XUV Laser Research Articles

Demonstration of a kHz-repetition-rate extreme ultraviolet laser at 41.8 nm.

We demonstrate the operation of a plasma-based extreme ultraviolet (XUV) laser at a 1 kHz repetition rate driven by infrared pump pulses of less than 20 mJ. The 41.8 nm laser pulses were generated in a Xe plasma created by optical-field ionization by the L1 Allegra laser at ELI Beamlines. The output power of the XUV laser lies in the few microwatt range, and the energy efficiency of this pumping scheme opens the way for further scaling in repetition rate and average power.

Emission control of entangled electrons in photoionisation of a hydrogen molecule

For photo-dissociation of a single hydrogen molecule (H2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$H_2$$\\end{document}) with combined XUV and IR laser pulses, we demonstrate optical control of the emission direction of the photoelectron with respect to the outgoing neutral fragment (the H-atom). Depending on the relative delay between the two laser fields, adjustable with sub-femtosecond time resolution, the photoelectron is emitted into the same hemisphere as the H-atom or opposite. This emission asymmetry is a result of entanglement of the two-electron final-state involving the spatially separated bound and emitted electron.

Imagining density distribution of molecular orbitals in IR+XUV co-rotating circular laser fields by frequency-domain theory

Abstract We have investigated the angle-resolved ATI spectrum of oriented molecules in the IR+XUV co-rotating circular laser fields. According to the different roles of IR and XUV laser in the ionization process, we purposefully adjust the photon energy of XUV and the intensity of IR laser to make the ionization spectrum of the molecule distributed in a suitable momentum region. Moreover, under the same laser conditions, the background fringes in the ionization spectrum of the molecule can be removed by using the ionization spectrum of the atom with the same ionization energy as the molecule, so that the molecular orbital density distribution in the suitable momentum region can be obtained. That is, for any unknown molecule, as long as the ionization energy of the molecule can be measured, the density distribution of the molecular orbital can be imaged in a definite momentum region by adjusting the laser field conditions, which may shed light on the experimental detection of molecular orbitals.

Sub-50 fs temporal resolution in an FEL-optical laser pump-probe experiment at FLASH2.

High temporal resolution is essential for ultra-fast pump-probe experiments. Arrival time jitter and drift measurements, as well as their control, become critical especially when combining XUV or X-ray free-electron lasers (FELs) with optical lasers due to the large scale of such facilities and their distinct pulse generation processes. This paper presents the application of a laser pulse arrival time monitor that actively corrects the arrival time of an optical laser relative to the FEL's main optical clock. Combined with post-analysis single pulse jitter correction this new approach improves the temporal resolution for pump-probe experiments significantly. Benchmark measurements on photo-ionization of xenon atoms performed at FLASH beamline FL26, demonstrate a sub-50 fs FWHM overall temporal resolution.

Dipole instability in molecules irradiated by XUV pulses

We study the response of small covalent molecules to XUV laser pulses. The theoretical description relies on a real-time and real-space Time-Dependent Density Functional Theory (TDDFT) approach at the level of the local density approximation complemented by an efficient self-interaction correction. We observe the development of a dipole instability well after the laser pulse has died out. We find that this instability mechanism is robust with respect to ionic motion, to a wide variety of laser characteristics and to the inclusion of incoherent correlations at the level of a relaxation time ansatz. To rule out any potential numerical effects, we use two independent computational implementations of the TDDFT approach. A comparison of the various laser parameters together with the widely used model approach consisting in an instantaneous hole excitation shows the generic character of this instability in terms of the level depletion of a deep lying electron state. An experimental verification of the phenomenon is proposed in terms of a time-resolved measurement of the photoelectron spectrum.

Photoelectron momentum distribution of hydrogen atoms in a superintense ultrashort high-frequency pulse

We use a numerically solved time-dependent Schrödinger equation for calculating the photoelectron momentum distribution of ground-state hydrogen atoms in the presence of superintense ultrashort high-frequency pulses. It is demonstrated that the dynamic interference effect within a superintense XUV laser beam has the ability to significantly alter the photoelectron momentum distribution. In our work, a clearly visible dynamic interference pattern is observed when hydrogen atoms are exposed to a superintense circularly polarized laser pulse with a photon energy of ℏω = 53.605 eV, which has previously been found for linearly polarized pulses or the weakly bounded model H− system for circularly polarized pulses. Angular-distorted interference arises for linear superintense XUV pulses of similar intensity. The significant differences in photoelectron momentum distributions that have been seen by linearly and circularly polarized XUV pulses are caused by the Coulomb rescattering phenomenon.

Spectra and Predissociation Dynamics of the 4pσB''1∑u+ and 4pπD' 1Πu States of D2.

The spectra and predissociation dynamics of the Rydberg states 4pσB″1∑u+(υ'=2) and 4pπD'1Πu±(υ'=1) of the D2 molecule have been studied by a combination of XUV laser pump, UV laser probe, and velocity map imaging methods. The D(2l) fragment yield spectrum and the D2 excitation spectrum have been measured. Although the predissociation rates were found to be small and comparable to the radiative decay rates, the D(2l) fragments were observed for all the Rydberg states. The measured D(2l) fragment angular distributions are in good agreement with a reported model that has an analytical formula for anisotropy parameters. The results demonstrated that the 4pσB″1∑u+ and 4pπD'1Π u+ states dissociate via direct and indirect coupling with an 1∑u+ state, respectively. For predissociation of the 4pσB″1∑u+ state, the experimental data showed that D(1s) + D(2s) is the main channel, supporting previous theoretical predictions that the predissociation occurs by coupling to the vibrational continuum of the 3pσB'1∑u+ state.

Signatures of stabilization in the angle-resolved photoemission by an ultrashort intense XUV laser pulse

We explore features of stabilization in the angle-resolved spectra of photoelectrons when ground-state atomic hydrogen is exposed to an intense ($\ensuremath{\sim}{10}^{18} \mathrm{W}/{\mathrm{cm}}^{2}$) linearly polarized ultrashort XUV pulse. In this regime, atomic stabilization can be traced to the temporal destructive interference between wave packets released by the intense field at different instants of time. By a comparison between the numerical solution of the time-dependent Schr\"odinger equation and a semianalytical model, we can identify the observed oscillations in the momentum distribution and the suppression of photoemission into the laser polarization direction as a direct result of stabilization. We also explore the effect of nondipole corrections on the angular distribution.

Time-resolved relaxation and fragmentation of polycyclic aromatic hydrocarbons investigated in the ultrafast XUV-IR regime

Polycyclic aromatic hydrocarbons (PAHs) play an important role in interstellar chemistry and are subject to high energy photons that can induce excitation, ionization, and fragmentation. Previous studies have demonstrated electronic relaxation of parent PAH monocations over 10–100 femtoseconds as a result of beyond-Born-Oppenheimer coupling between the electronic and nuclear dynamics. Here, we investigate three PAH molecules: fluorene, phenanthrene, and pyrene, using ultrafast XUV and IR laser pulses. Simultaneous measurements of the ion yields, ion momenta, and electron momenta as a function of laser pulse delay allow a detailed insight into the various molecular processes. We report relaxation times for the electronically excited PAH*, PAH+* and PAH2+* states, and show the time-dependent conversion between fragmentation pathways. Additionally, using recoil-frame covariance analysis between ion images, we demonstrate that the dissociation of the PAH2+ ions favors reaction pathways involving two-body breakup and/or loss of neutral fragments totaling an even number of carbon atoms.

Influence of polarization directions of the IR+XUV two-color laser fields on angle-resolved photoelectron energy spectrum.

By employing the frequency-domain theory, we investigate the influence of polarization directions on angle-resolved photoelectron energy spectrum in the above-threshold ionization (ATI) process of atoms exposed to the IR+XUV two-color laser fields, which shows the multiplateau structures. When the ionized electron is emitted along the IR laser's polarization direction, the width of each plateau keeps a certain energy range, and the jet structures and main lobes are determined by both the emission angle relative to the polarization direction of the XUV laser field and the number of the XUV photons absorbed by the electron. While when the ionized electron is emitted along the XUV laser's polarization direction, the width of each plateau depends on the polarization direction of the IR laser field, and the angular distribution of the ionized electron exhibits the isotropic characteristics. These results show that the ATI spectrum may be effectively controlled by changing the angle between the two laser fields' polarization directions.

Laser-assisted photoionization: Streaking, sideband, and pulse-train cases

We present a theoretical study of atomic laser-assisted photoionization emission (LAPE). We consider an atom driven by a linearly polarized XUV laser in two different scenarios: i) a single attosecond pulse (in both the streaking and sideband regimes) and ii) an attosecond pulse train. The process takes place assisted by a linearly polarized infrared (IR) laser field. In all these cases the energy and angle-resolved photoelectron spectrum (PES) is determined by a leading contribution, related to the intracycle factor [Gramajo et al., J. Phys. B 51, 055603 (2018)], complemented by other ones, derived from the periodicity and symmetry properties of the dipole transition matrix with respect to the IR field. Each of these terms imprint particular features in the PES that can be straightforwardly understood in terms of generalized energy conservation laws. We investigate in detail these PES structures, in particular, for the case of argon initially in the 3s quantum state. Our theoretical scheme, based on the strong-field approximation (SFA), can be applied, however, to other atomic species and field configurations as well.

Theoretical investigation of an intense ultrashort xuv pulse-induced Auger cascade in krypton

This paper theoretically investigates an Auger cascade triggered upon exposure of a Kr atom to an intense ultrashort xuv laser. In the first step of the cascade, Auger electrons are released from a coherent superposition of core-excited states to form an intermediate ionic state, which further decays through the Auger process to form a double ionic final state. Results show that the interference due to electrons released from a coherent superposition of closely spaced core-excited states, in their common continuum, in the first step of the cascade significantly modifies the line shape of the coincidence spectrum of the two Auger electrons of the cascade. The variation of the laser peak intensity not only influences the peak magnitude and linewidth but also induces a peak shift in the coincidence electron spectrum; further, the high-intensity-induced Rabi oscillations between ground and core-excited state induce line-shape splitting in the coincidence electron spectrum of the two Auger electrons, thereby manifesting the effect of Rabi oscillations even for the second Auger electron of the cascade. This work shows that the inter-related decay widths of the core-excited state and intermediate ionic state significantly influence the line shape of the coincidence electron spectrum of a cascade decay. The angular distribution of the second electron released from the cascade system of krypton is also investigated.

Bound-State Electron Dynamics Driven by Near-Resonantly Detuned Intense and Ultrashort Pulsed XUV Fields

We report on numerical results revealing line-shape asymmetry changes of electronic transitions in atoms near-resonantly driven by intense extreme-ultraviolet (XUV) electric fields by monitoring their transient absorption spectrum after transmission through a moderately dense atomic medium. Our numerical model utilizes ultrashort broadband XUV laser pulses varied in their intensity (1014–1015 W/cm2) and detuning nearly out of resonance for a quantitative evaluation of the absorption line-shape asymmetry. It will be shown how transient energy shifts of the bound electronic states can be linked to these asymmetry changes in the case of an ultrashort XUV driving pulse temporally shorter than the lifetime of the resonant excitation, and how the asymmetry can be controlled by the near-resonant detuning of the XUV pulse. In the case of a two-level system, the numerical model is compared to an analytical calculation, which helps to uncover the underlying mechanism for the detuning- and intensity-induced line-shape modification and links it to the generalized Rabi frequency. To further apply the numerical model to recent experimental results of the near-resonant dressing of the 2s2p doubly excited state in helium by an ultrashort XUV free-electron laser pulse we extend the two-level model with an ionization continuum, thereby enabling the description of transmission-type (Fraunhofer-like) transient absorption of a strongly laser-coupled autoionizing state.

Autoionization dynamics of helium nanodroplets resonantly excited by intense XUV laser pulses

The ionization dynamics of helium droplets irradiated by intense, femtosecond extreme ultraviolet (XUV) pulses is investigated in detail by photoelectron spectroscopy. Helium droplets are resonantly excited to atomic-like 2p states with a photon energy of 21.5 eV and autoionize by interatomic Coulombic decay (ICD). A complex evolution of the electron spectra as a function of droplet size (250 to 106 He atoms per droplet) and XUV intensity (109–1012 W cm−2) is observed, ranging from narrow atomic-like peaks that are due to binary autoionization, to an unstructured feature characteristic of electron emission from a nanoplasma. The experimental results are analyzed and interpreted with the help of a numerical simulation based on rate equations taking into account all relevant processes—multi-step ionization, electronic relaxation, ICD, secondary inelastic collisions, desorption of electronically excited atoms, and collective autoionization (CAI).

Parametric attosecond pulse amplification far from the ionization threshold from high order harmonic generation in He.

Parametric amplification of attosecond coherent pulses around 100 eV at the single-atom level is demonstrated for the first time by using the 3D time-dependent Schrödinger equation in high-harmonic generation processes from excited states of He+. We present the attosecond dynamics of the amplification process far from the ionization threshold and resolve the physics behind it. The amplification of a particular central photon energy requires the seed XUV pulses to be perfectly synchronized in time with the driving laser field for stimulated recombination to the He+ ground state and is only produced in a few specific laser cycles in agreement with the experimental measurements. Our simulations show that the amplified photon energy region can be controlled by varying the peak intensity of the laser field. Our results pave the way to the realization of compact attosecond pulse intense XUV lasers with broad applications.

Attosecond transient absorption spectroscopy without inversion symmetry

Transient absorption is a very powerful observable in attosecond experiments on atoms, molecules and solids and is frequently used in experiments employing phase-locked few-cycle infrared and XUV laser pulses derived from high harmonic generation. We show numerically and analytically that in non-centrosymmetric systems, such as many polyatomic molecules, which-way interference enabled by the lack of parity conservation leads to new spectral absorption features, which directly reveal the laser electric field. The extension of attosecond transient absorption spectroscopy (ATAS) to such targets hence becomes sensitive to global and local inversion symmetry. We anticipate that ATAS will find new applications in non-centrosymmetric systems, in which the carrier-to-envelope phase of the infrared pulse becomes a relevant parameter and in which the orientation of the sample and the electronic symmetry of the molecule can be addressed.

High repetition rate ultrafast laser technology for drivinghigh-order harmonic generation

The interaction of ultra-intense and ultrafast laser pulses with matter has paved the way for generation of ultrashort pulses in the attosecond timescale and coherent radiation in the extreme ultraviolet (XUV) and soft X-ray region at a table-top size. On one hand, it provides attosecond temporal resolution and atomic spatial resolution for ultrafast electron dynamics in atoms, molecules and condensed matters; on the other hand, it inspires the great potential of optical frequency comb (OFC) in precise measurements of transition frequencies and hyperfine splitting, a verification of basic physical constants such as the Rydberg constant, and the research on precise optical atomic clocks. With the expansion of the above-mentioned scientific researches, the repetition rate of the driving source has become the key factor to break through the measurement accuracy obstacles. Therefore, the research on high-repetition rate ultrafast laser technology for driving high-order harmonic generation (HHG) commences. In this paper, the high repetition rate HHG as the starting point, the method applied, the main parameter indices and the technical difficulties of the driving source are summarized. This paper can be a reference for the future development of high-repetition rate ultrafast laser technology for driving HHG. First of all, the research involving femtosecond enhancement cavity (fsEC) using passive amplification is introduced. The repetition rate of the high-order harmonics as obtained from this method has exceeded 100 MHz, which serves as the main technical means to obtain OFC in the XUV region. However, key issues, such as dispersion control of the high-precision resonator and stabilization of femtosecond laser frequency, are still the focus for future research and the current application difficulties of fsEC technology. Secondly, the research on Ti:sapphire laser system, ytterbium (Yb) doped fiber laser system and Yb all-solid-state laser system are reviewed. The few-cycle femtosecond pulse output by the Ti:sapphire laser system has an extremely high peak power, and it is the main driving source for generation of high-order harmonics and isolated attosecond pulses. However, constrained by the average power of the Ti:sapphire laser system, it is difficult to increase the repetition rate of XUV laser pulses to hundreds kilohertz or even megahertz. The thin-disk laser oscillator and the Yb fiber amplifier can produce femtosecond pulses with high average power. By means of nonlinear spectral broadening and pulse compression, an extreme ultraviolet radiation with a repetition rate of 10 MHz has been obtained in the HHG process of gases, yet the system structure remains complex and the technical difficulty is still high. Meanwhile, the Yb all-solid-state femtosecond amplifier, which can directly produce ultrashort pulses with high peak power, serves as an ideal driving source to realize HHG with the repetition rate on the MHz level. In view of the development trends of high-repetition rate driving sources, and based on the research experience on high-power Yb femtosecond lasers of our group, we propose high repetition rate ultrashort all-solid-state Yb amplifier for driving MHz repetition rate HHG with sub -40 fs all-solid-state Yb oscillator as seed and key techniques of gain narrowing suppression, precise dispersion control and efficient thermal management. Finally, the parameters of various technical means in passive amplification and gain amplification, as well as the key problems that need to be solved, are summarized, and the development trends of high repetition rate ultrafast laser technology for driving HHG are prospected.

Photoelectron holography of the H2+ molecule

We investigate the photoelectron spectrum of the H2+ target induced by few-cycle XUV laser pulses using first principle calculations. In the photoelectron spectrum, by performing calculations for different internuclear separations, we investigate how the structure of the target is influencing the spatial interference pattern. This interference pattern is created by the coherent superposition of electronic wave packets emitted at the same time, but following different paths. We find that the location of the interference minima in the spectra is dominantly determined by the target’s ionization energy, however, by comparing the H2+ results with model calculations with spherically symmetric potentials, clear differences were observed for the molecular potential relative to the central potentials. Next to the main feature (spatial interference) we have also identified the traces of the two-center interference in the photoelectron spectrum, however, these were mainly washed out due to the complex electronic wave packet dynamics that occurs during the interaction with the considered laser field.Graphical abstract

Correction to: XUV laser mass spectrometry for nano-scale 3D elemental profiling of functional thin films

Correction to: XUV laser mass spectrometry for nano-scale 3D elemental profiling of functional thin films

Differential pumping unit for windowless coupling of laser beams to ultra high vacuum

Current laser technology enables table-top high flux XUV sources with photon energies from several tens to several hundreds of eV via high harmonic generation in noble gases. Here we present a compact versatile coupling unit to establish windowless, and thus lossless coupling of such light sources to ultra high vacuum (UHV). The particular coupling unit has been developed to couple a XUV laser source to a heavy ion storage ring. Three-stage differential pumping allows to reduce the input pressure of ~10−6 mbar down to the 10−12 mbar range at the output. The unit particularly reduces the partial pressure of argon, which is used to generate the XUV radiation, by 6 orders of magnitude. Measurements of the pressure distribution inside the different chambers agree well with theoretical simulations.In principle, this unit can also serve for other purposes, where a windowless vacuum coupling is needed, with a transition from High Vacuum (HV) levels to deep UHV, such as coupling to cryogenically cooled detectors, ion traps or to photoelectron emission spectroscopy experiments.