Publications
174 results found
Hajivassiliou G, Kassapis M, Tisch JWG, 2023, Rapid retrieval of femtosecond and attosecond pulses from streaking traces using convolutional neural networks, New Journal of Physics, Vol: 25, ISSN: 1367-2630
Attosecond streaking is a powerful and versatile technique that allows the full-field characterisation of femtosecond to attosecond optical pulses. It has been instrumental in the verification of attosecond pulse generation and probing of ultrafast dynamics in matter. Recently, machine learning (ML) has been applied to retrieve the fields from streaking data (White and Chang 2019 Opt. Express27 4799; Zhu et al 2020 Sci. Rep.10 5782; Brunner et al 2022 Opt. Express30 15669–84). This offers a number of advantages compared with traditional iterative algorithms, including faster processing and better resilience to noise. Here, we implement a ML approach based on convolutional neural networks and limit the search to physically realistic pulses that can be specified with a small number of parameters. This leads to substantial reductions in both training and retrieval times, enabling near kHz retrieval rates. We examine how the retrieval performance is affected by noise, and for the first time in this context, study the effect of missing data. We show that satisfactory retrievals are still possible with signal to noise ratios as low as 10, and with up to $40\%$ of data missing.
Maimaris M, Pettipher AJ, Azzouzi M, et al., 2022, Sub-10-fs observation of bound exciton formation in organic optoelectronic devices, Nature Communications, Vol: 13, ISSN: 2041-1723
Fundamental mechanisms underlying exciton formation in organic semiconductors are complex and elusive as it occurs on ultrashort sub-100-fs timescales. Some fundamental aspects of this process, such as the evolution of exciton binding energy, have not been resolved in time experimentally. Here, we apply a combination of sub-10-fs Pump-Push-Photocurrent, Pump-Push-Photoluminescence, and Pump-Probe spectroscopies to polyfluorene devices to track the ultrafast formation of excitons. While Pump-Probe is sensitive to the total concentration of excited states, Pump-Push-Photocurrent and Pump-Push-Photoluminescence are sensitive to bound states only, providing access to exciton binding dynamics. We find that excitons created by near-absorption-edge photons are intrinsically bound states, or become such within 10 fs after excitation. Meanwhile, excitons with a modest >0.3 eV excess energy can dissociate spontaneously within 50 fs before acquiring bound character. These conclusions are supported by excited-state molecular dynamics simulations and a global kinetic model which quantitatively reproduce experimental data.
Maimaris M, Pettipher AJ, Azzouzi M, et al., 2022, Sub-10fs Photocurrent and Photoluminescence Action Spectroscopies of Organic Optoelectronic Devices Reveals Ultrafast Formation of Bound Excitonic States
We apply ultrafast pump-push-photocurrent and pump-push-photoluminescence spectroscopies to polyfluorene organic diode to track in time the bound exciton formation. ‘Cold’-excitons become bound within 10-fs while ‘hot’-excitons can dissociate spontaneously within 50-fs before acquiring bound character.
Barillot T, Alexander O, Cooper B, et 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.
Danson CN, White M, Barr JRM, et al., 2021, A history of high-power laser research and development in the United Kingdom, High Power Laser Science and Engineering, Vol: 9, Pages: 1-86, ISSN: 2095-4719
The first demonstration of laser action in ruby was made in 1960 by T. H. Maiman of Hughes Research Laboratories, USA. Many laboratories worldwide began the search for lasers using different materials, operating at different wavelengths. In the UK, academia, industry and the central laboratories took up the challenge from the earliest days to develop these systems for a broad range of applications. This historical review looks at the contribution the UK has made to the advancement of the technology, the development of systems and components and their exploitation over the last 60 years.
Weaver B, Greening D, Tisch J, et al., 2020, Generation and measurement of isolated attosecond pulses with enhanced flux using a two colour synthesized laser field, Optics Express, Vol: 28, Pages: 23329-23337, ISSN: 1094-4087
We have generated isolated attosecond pulses and performed attosecond streaking measurements using a two-colour synthesized laser field consisting of a strong near-infrared few-cycle pulse and a weaker multi-cycle pulse centred at 400 nm. An actively stabilized interferometer was used to coherently combine the two pulses. Using attosecond streaking we characterised the electric fields of the two pulses and accurately retrieved the spectrum of the multi-cycle pulse. We demonstrated a two-fold increase in the flux of isolated attosecond pulses produced and show that their duration was minimally affected by the presence of the weaker field due to spectral filtering by a multilayer mirror.
Crespo H, Witting T, Canhota M, et al., 2020, In-situ temporal measurement of ultrashort laser pulses at full power during high-intensity laser-matter interactions, Optica, Vol: 7, Pages: 995-1002, ISSN: 2334-2536
In laser-matter interaction experiments it is of paramount importance to be able tocharacterise the laser pulse on target (in-situ) and at full power. This allows pulse optimisationand meaningful comparison with theory, and can shed fundamental new light on pulse distortionsoccurring in or on the target. Here we introduce and demonstrate a new technique based ondispersion scan using the concurrent third harmonic emission from the target that permits the full(amplitude and phase), in-situ, in-parallel characterisation of ultrashort laser pulses in a gas orsolid target over a very wide intensity range that encompasses the1013−1015W cm−2regime ofhigh harmonic generation and other important strong field phenomena, with possible extensionto relativistic intensities also presently inaccessible to other diagnostics.
Johnson AS, Austin DR, Wood DA, et al., 2020, High-flux soft x-ray harmonic generation from ionization-shaped few-cycle laser pulses (vol 4, eaar3761, 2018), SCIENCE ADVANCES, Vol: 6, ISSN: 2375-2548
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
Debrah DA, Stewart GA, Basnayake G, et al., 2019, Direct in-situ single-shot measurements of the absolute carrier-envelope phases of ultrashort pulses, Optics Letters, Vol: 44, Pages: 3582-3585, ISSN: 0146-9592
Many important physical processes such as nonlinear optics and coherent control are highly sensitive to the absolute carrier-envelope phase (CEP) of driving ultrashort laser pulses. This makes the measurement of CEP immensely important in relevant fields. Even though relative CEPs can be measured with a few existing technologies, the estimate of the absolute CEP is not straightforward and always requires theoretical inputs. Here, we demonstrate a novel in-situ technique based on angular streaking that can achieve such a goal without complicated calibration procedures. Single-shot measurements of the absolute CEP have been achieved with an estimated precision of 0.19 radians.
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
Crespo H, Canhota M, Witting T, et al., 2019, Direct measurement of intense sub-4-fs pulses in a gas target by 3rd-harmonic dispersion-scan, 21st International Conference on Ultrafast Phenomena (UP), Publisher: E D P SCIENCES, ISSN: 2100-014X
Mecseki K, Hoeppner H, Buescher M, et al., 2018, Hard X-ray induced fast secondary electron cascading processes in solids, APPLIED PHYSICS LETTERS, Vol: 113, ISSN: 0003-6951
Recent studies confirmed that the materials used in the extreme UV and soft X-ray regime for precise characterization of intense free-electron laser pulses (e.g., Si3N4) do not work efficiently in the hard X-ray regime, which is due to the fact that the impact of a hard X-ray photon is followed by a series of electron cascading processes. Following theoretical indication, we show that this limitation can be circumvented and the cascading time can be significantly reduced if the X-ray photon energy is double the ionization energy. We investigate an alternative material for pulse diagnostics, SnO2, using the Linac Coherent Light Source at photon energies of 5 keV and 9 keV. We prove the validity of the concept and show that it has a large potential for practical applications. By applying the proposed criteria, the temporal accuracy of the non-invasive pulse diagnostic tools can be improved in current and emerging hard X-ray facilities.
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.
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.
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.
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.
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
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.
Ciappina MF, Perez-Hernandez JA, Landsman AS, et al., 2017, Attosecond physics at the nanoscale, Publisher: IOP PUBLISHING LTD
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Mercer IP, Witting T, Driver T, et al., 2017, Angle-resolved coherent wave mixing using a 4 fs ultra-broad bandwidth laser, Optics Letters, Vol: 42, Pages: 859-862, ISSN: 0146-9592
We demonstrate angle-resolved coherent (ARC) wave mixing using 4 fs light pulses derived from a laser source that spans 550–1000 nm. We believe this to be the shortest pulse duration used to date in coherent multi-dimensional spectroscopy. The marriage of this ultra-broad band, few-cycle coherent source with the ARC technique will permit new investigations of the interplay between energy transfers and quantum superposition states spanning 8200 cm−1. We applied this configuration to measurements on the photosynthetic low light (LL) complex from Rhodopseudomonas palustris in solution at ambient temperature. We observe bi-exponential population dynamics for energy transfer across 5500 cm−1 (0.65 eV), which we attribute to energy transfer from the transition of bacteriochlorophylls to the B850 pigment of the complex. We believe for the first time, to the best of our knowledge, we demonstrate that ARC maps can be recorded using a single laser pulse.
Schuette B, Ye P, Patchkovskii S, et al., 2016, Strong-field ionization of clusters using two-cycle pulses at 1.8 μm, Scientific Reports, Vol: 6, ISSN: 2045-2322
The interaction of intense laser pulses with nanoscale particles leads to the production of high-energy electrons, ions, neutral atoms, neutrons and photons. Up to now, investigations have focused on near-infrared to X-ray laser pulses consisting of many optical cycles. Here we study strong-field ionization of rare-gas clusters (103 to 105 atoms) using two-cycle 1.8 μm laser pulses to access a new interaction regime in the limit where the electron dynamics are dominated by the laser field and the cluster atoms do not have time to move significantly. The emission of fast electrons with kinetic energies exceeding 3 keV is observed using laser pulses with a wavelength of 1.8 μm and an intensity of 1 × 1015 W/cm2, whereas only electrons below 500 eV are observed at 800 nm using a similar intensity and pulse duration. Fast electrons are preferentially emitted along the laser polarization direction, showing that they are driven out from the cluster by the laser field. In addition to direct electron emission, an electron rescattering plateau is observed. Scaling to even longer wavelengths is expected to result in a highly directional current of energetic electrons on a few-femtosecond timescale.
Austin DR, witting T, sebastien J, et al., 2016, Spatio-temporal characterization of intense few-cycle 2 μm pulses, Optics Express, Vol: 24, Pages: 24786-24798, ISSN: 1094-4087
We present a variant of spatially encoded spectral shearing interferometry for measuring two-dimensional spatio-temporal slices of few-cycle pulses centered around 2μm. We demonstrate experimentally that the device accurately retrieves the pulse-front tilt caused by angular dispersion of two-cycle pulses. We then use the technique to characterize 500–650 μJ pulses from a hollow fiber pulse compressor, with durations as short as 7.1 fs (1.3 optical cycles).
Witting T, Greening D, Walke D, et al., 2016, Time-domain ptychography of over-octave-spanning laser pulses in the single-cycle regime., Optics Letters, Vol: 41, Pages: 4218-4221, ISSN: 1539-4794
We report, to the best of our knowledge, the first application of time-domain ptychography for the characterization of few-cycle laser pulses. Our method enables zero-additional phase measurements of over-octave-spanning laser pulses in the single cycle regime. The spectral phase is recovered using a robust ptychography algorithm that requires no input apart from the measured data trace. In addition to numerical tests, we validate our new device experimentally by reconstructing the complex electric field of a 1.5 cycle laser pulse with a bandwidth spanning 490 to 1060 nm. We further check the accuracy of our device by comparing the measured phases of octave-spanning chirped pulses to the known dispersion of fused silica glass.
Witting T, Austin DR, Barillot T, et al., 2016, Self-referenced characterization of space-time couplings in near-single-cycle laser pulses, Optics Letters, Vol: 41, Pages: 2382-2385, ISSN: 1539-4794
We report on the characterization of space–time couplingsin high-energy sub-2-cycle 770 nm laser pulses using a selfreferencingsingle-frame method. Using spatially encodedarrangement filter-based spectral phase interferometry fordirect electric field reconstruction, we characterize fewcyclepulses with a wavefront rotation of 2.8 × 1011 rev∕s(1.38 mrad per half-cycle) and pulses with pulse fronttilts ranging from −0.33 fs∕μm to −3.03 fs∕μm in thefocus.
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