Publications
317 results found
Longetti L, Randulova M, Ojeda J, et al., 2020, Photoemission from non-polar aromatic molecules in the gas and liquid phase, PHYSICAL CHEMISTRY CHEMICAL PHYSICS, Vol: 22, Pages: 3965-3974, ISSN: 1463-9076
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- Citations: 5
Driver T, Li S, Champenois EG, et al., 2020, Attosecond transient absorption spooktroscopy: a ghost imaging approach to ultrafast absorption spectroscopy, Physical Chemistry Chemical Physics, Vol: 22, Pages: 2704-2712, ISSN: 1463-9076
The recent demonstration of isolated attosecond pulses from an X-ray free-electron laser (XFEL) opens the possibility for probing ultrafast electron dynamics at X-ray wavelengths. An established experimental method for probing ultrafast dynamics is X-ray transient absorption spectroscopy, where the X-ray absorption spectrum is measured by scanning the central photon energy and recording the resultant photoproducts. The spectral bandwidth inherent to attosecond pulses is wide compared to the resonant features typically probed, which generally precludes the application of this technique in the attosecond regime. In this paper we propose and demonstrate a new technique to conduct transient absorption spectroscopy with broad bandwidth attosecond pulses with the aid of ghost imaging, recovering sub-bandwidth resolution in photoproduct-based absorption measurements.
Duris J, Li S, Driver T, et al., 2020, Tunable isolated attosecond x-ray pulses with Gigawatt peak power from a free-electron laser, Nature Photonics, Vol: 14, Pages: 30-36, ISSN: 1749-4885
The quantum mechanical motion of electrons in molecules and solids occurs on the sub-femtosecond timescale. Consequently, the study of ultrafast electronic phenomena requires thegeneration of laser pulses shorter than 1 fs and of sufficient intensity to interact with their targetwith high probability. Probing these dynamics with atomic-site specificity requires the extensionof sub-femtosecond pulses to the soft X-ray spectral region. Here we report the generation of iso-lated soft X-ray attosecond pulses with an X-ray free-electron laser. Our source has a pulse energythat is a million times larger than any other source of isolated attosecond pulses in the soft X-rayspectral region, with a peak power exceeding 100 GW. This unique combination of high intensity,high photon energy and short pulse duration enables the investigation of electron dynamics withX-ray non-linear spectroscopy and single-particle imaging, unlocking a path towards a new era ofattosecond science.
Pettipher AJ, Weaver B, Greening D, et al., 2020, Optimisation of a Femtosecond Pulse Synthesiser for High Harmonic Generation Using the Semi-Classical Model, Conference on Lasers and Electro-Optics (CLEO), Publisher: IEEE, ISSN: 2160-9020
Berrah N, Sanchez-Gonzalez A, Jurek Z, et al., 2019, Femtosecond-resolved observation of the fragmentation of buckminsterfullerene following X-ray multiphoton ionization (vol 15, pg 1279, 2019), NATURE PHYSICS, Vol: 15, Pages: 1301-1301, ISSN: 1745-2473
Berrah N, Sanchez-Gonzalez A, Jurek Z, et al., 2019, Femtosecond-resolved observation of the fragmentation of buckminsterfullerene following X-ray multiphoton ionization, Nature Physics, Vol: 15, Pages: 1279-1283, ISSN: 1745-2473
X-ray free-electron lasers have, over the past decade, opened up the possibility of understanding the ultrafast response of matter to intense X-ray pulses. In earlier research on atoms and small molecules, new aspects of this response were uncovered, such as rapid sequences of inner-shell photoionization and Auger ionization. Here, we studied a larger molecule, buckminsterfullerene (C60), exposed to 640 eV X-rays, and examined the role of chemical effects, such as chemical bonds and charge transfer, on the fragmentation following multiple ionization of the molecule. To provide time resolution, we performed femtosecond-resolved X-ray pump/X-ray probe measurements, which were accompanied by advanced simulations. The simulations and experiment reveal that despite substantial ionization induced by the ultrashort (20 fs) X-ray pump pulse, the fragmentation of C60 is considerably delayed. This work uncovers the persistence of the molecular structure of C60, which hinders fragmentation over a timescale of hundreds of femtoseconds. Furthermore, we demonstrate that a substantial fraction of the ejected fragments are neutral carbon atoms. These findings provide insights into X-ray free-electron laser-induced radiation damage in large molecules, including biomolecules.
Johnson AS, Avni T, Larsen E, et al., 2019, Attosecond soft X-ray high harmonic generation, PHILOSOPHICAL TRANSACTIONS OF THE ROYAL SOCIETY A-MATHEMATICAL PHYSICAL AND ENGINEERING SCIENCES, Vol: 377, ISSN: 1364-503X
Marangos JP, 2019, The measurement of ultrafast electronic and structural dynamics with X-rays, PHILOSOPHICAL TRANSACTIONS OF THE ROYAL SOCIETY A-MATHEMATICAL PHYSICAL AND ENGINEERING SCIENCES, Vol: 377, ISSN: 1364-503X
Drmota P, Marangos J, Greening D, et al., 2019, Investigation of valence band reconstruction methods for attosecond streaking data from surfaces, Optics Express, Vol: 27, Pages: 9394-9402, ISSN: 1094-4087
We analyze simulated streaked valence band photoemission with atomic streaking theory-based reconstruction methods to investigate the differences between atomic gas-phase streaking and valence band surface streaking. The careful distinction between atomic and surface streaking is a prerequisite to justify the application of atomic streaking theory-based reconstruction methods to surface streaking measurements. We show that neglecting the band structure underestimates the width of reconstructed photoelectron wavepackets, consistent with the Fourier transform limit of the band spectrum. We find that a fit of Gaussian wavepackets within the description of atomic streaking is adequate to a limited extent. Systematic errors that depend on the near-infrared skin depth, an inherently surface-specific property, are present in temporal widths of wavepackets reconstructed with atomic streaking theory-based methods.
Marangos J, 2019, Direct characterisation of tuneable few-femtosecond dispersive-wave pulses in the deep UV, Optics Letters, Vol: 44, Pages: 731-734, ISSN: 0146-9592
Dispersive wave emission (DWE) in gas-filled hollowcore dielectric waveguides is a promising source of tuneable coherent and broadband radiation, but so far the generation of few-femtosecond pulses usingthis technique has not been demonstrated. Using invacuum frequency-resolved optical gating, we directly characterise tuneable 3 fs pulses in the deep ultraviolet generated via DWE. Through numerical simulations, we identify that the use of a pressure gradient in the waveguide is critical for the generation of short pulses
Wyatt AS, Matía-Hernando P, Johnson AS, et al., 2019, Compression and amplification of SWIR single-cycle pulses for water window attosecond pulse generation
Schutte B, Peltz C, Austin DR, et al., 2018, Low-Energy Electron Emission in the Strong-Field Ionization of Rare Gas Clusters, PHYSICAL REVIEW LETTERS, Vol: 121, ISSN: 0031-9007
Johnson AS, Wood D, Austin DR, et al., 2018, Apparatus for soft x-ray table-top high harmonic generation, Review of Scientific Instruments, Vol: 89, ISSN: 0034-6748
There has been considerable recent interest in tabletop soft X-ray attosecond sources enabled by the new generation of intense, few-cycle laser sources at operating wavelengths longer than 800 nm. In our recent work [Johnson et al., Sci. Adv. 4(5), eaar3761 (2018)], we have demonstrated a new regime for the generation of X-ray attosecond pulses in the water window (284-540 eV) by high-harmonic generation, which resulted in soft X-ray fluxes of ≈109 photons/s and a maximum photon energy of 600 eV, an order of magnitude and 50 eV higher, respectively, than previously attained with few-cycle drivers. Here we present the key elements of our apparatus for the generation and detection of soft X-ray high harmonic radiation in the water window. Of critical importance is a differentially pumped gas target capable of supporting the multi-atmospheric pressures required to phase-match the high energy emission while strongly constraining the gas density, suppressing the effects of ionization and absorption outside the interaction region.
Johnson AS, Austin DR, Wood DA, et al., 2018, Correction for the Research Article: High-flux soft x-ray harmonic generation from ionization-shaped few-cycle laser pulses, Science Advances, Vol: 4, ISSN: 2375-2548
Laser-driven high-harmonic generation provides the only demonstrated route to generating stable, tabletop attosecondx-ray pulses but has low flux compared to other x-ray technologies. We show that high-harmonic generation can producehigher photon energies and flux by using higher laser intensities than are typical, strongly ionizing the medium andcreating plasma that reshapes the driving laser field. We obtain high harmonics capable of supporting attosecondpulses up to photon energies of 600 eV and a photon flux inside the water window (284 to 540 eV) 10 times higherthan previous attosecond sources. We demonstrate that operating in this regime is key for attosecond pulse generation in the x-ray range and will become increasingly important as harmonic generation moves to even longerwavelength driving fields.
Tisch JWG, Abdelrahman Z, Khokhlova M, et al., 2018, Chirp-control of resonant high-order harmonic generation in indium ablation plumes driven by intense few-cycle laser pulses, Optics Express, Vol: 26, Pages: 15745-15758, ISSN: 1094-4087
We have studied high-order harmonic generation (HHG) in an indium ablation plume driven by intense few-cycle laser pulses centered at 775 nm as a function of the frequency chirp of the laser pulse. We found experimentally that resonant emission lines between 19.7 eV and 22.3 eV (close to the 13th and 15th harmonic of the laser) exhibit a strong, asymmetric chirp dependence, with pronounced intensity modulations. The chirp dependence is reproduced by our numerical time-dependent Schrödinger equation simulations of a resonant HHG by the model indium ion. As demonstrated with our separate simulations of HHG within the strong field approximation, the resonance can be understood in terms of the chirp-dependent HHG photon energy coinciding with the energy of an autoionizing state to ground state transition with high oscillator strength. This supports the validity of the general theory of resonant four-step HHG in the few-cycle limit.
Johnson A, Austin D, Wood D, et al., 2018, High-flux soft x-ray harmonic generation from ionization-shaped few-cycle laser pulses, Science Advances, Vol: 4, ISSN: 2375-2548
Laser driven high harmonic generation provides the only demonstrated route to generatestable, tabletop attosecond X-ray pulses, but with low flux compared to other X-ray tech-nologies. Here we show that higher photon energies and flux can be obtained from highharmonic generation by using higher laser intensities than are typical, strongly ionizing themedium and creating plasma which reshapes the driving laser field. We obtain high harmon-ics capable of supporting attosecond pulses out to photon energies of 600 eV, and a photonflux inside the water window (284 eV to 540 eV) ten times higher than previous attosecondsources. We demonstrate that operating in this regime is key for attosecond pulse generationin the X-ray range, and will become increasingly important as harmonic generation movesto even longer wavelength driving fields.
Alharbi A, Boguslavskiy AE, Austin D, et al., 2018, Femtosecond Laser Mass Spectrometry and High Harmonic Spectroscopy of Xylene Isomers (vol 8, 3789, 2018), SCIENTIFIC REPORTS, Vol: 8, ISSN: 2045-2322
Matia-Hernando P, Witting T, Walke DJ, et al., 2018, Enhanced attosecond pulse generation in the vacuum ultraviolet using a two-colour driving field for high harmonic generation, Journal of Modern Optics, Vol: 65, Pages: 737-744, ISSN: 0950-0340
High-harmonic radiation in the extreme ultraviolet and soft X-ray spectral regions can be used to generate attosecond pulses and to obtain structural and dynamic information in atoms and molecules. However, these sources typically suffer from a limited photon flux. An additional issue at lower photon energies is the appearance of satellites in the time domain, stemming from insufficient temporal gating and the spectral filtering required for the isolation of attosecond pulses. Such satellites limit the temporal resolution. The use of multi-colour driving fields has been proven to enhance the harmonic yield and provide additional control, using the relative delays between the different spectral components for waveform shaping. We describe here a two-colour high-harmonic source that combines a few-cycle near-infrared pulse with a multi-cycle second harmonic pulse, with both relative phase and carrier-envelope phase stabilization. We observe strong modulations in the harmonic flux, and present simulations and experimental results supporting the suppression of satellites in sub-femtosecond pulses at 20 eV compared to the single colour field case, an important requirement for attosecond pump-probe measurements.
Alharbi A, Boguslavskiy AE, Austin D, et al., 2018, Femtosecond Laser Mass Spectrometry and High Harmonic Spectroscopy of Xylene Isomers, SCIENTIFIC REPORTS, Vol: 8, ISSN: 2045-2322
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- Citations: 5
Wyatt AS, Matía-Hernando P, Johnson AS, et al., 2018, Compression, amplification and characterization of few-cycle shortwavelength infrared pulses, ISSN: 0277-786X
We present a Ti:Sapphire pumped optical parametric amplifier for the simultaneous amplification and compression of sub-10fs ultrashort pulses centered at 1.7um; third-harmonic generation dispersion scan in bulk glass is used for temporal pulse characterization.
Sanchez-Gonzalez A, Johnson AS, Fitzpatrick A, et al., 2017, Coincidence timing of femtosecond optical pulses in an X-ray free electron laser, Journal of Applied Physics, Vol: 122, ISSN: 0021-8979
Femtosecond resolution pump-probe experiments are now routinely carried out at X-ray FreeElectron Lasers, enabled by the development of cross-correlation “time-tools” which correct thepicosecond-level jitter between the optical and X-ray pulses. These tools provide very accurate,<10 fs, measurement of the relative arrival time, but do not provide a measure of the absolutecoincidence time in the interaction. Cross-correlation experiments using transient reflectivity in acrystal are commonly used for this purpose, and to date no quantitative analysis of the accuracy orstability of absolute coincidence time determination has been performed. We have performed aquantitative analysis of coincidence timing at the SACLA facility through a cross-correlation of100 6 10 fs, 400 nm optical pulses with 7 fs, 10.5 keV X-ray pulses via transient reflectivity in acerium-doped yttrium aluminum garnet crystal. We have modelled and fit the transient reflectivity,which required a convolution with a 226 6 12 fs uncertainty that was believed to be dominated byX-ray and laser intensity fluctuations, or assuming an extinction depth of 13.3 lm greater than theliterature value of 66.7 lm. Despite this, we are able to determine the absolute coincidence time toan accuracy of 30 fs. We discuss the physical contributions to the uncertainty of coincidence timedetermination, which may include an uncharacterised offset delay in the development of transientreflectivity, including cascading Auger decays, secondary ionisation and cooling processes.Additionally, we present measurements of the intrinsic short-term and long-term drifts between theX-rays and the optical laser timing from time-tool analysis, which is dominated by a thermalexpansion of the 25 m optical path between tool and the interaction region, seen to be 60 fs overa period of 5 h.
Witting T, Greening G, Walke D, et al., 2017, Spatio-temporal characterization of optical waveforms, Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC), Publisher: IEEE
Pulse characterization technology to characterize space-time couplings exists but most technqiues are not suiatable for few-cycle pulses. A lot of recent work has been focussed on combining modern few-cycle pulse characterization methods with spatial phase measurements based on a filtered reference pulse.This work reviews current state of the art in this field and also show recent results and progress in the self-referenced characterization of near single cycle pulses with space-time couplings. Using SEA-F-SPIDER the spatio-temporal electric field of a laser pulse can be reconstructed without the need for a reference pulse. In the current implementation we can recover the field E(x,t) in one spatial dimension. Measurements of ultrafast wavefront rotation and pulse front tilt in pulses generated in hollow fiber compressors are presented. The paper also discusses application in OPCPA and NOPA systems where the amplified pulses are expected to carry a significant amount of space-time couplings. Furthermore, avenues towards the extension of the technique to both spatial dimensions for a complete reconstruction of E(x,y,t) are discussed.
McGrath F, Johnson AS, Austin DR, et al., 2017, An apparatus for quantitative high-harmonic generation spectroscopy in molecular vapours, REVIEW OF SCIENTIFIC INSTRUMENTS, Vol: 88, ISSN: 0034-6748
Witting T, Greening D, Walke D, et al., 2017, Near single-cycle pulse characterization with time-domain ptychography, Conference on Lasers and Electro-Optics (CLEO), Publisher: IEEE, ISSN: 2160-9020
Hutchison CDM, Cordon-Preciado V, Morgan RML, et al., 2017, X-ray Free Electron Laser Determination of Crystal Structures of Dark and Light States of a Reversibly Photoswitching Fluorescent Protein at Room Temperature., International Journal of Molecular Sciences, Vol: 18, ISSN: 1422-0067
The photochromic fluorescent protein Skylan-NS (Nonlinear Structured illumination variant mEos3.1H62L) is a reversibly photoswitchable fluorescent protein which has an unilluminated/ground state with an anionic and cis chromophore conformation and high fluorescence quantum yield. Photo-conversion with illumination at 515 nm generates a meta-stable intermediate with neutral trans-chromophore structure that has a 4 h lifetime. We present X-ray crystal structures of the cis (on) state at 1.9 Angstrom resolution and the trans (off) state at a limiting resolution of 1.55 Angstrom from serial femtosecond crystallography experiments conducted at SPring-8 Angstrom Compact Free Electron Laser (SACLA) at 7.0 keV and 10.5 keV, and at Linac Coherent Light Source (LCLS) at 9.5 keV. We present a comparison of the data reduction and structure determination statistics for the two facilities which differ in flux, beam characteristics and detector technologies. Furthermore, a comparison of droplet on demand, grease injection and Gas Dynamic Virtual Nozzle (GDVN) injection shows no significant differences in limiting resolution. The photoconversion of the on- to the off-state includes both internal and surface exposed protein structural changes, occurring in regions that lack crystal contacts in the orthorhombic crystal form.
Barillot TR, Matia-Hernando P, Greening D, et al., 2017, Towards XUV pump-probe experiments in the femtosecond to sub-femtosecond regime: New measurement of the helium two-photon ionization cross-section, Chemical Physics Letters, Vol: 683, Pages: 38-42, ISSN: 0009-2614
Non-linear photoionization of molecules in the 10–50 eV range is a prerequisite for pump-probe measurements with sub-femtosecond resolution, but hitherto has been limited to femtosecond resolution, low repetition rate and high photon flux laser systems. We demonstrate two-photon single ionization of helium atoms using 100 pJ, 1.34 fs pulses (main peak FWHM = 680 as) at 1 kHz repetition rate with a central photon energy of 19.6 eV. We obtained an exponent of 2:27 0:21 for the intensity dependence of the signal and a two-photon ionization cross-section of 5:0 0:5 x 10−50 cm4 s. Our work opens the possibility of attosecond pump-probe measurements of ultrafast molecular processes.
Galinis G, Strucka J, Barnard JCT, et al., 2017, Micrometer-thickness liquid sheet jets flowing in vacuum, Review of Scientific Instruments, Vol: 88, ISSN: 0034-6748
Thin liquid sheet jet flows in vacuum provide a new platform for performing experiments in the liquid phase, for example X-ray spectroscopy. Micrometer thickness, high stability, and optical flatness are the key characteristics required for successful exploitation of these targets. A novel strategy for generating sheet jets in vacuum is presented in this article. Precision nozzles were designed and fabricated using high resolution (0.2 μm) 2-photon 3D printing and generated 1.49 ±± 0.04 μm thickness, stable, and <λλ/20-flat jets in isopropanol under normal atmosphere and under vacuum at 5 ×× 10−1 mbar. The thin sheet technology also holds great promise for advancing the fields of high harmonic generation in liquids, laser acceleration of ions as well as other fields requiring precision and high repetition rate targets.
Sanchez Gonzalez A, Micaelli P, Olivier C, et al., 2017, Accurate prediction of X-ray pulse properties from a free-electron laser using machine learning, Nature Communications, Vol: 8, ISSN: 2041-1723
Free-electron lasers providing ultra-short high-brightness pulses of X-ray radiation have great potential for a wide impact on science, and are a critical element for unravelling the structural dynamics of matter. To fully harness this potential, we must accurately know the X-ray properties: intensity, spectrum and temporal profile. Owing to the inherent fluctuations in free-electron lasers, this mandates a full characterization of the properties for each and every pulse. While diagnostics of these properties exist, they are often invasive and many cannot operate at a high-repetition rate. Here, we present a technique for circumventing this limitation. Employing a machine learning strategy, we can accurately predict X-ray properties for every shot using only parameters that are easily recorded at high-repetition rate, by training a model on a small set of fully diagnosed pulses. This opens the door to fully realizing the promise of next-generation high-repetition rate X-ray lasers.
Fennel T, Marangos J, Mukamel S, et al., 2017, Call for papers: special issue on correlations in light-matter interactions, JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS, Vol: 50, ISSN: 0953-4075
Ciappina MF, Perez-Hernandez JA, Landsman AS, et al., 2017, Attosecond physics at the nanoscale, Publisher: IOP PUBLISHING LTD
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- Citations: 246
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