Α 10-gigawatt attosecond source for non-linear XUV optics and XUV-pump-XUV-probe studies

Channel separation of secondary generated radiation induced by orthogonal XUV and IR pulses

The secondary generated radiation induced by orthogonal linearly polarized extreme ultraviolet (XUV) and infrared (IR) pulses is analyzed for the spectral region of the second XUV harmonic. The polarization-filtering-based method is utilized to separate two spectrally overlapping and competing channels, which are the XUV second harmonic generation (SHG) by IR-dressed atom and XUV-assisted recombination channel of high-order harmonic generation in the IR field [Phys. Rev. A98, 063433 (2018)10.1103/PhysRevA.98.063433]. We demonstrate the use of the separated XUV SHG channel for accurately retrieving the IR-pulse waveform and find the range of IR-pulse intensities for which this retrieving is applicable.

J-Matrix time propagation of atomic hydrogen in attosecond fields

The J-Matrix approach for scattering is extended to the time-dependent Schrödinger equation (TDSE) for one electron atoms in external few cycle attosecond fields. To this purpose, the wave function is expanded in square integrable (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$L^2$$\end{document}L2) Sturmian functions and an equation system for the transition amplitudes is established. Outside the interaction zone, boundary conditions are imposed at the border in the \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$L^2$$\end{document}L2 function space. These boundary conditions correspond to outgoing waves (Siegert states) and minimize reflections at the \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$L^2$$\end{document}L2 boundary grid. Outgoing wave behaviour in the asymptotic region is achieved by employing Pollaczek functions. The method enables the treatment of light - atom interactions within arbitrary external fields. Using a partial wave decomposition, the coupled differential equation system is solved by a Runge-Kutta method. As a proof of the method ionization processes of atomic hydrogen in half and few cycle attosecond fields are examined. The electron energy spectrum is calculated and the numerical implementation will be presented. Different forms of the interaction operator are considered and the convergence behaviour is discussed. Results are compared to other studies which use independent approaches like finite difference methods. Remarkable agreement is achieved even with strong field strengths of the electromagnetic field. It is demonstrated that expanding in \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$L^2$$\end{document}L2 functions and imposing boundary conditions at the limit in the \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$L^2$$\end{document}L2 function space can be an advantageous alternative to conventional propagation methods using complex absorbing potentials or complex scaling.

Waveform retrieving of an isolated attosecond pulse using high-order harmonics generation of the superimposed infrared field

An all-optical method is suggested for the metrology of an isolated, pulse-to-pulse stabilized attosecond pulse. It is shown analytically that high-order harmonic generation (HHG) yield for an intense IR pulse and time-delayed attosecond pulse keeps encoded waveform of the attopulse, which can be decoded by the time delay measurements of the HHG yield. The retrieval method is demonstrated by modeling HHG from Ne atom within time-dependent Kohn-Sham equations. The application of the suggested method for monitoring the carrier-envelope phase of the attosecond pulse is discussed.

Selecting two-photon sequential ionization pathways in H2 through harmonic filtering

Recent experiments in gas-phase molecules have shown the versatility of using attosecond pulse trains combined with IR femtosecond pulses to track and control excitation and ionization yields on the attosecond timescale. The interplay between electron and nuclear motions drives the light-induced transitions favoring specific reaction paths, so that the time delay between the pulses can be used as the tracking parameter or as a control knob to manipulate the molecular dynamics. Here, we present ab initio simulations on the hydrogen molecule to demonstrate that by filtering the high harmonics in an attosecond pulse train one can quench or enhance specific quantum paths thus dictating the outcome of the reaction. It is then possible to discriminate the dominant sequential processes in two-photon ionization, as for example molecular excitation followed by ionization or the other way around. More interestingly, frequency filters can be employed to steer the one- and two-photon yields to favor electron emission in a specific direction.

Spectral filtering of high-order harmonics via optics-free focusing

Controlling the wavefront of an extreme ultraviolet (XUV) high-order harmonic beam during the generation process offers the capability of modifying the beam properties without resorting to any XUV optics. By characterizing the XUV intensity profile and wavefront, we quantitatively retrieve both the size and the position of the waist of each harmonic generated in an argon jet. We show that optics-free focusing can occur under specific generating conditions leading to XUV focii of micrometer size. We also demonstrate that each focus is located at distinct longitudinal positions. Using this remarkable XUV wavefront control combined with near focus spatial selection, we experimentally demonstrate efficient and adjustable spectral filtering of the XUV beam, along with a strong rejection of the fundamental beam, without using any XUV optics. The experimental results are compared with simulations providing the impact of the filtering on the temporal profile of the XUV field. It shows that the attosecond structure is preserved and that the beam is more homogeneous after the filtering, thereby reducing the longitudinal focii shift. This is a major step to achieve high XUV intensity and probing ultrafast processes with an improved resolution.

XUV pump-XUV probe transient absorption spectroscopy at FELs

The emergence of ultra-intense extreme-ultraviolet (XUV) and X-ray free-electron lasers (FELs) has opened the door for the experimental realization of non-linear XUV and X-ray spectroscopy techniques. Here we demonstrate an experimental setup for an all-XUV transient absorption spectroscopy method for gas-phase targets at the FEL. The setup combines a high spectral resolving power of E/ΔE ≈ 1500 with sub-femtosecond interferometric resolution, and covers a broad XUV photon-energy range between approximately 20 and 110 eV. We demonstrate the feasibility of this setup firstly on a neon target. Here, we intensity- and time-resolve key aspects of non-linear XUV-FEL light-matter interactions, namely the non-resonant ionization dynamics and resonant coupling dynamics of bound states, including XUV-induced Stark shifts of energy levels. Secondly, we show that this setup is capable of tracking the XUV-initiated dissociation dynamics of small molecular targets (oxygen and diiodomethane) with site-specific resolution, by measuring the XUV transient absorption spectrum. In general, benefitting from a single-shot detection capability, we show that the setup and method provides single-shot phase-locked XUV pulse pairs. This lays the foundation to perform, in the future, experiments as a function of the XUV interferometric time delay and the relative phase, which enables advanced coherent non-linear spectroscopy schemes in the XUV and X-ray spectral range.

Table-top extreme ultraviolet second harmonic generation

The lack of available table-top extreme ultraviolet (XUV) sources with high enough fluxes and coherence properties has limited the availability of nonlinear XUV and x-ray spectroscopies to free-electron lasers (FELs). Here, we demonstrate second harmonic generation (SHG) on a table-top XUV source by observing SHG near the Ti M_2,3 edge with a high-harmonic seeded soft x-ray laser. Furthermore, this experiment represents the first SHG experiment in the XUV. First-principles electronic structure calculations suggest the surface specificity and separate the observed signal into its resonant and nonresonant contributions. The realization of XUV-SHG on a table-top source opens up more accessible opportunities for the study of element-specific dynamics in multicomponent systems where surface, interfacial, and bulk-phase asymmetries play a driving role.

Enhanced XUV harmonics generation from diatomic gases using two orthogonally polarized laser fields

Enhanced high repetition rate coherent extreme ultraviolet (XUV) harmonics represent efficient probe of electron dynamics in atoms, molecules and solids. In this work, we used orthogonally-polarized two-color laser field to generate strong even and odd high order harmonics from molecular gas targets. The dynamics of odd and even harmonics from O2, and N2 gases were investigated by employing single- and two-color laser fields using the fundamental radiation and second harmonic of 1030 nm, 37 fs, 50 kHz pulses. The relative efficiencies of harmonics were analyzed as a function of the thickness of the barium borate crystal used for second harmonic generation. Defocusing-assisted phase matching conditions were achieved in N2 gas for different groups of XUV harmonics.

Isolated single-cycle extreme-ultraviolet pulses from undulator radiation

We propose a method to generate an isolated single-cycle pulse in the extreme-ultraviolet spectral region using a broadband conventional laser. The uncompressed laser pulse imprints a chirped sinusoid current profile onto a relativistic electron beam. As the beam propagates along a specifically tailored magnetic field of an undulator, it produces an isolated single-cycle pulse. For moderate laser intensities (0.2 mJ per pulse) and typical operating parameters of current electron accelerators, we predict a 26 as, 5 GW peak-power pulse spanning wavelengths down to 15 nm.