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12 pages, 1366 KiB

Open AccessFeature PaperArticle

by Tobias Schweizer, Michael H. Brügmann, Wolfram Helml, Nick Hartmann, Ryan Coffee and Thomas Feurer

Appl. Sci. 2018, 8(7), 1039; https://0-doi-org.brum.beds.ac.uk/10.3390/app8071039 - 26 Jun 2018

Cited by 4 | Viewed by 3797

Abstract

Dedicated simulations show that the application of time-domain ptychography to angular photo-electron streaking data allows shot-to-shot reconstruction of individual X-ray free electron laser pulses. Specifically, in this study, we use an extended ptychographic iterative engine to retrieve both the unknown X-ray pulse and [...] Read more.

Dedicated simulations show that the application of time-domain ptychography to angular photo-electron streaking data allows shot-to-shot reconstruction of individual X-ray free electron laser pulses. Specifically, in this study, we use an extended ptychographic iterative engine to retrieve both the unknown X-ray pulse and the unknown streak field. We evaluate the quality of reconstruction versus spectral resolution, signal-to-noise and sampling size of the spectrogram. Full article

(This article belongs to the Special Issue Extreme Time Scale Photonics)

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18 pages, 2757 KiB

Open AccessArticle

A Versatile Velocity Map Ion-Electron Covariance Imaging Spectrometer for High-Intensity XUV Experiments

by Linnea Rading, Jan Lahl, Sylvain Maclot, Filippo Campi, Hélène Coudert-Alteirac, Bart Oostenrijk, Jasper Peschel, Hampus Wikmark, Piotr Rudawski, Mathieu Gisselbrecht and Per Johnsson

Appl. Sci. 2018, 8(6), 998; https://0-doi-org.brum.beds.ac.uk/10.3390/app8060998 - 19 Jun 2018

Cited by 11 | Viewed by 5716

Abstract

We report on the design and performance of a velocity map imaging (VMI) spectrometer optimized for experiments using high-intensity extreme ultraviolet (XUV) sources such as laser-driven high-order harmonic generation (HHG) sources and free-electron lasers (FELs). Typically exhibiting low repetition rates and high single-shot [...] Read more.

We report on the design and performance of a velocity map imaging (VMI) spectrometer optimized for experiments using high-intensity extreme ultraviolet (XUV) sources such as laser-driven high-order harmonic generation (HHG) sources and free-electron lasers (FELs). Typically exhibiting low repetition rates and high single-shot count rates, such experiments do not easily lend themselves to coincident detection of photo-electrons and -ions. In order to obtain molecular frame or reaction channel-specific information, one has to rely on other correlation techniques, such as covariant detection schemes. Our device allows for combining different photo-electron and -ion detection modes for covariance analysis. We present the expected performance in the different detection modes and present the first results using an intense high-order harmonic generation (HHG) source. Full article

(This article belongs to the Special Issue Extreme Time Scale Photonics)

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11 pages, 5951 KiB

Open AccessArticle

Theoretical Study: High Harmonic Generation by Light Transients

by Maximilian Wendl, Maximilian Högner and Hanieh Fattahi

Appl. Sci. 2018, 8(5), 728; https://0-doi-org.brum.beds.ac.uk/10.3390/app8050728 - 05 May 2018

Cited by 6 | Viewed by 3443

Abstract

The dynamic of electron densities in matter upon the interaction with an intense, few-cycle electric field of light causes variety of nonlinear phenomena. Capturing the spatiotemporal dynamics of electrons calls for isolated attosecond pulses in the X-ray regime, with sufficient flux to allow [...] Read more.

The dynamic of electron densities in matter upon the interaction with an intense, few-cycle electric field of light causes variety of nonlinear phenomena. Capturing the spatiotemporal dynamics of electrons calls for isolated attosecond pulses in the X-ray regime, with sufficient flux to allow for: (i) attosecond pump–attosecond probe spectroscopy; or (ii) four-dimensional imaging. Light field synthesizers generate arbitrary sub-cycle, non-sinusoidal waveforms. They have a great potential to overcome the limitations of current laser sources and to extend attosecond pulses towards the X-ray regime. In this paper, we show theoretically how the achievable high-energy, high-power waveforms from current light field synthesizers can be optimized to enhance the harmonic yield at high photon energies and can serve as a promising source for scaling the photon energies of attosecond pulses. We demonstrate that the simulated optimized, non-sinusoidal waveform in this work can increase the photon flux of keV, attosecond pulses by five orders of magnitude compared to the achievable flux from longer wavelength sources and at similar photon energies. Full article

(This article belongs to the Special Issue Extreme Time Scale Photonics)

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10 pages, 3507 KiB

Open AccessArticle

Surface State Dynamics of Topological Insulators Investigated by Femtosecond Time- and Angle-Resolved Photoemission Spectroscopy

by Hamoon Hedayat, Davide Bugini, Hemian Yi, Chaoyu Chen, Xingjiang Zhou, Giulio Cerullo, Claudia Dallera and Ettore Carpene

Appl. Sci. 2018, 8(5), 694; https://0-doi-org.brum.beds.ac.uk/10.3390/app8050694 - 30 Apr 2018

Cited by 6 | Viewed by 4622

Abstract

Topological insulators (TI) are known for striking quantum phenomena associated with their spin-polarized topological surface state (TSS). The latter in particular forms a Dirac cone that bridges the energy gap between valence and conduction bands, providing a unique opportunity for prospective device applications. [...] Read more.

Topological insulators (TI) are known for striking quantum phenomena associated with their spin-polarized topological surface state (TSS). The latter in particular forms a Dirac cone that bridges the energy gap between valence and conduction bands, providing a unique opportunity for prospective device applications. In TI of the Bi_xSb_2−xTe_ySe_3−y (BSTS) family, stoichiometry determines the morphology and position of the Dirac cone with respect to the Fermi level. In order to engineer specific transport properties, a careful tuning of the TSS is highly desired. Therefore, we have systematically explored BSTS samples with different stoichiometries by time- and angle-resolved photoemission spectroscopy (TARPES). This technique provides snapshots of the electronic structure and discloses the carrier dynamics in surface and bulk states, providing crucial information for the design of electro-spin current devices. Our results reveal the central role of doping level on the Dirac cone structure and its femtosecond dynamics. In particular, an extraordinarily long TSS lifetime is observed when the the vertex of the Dirac cone lies at the Fermi level. Full article

(This article belongs to the Special Issue Extreme Time Scale Photonics)

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13 pages, 3147 KiB

Open AccessArticle

Disentangling Long Trajectory Contributions in Two-Colour High Harmonic Generation

by Cornelia Hofmann, Alexandra S. Landsman and Ursula Keller

Appl. Sci. 2018, 8(3), 341; https://0-doi-org.brum.beds.ac.uk/10.3390/app8030341 - 28 Feb 2018

Cited by 8 | Viewed by 6270

Abstract

This work investigates High Harmonic Generation (HHG) in gas targets, induced by intense two-colour driving fields. We compared classical trajectory Monte Carlo simulations based on a semiclassical model of strong field tunnel ionisation of helium to experimental HHG spectra, and investigated the relative [...] Read more.

This work investigates High Harmonic Generation (HHG) in gas targets, induced by intense two-colour driving fields. We compared classical trajectory Monte Carlo simulations based on a semiclassical model of strong field tunnel ionisation of helium to experimental HHG spectra, and investigated the relative contribution of long trajectories to low harmonic orders. This phenomenon can be found even when the experimental setup is chosen to suppress long trajectories and favour phase matching for short trajectories. Full article

(This article belongs to the Special Issue Extreme Time Scale Photonics)

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13 pages, 396 KiB

Open AccessArticle

Attosecond Time Delay in Photoionization of Noble-Gas and Halogen Atoms

by Liang-Wen Pi and Alexandra S. Landsman

Appl. Sci. 2018, 8(3), 322; https://0-doi-org.brum.beds.ac.uk/10.3390/app8030322 - 26 Feb 2018

Cited by 11 | Viewed by 4185

Abstract

Ultrafast processes are now accessible on the attosecond time scale due to the availability of ultrashort XUV laser pulses. Noble-gas and halogen atoms remain important targets due to their giant dipole resonance and Cooper minimum. Here, we calculate photoionization cross section, asymmetry parameter [...] Read more.

Ultrafast processes are now accessible on the attosecond time scale due to the availability of ultrashort XUV laser pulses. Noble-gas and halogen atoms remain important targets due to their giant dipole resonance and Cooper minimum. Here, we calculate photoionization cross section, asymmetry parameter and Wigner time delay using the time-dependent local-density approximation (TDLDA), which includes the electron correlation effects. Our results are consistent with experimental data and other theoretical calculations. The asymmetry parameter provides an extra layer of access to the phase information of the photoionization processes. We find that halogen atoms bear a strong resemblance on cross section, asymmetry parameter and time delay to their noble-gas neighbors. Our predicted time delay should provide a guidance for future experiments on those atoms and related molecules. Full article

(This article belongs to the Special Issue Extreme Time Scale Photonics)

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Review

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22 pages, 380 KiB

Open AccessFeature PaperReview

Attosecond-Resolved Electron Dynamics in Many-Electron Atoms: Quantitative Theory and Comparison with Measurements

by Cleanthes Anthony Nicolaides

Appl. Sci. 2018, 8(4), 533; https://0-doi-org.brum.beds.ac.uk/10.3390/app8040533 - 30 Mar 2018

Cited by 9 | Viewed by 3455

Abstract

A variety of processes originating from the interaction of atomic or molecular N-electron states with strong and/or hypershort radiation pulses can be understood quantitatively only by first determining with good accuracy the solutions of the many-electron time-dependent Schrödinger equation (METDSE) that describe the [...] Read more.

A variety of processes originating from the interaction of atomic or molecular N-electron states with strong and/or hypershort radiation pulses can be understood quantitatively only by first determining with good accuracy the solutions of the many-electron time-dependent Schrödinger equation (METDSE) that describe the corresponding physics. The METDSE is solvable nonperturbatively via the state-specific expansion approach (SSEA). SSEA solutions have been used, or can be used, for quantitative explanation and numerically reliable predictions of quantities that have been measured or are measurable in modern laser-driven experiments that can track, with hypershort (attosecond) time resolution, the effects of electron rearrangements in atoms and molecules. The calculations take into account in a transparent way the interplay between the phenomena and the electronic structures of the physically significant states in discrete and multichannel continuous spectra, including multiply- and inner-hole–excited resonance states. The discussion focuses on novel topics of time-resolved many-electron physics and includes a comparison of our predictions to recent quantitative measurements of attosecond-resolved generation of the profile of the

{(2 s 2 p)}^{1} P^{o}

doubly excited resonance state of helium during photoionization and of the relative time delay in photoemission of the (2s,2p) electrons of neon. Full article

(This article belongs to the Special Issue Extreme Time Scale Photonics)

28 pages, 2548 KiB

Open AccessReview

Imaging Electron Dynamics with Ultrashort Light Pulses: A Theory Perspective

by Daria Popova-Gorelova

Appl. Sci. 2018, 8(3), 318; https://0-doi-org.brum.beds.ac.uk/10.3390/app8030318 - 25 Feb 2018

Cited by 8 | Viewed by 4113

Abstract

A wide range of ultrafast phenomena in various atomic, molecular and condense matter systems is governed by electron dynamics. Therefore, the ability to image electronic motion in real space and real time would provide a deeper understanding of such processes and guide developments [...] Read more.

A wide range of ultrafast phenomena in various atomic, molecular and condense matter systems is governed by electron dynamics. Therefore, the ability to image electronic motion in real space and real time would provide a deeper understanding of such processes and guide developments of tools to control them. Ultrashort light pulses, which can provide unprecedented time resolution approaching subfemtosecond time scale, are perspective to achieve real-time imaging of electron dynamics. This task is challenging not only from an experimental view, but also from a theory perspective, since standard theories describing light-matter interaction in a stationary regime can provide erroneous results in an ultrafast case as demonstrated by several theoretical studies. We review the theoretical framework based on quantum electrodynamics, which has been shown to be necessary for an accurate description of time-resolved imaging of electron dynamics with ultrashort light pulses. We compare the results of theoretical studies of time-resolved nonresonant and resonant X-ray scattering, and time- and angle-resolved photoelectron spectroscopy and show that the corresponding time-resolved signals encode analogous information about electron dynamics. Thereby, the information about an electronic system provided by these time-resolved techniques is different from the information provided by their time-independent analogues. Full article

(This article belongs to the Special Issue Extreme Time Scale Photonics)

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