2021, Murillo-Sánchez, Marta L., Reitsma, Geert, Poullain, Sonia Marggi, Fernández-Milán, Pedro, González-Vázquez, Jesús, de Nalda, Rebeca, Martín, Fernando, Vrakking, Marc J. J., Kornilov, Oleg, Bañares, Luis

The time-resolved photodynamics of the methyl iodide cation (CH3I+) are investigated by means of femtosecond XUV-IR pump-probe spectroscopy. A time-delay-compensated XUV monochromator is employed to isolate a specific harmonic, the 9th harmonic of the fundamental 800 nm (13.95 eV, 88.89 nm), which is used as a pump pulse to prepare the cation in several electronic states. A time-delayed IR probe pulse is used to probe the dissociative dynamics on the first excited state potential energy surface. Photoelectrons and photofragment ions - and I+ - are detected by velocity map imaging. The experimental results are complemented with high level ab initio calculations for the potential energy curves of the electronic states of CH3I+ as well as with full dimension on-the-fly trajectory calculations on the first electronically excited state, considering the presence of the IR pulse. The and I+ pump-probe transients reflect the role of the IR pulse in controlling the photodynamics of CH3I+ in the state, mainly through the coupling to the ground state and to the excited state manifold. Oscillatory features are observed and attributed to a vibrational wave packet prepared in the state. The IR probe pulse induces a coupling between electronic states leading to a slow depletion of fragments after the cation is transferred to the ground states and an enhancement of I+ fragments by absorption of IR photons yielding dissociative photoionization. © 2021 The Author(s). Published by IOP Publishing Ltd on behalf of the Institute of Physics and Deutsche Physikalische Gesellschaft.

2020, Tailliez, C., Stathopulos, A., Skupin, S., Buožius, D., Babushkin, T., Vaičaitis, I., Bergé, L.

We study the influence of the polarization states of ionizing femtosecond two-color pulses on the emitted terahertz radiation in gases. A local-current model and plane-wave evaluations justify the previously-reported impact on the THz energy yield and a (almost) linearly-polarized THz field when using circularly-polarized laser harmonics. For such pump pulses, the THz yield is independent of the relative phase between the two colors. When the pump pulses have same helicity, the increase in the THz yield is associated with longer ionization sequences and higher electron transverse momenta acquired in the driving field. Reversely, for two color pulses with opposite helicity, the dramatic loss of THz power comes from destructive interferences driven by the highly symmetric response of the photocurrents lined up on the third harmonic of the fundamental pulse. While our experiments confirm an increased THz yield for circularly-polarized pumps of same helicity, surprisingly, the emitted THz radiation is not linearly-polarized. This effect is explained by means of comprehensive 3D numerical simulations highlighting the role of the spatial alignment and non-collinear propagation of the two colors. © 2020 The Author(s). Published by IOP Publishing Ltd on behalf of the Institute of Physics and Deutsche Physikalische Gesellschaft.

2021, Lécz, Zsolt, Andreev, Alexander, Kamperidis, C., Hafz, Nasr

Acceleration of electrons in laser-driven plasma wakefields has been extended up to the ∼8 GeV energy within a distance of tens of centimeters. However, in applications, requiring small energy spread within the electron bunch, only a small portion of the bunch can be used and often the low-energy electrons represent undesired background in the spectrum. We present a compact and tunable scheme providing clean and mono-energetic electron bunches with less than one percent energy spread and with central energy on the GeV level. It is a two-step process consisting of ionization injection with attosecond pulses and acceleration in a capillary plasma wave-guide. Semi-analytical theory and particle-in-cell simulations are used to accurately model the injection and acceleration steps.

2020, Khokhlova, M.A., Strelkov, V.V.

The efficient generation of the coherent XUV light via frequency conversion of intense laser drivers is a problem of both fundamental and technological importance. Increasing the intensity of the generated high harmonics by raising the intensity of the driving field works only up to a point: at high intensities, rapid ionisation of the medium limits the conversion efficiency. Considering the combined effect of the phase-matching and of the blue shift of the driving field during its propagation in a rapidly ionising medium, we show that the latter can be the dominant limiting mechanism. We introduce a new spatial scale, the blue-shift length, which sets the upper bound for the quadratic intensity growth of the generated harmonics. Moreover, we show that this seemingly fundamental restriction can be overcome by using an additional generating weak mid-IR field. For specific combinations of frequencies of the generating fields, the corresponding high-order frequency-mixing process does not suffer from the blue shift of the drivers and phase mismatch, and thus its efficiency grows quadratically with propagation distance. Our results thus open a new route for highly efficient generation of coherent XUV light. © 2020 The Author(s). Published by IOP Publishing Ltd on behalf of the Institute of Physics and Deutsche Physikalische Gesellschaft.

2020, Wituschek, Andreas, Kornilov, Oleg, Witting, Tobias, Maikowski, Laura, Stienkemeier, Frank, Vrakking, Marc J.J., Bruder, Lukas

The development of schemes for coherent nonlinear time-domain spectroscopy in the extreme-ultraviolet regime (XUV) has so far been impeded by experimental difficulties that arise at these short wavelengths. In this work we present a novel experimental approach, which facilitates the timing control and phase cycling of XUV pulse sequences produced by harmonic generation in rare gases. The method is demonstrated for the generation and high spectral resolution characterization of narrow-bandwidth harmonics (˜14 eV) in argon and krypton. Our technique simultaneously provides high phase stability and a pathway-selective detection scheme for nonlinear signals - both necessary prerequisites for all types of coherent nonlinear spectroscopy. © 2020 The Author(s). Published by IOP Publishing Ltd on behalf of the Institute of Physics and Deutsche Physikalische Gesellschaft.

2021, Lécz, Zsolt, Andreev, Alexander, Kamperidis, Christos, Hafz, Nasr

Acceleration of electrons in laser-driven plasma wakefields has been extended up to the 10 GeV energy within a distance of 10s of centimeters. However, in applications, requiring small energy spread within the electron bunch, only a small portion of the bunch can be used and often the low-energy electrons represent undesired background in the spectrum. We present a compact and tunable scheme providing clean and mono-energetic electron bunches with less than one percent energy spread and with central energy on the GeV level. It is a two-step process consisting of ionization injection with attosecond pulses and acceleration in a capillary plasma wave-guide. Semi-analytical theory and particle-in-cell simulations are used to accurately model the injection and acceleration steps.