Imperial College London

Dr Allan Johnson

Faculty of Natural SciencesDepartment of Physics

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allan.johnson13 CV

 
 
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Blackett LaboratorySouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
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56 results found

Bhattacharya U, Chaudhary S, Grass T, Johnson AS, Wall S, Lewenstein Met al., 2022, Fermionic Chern insulator from twisted light with linear polarization, PHYSICAL REVIEW B, Vol: 105, ISSN: 2469-9950

Journal article

Pastor E, Moreno-Mencia D, Monti M, Johnson AS, Fleischmann N, Wang C, Shi Y, Liu X, Mazzone DG, Dean MPM, Wall Set al., 2022, Nonthermal breaking of magnetic order via photogenerated spin defects in the spin-orbit coupled insulator Sr3Ir2O7, PHYSICAL REVIEW B, Vol: 105, ISSN: 2469-9950

Journal article

Perez-Salinas D, Johnson AS, Prabhakaran D, Wall Set al., 2022, Multi-mode excitation drives disorder during the ultrafast melting of a C4-symmetry-broken phase, NATURE COMMUNICATIONS, Vol: 13

Journal article

Amuah EB, Johnson AS, Wall SE, 2021, An achromatic pump-probe setup for broadband, few-cycle ultrafast spectroscopy in quantum materials, REVIEW OF SCIENTIFIC INSTRUMENTS, Vol: 92, ISSN: 0034-6748

Journal article

Barillot T, Alexander O, Cooper B, Driver T, Garratt D, Li S, Al Haddad A, Sanchez-Gonzalez A, Agåker M, Arrell C, Bearpark MJ, Berrah N, Bostedt C, Bozek J, Brahms C, Bucksbaum PH, Clark A, Doumy G, Feifel R, Frasinski LJ, Jarosch S, Johnson AS, Kjellsson L, Kolorenč P, Kumagai Y, Larsen EW, Matia-Hernando P, Robb M, Rubensson J-E, Ruberti M, Sathe C, Squibb RJ, Tan A, Tisch JWG, Vacher M, Walke DJ, Wolf TJA, Wood D, Zhaunerchyk V, Walter P, Osipov T, Marinelli A, Maxwell TJ, Coffee R, Lutman AA, Averbukh V, Ueda K, Cryan JP, Marangos JPet al., 2021, Correlation-driven transient hole dynamics resolved in space and time in the isopropanol molecule, Physical Review X, Vol: 11, Pages: 1-15, ISSN: 2160-3308

The possibility of suddenly ionized molecules undergoing extremely fast electron hole (or, hole)dynamics prior to significant structural change was first recognized more than 20 years ago andtermed charge migration. The accurate probing of ultrafast electron hole dynamics requires measurements that have both sufficient temporal resolution and can detect the localization of a specifichole within the molecule. We report an investigation of the dynamics of inner valence hole states inisopropanol where we use an x-ray pump/x-ray probe experiment, with site and state-specific probing of a transient hole state localized near the oxygen atom in the molecule, together with an abinitio theoretical treatment. We record the signature of transient hole dynamics and make the firsttentative observation of dynamics driven by frustrated Auger-Meitner transitions. We verify thatthe effective hole lifetime is consistent with our theoretical prediction. This state-specific measurement paves the way to widespread application for observations of transient hole dynamics localizedin space and time in molecules and thus to charge transfer phenomena that are fundamental inchemical and material physics.

Journal article

Johnson AS, Valls Conesa J, Vidas L, Perez-Salinas D, Gunther CM, Pfau B, Hallman KA, Haglund RF, Eisebitt S, Wall Set al., 2021, Quantitative hyperspectral coherent diffractive imaging spectroscopy of a solid-state phase transition in vanadium dioxide, SCIENCE ADVANCES, Vol: 7, ISSN: 2375-2548

Journal article

Mazzone DG, Meyers D, Cao Y, Vale JG, Dashwood CD, Shi Y, James AJA, Robinson NJ, Lin J, Thampy V, Tanaka Y, Johnson AS, Miao H, Wang R, Assefa TA, Kim J, Casa D, Mankowsky R, Zhu D, Alonso-Mori R, Song S, Yavas H, Katayama T, Yabashi M, Kubota Y, Owada S, Liu J, Yang J, Konik RM, Robinson IK, Hill JP, McMorrow DF, Forst M, Wall S, Liu X, Dean MPMet al., 2021, Laser-induced transient magnons in Sr3Ir2O7 throughout the Brillouin zone, PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA, Vol: 118, ISSN: 0027-8424

Journal article

Austin D, Johnson A, McGrath F, Wood D, Miseikis L, Siegel T, Hawkins P, Harvey A, Mašín Z, Patchkovskii S, Vacher M, Malhado JP, Ivanov M, Smirnova O, Marangos Jet al., 2021, Extracting sub-cycle electronic and nuclear dynamics from high harmonic spectra, Scientific Reports, Vol: 11, ISSN: 2045-2322

We present a new methodology for measuring few-femtosecond electronic and nuclear dynamics in both atoms and polyatomic molecules using multidimensional high harmonic generation (HHG) spectroscopy measurements, in which the spectra are recorded as a function of the laser intensity to form a two-dimensional data set. The method is applied to xenon atoms and to benzene molecules, the latter exhibiting significant fast nuclear dynamics following ionization. We uncover the signature of the sub-cycle evolution of the returning electron flux in strong-field ionized xenon atoms, implicit in the strong field approximation but not previously observed directly. We furthermore extract the nuclear autocorrelation function in strong field ionized benzene cations, which is determined to have a decay of τ0=4±1 fs, in good agreement with the τ0=3.5 fs obtained from direct dynamics variational multi-configuration Gaussian calculations. Our method requires minimal assumptions about the system, and is applicable even to un-aligned polyatomic molecules.

Journal article

Vidas L, Schick D, Martinez E, Perez-Salinas D, Ramos-Alvarez A, Cichy S, Batlle-Porro S, Johnson AS, Hallman KA, Haglund RF, Wall Set al., 2020, Does Vo(2) Host a Transient Monoclinic Metallic Phase?, PHYSICAL REVIEW X, Vol: 10, ISSN: 2160-3308

Journal article

Rodriguez-Sune L, Scalora M, Johnson AS, Cojocaru C, Akozbek N, Coppens ZJ, Perez-Salinas D, Wall S, Trull Jet al., 2020, Experimental and theoretical study of second harmonic generation from an ITO nanolayer

We present an experimental study of second harmonic generation from a 20nm-thick indium tin oxide layer in the proximity of its epsilon-near-zero condition. We report measurements of second harmonic efficiencies as a function of the input wavelength and angle of incidence close to the epsilon-near-zero crossing point. We perform theoretical simulations using a hydrodynamical model that is able to capture all major physical mechanisms driving the nonlinear electrodynamic behaviour of conductive oxides. The experimental and theoretical results are compared and a very good quantitative and qualitative agreement is found.

Conference paper

Johnson AS, Amuah EB, Brahms C, Wall Set al., 2020, Measurement of 10 fs pulses across the entire Visible to Near-Infrared Spectral Range, SCIENTIFIC REPORTS, Vol: 10, ISSN: 2045-2322

Journal article

Rodriguez-Sune L, Scalora M, Johnson AS, Cojocaru C, Akozbek N, Coppens ZJ, Perez-Salinas D, Wall S, Trull Jet al., 2020, Study of second and third harmonic generation from an indium tin oxide nanolayer: Influence of nonlocal effects and hot electrons, APL PHOTONICS, Vol: 5, ISSN: 2378-0967

Journal article

Rodriguez-Suné L, Scalora M, Johnson AS, Cojocaru C, Akozbek N, Coppens ZJ, Perez-Salinas D, Wall S, Trull Jet al., 2020, Second harmonic generation from an ITO nanolayer: Experiment versus theory, ISSN: 0277-786X

We report a comparative experimental and theoretical study of second harmonic generation from a 20nm-thick indium tin oxide nanolayer in the proximity of the epsilon-near-zero condition. We record the efficiency of the second harmonic signal both as a function of wavelength as well as of the angle of incidence around the epsilon-near-zero crossing point. We compare our experimental results with numerical simulations based on a hydrodynamical model able to capture all major physical mechanisms driving the electrodynamic behavior of conductive oxide layers, with unique aspects of the different nonlinear sources. We found a very good quantitative and qualitative agreement between experiment and theory.

Conference paper

Johnson AS, Avni T, Larsen E, Austin DR, Marangos JPet 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

Journal article

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

Journal article

Wyatt AS, Matía-Hernando P, Johnson AS, Alexander O, Chapman R, Cacho C, Austin DR, Tisch JWG, Marangos JP, Springate Eet al., 2019, Compression and amplification of SWIR single-cycle pulses for water window attosecond pulse generation

Conference paper

Johnson AS, Wood D, Austin DR, Brahms C, Gregory A, Holzner KB, Jarosch S, Larsen EW, Parker S, Struber C, Ye P, Tisch JWG, Marangos JPet 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.

Journal article

Johnson AS, Austin DR, Wood DA, Brahms C, Gregory A, Holzner KB, Jarosch S, Larsen EW, Parker S, Struber CS, Ye P, Tisch JWG, Marangos JPet 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.

Journal article

Johnson A, Austin D, Wood D, Brahms M, Gregory A, Holzner K, Jarosch S, Larsen E, Parker S, Struber C, Ye P, Tisch J, Marangos JPet 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.

Journal article

Alharbi A, Boguslavskiy AE, Austin D, Thire N, Wood D, Hawkins P, McGrath F, Johnson AS, Lopez-Quintas I, Schmidt B, Legare F, Marangos JP, Anh-Thu L, Bhardwaj Ret al., 2018, Femtosecond Laser Mass Spectrometry and High Harmonic Spectroscopy of Xylene Isomers (vol 8, 3789, 2018), SCIENTIFIC REPORTS, Vol: 8, ISSN: 2045-2322

Journal article

Alharbi A, Boguslavskiy AE, Austin D, Thire N, Wood D, Hawkins P, McGrath F, Johnson AS, Lopez-Quintas I, Schmidt B, Legare F, Marangos JP, Le A-T, Bhardwaj Ret al., 2018, Femtosecond Laser Mass Spectrometry and High Harmonic Spectroscopy of Xylene Isomers, SCIENTIFIC REPORTS, Vol: 8, ISSN: 2045-2322

Journal article

Wyatt AS, Matía-Hernando P, Johnson AS, Chapman RT, Cacho C, Austin DR, Marangos JP, Tisch JWG, Springate Eet 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.

Conference paper

Sanchez-Gonzalez A, Johnson AS, Fitzpatrick A, Hutchison CDM, Fare C, Cordon-Preciado V, Dorlhiac G, Ferreira JL, Morgan RM, Marangos JP, Owada S, Nakane T, Tanaka R, Tono K, Iwata S, van Thor JJet 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.

Journal article

McGrath F, Johnson AS, Austin DR, Hawkins P, Wood D, Miseikis L, Simpson ER, Castillejo M, Torres R, Parker S, Siegel T, Marangos JPet al., 2017, An apparatus for quantitative high-harmonic generation spectroscopy in molecular vapours, REVIEW OF SCIENTIFIC INSTRUMENTS, Vol: 88, ISSN: 0034-6748

Journal article

Hutchison CDM, Cordon-Preciado V, Morgan RML, Nakane T, Ferreira J, Dorlhiac G, Sanchez-Gonzalez A, Johnson AS, Fitzpatrick A, Fare C, Marangos JP, Yoon CH, Hunter MS, DePonte DP, Boutet S, Owada S, Tanaka R, Tono K, Iwata S, van Thor JJet 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.

Journal article

Sanchez Gonzalez A, Micaelli P, Olivier C, Barillot TR, Ilchen I, Lutman AA, Marinelli A, Maxwell T, Achner A, Agåker M, Berrah N, Bostedt C, Bozek JD, Buck J, Bucksbaum PH, Carron Montero S, Cooper B, Cryan JP, Dong M, Feifel R, Frasinski LJ, Fukuzawa H, Galler A, Hartmann G, Hartmann N, Helml W, Johnson AS, Knie A, Lindahl AO, Liu J, Motomura K, Mucke M, O'Grady C, Rubensson J-E, Simpson ER, Squibb RJ, Såthe C, Ueda K, Vacher M, Walke DJ, Zhaunerchyk V, Coffee RN, Marangos JPet 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.

Journal article

Austin DR, McGrath F, Miseikis L, Wood D, Hawkins P, Johnson AS, Vacher M, Mašín Z, Harvey A, Ivanov M, Smirnova O, Marangos JPet al., 2016, Role of tunnel ionization in high harmonic generation from substituted benzenes., Faraday Discussions, Vol: 194, Pages: 349-368, ISSN: 1364-5498

We theoretically study high-harmonic generation in toluene, ortho-xylene and fluorobenzene driven by a 1.8 μm ultrashort pulse. We find that the chemical substitutions have a strong influence on the amplitude and phase of the emission from the highest occupied molecular orbital, despite having a small influence on the orbital itself. We show that this influence is due to the tunnel ionization step, which depends critically on the sign and amplitude of the asymptotic part of the wave function. We discuss how these effects would manifest in phase-sensitive high-harmonic generation spectroscopy experiments.

Journal article

Decleva P, Orr-Ewing AJ, Kowalewski M, Kornilov O, Marangos JP, Worner HJ, Johnson AS, Forbes R, Rolles D, Townsend D, Schalk O, Mai S, Penfold TJ, Miller RJD, Centurion M, Ueda K, Domcke W, Weber PM, Baeck KK, Travnikova O, Liekhus-Schmaltz C, Figueira JAP, Neumark NDM, Gessner O, Stolow A, Rudenko A, Mishra PK, Kirrander A, Dowek D, Martin F, Vibok A, Minitti MP, Stankus B, Burger Cet al., 2016, Structural dynamics: general discussion, FARADAY DISCUSSIONS, Vol: 194, Pages: 583-620, ISSN: 1359-6640

Journal article

Milne CJ, Weber PM, Kowalewski M, Marangos JP, Johnson AS, Forbes R, Worner HJ, Rolles D, Townsend D, Schalk O, Mai S, Vacher M, Miller RJD, Centurion M, Vibok A, Domcke W, Cireasa R, Ueda K, Bencivenga F, Neumark DM, Stolow A, Rudenko A, Kirrander A, Dowek D, Martin F, Ivanov M, Dahlstrom JM, Dudovich N, Mukamel S, Sanchez-Gonzalez A, Minitti MP, Austin DR, Kimberg V, Masin Zet al., 2016, Attosecond processes and X-ray spectroscopy: general discussion, Faraday Discussions, Vol: 194, Pages: 427-462, ISSN: 1359-6640

Journal article

Orr-Ewing AJ, Verlet JRR, Penfold TJ, Minns RS, Minitti MP, Solling TI, Schalk O, Kowalewski M, Marangos JP, Robb MA, Johnson AS, Worner HJ, Shalashilin DV, Miller RJD, Domcke W, Ueda K, Weber PM, Cireasa R, Vacher M, Roberts GM, Decleva P, Bencivenga F, Neumark DM, Gessner O, Stolow A, Mishra PK, Polyak I, Baeck KK, Kirrander A, Dowek D, Jimenez-Galan A, Martin F, Mukamel S, Sekikawa T, Gelin MF, Townsend D, Makhov DV, Neville SPet al., 2016, Electronic and non-adiabatic dynamics: general discussion, Faraday Discussions, Vol: 194, Pages: 209-257, ISSN: 1359-6640

Journal article

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