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- ItemX-ray emission from stainless steel foils irradiated by femtosecond petawatt laser pulses(Bristol : IOP Publ., 2018) Alkhimova, M.A.; Faenov, A.Ya.; Pikuz, T.A.; Skobelev, I.Yu.; Pikuz, S.A.; Nishiuchi, M.; Sakaki, H.; Pirozhkov, A.S.; Sagisaka, S.; Dover, N.P.; Kondo, Ko.; Ogura, K.; Fukuda, Y.; Kiriyama, H.; Esirkepov, T.; Bulanov, S V.; Andreev, A.; Kando, M.; Zhidkov, A.; Nishitani, K.; Miyahara, T.; Watanabe, Y.; Kodama, R.; Kondo, K.We report about nonlinear growth of x-ray emission intensity emitted from plasma generated by femtosecond petawatt laser pulses irradiating stainless steel foils. X-ray emission intensity increases as ∼ I 4.5 with laser intensity I on a target. High spectrally resolved x-ray emission from front and rear surfaces of 5 μm thickness stainless steel targets were obtained at the wavelength range 1.7-2.1 Å, for the first time in experiments at femtosecond petawatt laser facility J-KAREN-P. Total intensity of front x-ray spectra three times dominates to rear side spectra for maximum laser intensity I ≈ 3.21021 W/cm2. Growth of x-ray emission is mostly determined by contribution of bremsstrahlung radiation that allowed estimating bulk electron plasma temperature for various magnitude of laser intensity on target.
- ItemLooking inside the tunnelling barrier: II. Co- and counter-rotating electrons at the ‘tunnelling exit’(Bristol : IOP Publ., 2018-08-03) Kaushal, Jivesh; Smirnova, OlgaThe initial conditions for electron trajectories at the exit from the tunnelling barrier are often used in strong field models, for example to bridge the first and the second steps of the three-step model celebrated in this issue. Since the analytical R-matrix theory does not rely on the three-step model or the concept of the tunnelling barrier in coordinate space, obtaining the initial conditions for electron trajectories at the barrier exit is, strictly speaking, not necessary to calculate standard observables. Not necessary, but possible—especially when motivated by the occasion of this issue. The opportunity to evaluate such initial conditions emerges as a corollary of analysing sub-barrier kinematics, which includes the interplay of laser and Coulomb fields on the sub-cycle scale (see the companion paper I). We apply our results to discuss the difference in such initial conditions for co- and counter-rotating electrons liberated during strong field ionisation. We derive quantum orbits and classical trajectories describing ionization dynamics of co- and counter-rotating electrons in long-range potentials.
- ItemTerahertz magnetic field enhancement in an asymmetric spiral metamaterial(Bristol : IOP Publ., 2018-10-25) Polley, Debanjan; Hagström, Nanna Zhou; Schmising, Clemens von Korff; Eisebitt, Stefan; Bonetti, StefanoWe use finite element simulations in both the frequency and the time-domain to study the terahertz resonance characteristics of a metamaterial (MM) comprising a spiral connected to a straight arm. The MM acts as a RLC circuit whose resonance frequency can be precisely tuned by varying the characteristic geometrical parameters of the spiral: inner and outer radius, width and number of turns. We provide a simple analytical model that uses these geometrical parameters as input to give accurate estimates of the resonance frequency. Finite element simulations show that linearly polarized terahertz radiation efficiently couples to the MM thanks to the straight arm, inducing a current in the spiral, which in turn induces a resonant magnetic field enhancement at the center of the spiral. We observe a large (approximately 40 times) and uniform (over an area of ∼10 μm2) enhancement of the magnetic field for narrowband terahertz radiation with frequency matching the resonance frequency of the MM. When a broadband, single-cycle terahertz pulse propagates towards the MM, the peak magnetic field of the resulting band-passed waveform still maintains a six-fold enhancement compared to the peak impinging field. Using existing laser-based terahertz sources, our MM design allows to generate magnetic fields of the order of 2 T over a time scale of several picoseconds, enabling the investigation of nonlinear ultrafast spin dynamics in table-top experiments. Furthermore, our MM can be implemented to generate intense near-field narrowband, multi-cycle electromagnetic fields to study generic ultrafast resonant terahertz dynamics in condensed matter.
- ItemPhase- and intensity-resolved measurements of above threshold ionization by few-cycle pulses(Bristol : IOP Publ., 2018-06-11) Kübel, M.; Arbeiter, M.; Burger, C.; Kling, Nora G.; Pischke, T.; Moshammer, R.; Fennel, T.; Kling, M.F.; Bergues, B.We investigate the carrier-envelope phase (CEP) and intensity dependence of the longitudinal momentum distribution of photoelectrons resulting from above threshold ionization of argon by few-cycle laser pulses. The intensity of the pulses with a center wavelength of 750 nm is varied in a range between 0.7 × 1014 and . Our measurements reveal a prominent maximum in the CEP-dependent asymmetry at photoelectron energies of 2 U P (U P being the ponderomotive potential), that is persistent over the entire intensity range. Further local maxima are observed around 0.3 and 0.8 U P. The experimental results are in good agreement with theoretical results obtained by solving the three-dimensional time-dependent Schrödinger equation. We show that for few-cycle pulses, the amplitude of the CEP-dependent asymmetry provides a reliable measure for the peak intensity on target. Moreover, the measured asymmetry amplitude exhibits an intensity-dependent interference structure at low photoelectron energy, which could be used to benchmark model potentials for complex atoms.
- ItemX-ray spectroscopy of super-intense laser-produced plasmas for the study of nonlinear processes. Comparison with PIC simulations(Bristol : IOP Publ., 2017) Dalimier, E.; Ya Faenov, A.; Oks, E.; Angelo, P.; Pikuz, T.A.; Fukuda, Y.; Andreev, A.; Koga, J.; Sakaki, H.; Kotaki, H.; Pirozhkov, A.; Hayashi, Y.; Skobelev, I.Yu.; Pikuz, S.A.; Kawachi, T.; Kando, M.; Kondo, K.; Zhidkov, A.; Tubman, E.; Butler, N.M.H.; Dance, R.J.; Alkhimova, M.A.; Booth, N.; Green, J.; Gregory, C.; McKenna, P.; Woolsey, N.; Kodama, R.We present X-ray spectroscopic diagnostics in femto-second laser-driven experiments revealing nonlinear phenomena caused by the strong coupling of the laser radiation with the created plasma. Among those nonlinear phenomena, we found the signatures of the Two Plasmon Decay (TPD) instability in a laser-driven CO2 cluster-based plasma by analyzing the Langmuir dips in the profile of the O VIII Lyϵ line, caused by the Langmuir waves created at the high laser intensity 3 1018Wcm-2. With similar laser intensities, we reveal also the nonlinear phenomenon of the Second Harmonic Generation (SHG) of the laser frequency by analyzing the nonlinear phenomenon of satellites of Lyman δ and ϵ lines of Ar XVII. In the case of relativistic laser-plasma interaction we discovered the Parametric Decay Instability (PDI)-induced ion acoustic turbulence produced simultaneously with Langmuir waves via irradiation of thin Si foils by laser intensities of 1021Wcm-2.
- ItemIntracycle interference in ionization of Ar by a laser assisted XUV pulse(Bristol : IOP Publ., 2017) Arbó, D.G.; López, S. D.; Kubin, M.; Hummert, J.; Vrakking, M.J.J.; Kornilov, O.Synopsis We present a theoretical and experimental study of the subcycle interference in laser assisted XUV ionization of Ar atoms. Averaging over the focal volume happens to blur the intracycle interference, which thus cannot be measured directly. We show that even at these conditions, the intracycle interference can be obtained through the subtraction of two different angle and energy-resolved distributions at slightly different laser intensities.
- ItemCharacterization of self-modulated electron bunches in an argon plasma(Bristol : IOP Publ., 2018) Gross, M.; Lishilin, O.; Loisch, G.; Boonpornprasert, P.; Chen, Y.; Engel, J.; Good, J.; Huck, H.; Isaev, I.; Krasilnikov, M.; Li, X.; Niemczyk, R.; Oppelt, A.; Qian, H.; Renier, Y.; Stephan, F.; Zhao, Q.; Brinkmann, R.; Martinez de la Ossa, A.; Osterhoff, J.; Grüner, F.J.; Mehrling, T.; Schroeder, C.B.; Will, I.The self-modulation instability is fundamental for the plasma wakefield acceleration experiment of the AWAKE (Advanced Wakefield Experiment) collaboration at CERN where this effect is used to generate proton bunches for the resonant excitation of high acceleration fields. Utilizing the availability of flexible electron beam shaping together with excellent diagnostics including an RF deflector, a supporting experiment was set up at the electron accelerator PITZ (Photo Injector Test facility at DESY, Zeuthen site), given that the underlying physics is the same. After demonstrating the effect [1] the next goal is to investigate in detail the self-modulation of long (with respect to the plasma wavelength) electron beams. In this contribution we describe parameter studies on self-modulation of a long electron bunch in an argon plasma. The plasma was generated with a discharge cell with densities in the 1013 cm-3 to 1015 cm-3 range. The plasma density was deduced from the plasma wavelength as indicated by the self-modulation period. Parameter scans were conducted with variable plasma density and electron bunch focusing.
- ItemHigh-order harmonic generation by polyatomic molecules(Bristol : IOP Publ., 2017) Odžak, S.; Hasović, E.; Milošević, D.B.We present a theory of high-order harmonic generation by arbitrary polyatomic molecules based on the molecular strong-field approximation (MSFA) in the framework of the S-matrix theory. A polyatomic molecule is modeled by an (N + 1)-particle system, which consists of N heavy atomic (ionic) centers and an electron. We derived various versions (with or without the dressing of the initial and/or final molecular state) of the MSFA. The general expression for the T-matrix element takes a simple form for neutral polyatomic molecules. We show the existence of the interference minima in the harmonic spectrum and explain these minima as a multiple-slit type of interference. This is illustrated by numerical examples for the nitrous oxide (N2O) molecule exposed to strong linearly polarized laser field.
- ItemElectron Rescattering in a Bicircular Laser Field(Bristol : IOP Publ., 2017) Hasović, E.; Becker, W.; Milošević, D.B.We investigate high-order above-threshold ionization (HATI) of krypton atoms by a bicircular laser field, which consists of two coplanar co- or counter-rotating circularly polarized fields of frequencies rw and sw. We show that the photoelectron spectra in the HATI process, presented in the momentum plane, exhibit the same discrete rotational symmetry as the driving field. We also analyze HATI spectra for various combinations of the intensities of two field components for co- and counter-rotating fields. We find that the appearance of high-energy plateau for the counter-rotating case is vary sensitive to the laser intensity ratio, while the plateau is always absent for the co-rotating bicircular field.
- ItemAbove-threshold ionization in a bicircular field: Quantum orbits unfolding in a plane(Bristol : IOP Publ., 2017) Becker, W.; Milošević, D.B.Above-threshold ionization (ATI) of atoms by a strong bicircular laser field is investigated using the strong-field approximation and the quantum-orbit theory. The bicircular field consists of two coplanar counterrotating circularly polarized fields with a frequency ratio of 2:1. The velocity map of the angle-resolved ATI spectra, both for direct and rescattered electrons, reflects the shape of a parametric plot of the bicircular field and its symmetries. It is shown that the main characteristics of the ATI spectra can be explained using only a few quantum orbits having short travel times. We also analyze a recently discovered [Phys. Rev. A 93, 052402(R) (2016)] bicircular-field-induced spin asymmetry of the ATI electrons and show that the momentum dependence of the spin-asymmetry parameter is stronger for longer wavelengths.
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