe to detect the dynamical ac Stark shifts. Our results indicate that the accidental degeneracy of the hydrogen excited states is removed while they are dressed by the IR field, leading to large ac Stark shifts. Furthermore, we observe the Autler-Townes doublets for the n=2n=2n=2 and 3 levels using the 656 nm dressing field, but their separation does not agree with the prediction by the conventional three-level model that neglects the dynamical ac Stark shifts.