# Publications by Prof. Dr. Matt Zepf

All publications of HI Jena

## 2019

F. Karbstein, A. Blinne, H. Gies, and M. Zepf
Boosting Quantum Vacuum Signatures by Coherent Harmonic Focusing
Physical Review Letters 123, 091802 (2019)

Abstract: We show that coherent harmonic focusing provides an efficient mechanism to boost all-optical signatures of quantum vacuum nonlinearity in the collision of high-intensity laser fields, thereby offering a promising route to their first experimental detection. Assuming two laser pulses of given parameters at our disposal, we demonstrate a substantial increase of the number of signal photons measurable in experiments where one of the pulses undergoes coherent harmonic focusing before it collides with the fundamental-frequency pulse. Imposing a quantitative criterion to discern the signal photons from the background of the driving laser photons and accounting for the finite purity of polarization filtering, we find that signal photons arising from inelastic scattering processes constitute a promising signature. By contrast, quasielastic contributions which are conventionally assumed to form the most prospective signal remain background dominated. Our findings may result in a paradigm shift concerning which photonic signatures of quantum vacuum nonlinearity are accessible in experiment.

A. Blinne, H. Gies, F. Karbstein, C. Kohlfürst, and M. Zepf
Photon-Photon Scattering at the High-Intensity Frontier: Paraxial Beams
Journal of Physics: Conference Series 1206, 012016 (2019)

Abstract: Our goal is to study optical signatures of quantum vacuum nonlinearities in strong macroscopic electromagnetic fields provided by high-intensity laser beams. The vacuum emission scheme is perfectly suited for this task as it naturally distinguishes between incident laser beams, described as classical electromagnetic fields driving the effect, and emitted signal photons encoding the signature of quantum vacuum nonlinearity. Using the Heisenberg-Euler effective action, our approach allows for a reliable study of photonic signatures of QED vacuum nonlinearity in the parameter regimes accessible by all-optical high-intensity laser experiments. To this end, we employ an efficient, flexible numerical algorithm, which allows for a detailed study of the signal photons emerging in the collision of focused paraxial high-intensity laser pulses. Due to the high accuracy of our numerical solutions we predict the total number of signal photons, but also have full access to the signal photons’ characteristics, including their spectrum, propagation directions and polarizations. We discuss setups offering an excellent background-to-noise ratio, thus providing an important step towards the experimental verification of quantum vacuum nonlinearities.

A. Blinne, H. Gies, F. Karbstein, C. Kohlfürst, and M. Zepf
The Vacuum Emission Picture Beyond Paraxial Approximation
Journal of Physics: Conference Series 1206, 012017 (2019)

Abstract: Optical signatures of the effective nonlinear couplings among electromagnetic fields in the quantum vacuum can be conveniently described in terms of stimulated photon emission processes induced by strong classical, space-time dependent electromagnetic fields. Recent studies have adopted this approach to study collisions of Gaussian laser pulses in paraxial approximation. The present study extends these investigations beyond the paraxial approximation by using an efficient numerical solver for the classical input fields. This new numerical code allows for a consistent theoretical description of optical signatures of QED vacuum nonlinearities in generic electromagnetic fields governed by Maxwell’s equations in the vacuum, such as manifestly non-paraxial laser pulses. Our code is based on a locally constant field approximation of the Heisenberg-Euler effective Lagrangian. As this approximation is applicable for essentially all optical high-intensity laser experiments, our code is capable of calculating signal photon emission amplitudes in completely generic input field configurations, limited only by numerical cost.

A. Blinne, S. Kuschel, S. Tietze, and M. Zepf
Efficient retrieval of phase information from real-valued electromagnetic field data
Journal of Computational Physics: X 1, 100019 (2019)

Abstract: While analytic calculations may give access to complex-valued electromagnetic field data which allow trivial access to envelope and phase information, the majority of numeric codes uses a real-valued representation. This typically increases the performance and reduces the memory footprint, albeit at a price: In the real-valued case it is much more difficult to extract envelope and phase information, even more so if counter propagating waves are spatially superposed. A novel method for the analysis of real-valued electromagnetic field data is presented in this paper. We show that, by combining the real-valued electric and magnetic field at a single point in time, we can directly reconstruct the full information of the electromagnetic fields in the form of complex-valued spectral coefficients (k→-space) at a low computational cost of only three Fourier transforms. The method allows for counter propagating plane waves to be accurately distinguished as well as their complex spectral coefficients, i.e. spectral amplitudes and spectral phase to be calculated. From these amplitudes, the complex-valued electromagnetic fields and also the complex-valued vector potential can be calculated from which information about spatiotemporal phase and amplitude is readily available. Additionally, the complex fields allow for efficient vacuum propagation allowing to calculate far field data or boundary input data from near field data. An implementation of the new method is available as part of PostPic1, a data analysis toolkit written in the Python programming language.

A. Blinne, H. Gies, F. Karbstein, C. Kohlfürst, and M. Zepf
All-optical signatures of quantum vacuum nonlinearities in generic laser fields
Physical Review D 99, 016006 (2019)

Abstract: All-optical experiments at the high-intensity frontier offer a promising route to unprecedented precision tests of quantum electrodynamics in strong macroscopic electromagnetic fields. So far, most theoretical studies of all-optical signatures of quantum vacuum nonlinearity are based on simplifying approximations of the beam profiles and pulse shapes of the driving laser fields. Since precision tests require accurate quantitative theoretical predictions, we introduce an efficient numerical tool facilitating the quantitative theoretical study of all-optical signatures of quantum vacuum nonlinearity in generic laser fields. Our approach is based on the vacuum emission picture, and makes use of the fact that the dynamics of the driving laser fields are to an excellent approximation governed by classical Maxwell theory in vacuum. In combination with a Maxwell solver, which self-consistently propagates any given laser field configuration, this allows for accurate theoretical predictions of photonic signatures of vacuum nonlinearity in high-intensity laser experiments from first principles. We employ our method to simulate photonic signatures of quantum vacuum nonlinearity in laser pulse collisions involving a few-cycle pulse, and show that the angular and spectral distributions of the emitted signal photons deviate from those of the driving laser beams.

## 2018

K. T. Behm, J. M. Cole, A. S. Joglekar, E. Gerstmayr, J. C. Wood, C. D. Baird, T. G. Blackburn, M. Duff, C. Harvey, A. Ilderton, S. Kuschel, S. P. D. Mangles, M. Marklund, P. McKenna, C. D. Murphy, Z. Najmudin, K. Poder, C. P. Ridgers, G. Sarri, G. M. Samarin, D. Symes, J. Warwick, M. Zepf, K. Krushelnick, and A. G. R. Thomas
A spectrometer for ultrashort gamma-ray pulses with photon energies greater than 10 MeV
Review of Scientific Instruments 89, 113303 (2018)

Abstract: We present a design for a pixelated scintillator based gamma-ray spectrometer for non-linear inverse Compton scattering experiments. By colliding a laser wakefield accelerated electron beam with a tightly focused, intense laser pulse, gamma-ray photons up to 100 MeV energies and with few femtosecond duration may be produced. To measure the energy spectrum and angular distribution, a 33 × 47 array of cesium-iodide crystals was oriented such that the 47 crystal length axis was parallel to the gamma-ray beam and the 33 crystal length axis was oriented in the vertical direction. Using an iterative deconvolution method similar to the YOGI code, modeling of the scintillator response using GEANT4 and fitting to a quantum Monte Carlo calculated photon spectrum, we are able to extract the gamma ray spectra generated by the inverse Compton interaction.

S. Kuschel, M. B. Schwab, M. Yeung, D. Hollatz, A. Seidel, W. Ziegler, A. Sävert, M. C. Kaluza, and M. Zepf
Controlling the Self-Injection Threshold in Laser Wakefield Accelerators
Physical Review Letters 121, 154801 (2018)

Abstract: Controlling the parameters of a laser plasma accelerated electron beam is a topic of intense research with a particular focus placed on controlling the injection phase of electrons into the accelerating structure from the background plasma. An essential prerequisite for high-quality beams is dark-current free acceleration (i.e., no electrons accelerated beyond those deliberately injected). We show that small-scale density ripples in the background plasma are sufficient to cause the uncontrolled (self-)injection of electrons. Such ripples can be as short as ∼50  μm and can therefore not be resolved by standard interferometry. Background free injection with substantially improved beam characteristics (divergence and pointing) is demonstrated in a gas cell designed for a controlled gas flow. The results are supported by an analytical theory as well as 3D particle in cell simulations.

G. M. Samarin, M. Zepf, and G. Sarri
Radiation reaction studies in an all-optical set-up: experimental limitations
Journal of Modern Optics 65, 1362 (2018)

Abstract: The recent development of ultra-high intensity laser facilities is finally opening up the possibility of studying high-field quantum electrodynamics in the laboratory. Arguably, one of the central phenomena in this area is that of quantum radiation reaction experienced by an ultra-relativistic electron beam as it propagates through the tight focus of a laser beam. In this paper, we discuss the major experimental challenges that are to be faced in order to extract meaningful and quantitative information from this class of experiments using existing and near-term laser facilities.

W. Schumaker, T. Liang, R. Clarke, J. M. Cole, G. Grittani, S. Kuschel, S. P. D. Mangles, Z. Najmudin, K. Poder, G. Sarri, D. Symes, A. G. R. Thomas, M. Vargas, M. Zepf, and K. Krushelnick
Making pions with laser light
New Journal of Physics 20, 073008 (2018)

Abstract: The interaction of high intensity short pulse laser beams with plasmas can accelerate electrons to energies in excess of a GeV. These electron beams can subsequently be used to generate short-lived particles such as positrons, muons, and pions. In recent experiments, we have made the first measurements of pion production using 'all optical' methods. In particular, we have demonstrated that the interaction of bremsstrahlung generated by laser driven electron beams with aluminum atoms can produce the long lived isotope of magnesium (²⁷Mg) which is a signature for pion (π⁺) production and subsequent muon decay. Using a 300 TW laser pulse, we have measured the generation of 150 ± 50 pions per shot. We also show that the energetic electron beam is a source of an intense, highly directional neutron beam resulting from (γ, n) reactions which contributes to the ²⁷Mg measurement as background via the (n, p) process.

K. Poder, M. Tamburini, G. Sarri, A. D. Piazza, S. Kuschel, C. Baird, K. Behm, S. Bohlen, J. Cole, D. Corvan, M. Duff, E. Gerstmayr, C. Keitel, K. Krushelnick, S. Mangles, P. McKenna, C. Murphy, Z. Najmudin, C. Ridgers, G. Samarin, D. Symes, A. Thomas, J. Warwick, and M. Zepf
Experimental Signatures of the Quantum Nature of Radiation Reaction in the Field of an Ultraintense Laser
Physical Review X 8, 031004 (2018)

Abstract: The description of the dynamics of an electron in an external electromagnetic field of arbitrary intensity is one of the most fundamental outstanding problems in electrodynamics. Remarkably, to date, there is no unanimously accepted theoretical solution for ultrahigh intensities and little or no experimental data. The basic challenge is the inclusion of the self-interaction of the electron with the field emitted by the electron itself—the so-called radiation reaction force. We report here on the experimental evidence of strong radiation reaction, in an all-optical experiment, during the propagation of highly relativistic electrons (maximum energy exceeding 2 GeV) through the field of an ultraintense laser (peak intensity of 4×10²⁰  W/cm²). In their own rest frame, the highest-energy electrons experience an electric field as high as one quarter of the critical field of quantum electrodynamics and are seen to lose up to 30% of their kinetic energy during the propagation through the laser field. The experimental data show signatures of quantum effects in the electron dynamics in the external laser field, potentially showing departures from the constant cross field approximation.

M. M. Günther, A. V. Volotka, M. Jentschel, S. Fritzsche, Th. Stöhlker, P. G. Thirolf, and M. Zepf
Dispersive refraction of different light to heavy materials at MeV γ-ray energies
Physical Review A 97, 063843 (2018)

Abstract: The dispersive behavior of materials with atomic charge numbers varying from Z=4 (beryllium, Be) to Z=82 (lead, Pb) was investigated experimentally and theoretically at gamma-ray energies up to 2 MeV. The experiment was performed at the double-crystal gamma spectrometer GAMS6 of the Institut Laue-Langevin in Grenoble. The experimental results were compared with theoretical calculations which account for all major elastic processes involved. Overall, we found a good agreement between theory and experiment. We find that, for the development of refractive optics at $\gamma$-ray energies beyond those currently in use, high-Z materials become increasingly attractive compared to the beryllium lens-stacks used at x-ray energies.

G. A. Becker, S. Tietze, S. Keppler, J. Reislöhner, J. H. Bin, L. Bock, F.-E. Brack, J. Hein, M. Hellwing, P. Hilz, M. Hornung, A. Kessler, S. D. Kraft, S. Kuschel, H. Liebetrau, W. Ma, J. Polz, H.-P. Schlenvoigt, F. Schorcht, M. B. Schwab, A. Seidel, K. Zeil, U. Schramm, M. Zepf, J. Schreiber, S. Rykovanov, and M. C. Kaluza
Ring-like spatial distribution of laser accelerated protons in the ultra-high-contrast TNSA-regime
Plasma Physics and Controlled Fusion 60, 055010 (2018)

Abstract: The spatial distribution of protons accelerated from submicron-thick plastic foil targets using multi-terawatt, frequency-doubled laser pulses with ultra-high temporal contrast has been investigated experimentally. A very stable, ring-like beam profile of the accelerated protons, oriented around the target’s normal direction has been observed. The ring’s opening angle has been found to decrease with increasing foil thicknesses. Two-dimensional particle-in-cell simulations reproduce our results indicating that the ring is formed during the expansion of the proton density distribution into the vacuum as described by the mechanism of target-normal sheath acceleration. Here—in addition to the longitudinal electric fields responsible for the forward acceleration of the protons—a lateral charge separation leads to transverse field components accelerating the protons in the lateral direction.

A. Blinne, D. Schinkel, S. Kuschel, N. Elkina, S. G. Rykovanov, and M. Zepf
A systematic approach to numerical dispersion in Maxwell solvers
Computer Physics Communications 224, 273 (2018)

Abstract: The finite-difference time-domain (FDTD) method is a well established method for solving the time evolution of Maxwell’s equations. Unfortunately the scheme introduces numerical dispersion and therefore phase and group velocities which deviate from the correct values. The solution to Maxwell’s equations in more than one dimension results in non-physical predictions such as numerical dispersion or numerical Cherenkov radiation emitted by a relativistic electron beam propagating in vacuum. Improved solvers, which keep the staggered Yee-type grid for electric and magnetic fields, generally modify the spatial derivative operator in the Maxwell–Faraday equation by increasing the computational stencil. These modified solvers can be characterized by different sets of coefficients, leading to different dispersion properties. In this work we introduce a norm function to rewrite the choice of coefficients into a minimization problem. We solve this problem numerically and show that the minimization procedure leads to phase and group velocities that are considerably closer to c as compared to schemes with manually set coefficients available in the literature. Depending on a specific problem at hand (e.g. electron beam propagation in plasma, high-order harmonic generation from plasma surfaces, etc.), the norm function can be chosen accordingly, for example, to minimize the numerical dispersion in a certain given propagation direction. Particle-in-cell simulations of an electron beam propagating in vacuum using our solver are provided.

B. Lei, J. Wang, V. Kharin, M. Zepf, and S. Rykovanov
γ-Ray Generation from Plasma Wakefield Resonant Wiggler
Physical Review Letters 120, 134801 (2018)

Abstract: A flexible gamma-ray radiation source based on the resonant laser-plasma wakefield wiggler is proposed. The wiggler is achieved by inducing centroid oscillations of a short laser pulse in a plasma channel. Electrons (self-)injected in such a wakefield experience both oscillations due to the transverse electric fields and energy gain due to the longitudinal electric field. The oscillations are significantly enhanced when the laser pulse centroid oscillations are in resonance with the electron betatron oscillations, extending the radiation spectrum to the gamma-ray range. The polarization of the radiation can be easily controlled by adjusting the injection of the laser pulse into the plasma channel.

M. Taylor, M. Coughlan, G. Nersisyan, L. Senje, D. Jung, F. Currell, D. Riley, C. L. S. Lewis, M. Zepf, and B. Dromey
Probing ultrafast proton induced dynamics in transparent dielectrics
Plasma Physics and Controlled Fusion 60, 054004 (2018)

Abstract: A scheme has been developed permitting the spatial and temporal characterisation of ultrafast dynamics induced by laser driven proton bursts in transparent dielectrics. Advantage is taken of the high degree of synchronicity between the proton bursts generated during laser-foil target interactions and the probing laser to provide the basis for streaking of the dynamics. Relaxation times of electrons (<10⁻¹² s) are measured following swift excitation across the optical band gap for various glass samples. A temporal resolution of <500 fs is achieved demonstrating that these ultrafast dynamics can be characterized on a single-shot basis.

J. H. Bin, M. Yeung, Z. Gong, H. Y. Wang, C. Kreuzer, M. L. Zhou, M. J. V. Streeter, P. S. Foster, S. Cousens, B. Dromey, J. Meyer-ter-Vehn, M. Zepf, and J. Schreiber
Enhanced Laser-Driven Ion Acceleration by Superponderomotive Electrons Generated from Near-Critical-Density Plasma
Physical Review Letters 120, 074801 (2018)

Abstract: We report on the experimental studies of laser driven ion acceleration from a double-layer target where a near-critical density target with a few-micron thickness is coated in front of a nanometer-thin diamondlike carbon foil. A significant enhancement of proton maximum energies from 12 to ∼30  MeV is observed when a relativistic laser pulse impinges on the double-layer target under linear polarization. We attributed the enhanced acceleration to superponderomotive electrons that were simultaneously measured in the experiments with energies far beyond the free-electron ponderomotive limit. Our interpretation is supported by two-dimensional simulation results.

J. M. Cole, K. T. Behm, E. Gerstmayr, T. G. Blackburn, J. C. Wood, C. D. Baird, M. J. Duff, C. Harvey, A. Ilderton, A. S. Joglekar, K. Krushelnick, S. Kuschel, M. Marklund, P. McKenna, C. D. Murphy, K. Poder, C. P. Ridgers, G. M. Samarin, G. Sarri, D. R. Symes, A. G. R. Thomas, J. Warwick, M. Zepf, Z. Najmudin, and S. P. D. Mangles
Experimental Evidence of Radiation Reaction in the Collision of a High-Intensity Laser Pulse with a Laser-Wakefield Accelerated Electron Beam
Physical Review X 8, 011020 (2018)

Abstract: The dynamics of energetic particles in strong electromagnetic fields can be heavily influenced by the energy loss arising from the emission of radiation during acceleration, known as radiation reaction. When interacting with a high-energy electron beam, today’s lasers are sufficiently intense to explore the transition between the classical and quantum radiation reaction regimes. We present evidence of radiation reaction in the collision of an ultrarelativistic electron beam generated by laser-wakefield acceleration (ϵ>500  MeV) with an intense laser pulse (a₀>10). We measure an energy loss in the postcollision electron spectrum that is correlated with the detected signal of hard photons (γ rays), consistent with a quantum description of radiation reaction. The generated γ rays have the highest energies yet reported from an all-optical inverse Compton scattering scheme, with critical energy ϵ꜀ᵣᵢₜ>30  MeV.

## 2017

J. W. Wang, C. B. Schroeder, R. Li, M. Zepf, and S. G. Rykovanov
Plasma channel undulator excited by high-order laser modes
Scientific Reports 7, 16884 (2017)

Abstract: The possibility of utilizing plasma undulators and plasma accelerators to produce compact ultraviolet and X-ray sources, has attracted considerable interest for a few decades. This interest has been driven by the great potential to decrease the threshold for accessing such sources, which are mainly provided by a few dedicated large-scale synchrotron or free-electron laser (FEL) facilities. However, the broad radiation bandwidth of such plasma devices limits the source brightness and makes it difficult for the FEL instability to develop. Here, using multi-dimensional particle-in-cell (PIC) simulations, we demonstrate that a plasma undulator generated by the beating of a mixture of high-order laser modes propagating inside a plasma channel, leads to a few percent radiation bandwidth. The strength of the undulator can reach unity, the period can be less than a millimeter, and the number of undulator periods can be significantly increased by a phase locking technique based on the longitudinal tapering. Polarization control of such an undulator can be achieved by appropriately choosing the phase of the modes. According to our results, in the fully beam loaded regime, the electron current in the plasma undulator can reach 0.3\^aEUR%0kA level, making such an undulator a potential candidate towards a table-top FEL.

Y. Zhang, B. Qiao, X. Xu, H. Chang, H. Lu, C. Zhou, H. Zhang, S. Zhu, M. Zepf, and X. He
Intense attosecond pulses from laser-irradiated near-critical-density plasmas
Optics Express 25, 29058 (2017)

Abstract: A novel practical and efficient way of obtaining intense attosecond pulses is proposed, where the near-critical-density (NCD) plasma target satisfying n₀/a₀n꜀ ≈ 1 is used. The unique interaction dynamics in NCD plasmas have been identified theoretically and by particle-in-cell simulations, which show that three distinct dense electron nanobunches are formed each half a laser cycle and two of them can induce intense attosecond pulses in respectively the reflected and the transmitted directions by the so-called “coherent synchrotron emission” (CSE) mechanism [experimentally confirmed in Nat. Phys. 8, 804 (2012)]. Comparing with CSE in solids, here not only the required stringent conditions on laser and target are relaxed, but also the radiation intensities are enhanced by two orders of magnitude. It is shown that relativistically intense attosecond X-ray pulses with intensity 10¹⁹ W/cm² and duration ~50 as can be robustly obtained in both directions by currently available driving lasers at intensities of 10²⁰ W/cm².

G. Sarri, J. Warwick, W. Schumaker, K. Poder, J. Cole, D. Doria, T. Dzelzainis, K. Krushelnick, S. Kuschel, S. P. D. Mangles, Z. Najmudin, L. Romagnani, G. M. Samarin, D. Symes, A. G. R. Thomas, M. Yeung, and M. Zepf
Spectral and spatial characterisation of laser-driven positron beams
Plasma Physics and Controlled Fusion 59, 014015 (2017)

Abstract: The generation of high-quality relativistic positron beams is a central area of research in experimental physics, due to their potential relevance in a wide range of scientific and engineering areas, ranging from fundamental science to practical applications. There is now growing interest in developing hybrid machines that will combine plasma-based acceleration techniques with more conventional radio-frequency accelerators, in order to minimise the size and cost of these machines. Here we report on recent experiments on laser-driven generation of high-quality positron beams using a relatively low energy and potentially table-top laser system. The results obtained indicate that current technology allows to create, in a compact setup, positron beams suitable for injection in radio-frequency accelerators.

G. M. Samarin, M. Zepf, and G. Sarri
Radiation reaction studies in an all-optical set-up: experimental limitations
Journal of Modern Optics 64, 2281 (2017)

Abstract: The recent development of ultra-high intensity laser facilities is finally opening up the possibility of studying high-field quantum electrodynamics in the laboratory. Arguably, one of the central phenomena in this area is that of quantum radiation reaction experienced by an ultra-relativistic electron beam as it propagates through the tight focus of a laser beam. In this paper, we discuss the major experimental challenges that are to be faced in order to extract meaningful and quantitative information from this class of experiments using existing and near-term laser facilities.

C. Scullion, D. Doria, L. Romagnani, A. Sgattoni, K. Naughton, D. R. Symes, P. McKenna, A. Macchi, M. Zepf, S. Kar, and M. Borghesi
Polarization Dependence of Bulk Ion Acceleration from Ultrathin Foils Irradiated by High-Intensity Ultrashort Laser Pulses
Physical Review Letters 119, 054801 (2017)

Abstract: The acceleration of ions from ultrathin (10–100 nm) carbon foils has been investigated using intense (∼6×10²⁰ W cm⁻²) ultrashort (45 fs) laser pulses, highlighting a strong dependence of the ion beam parameters on the laser polarization, with circularly polarized (CP) pulses producing the highest energies for both protons and carbons (25−30  MeV/nucleon); in particular, carbon ion energies obtained employing CP pulses were significantly higher (∼2.5 times) than for irradiations employing linearly polarized pulses. Particle-in-cell simulations indicate that radiation pressure acceleration becomes the dominant mechanism for the thinnest targets and CP pulses.

A. Alejo, A. G. Krygier, H. Ahmed, J. T. Morrison, R. J. Clarke, J. Fuchs, A. Green, J. S. Green, D. Jung, A. Kleinschmidt, Z. Najmudin, H. Nakamura, P. Norreys, M. Notley, M. Oliver, M. Roth, L. Vassura, M. Zepf, M. Borghesi, R. R. Freeman, and S. Kar
High flux, beamed neutron sources employing deuteron-rich ion beams from D2O-ice layered targets
Plasma Physics and Controlled Fusion 59, 064004 (2017)

Abstract: A forwardly-peaked bright neutron source was produced using a laser-driven, deuteron-rich ion beam in a pitcher-catcher scenario. A proton-free ion source was produced via target normal sheath acceleration from Au foils having a thin layer of D₂O ice at the rear side, irradiated by sub-petawatt laser pulses (~200 J, ~750 fs) at peak intensity ∼2×10²⁰ W cm⁻². The neutrons were preferentially produced in a beam of ~70° FWHM cone along the ion beam forward direction, with maximum energy up to ~40 MeV and a peak flux along the axis ∼2×10⁹ n sr⁻¹ for neutron energy above 2.5 MeV. The experimental data is in good agreement with the simulations carried out for the d(d,n)³He reaction using the deuteron beam produced by the ice-layered target.

M. M. Günther, M. Jentschel, A. J. Pollitt, P. G. Thirolf, and M. Zepf
Refractive-index measurement of Si at γ-ray energies up to 2 MeV
Physical Review A 95, 053864 (2017)

Abstract: The refractive index of silicon at γ-ray energies from 181 to 1959 keV was investigated using the GAMS6 double crystal spectrometer and found to follow the predictions of the classical scattering model. This is in contrast to earlier measurements on the GAMS5 spectrometer, which suggested a sign change in the refractive index for photon energies above 500 keV. We present a reevaluation of the original data from 2011 as well as data from a 2013 campaign in which we show that systematic errors due to diffraction effects of the prism can explain the earlier data.

L. Senje, M. Coughlan, D. Jung, M. Taylor, G. Nersisyan, D. Riley, C. L. S. Lewis, O. Lundh, C.-G. Wahlström, M. Zepf, and B. Dromey
Experimental investigation of picosecond dynamics following interactions between laser accelerated protons and water
Applied Physics Letters 110, 104102 (2017)

Abstract: We report direct experimental measurements with picosecond time resolution of how high energy protons interact with water at extreme dose levels (kGy), delivered in a single pulse with the duration of less than 80 ps. The unique synchronisation possibilities of laser accelerated protons with an optical probe pulse were utilized to investigate the energy deposition of fast protons in water on a time scale down to only a few picoseconds. This was measured using absorbance changes in the water, induced by a population of solvated electrons created in the tracks of the high energy protons. Our results indicate that for sufficiently high doses delivered in short pulses, intertrack effects will affect the yield of solvated electrons. The experimental scheme allows for investigation of the ultrafast mechanisms occurring in proton water radiolysis, an area of physics especially important due to its relevance in biology and for proton therapy.

H. X. Chang, B. Qiao, T. W. Huang, Z. Xu, C. T. Zhou, Y. Q. Gu, X. Q. Yan, M. Zepf, and X. T. He
Brilliant petawatt gamma-ray pulse generation in quantum electrodynamic laser-plasma interaction
Scientific Reports 7, 45031 (2017)

Abstract: We show a new resonance acceleration scheme for generating ultradense relativistic electron bunches in helical motions and hence emitting brilliant vortical γ-ray pulses in the quantum electrodynamic (QED) regime of circularly-polarized (CP) laser-plasma interactions. Here the combined effects of the radiation reaction recoil force and the self-generated magnetic fields result in not only trapping of a great amount of electrons in laser-produced plasma channel, but also significant broadening of the resonance bandwidth between laser frequency and that of electron betatron oscillation in the channel, which eventually leads to formation of the ultradense electron bunch under resonant helical motion in CP laser fields. Three-dimensional PIC simulations show that a brilliant γ-ray pulse with unprecedented power of 6.7 PW and peak brightness of 10²⁵ photons/s/mm²/mrad²/0.1% BW (at 15 MeV) is emitted at laser intensity of 1.9 × 10²³ W/cm².

D. Adolph, M. Möller, J. Bierbach, M. Schwab, A. Sävert, M. Yeung, A. M. Sayler, M. Zepf, M. C. Kaluza, and G. G. Paulus
Real-time, single-shot, carrier-envelope-phase measurement of a multi-terawatt laser
Applied Physics Letters 110, 081105 (2017)

Abstract: We present the single-shot carrier-envelope phase (CEP) determination of a 1 Hz, multi-terawatt (TW) laser system with a setup based on spectral broadening in a hollow-core fiber and a stereographic measurement of the energy-dependent above-threshold ionization plateau. The latter is extremely sensitive to variations in CEP. As compared to the f-2f interferometers, this technique reduces the uncertainties due to the shot-to-shot intensity fluctuations, which are prevalent in the TW laser systems. The experimental results pave the way towards the investigation and control over CEP-sensitive processes at ultra-high intensities.

## 2016

M. Yeung, S. Rykovanov, J. Bierbach, L. Li, E. Eckner, S. Kuschel, A. Woldegeorgis, C. Rödel, A. Sävert, G. G. Paulus, M. Coughlan, B. Dromey, and M. Zepf
Experimental observation of attosecond control over relativistic electron bunches with two-colour fields
Nature Photonics 32, 11 (2016)

Abstract: Energy coupling during relativistically intense laser–matter interactions is encoded in the attosecond motion of strongly driven electrons at the pre-formed plasma–vacuum boundary. Studying and controlling this motion can reveal details about the microscopic processes that govern a vast array of light–matter interaction phenomena, including those at the forefront of extreme laser–plasma science such as laser-driven ion acceleration, bright attosecond pulse generation and efficient energy coupling for the generation and study of warm dense matter. Here we experimentally demonstrate that by precisely adjusting the relative phase of an additional laser beam operating at the second harmonic of the driving laser it is possible to control the trajectories of relativistic electron bunches formed during the interaction with a solid target at the attosecond scale. We observe significant enhancements in the resulting high-harmonic yield, suggesting potential applications for sources of ultra-bright, extreme ultraviolet attosecond radiation to be used in atomic and molecular pump–probe experiments.

T. Tanikawa, A. Hage, M. Kuhlmann, J. Gonschior, S. Grunewald, E. Plönjes, S. Düsterer, G. Brenner, S. Dziarzhytski, M. Braune, M. Brachmanski, Z. Yin, F. Siewert, T. Dzelzainis, B. Dromey, M. Prandolini, F. Tavella, M. Zepf, and B. Faatz
First observation of SASE radiation using the compact wide-spectral-range XUV spectrometer at FLASH2
Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 830, 170 (2016)

Abstract: The Free-electron LASer in Hamburg (FLASH) has been extended with a new undulator line FLASH2 in 2014. A compact grazing-incident wide-spectral-range spectrometer based on spherical-variable-line-spacing (SVLS) gratings in the extreme ultraviolet (XUV) region was constructed to optimize and characterize the free-electron laser (FEL) performance at FLASH2. The spectrometer is equipped with three different concave SVLS gratings covering a spectral range from 1 to 62 nm to analyze the spectral characteristics of the XUV radiation. Wavelength calibration and evaluation of the spectral resolution were performed at the plane grating monochromator beamline PG2 at FLASH1 before the installation at FLASH2, and compared with analytical simulations. The first light using self-amplified spontaneous emission from FLASH2 was observed by the spectrometer during a simultaneous operation of both undulator lines — FLASH1 and FLASH2. In addition, the spectral resolution of the spectrometer was evaluated by comparing the measured spectrum from FLASH2 with FEL simulations.

S. Kuschel, D. Hollatz, T. Heinemann, O. Karger, M. B. Schwab, D. Ullmann, A. Knetsch, A. Seidel, C. Rödel, M. Yeung, M. Leier, A. Blinne, H. Ding, T. Kurz, D. J. Corvan, A. Sävert, S. Karsch, M. C. Kaluza, B. Hidding, and M. Zepf
Demonstration of passive plasma lensing of a laser wakefield accelerated electron bunch
Physical Review Accelerators and Beams 19, 071301 (2016)

Abstract: We report on the first demonstration of passive all-optical plasma lensing using a two-stage setup. An intense femtosecond laser accelerates electrons in a laser wakefield accelerator (LWFA) to 100 MeV over millimeter length scales. By adding a second gas target behind the initial LWFA stage we introduce a robust and independently tunable plasma lens. We observe a density dependent reduction of the LWFA electron beam divergence from an initial value of 2.3 mrad, down to 1.4 mrad (rms), when the plasma lens is in operation. Such a plasma lens provides a simple and compact approach for divergence reduction well matched to the mm-scale length of the LWFA accelerator. The focusing forces are provided solely by the plasma and driven by the bunch itself only, making this a highly useful and conceptually new approach to electron beam focusing. Possible applications of this lens are not limited to laser plasma accelerators. Since no active driver is needed the passive plasma lens is also suited for high repetition rate focusing of electron bunches. Its understanding is also required for modeling the evolution of the driving particle bunch in particle driven wake field acceleration.

S. Kar, H. Ahmed, R. Prasad, M. Cerchez, S. Brauckmann, B. Aurand, G. Cantono, P. Hadjisolomou, C. Lewis, A. Macchi, G. Nersisyan, A. Robinson, A. Schroer, M. Swantusch, M. Zepf, O. Willi, and M. Borghesi
Guided post-acceleration of laser-driven ions by a miniature modular structure
Nature Communications 7, 10792 (2016)

Abstract: All-optical approaches to particle acceleration are currently attracting a significant research effort internationally. Although characterized by exceptional transverse and longitudinal emittance, laser-driven ion beams currently have limitations in terms of peak ion energy, bandwidth of the energy spectrum and beam divergence. Here we introduce the concept of a versatile, miniature linear accelerating module, which, by employing laser-excited electromagnetic pulses directed along a helical path surrounding the laser-accelerated ion beams, addresses these shortcomings simultaneously. In a proof-of-principle experiment on a university-scale system, we demonstrate post-acceleration of laser-driven protons from a flat foil at a rate of 0.5 GeV/m, already beyond what can be sustained by conventional accelerator technologies, with dynamic beam collimation and energy selection. These results open up new opportunities for the development of extremely compact and cost-effective ion accelerators for both established and innovative applications.

B. Dromey, M. Coughlan, L. Senje, M. Taylor, S. Kuschel, B. Villagomez-Bernabe, R. Stefanuik, G. Nersisyan, L. Stella, J. Kohanoff, M. Borghesi, F. Currell, D. Riley, D. Jung, C.-G. Wahlström, C. L. S. Lewis, and M. Zepf
Picosecond metrology of laser-driven proton bursts
Nature Communications 7, 10642 (2016)

Abstract: Tracking primary radiation-induced processes in matter requires ultrafast sources and high precision timing. While compact laser-driven ion accelerators are seeding the development of novel high instantaneous flux applications, combining the ultrashort ion and laser pulse durations with their inherent synchronicity to trace the real-time evolution of initial damage events has yet to be realized. Here we report on the absolute measurement of proton bursts as short as 3.5±0.7 ps from laser solid target interactions for this purpose. Our results verify that laser-driven ion acceleration can deliver interaction times over a factor of hundred shorter than those of state-of-the-art accelerators optimized for high instantaneous flux. Furthermore, these observations draw ion interaction physics into the field of ultrafast science, opening the opportunity for quantitative comparison with both numerical modelling and the adjacent fields of ultrafast electron and photon interactions in matter.

J. W. Wang, W. Yu, M. Y. Yu, H. Xu, J. J. Ju, S. X. Luan, M. Murakami, M. Zepf, and S. Rykovanov
High-energy-density electron beam from interaction of two successive laser pulses with subcritical-density plasma
Physical Review Accelerators and Beams 19, 021301 (2016)

Abstract: It is shown by particle-in-cell simulations that a narrow electron beam with high energy and charge density can be generated in a subcritical-density plasma by two consecutive laser pulses. Although the first laser pulse dissipates rapidly, the second pulse can propagate for a long distance in the thin wake channel created by the first pulse and can further accelerate the preaccelerated electrons therein. Given that the second pulse also self-focuses, the resulting electron beam has a narrow waist and high charge and energy densities. Such beams are useful for enhancing the target-back space-charge field in target normal sheath acceleration of ions and bremsstrahlung sources, among others.

S. Cousens, B. Reville, B. Dromey, and M. Zepf
Temporal Structure of Attosecond Pulses from Laser-Driven Coherent Synchrotron Emission
Physical Review Letters 116, 083901 (2016)

Abstract: The microscopic dynamics of laser-driven coherent synchrotron emission transmitted through thin foils are investigated using particle-in-cell simulations. For normal incidence interactions, we identify the formation of two distinct electron nanobunches from which emission takes place each half-cycle of the driving laser pulse. These emissions are separated temporally by 130 as and are dominant in different frequency ranges, which is a direct consequence of the distinct characteristics of each electron nanobunch. This may be exploited through spectral filtering to isolate these emissions, generating electromagnetic pulses of duration ~70 as.

I. C. E. Turcu, F. Negoita, D. A. Jaroszynski, P. Mckenna, S. Balascuta, D. Ursescu, I. Dancus, M. O. Cernaianu, M. V. Tataru, P. Ghenuche, D. Stutman, A. Boianu, M. Risca, M. Toma, C. Petcu, G. Acbas, S. R. Yoffe, A. Noble, B. Ersfeld, E. Brunetti, R. Capdessus, C. Murphy, C. P. Ridgers, D. Neely, S. P. D. Mangles, R. J. Gray, A. G. R. Thomas, J. G. Kirk, A. Ilderton, M. Marklund, D. F. Gordon, B. Hafizi, D. Kaganovich, J. P. Palastro, E. D'Humieres, M. Zepf, G. Sarri, H. Gies, F. Karbstein, J. Schreiber, G. G. Paulus, B. Dromey, C. Harvey, A. Di Piazza, C. H. Keitel, M. C. Kaluza, S. Gales, and N. V. Zamfir
High Field Physics and QED Experiments at ELI-NP
Romanian Reports In Physics 68, S145 (2016)

Abstract: ELI-NP facility will enable for the first time the use of two 10 PW laser beams for quantum electrodynamics (QED) experiments. The first beam will accelerate electrons to relativistic energies. The second beam will subject relativistic electrons to the strong electromagnetic field generating QED processes: intense gamma ray radiation and electron-positron pair formation. The laser beams will be focused to intensities above 10^21 W/cm^2 and reaching 10^22–10^23 W/cm^2 for the first time. We propose to use this capability to investigate new physical phenomena at the interfaces of plasma, nuclear and particle physics at ELI-NP. This High Power Laser System Technical Design Report (HPLS-TDR2) presents the experimental area E6 at ELI-NP for investigating high field physics and quantum electrodynamics and the production of electron-positron-pairs and of energetic gamma-rays. The scientific community submitted 12 commissioning runs for E6 interaction chamber with two 10 PW laser beams and one proposal for the CETAL interaction chamber with 1 PW laser. The proposals are representative of the international high field physics community being written by 48 authors from 14 European and US organizations. The proposals are classified according to the science area investigated into: Radiation Reaction Physics: Classical and Quantum; Compton and Thomson Scattering Physics: Linear and Non Linear Regimes; QED in Vacuum; Atoms in Extreme Fields. Two pump-probe colliding 10 PW laser beams are proposed for the E6 interaction chamber. The focused pump laser beam accelerates the electrons to relativistic energies. The accelerated electron bunches interact with the very high electro-magnetic field of the focused probe laser beam. We propose two main types of experiments with: (a) gas targets in which the pump laser-beam is focused by a long focal length mirror and drives a wakefield in which the electron bunch is accelerated to multi-GeV energies and then exposed to the EM field of the probe laser which is tightly focused; (b) solid targets in which both the pump and probe laser beams are focused on the solid target, one accelerating the electrons in the solid and the other, delayed, providing the high electric field to which the relativistic electrons are subjected. We propose four main focusing configurations for the pump and probe laser beams, two for each type of target: counter-propagating 10 PW focused laser beams and the two 10 PW laser beams focused in the same direction. For solid targets we propose an additional configuration with plasma-mirror on the pump laser beam: the plasma mirror placed between the focusing mirror and target. It is proposed that the 10 PW laser beams will have polarization control and focus control by means of adaptive optics. Initially only one 10 PW may have polarization control and adaptive optics. In order to accommodate the two laser beams and diagnostics the proposed interaction chamber is quasi-octagonal with a diameter of 4.5 m. A large electron-spectrometer is proposed for multi-GeV electrons. Other diagnostics are requested for: gamma-rays, electrons and positrons, protons and ions, plasma characterization, transmitted and reflected laser beam. Targets will be provided by the ELI-NP Target Laboratory or purchased. The E6 experiments and diagnostics will benefit from the ELI-NP Electronics Laboratory, the Workshop and the Optics Laboratory. In order to ensure radiation-protection, a large beam-dump is planned for both multi-GeV electrons and multi-100 MeV protons.

## 2015

M. Hornung, G. Becker, A. Seidel, J. Reislöhner, H. Liebetrau, L. Bock, S. Keppler, A. Kessler, M. Zepf, J. Hein, and M. Kaluza
Generation of 25-TW Femtosecond Laser Pulses at 515 nm with Extremely High Temporal Contrast
Applied Sciences 5, 1970 (2015)

Abstract: We report on the frequency doubling of femtosecond laser pulses at 1030 nm center wavelength generated from the fully diode-pumped laser system POLARIS. The newly generated pulses at a center wavelength of 515 nm have a pulse energy of 3 J with a pulse duration of 120 fs. On the basis of initially ultra-high contrast seed pulses we expect a temporal intensity contrast better 10^17 200 ps before the peak of the main pulse. We analyzed the temporal intensity contrast from milliseconds to femtoseconds with a dynamic range covering more than 20 orders of magnitude. The pulses were focussed with a f/2-focussing parabola resulting in a peak intensity exceeding 10^20 W/cm2. The peak power and intensity are to the best of our knowledge the highest values for 515 nm-laser-pulses achieved so far.

D. Doria, S. Kar, H. Ahmed, A. Alejo, J. Fernandez, M. Cerchez, R. J. Gray, F. Hanton, D. A. MacLellan, P. McKenna, Z. Najmudin, D. Neely, L. Romagnani, J. A. Ruiz, G. Sarri, C. Scullion, M. Streeter, M. Swantusch, O. Willi, M. Zepf, and M. Borghesi
Calibration of BAS-TR image plate response to high energy (3-300 MeV) carbon ions
Review of Scientific Instruments 86, 123302 (2015)

Abstract: The paper presents the calibration of Fuji BAS-TR image plate (IP) response to high energy carbon ions of different charge states by employing an intense laser-driven ion source, which allowed access to carbon energies up to 270 MeV. The calibration method consists of employing a Thomson parabola spectrometer to separate and spectrally resolve different ion species, and a slotted CR-39 solid state detector overlayed onto an image plate for an absolute calibration of the IP signal. An empirical response function was obtained which can be reasonably extrapolated to higher ion energies. The experimental data also show that the IP response is independent of ion charge states.

H. X. Chang, B. Qiao, Z. Xu, X. R. Xu, C. T. Zhou, X. Q. Yan, S. Z. Wu, M. Borghesi, M. Zepf, and X. T. He
Generation of overdense and high-energy electron-positron-pair plasmas by irradiation of a thin foil with two ultraintense lasers
Physical Review E 92, 053107 (2015)

Abstract: A scheme for enhanced quantum electrodynamics (QED) production of electron-positron-pair plasmas is proposed that uses two ultraintense lasers irradiating a thin solid foil from opposite sides. In the scheme, under a proper matching condition, in addition to the skin-depth emission of γ-ray photons and Breit-Wheeler creation of pairs on each side of the foil, a large number of high-energy electrons and photons from one side can propagate through it and interact with the laser on the other side, leading to much enhanced γ-ray emission and pair production. More importantly, the created pairs can be collected later and confined to the center by opposite laser radiation pressures when the foil becomes transparent, resulting in the formation of unprecedentedly overdense and high-energy pair plasmas. Two-dimensional QED particle-in-cell simulations show that electron-positron-pair plasmas with overcritical density 10²² cm⁻³ and a high energy of 100s of MeV are obtained with 10 PW lasers at intensities 10²³ W/cm², which are of key significance for laboratory astrophysics studies.

M. Yeung, J. Bierbach, E. Eckner, S. Rykovanov, S. Kuschel, A. Sävert, M. Förster, C. Rödel, G. Paulus, S. Cousens, M. Coughlan, B. Dromey, and M. Zepf
Noncollinear Polarization Gating of Attosecond Pulse Trains in the Relativistic Regime
Physical Review Letters 115, 193903 (2015)

Abstract: High order harmonics generated at relativistic intensities have long been recognized as a route to the most powerful extreme ultraviolet pulses. Reliably generating isolated attosecond pulses requires gating to only a single dominant optical cycle, but techniques developed for lower power lasers have not been readily transferable. We present a novel method to temporally gate attosecond pulse trains by combining noncollinear and polarization gating. This scheme uses a split beam configuration which allows pulse gating to be implemented at the high beam fluence typical of multi-TW to PW class laser systems. Scalings for the gate width demonstrate that isolated attosecond pulses are possible even for modest pulse durations achievable for existing and planned future ultrashort high-power laser systems. Experimental results demonstrating the spectral effects of temporal gating on harmonic spectra generated by a relativistic laser plasma interaction are shown.

F. Karbstein, H. Gies, M. Reuter, and M. Zepf
Vacuum birefringence in strong inhomogeneous electromagnetic fields
Physical Review D 92, 071301 (2015)

Abstract: Birefringence is one of the fascinating properties of the vacuum of quantum electrodynamics (QED) in strong electromagnetic fields. The scattering of linearly polarized incident probe photons into a perpendicularly polarized mode provides a distinct signature of the optical activity of the quantum vacuum and thus offers an excellent opportunity for a precision test of nonlinear QED. Precision tests require accurate predictions and thus a theoretical framework that is capable of taking the detailed experimental geometry into account. We derive analytical solutions for vacuum birefringence which include the spatio-temporal field structure of a strong optical pump laser field and an x-ray probe. We show that the angular distribution of the scattered photons depends strongly on the interaction geometry and find that scattering of the perpendicularly polarized scattered photons out of the cone of the incident probe x-ray beam is the key to making the phenomenon experimentally accessible with the current generation of FEL/high-field laser facilities.

H. Y. Wang, X. Q. Yan, and M. Zepf
Signatures of quantum radiation reaction in laser-electron-beam collisions
Physics of Plasmas 22, 093103 (2015)

Abstract: Electron dynamics in the collision of an electron beam with a high-intensity focused ultrashort laser pulse are investigated using three-dimensional QED particle-in-cell (PIC) simulations, and the results are compared with those calculated by classical Landau and Lifshitz PIC simulations. Significant differences are observed from the angular dependence of the electron energy distribution patterns for the two different approaches, because photon emission is no longer well approximated by a continuous process in the quantum radiation-dominated regime. The stochastic nature of photon emission results in strong signatures of quantum radiation-reaction effects under certain conditions. We show that the laser spot size and duration greatly influence these signatures due to the competition of QED effects and the ponderomotive force, which is well described in the classical approximation. The clearest signatures of quantum radiation reaction are found in the limit of large laser spots and few cycle pulse durations.

G. Sarri, M. E. Dieckmann, I. Kourakis, A. Di Piazza, B Reville, C. H. Keitel, and M. Zepf
Overview of laser-driven generation of electron-positron beams
Journal of Plasma Physics 81, 23 (2015)

Abstract: Electron-positron (e-p) plasmas are widely thought to be emitted, in the form of ultra-relativistic winds or collimated jets, by some of the most energetic or powerful objects in the Universe, such as black-holes, pulsars, and quasars. These phenomena represent an unmatched astrophysical laboratory to test physics at its limit and, given their immense distance from Earth (some even farther than several billion light years), they also provide a unique window on the very early stages of our Universe. However, due to such gigantic distances, their properties are only inferred from the indirect interpretation of their radiative signatures and from matching numerical models: their generation mechanism and dynamics still pose complicated enigmas to the scientific community. Small-scale reproductions in the laboratory would represent a fundamental step towards a deeper understanding of this exotic state of matter. Here we present recent experimental results concerning the laser-driven production of ultra-relativistic e-p beams. In particular, we focus on the possibility of generating beams that present charge neutrality and that allow for collective effects in their dynamics, necessary ingredients for the testing pair-plasma physics in the laboratory. A brief discussion of the analytical and numerical modelling of the dynamics of these plasmas is also presented in order to provide a summary of the novel plasma physics that can be accessed with these objects. Finally, general considerations on the scalability of laboratory plasmas up to astrophysical scenarios are given.

J. H. Bin, W. J. Ma, H. Y. Wang, M. J. V. Streeter, C. Kreuzer, D. Kiefer, M. Yeung, S. Cousens, P. S. Foster, B. Dromey, X. Q. Yan, R. Ramis, J. Meyer-ter-Vehn, M. Zepf, and J. Schreiber
Ion Acceleration Using Relativistic Pulse Shaping in Near-Critical-Density Plasmas
Physical Review Letters 115, 064801 (2015)

Abstract: Ultraintense laser pulses with a few-cycle rising edge are ideally suited to accelerating ions from ultrathin foils, and achieving such pulses in practice represents a formidable challenge. We show that such pulses can be obtained using sufficiently strong and well-controlled relativistic nonlinearities in spatially well-defined near-critical-density plasmas. The resulting ultraintense pulses with an extremely steep rising edge give rise to significantly enhanced carbon ion energies consistent with a transition to radiation pressure acceleration.

S. R. Mirfayzi, S. Kar, H. Ahmed, A. G. Krygier, A. Green, A. Alejo, R. Clarke, R. R. Freeman, J. Fuchs, D. Jung, A. Kleinschmidt, J. T. Morrison, Z. Najmudin, H. Nakamura, P. Norreys, M. Oliver, M. Roth, L. Vassura, M. Zepf, and M. Borghesi
Calibration of time of flight detectors using laser-driven neutron source
Review of Scientific Instruments 86, 073308 (2015)

Abstract: Calibration of three scintillators (EJ232Q, BC422Q, and EJ410) in a time-of-flight arrangement using a laser drive-neutron source is presented. The three plastic scintillator detectors were calibrated with gamma insensitive bubble detector spectrometers, which were absolutely calibrated over a wide range of neutron energies ranging from sub-MeV to 20 MeV. A typical set of data obtained simultaneously by the detectors is shown, measuring the neutron spectrum emitted from a petawatt laser irradiated thin foil.

J. Bierbach, M. Yeung, E. Eckner, C. Roedel, S. Kuschel, M. Zepf, and G. G. Paulus
Long-term operation of surface high-harmonic generation from relativistic oscillating mirrors using a spooling tape
Optics Express 23, 12321 (2015)

Abstract: Surface high-harmonic generation in the relativistic regime is demonstrated as a source of extreme ultra-violet (XUV) pulses with extended operation time. Relativistic high-harmonic generation is driven by a frequency-doubled high-power Ti:Sapphire laser focused to a peak intensity of 3·10^19 W/cm2 onto spooling tapes. We demonstrate continuous operation over up to one hour runtime at a repetition rate of 1 Hz. Harmonic spectra ranging from 20 eV to 70 eV (62 nm to 18 nm) were consecutively recorded by an XUV spectrometer. An average XUV pulse energy in the µJ range is measured. With the presented setup, relativistic surface high-harmonic generation becomes a powerful source of coherent XUV pulses that might enable applications in, e.g. attosecond laser physics and the seeding of free-electron lasers, when the laser issues causing 80-% pulse energy fluctuations are overcome.

A. G. Krygier, J. T. Morrison, S. Kar, H. Ahmed, A. Alejo, R. Clarke, J. Fuchs, A. Green, D. Jung, A. Kleinschmidt, Z. Najmudin, H. Nakamura, P. Norreys, M. Notley, M. Oliver, M. Roth, L. Vassura, M. Zepf, M. Borghesi, and R. R. Freeman
Selective deuterium ion acceleration using the Vulcan petawatt laser
Physics of Plasmas 22, 053102 (2015)

Abstract: We report on the successful demonstration of selective acceleration of deuterium ions by target-normal sheath acceleration (TNSA) with a high-energy petawatt laser. TNSA typically produces a multi-species ion beam that originates from the intrinsic hydrocarbon and water vaporcontaminants on the target surface. Using the method first developed by Morrison et al., an ion beam with >99% deuterium ions and peak energy 14 MeV/nucleon is produced with a 200 J, 700 fs, >10²⁰ W/cm² laser pulse by cryogenically freezing heavy water (D₂O) vapor onto the rear surface of the target prior to the shot. Within the range of our detectors (0°-8.5°), we find laser-to-deuterium-ion energy conversion efficiency of 4.3% above 0.7 MeV/nucleon while a conservative estimate of the total beam gives a conversion efficiency of 9.4%.

H. Y. Wang, B. Liu, X. Q. Yan, and M. Zepf
Gamma-ray emission in near critical density plasmas at laser intensities of 10^21 W/cm2
Physics of Plasmas 22, 033102 (2015)

Abstract: We study synchrotron radiation emission from laser interaction with near critical density (NCD) plasmas at intensities of 10²¹ W/cm² using three-dimensional particle-in-cell simulations. It is found that the electron dynamics depend on the laser shaping process in NCD plasmas, and thus the angular distribution of the emitted photons changes as the laser pulse evolves in space and time. The final properties of the resulting synchrotron radiation, such as its overall energy, the critical photon energy, and the radiation angular distribution, are strongly affected by the laser polarization and plasma density. By using a 420 TW/50 fs laser pulse at the optimal plasma density (~1 nc), about 10⁸ photons/0.1% bandwidth are produced at multi-MeV photon energies, providing a route to ultraintense, femtosecond gamma ray pulses.

T. Hahn, J. Bierbach, C. Rödel, D. Hemmers, M. Yeung, B. Dromey, S. Fuchs, A. Galestian, S. Kuschel, M. Zepf, G. Paulus, and G. Pretzler
Broadband XUV polarimetry of high harmonics from plasma surfaces using multiple Fresnel reflections
Applied Physics B 118, 241 (2015)

Abstract: High-harmonic generation (HHG) by nonlinear interaction of intense laser pulses with gases or plasma surfaces is the most prominent way of creating highly coherent extreme ultraviolet (EUV/XUV) pulses. In the last years, several scientific applications have been found which require the measurement of the polarization of the harmonic radiation. We present a broadband XUV polarimeter based on multiple Fresnel reflections providing an extinction rate of 5–25 for 17–45 nm which is particularly suited for surface harmonics. The device has first been tested at a gas harmonic source providing linearly polarized XUV radiation. In a further experiment using HHG from plasma surfaces, the XUV polarimeter allowed a polarization measurement of high harmonic radiation from plasma surfaces for the first time which reveals a linear polarization state as predicted for our generation parameters. The generation and control of intense polarized XUV pulses - together with the availability of broadband polarizers in the XUV - open the way for a series of new experiments. For instance, dichroism in the XUV, elliptically polarized harmonics from aligned molecules, or the selection rules of relativistic surface harmonics can be studied with the broadband XUV polarimeter.

H. Y. Wang, X. Q. Yan, and M. Zepf
High-energy monoenergetic proton beams from two stage acceleration with a slow laser pulse
Physical Review ST - Accelerators and Beams 18, 021302 (2015)

Abstract: We present a new regime to generate high-energy quasimonoenergetic proton beams in a "slow-pulse" regime, where the laser group velocity vg<c is reduced by an extended near-critical density plasma. In this regime, for properly matched laser intensity and group velocity, ions initially accelerated by the light sail (LS) mode can be further trapped and reflected by the snowplough potential generated by the laser in the near-critical density plasma. These two acceleration stages are connected by the onset of Rayleigh-Taylor-like (RT) instability. The usual ion energy spectrum broadening by RT instability is controlled and high quality proton beams can be generated. It is shown by multidimensional particle-in-cell simulation that quasimonoenergetic proton beams with energy up to hundreds of MeV can be generated at laser intensities of 10²¹ W/cm².

## 2014

W. Ma, J. Bin, H. Wang, M. Yeung, C. Kreuzer, M. Streeter, P. Foster, S. Cousens, D. Kiefer, B. Dromey, X. Yan, J. Meyer-ter-Vehn, M. Zepf, and J. Schreiber
Bright Subcycle Extreme Ultraviolet Bursts from a Single Dense Relativistic Electron Sheet
Physical Review Letters 113, 235002 (2014)

Abstract: Double-foil targets separated by a low density plasma and irradiated by a petawatt-class laser are shown to be a copious source of coherent broadband radiation. Simulations show that a dense sheet of relativistic electrons is formed during the interaction of the laser with the tenuous plasma between the two foils. The coherent motion of the electron sheet as it transits the second foil results in strong broadband emission in the extreme ultraviolet, consistent with our experimental observations.

G. Sarri, D. Corvan, W. Schumaker, J. Cole, A. Di Piazza, H. Ahmed, C. Harvey, C. Keitel, K. Krushelnick, S. Mangles, Z. Najmudin, D. Symes, A. Thomas, M. Yeung, Z. Zhao, and M. Zepf
Ultrahigh Brilliance Multi-MeV γ-Ray Beams from Nonlinear Relativistic Thomson Scattering
Physical Review Letters 113, 224801 (2014)

Abstract: We report on the generation of a narrow divergence (θγ<2.5  mrad), multi-MeV (Emax≈18  MeV) and ultrahigh peak brilliance (>1.8×10^20  photons s^−1 mm^−2  mrad^−2 0.1% BW) γ-ray beam from the scattering of an ultrarelativistic laser-wakefield accelerated electron beam in the field of a relativistically intense laser (dimensionless amplitude a0≈2). The spectrum of the generated γ-ray beam is measured, with MeV resolution, seamlessly from 6 to 18 MeV, giving clear evidence of the onset of nonlinear relativistic Thomson scattering. To the best of our knowledge, this photon source has the highest peak brilliance in the multi-MeV regime ever reported in the literature.

L. Senje, M. Yeung, B. Aurand, S. Kuschel, C. Rödel, F. Wagner, K. Li, B. Dromey, V. Bagnoud, P. Neumayer, M. Roth, C.-G. Wahlström, M. Zepf, T. Kuehl, and D. Jung
Diagnostics for studies of novel laser ion acceleration mechanisms
Review of Scientific Instruments 85, 113302 (2014)

Abstract: Diagnostic for investigating and distinguishing different laser ion acceleration mechanisms has been developed and successfully tested. An ion separation wide angle spectrometer can simultaneously investigate three important aspects of the laser plasma interaction: (1) acquire angularly resolved energy spectra for two ion species, (2) obtain ion energy spectra for multiple species, separated according to their charge to mass ratio, along selected axes, and (3) collect laser radiation reflected from and transmitted through the target and propagating in the same direction as the ion beam. Thus, the presented diagnostic constitutes a highly adaptable tool for accurately studying novel acceleration mechanisms in terms of their angular energy distribution, conversion efficiency, and plasma density evolution.

A. Alejo, S. Kar, H. Ahmed, A. G. Krygier, D. Doria, R. Clarke, J. Fernandez, R. R. Freeman, J. Fuchs, A. Green, J. S. Green, D. Jung, A. Kleinschmidt, C. L. S. Lewis, J. T. Morrison, Z. Najmudin, H. Nakamura, G. Nersisyan, P. Norreys, M. Notley, M. Oliver, M. Roth, J. A. Ruiz, L. Vassura, M. Zepf, and M. Borghesi
Characterisation of deuterium spectra from laser driven multi-species sources by employing differentially filtered image plate detectors in Thomson spectrometers
Review of Scientific Instruments 85, 093303 (2014)

Abstract: A novel method for characterising the full spectrum of deuteron ions emitted by laser driven multi-species ion sources is discussed. The procedure is based on using differential filtering over the detector of a Thompson parabola ion spectrometer, which enables discrimination of deuterium ions from heavier ion species with the same charge-to-mass ratio (such as C^(6+), O^(8+), etc.). Commonly used Fuji Image plates were used as detectors in the spectrometer, whose absolute response to deuterium ions over a wide range of energies was calibrated by using slotted CR-39 nuclear track detectors. A typical deuterium ion spectrum diagnosed in a recent experimental campaign is presented, which was produced from a thin deuterated plastic foil target irradiated by a high power laser.

H. Y. Wang, X. Q. Yan, and M. Zepf
Collimated proton acceleration in light sail regime with a tailored pinhole target
Physics of Plasmas 21, 063113 (2014)

Abstract: A scheme for producing collimated protons from laser interactions with a diamond-like-carbon + pinhole target is proposed. The process is based on radiation pressure acceleration in the multi-species light-sail regime [B. Qiao et al., Phys. Rev. Lett. 105, 155002 (2010); T. P. Yu et al., Phys. Rev. Lett. 105, 065002 (2010)]. Particle-in-cell simulations demonstrate that transverse quasistatic electric field at TV/m level can be generated in the pinhole. The transverse electric field suppresses the transverse expansion of protons effectively, resulting in a higher density and more collimated proton beam compared with a single foil target. The dependence of the proton beam divergence on the parameters of the pinhole is also investigated.

D. J. Corvan, G. Sarri, and M. Zepf
Design of a compact spectrometer for high-flux MeV gamma-ray beams
Review of Scientific Instruments 85, 6 (2014)

Abstract: A novel design for a compact gamma-ray spectrometer is presented. The proposed system allows for spectroscopy of high-flux multi-MeV gamma-ray beams with MeV energy resolution in a compact design. In its basic configuration, the spectrometer exploits conversion of gamma-rays into electrons via Compton scattering in a low-Z material. The scattered electron population is then spectrally resolved using a magnetic spectrometer. The detector is shown to be effective for gamma-ray energies between 3 and 20 MeV. The main properties of the spectrometer are confirmed by Monte Carlo simulations.

M. Yeung, B. Dromey, S. Cousens, T. Dzelzainis, D. Kiefer, J. Schreiber, J. Bin, W. Ma, C. Kreuzer, J. Meyer-ter-Vehn, M. Streeter, P. Foster, S. Rykovanov, and M. Zepf
Dependence of Laser-Driven Coherent Synchrotron Emission Efficiency on Pulse Ellipticity and Implications for Polarization Gating
Physical Review Letters 112, 123902 (2014)

Abstract: The polarization dependence of laser-driven coherent synchrotron emission transmitted through thin foils is investigated experimentally. The harmonic generation process is seen to be almost completely suppressed for circular polarization opening up the possibility of producing isolated attosecond pulses via polarization gating. Particle-in-cell simulations suggest that current laser pulses are capable of generating isolated attosecond pulses with high pulse energies.

D. Gwynne, S. Kar, D. Doria, H. Ahmed, M. Cerchez, J. Fernandez, R. J. Gray, J. S. Green, F. Hanton, D. A. MacLellan, P. McKenna, Z. Najmudin, D. Neely, J. A. Ruiz, A. Schiavi, M. Streeter, M. Swantusch, O. Willi, M. Zepf, and M. Borghesi
Modified Thomson spectrometer design for high energy, multi-species ion sources
Review of Scientific Instruments 85, 033304 (2014)

Abstract: A modification to the standard Thomson parabola spectrometer is discussed, which is designed to measure high energy (tens of MeV/nucleon), broad bandwidth spectra of multi-species ions accelerated by intense laser plasma interactions. It is proposed to implement a pair of extended, trapezoidal shaped electric plates, which will not only resolve ion traces at high energies, but will also retain the lower energy part of the spectrum. While a longer (along the axis of the undeflected ion beam direction) electric plate design provides effective charge state separation at the high energy end of the spectrum, the proposed new trapezoidal shape will enable the low energy ions to reach the detector, which would have been clipped or blocked by simply extending the rectangular plates to enhance the electrostatic deflection.

H. Y. Wang, C. Lin, B. Liu, Z. M. Sheng, H. Y. Lu, W. J. Ma, J. H. Bin, J. Schreiber, X. T. He, J. E. Chen, M. Zepf, and X. Yan
Laser-driven three-stage heavy-ion acceleration from relativistic laser-plasma interaction
Physical Review E 89, 013107 (2014)

Abstract: A three-stage heavy ion acceleration scheme for generation of high-energy quasimonoenergetic heavy ion beams is investigated using two-dimensional particle-in-cell simulation and analytical modeling. The scheme is based on the interaction of an intense linearly polarized laser pulse with a compound two-layer target (a front heavy ion layer + a second light ion layer). We identify that, under appropriate conditions, the heavy ions preaccelerated by a two-stage acceleration process in the front layer can be injected into the light ion shock wave in the second layer for a further third-stage acceleration. These injected heavy ions are not influenced by the screening effect from the light ions, and an isolated high-energy heavy ion beam with relatively low-energy spread is thus formed. Two-dimensional particle-in-cell simulations show that ∼100 MeV/u quasimonoenergetic Fe^24+ beams can be obtained by linearly polarized laser pulses at intensities of 1.1×10^21W/cm2.

## 2013

S. Kar, K. F. Kakolee, M. Cerchez, D. Doria, A. Macchi, P. McKenna, D. Neely, J. Osterholz, K. Quinn, B. Ramakrishna, G. Sarri, O. Willi, X. H. Yuan, M. Zepf, and M. Borghesi
Experimental investigation of hole boring and light sail regimes of RPA by varying laser and target parameters
Plasma Physics and Controlled Fusion 55, 124030 (2013)

Abstract: Temporal evolution of plasma jets from micrometre-scale thick foils following the interaction of intense (3 × 10 20 W cm^−2 ) laser pulses is studied systematically by time resolved optical interferometry. The fluid velocity in the plasma jets is determined by comparing the data with 2D hydrodynamic simulation, which agrees with the expected hole-boring (HB) velocity due to the laser radiation pressure. The homogeneity of the plasma density across the jets has been found to be improved substantially when irradiating the laser at circular polarization compared to linear polarization. While overdense plasma jets were formed efficiently for micrometre thick targets, decreasing the target areal density and/or increasing the irradiance on the target have provided indication of transition from the ‘HB’ to the ‘light sail (LS)’ regime of RPA, characterized by the appearance of narrow-band spectral features at several MeV/nucleon in proton and carbon spectra.

G. Sarri, W. Schumaker, A. D. Piazza, K. Poder, J. M. Cole, M. Vargas, D. Doria, S. Kushel, B. Dromey, G. Grittani, L. Gizzi, M. E. Dieckmann, A. Green, V. Chvykov, A. Maksimchuk, V. Yanovsky, Z. H. He, B. X. Hou, J. A. Nees, S. Kar, Z. Najmudin, A. G. R. Thomas, C. H. Keitel, K. Krushelnick, and M. Zepf
Laser-driven generation of collimated ultra-relativistic positron beams
Plasma Physics and Controlled Fusion 55, 124017 (2013)

Abstract: We report on recent experimental results concerning the generation of collimated (divergence of the order of a few mrad) ultra-relativistic positron beams using a fully optical system. The positron beams are generated exploiting a quantum-electrodynamic cascade initiated by the propagation of a laser-accelerated, ultra-relativistic electron beam through high- Z solid targets. As long as the target thickness is comparable to or smaller than the radiation length of the material, the divergence of the escaping positron beam is of the order of the inverse of its Lorentz factor. For thicker solid targets the divergence is seen to gradually increase, due to the increased number of fundamental steps in the cascade, but it is still kept of the order of few tens of mrad, depending on the spectral components in the beam. This high degree of collimation will be fundamental for further injection into plasma-wakefield afterburners.

G. Sarri, W. Schumaker, A. Di Piazza, M. Vargas, B. Dromey, M. E. Dieckmann, V. Chvykov, A. Maksimchuk, V. Yanovsky, Z. H. He, B. X. Hou, J. A. Nees, A. G. R. Thomas, C. H. Keitel, M. Zepf, and K. Krushelnick
Table-Top Laser-Based Source of Femtosecond, Collimated, Ultrarelativistic Positron Beams
Physical Review Letters 110, 255002 (2013)

Abstract: The generation of ultrarelativistic positron beams with short duration (τ_e+≃30  fs), small divergence (θ_e+≃3  mrad), and high density (n_e+≃10^14–10^15  cm−3) from a fully optical setup is reported. The detected positron beam propagates with a high-density electron beam and γ rays of similar spectral shape and peak energy, thus closely resembling the structure of an astrophysical leptonic jet. It is envisaged that this experimental evidence, besides the intrinsic relevance to laser-driven particle acceleration, may open the pathway for the small-scale study of astrophysical leptonic jets in the laboratory.

D. Kiefer, M. Yeung, T. Dzelzainis, P. S. Foster, S. G. Rykovanov, C. L. S. Lewis, R. S. Marjoribanks, H. Ruhl, D. Habs, J. Schreiber, M. Zepf, and B. Dromey
Relativistic electron mirrors from nanoscale foils for coherent frequency upshift to the extreme ultraviolet
Nature Communications 4, 1763 (2013)

Abstract: Reflecting light from a mirror moving close to the speed of light has been envisioned as a route towards producing bright X-ray pulses since Einstein’s seminal work on special relativity. For an ideal relativistic mirror, the peak power of the reflected radiation can substantially exceed that of the incident radiation due to the increase in photon energy and accompanying temporal compression. Here we demonstrate for the first time that dense relativistic electron mirrors can be created from the interaction of a high-intensity laser pulse with a freestanding, nanometre-scale thin foil. The mirror structures are shown to shift the frequency of a counter-propagating laser pulse coherently from the infrared to the extreme ultraviolet with an efficiency > 10^4 times higher than in the case of incoherent scattering. Our results elucidate the reflection process of laser-generated electron mirrors and give clear guidance for future developments of a relativistic mirror structure.

M. Yeung, B. Dromey, D. Adams, S. Cousens, R. Hörlein, Y. Nomura, G. D. Tsakiris, and M. Zepf
Beaming of High-Order Harmonics Generated from Laser-Plasma Interactions
Physical Review Letters 110, 16 (2013)

Abstract: Beam divergences of high-order extreme ultraviolet harmonics from intense laser interactions with steep plasma density gradients are studied through experiment and Fourier analysis of the harmonic spatial phase. We show that while emission due to the relativistically oscillating mirror mechanism can be explained by ponderomotive surface denting, in agreement with previous results, the divergence of the emission due to the coherent wake emission mechanism requires a combination of the dent phase and an intrinsic emission phase. The temporal dependence of the divergences for both mechanisms is highlighted while it is also shown that the coherent wake emission divergence can be small in circumstances where the phase terms compensate each other.

S. Kahaly, S. Monchocé, H. Vincenti, T. Dzelzainis, B. Dromey, M. Zepf, Ph. Martin, and F. Quéré
Direct Observation of Density-Gradient Effects in Harmonic Generation from Plasma Mirrors
Physical Review Letters 110, 175001 (2013)

Abstract: High-order harmonics and attosecond pulses of light can be generated when ultraintense, ultrashort laser pulses reflect off a solid-density plasma with a sharp vacuum interface, i.e., a plasma mirror. We demonstrate experimentally the key influence of the steepness of the plasma-vacuum interface on the interaction, by measuring the spectral and spatial properties of harmonics generated on a plasma mirror whose initial density gradient scale length L is continuously varied. Time-resolved interferometry is used to separately measure this scale length.

M. Yeung, B. Dromey, C. Rödel, J. Bierbach, M. Wünsche, G. Paulus, T. Hahn, D. Hemmers, C. Stelzmann, G. Pretzler, and M. Zepf
Near-monochromatic high-harmonic radiation from relativistic laser-plasma interactions with blazed grating surfaces
New Journal of Physics 15, 025042 (2013)

Abstract: Intense, femtosecond laser interactions with blazed grating targets are studied through experiment and particle-in-cell (PIC) simulations. The high harmonic spectrum produced by the laser is angularly dispersed by the grating leading to near-monochromatic spectra emitted at different angles, each dominated by a single harmonic and its integer-multiples. The spectrum emitted in the direction of the third-harmonic diffraction order is measured to contain distinct peaks at the 9th and 12th harmonics which agree well with two-dimensional PIC simulations using the same grating geometry. This confirms that surface smoothing effects do not dominate the far-field distributions for surface features with sizes on the order of the grating grooves whilst also showing this to be a viable method of producing near-monochromatic, short-pulsed extreme-ultraviolet radiation.

## 2012

B. Dromey, S. Rykovanov, M. Yeung, R. Hörlein, D. Jung, D. C. Gautier, T. Dzelzainis, D. Kiefer, S. Palaniyppan, R. Shah, J. Schreiber, H. Ruhl, J. C. Fernandez, C. L. S. Lewis, M. Zepf, and B. M. Hegelich
Coherent synchrotron emission from electron nanobunches formed in relativistic laser-plasma interactions
Nature Physics 8, 804 (2012)

Abstract: Extreme ultraviolet (XUV) and X-ray harmonic spectra produced by intense laser - solid interactions have, so far, been consistent with Doppler upshifted reflection from collective relativistic plasma oscillations - the relativistically oscillating mirror mechanism. Recent theoretical work, however, has identified a new interaction regime in which dense electron nanobunches are formed at the plasma–vacuum boundary resulting in coherent XUV radiation by coherent synchrotron emission (CSE). Our experiments enable the isolation of CSE from competing processes, demonstrating that electron nanobunch formation does indeed occur. We observe spectra with the characteristic spectral signature of CSE - a slow decay of intensity, I, with high-harmonic order, n, as I(n) ~ n^(−1.62) before a rapid efficiency rollover. Particle-in-cell code simulations reveal how den se nanobunches of electrons are periodically formed and accelerated during normal-incidence interactions with ultrathin foils and result in CSE in the transmitted direction. This observation of CSE presents a route to high-energy XUV pulses and offers a new window on understanding ultrafast energy coupling during intense laser - solid density interactions.

S. Kar, K. F. Kakolee, B. Qiao, A. Macchi, M. Cerchez, D. Doria, M. Geissler, P. McKenna, D. Neely, J. Osterholz, R. Prasad, K. Quinn, B. Ramakrishna, G. Sarri, O. Willi, X. Y. Yuan, M. Zepf, and M. Borghesi
Ion Acceleration in Multispecies Targets Driven by Intense Laser Radiation Pressure
Physical Review Letters 109, 185006 (2012)

Abstract: The acceleration of ions from ultrathin foils has been investigated by using 250 TW, subpicosecond laser pulses, focused to intensities of up to 3 × 10^(20)  W cm^(-2). The ion spectra show the appearance of narrow-band features for protons and carbon ions peaked at higher energies (in the 5 - 10  MeV/nucleon range) and with significantly higher flux than previously reported. The spectral features and their scaling with laser and target parameters provide evidence of a multispecies scenario of radiation pressure acceleration in the light sail mode, as confirmed by analytical estimates and 2D particle-in-cell simulations. The scaling indicates that monoenergetic peaks with more than 100  MeV/nucleon are obtainable with moderate improvements of the target and laser characteristics, which are within reach of ongoing technical developments.

C. Rödel, D. an der Brügge, J. Bierbach, M. Yeung, T. Hahn, B. Dromey, S. Herzer, S. Fuchs, A. Pour, E. Eckner, M. Behmke, M. Cerchez, O. Jäckel, D. Hemmers, T. Toncian, M. C. Kaluza, A. Belyanin, G. Pretzler, O. Willi, A. Pukhov, M. Zepf, and G. G. Paulus
Harmonic Generation from Relativistic Plasma Surfaces in Ultrasteep Plasma Density Gradients
Physical Review Letters 109, 125002 (2012)

Abstract: Harmonic generation in the limit of ultrasteep density gradients is studied experimentally. Observations reveal that, while the efficient generation of high order harmonics from relativistic surfaces requires steep plasma density scale lengths (L_p/λ < 1), the absolute efficiency of the harmonics declines for the steepest plasma density scale length L_p → 0, thus demonstrating that near-steplike density gradients can be achieved for interactions using high-contrast high-intensity laser pulses. Absolute photon yields are obtained using a calibrated detection system. The efficiency of harmonics reflected from the laser driven plasma surface via the relativistic oscillating mirror was estimated to be in the range of 10^{-4} – 10^{-6} of the laser pulse energy for photon energies ranging from 20 – 40 eV, with the best results being obtained for an intermediate density scale length.

A. Willner, A. Hage, R. Riedel, I. Grguraš, A. Simoncig, M. Schulz, T. Dzelzainis, H. Höppner, S. Huber, M. J. Prandolini, B. Dromey, M. Zepf, A. L. Cavalieri, and F. Tavella
Coherent spectral enhancement of carrier-envelope-phase stable continua with dual-gas high harmonic generation
Optics Letters 37, 3672 (2012)

Abstract: Attosecond science is enabled by the ability to convert femtosecond near-infrared laser light into coherent harmonics in the extreme ultraviolet spectral range. While attosecond sources have been utilized in experiments that have not demanded high intensities, substantially higher photon flux would provide a natural link to the next significant experimental breakthrough. Numerical simulations of dual-gas high harmonic generation indicate that the output in the cutoff spectral region can be selectively enhanced without disturbing the single-atom gating mechanism. Here, we summarize the results of these simulations and present first experimental findings to support these predictions.

J. Bierbach, C. Rödel, M. Yeung, B. Dromey, T. Hahn, A. Pour, S. Fuchs, A. E. Paz, S. Herzer, S. Kuschel, O. Jäckel, M. C. Kaluza, G. Pretzler, M. Zepf, and G. G. Paulus
Generation of 10 µW relativistic surface high-harmonic radiation at a repetition rate of 10 Hz
New Journal of Physics 14, 065005 (2012)

Abstract: Experimental results on relativistic surface HHG at a repetition rate of 10 Hz are presented. Average powers in the 10 μW range are generated in the spectral range of 51 to 26 nm (24 - 48 eV). The surface harmonic radiation is produced by focusing the second-harmonic of a high-power laser onto a rotating glass surface to moderately relativistic intensities of 3 × 10^{19} W cm^{−2}. The harmonic emission exhibits a divergence of 26 mrad. Together with absolute photon numbers recorded by a calibrated spectrometer, this allows for the determination of the extreme ultraviolet (XUV) yield. The pulse energies of individual harmonics are reaching up to the μJ level, equivalent to an efficiency of 10^{−5}. The capability of producing stable and intense high-harmonic radiation from relativistic surface plasmas may facilitate experiments on nonlinear ionization or the seeding of free-electron lasers.

## 2011

A. Willner, F. Tavella, M. Yeung, T. Dzelzainis, C. Kamperidis, M. Bakarezos, D. Adams, R. Riedel, M. Schulz, M. C. Hoffmann, W. Hu, J. Rossbach, M. Drescher, V. S. Yakovlev, N. A. Papadogiannis, M. Tatarakis, B. Dromey, and M. Zepf
Efficient control of quantum paths via dual-gas high harmonic generation
New Journal of Physics 13, 113001 (2011)

Abstract: The accurate control of the relative phase of multiple distinct sources of radiation produced by high harmonic generation is of central importance in the continued development of coherent extreme UV (XUV) and attosecond sources. Here, we present a novel approach which allows extremely accurate phase control between multiple sources of high harmonic radiation generated within the Rayleigh range of a single-femtosecond laser pulse using a dual-gas, multi-jet array. Fully ionized hydrogen acts as a purely passive medium and allows highly accurate control of the relative phase between each harmonic source. Consequently, this method allows quantum path selection and rapid signal growth via the full coherent superposition of multiple HHG sources (the so-called quasi-phase-matching). Numerical simulations elucidate the complex interplay between the distinct quantum paths observed in our proof-of-principle experiments.

A. Willner, F. Tavella, M. Yeung, T. Dzelzainis, C. Kamperidis, M. Bakarezos, D. Adams, M. Schulz, R. Riedel, M. C. Hoffmann, W. Hu, J. Rossbach, M. Drescher, N. A. Papadogiannis, M. Tatarakis, B. Dromey, and M. Zepf
Coherent Control of High Harmonic Generation via Dual-Gas Multijet Arrays
Physical Review Letters 107, 175002 (2011)

Abstract: High harmonic generation (HHG) is a central driver of the rapidly growing field of ultrafast science. We present a novel quasiphase-matching (QPM) concept with a dual-gas multijet target leading, for the first time, to remarkable phase control between multiple HHG sources (>2) within the Rayleigh range. The alternating jet structure with driving and matching zones shows perfect coherent buildup for up to six QPM periods. Although not in the focus of the proof-of-principle studies presented here, we achieved competitive conversion efficiencies already in this early stage of development.

## 2010

C. Bellei, S. R. Nagel, S. Kar, A. Henig, S. Kneip, C. Palmer, A. Sävert, L. Willingale, D. Carroll, B. Dromey, J. S. Green, K. Markey, P. Simpson, R. J. Clarke, H. Lowe, D. Neely, C. Spindloe, M. Tolley, M. C. Kaluza, S. P. D. Mangles, P. McKenna, P. A. Norreys, J. Schreiber, M. Zepf, J. R. Davies, K. Krushelnick, and Z. Najmudin
Micron-scale fast electron filaments and recirculation determined from rear-side optical emission in high-intensity laser-solid interactions
New Journal of Physics 12, 073016 (2010)

Abstract: The transport of relativistic electrons generated in the interaction of petawatt class lasers with solid targets has been studied through measurements of the second harmonic optical emission from their rear surface. The high degree of polarization of the emission indicates that it is predominantly optical transition radiation (TR). A halo that surrounds the main region of emission is also polarized and is attributed to the effect of electron recirculation. The variation of the polarization state and intensity of radiation with the angle of observation indicates that the emission of TR is highly directional and provides evidence for the presence of mu m-size filaments. A brief discussion on the possible causes of such a fine electron beam structure is given.