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- ItemHigh harmonic interferometry of the Lorentz force in strong mid-infrared laser fields([Bad Honnef] : Dt. Physikalische Ges., 2018-05-11) Pisanty, Emilio; Hickstein, Daniel D.; Galloway, Benjamin R.; Durfee, Charles G.; Kapteyn, Henry C.; Murnane, Margaret M.; Ivanov, MishaThe interaction of intense mid-infrared laser fields with atoms and molecules leads to a range of new opportunities, from the production of bright, coherent radiation in the soft x-ray range, to imaging molecular structures and dynamics with attosecond temporal and sub-angstrom spatial resolution. However, all these effects, which rely on laser-driven recollision of an electron removed by the strong laser field and its parent ion, suffer from the rapidly increasing role of the magnetic field component of the driving pulse: the associated Lorentz force pushes the electrons off course in their excursion and suppresses all recollision-based processes, including high harmonic generation as well as elastic and inelastic scattering. Here we show how the use of two non-collinear beams with opposite circular polarizations produces a forwards ellipticity which can be used to monitor, control, and cancel the effect of the Lorentz force. This arrangement can thus be used to re-enable recollision-based phenomena in regimes beyond the long-wavelength breakdown of the dipole approximation, and it can be used to observe this breakdown in high harmonic generation using currently available light sources.
- ItemCoulomb time delays in high harmonic generation([Bad Honnef] : Dt. Physikalische Ges., 2017-02-02) Torlina, Lisa; Smirnova, OlgaMeasuring the time it takes to remove an electron from an atom or molecule during photoionization has been the focus of a number of recent experiments using newly developed attosecond spectroscopies. The interpretation of such measurements, however, depends critically on the measurement protocol and the specific observables available in each experiment. One such protocol relies on high harmonic generation. In this paper, we derive rigorous and general expressions for ionisation and recombination times in high harmonic generation experiments. We show that these times are different from, but related to, ionisation times measured in photoelectron spectroscopy: that is, those obtained using the attosecond streak camera, RABBITT and attoclock methods. We then proceed to use the analytical R-matrix theory to calculate these times and compare them with experimental values.
- ItemLaser-induced extreme magnetic field in nanorod targets([Bad Honnef] : Dt. Physikalische Ges., 2018-03-27) Lécz, Zsolt; Andreev, AlexanderThe application of nano-structured target surfaces in laser-solid interaction has attracted significant attention in the last few years. Their ability to absorb significantly more laser energy promises a possible route for advancing the currently established laser ion acceleration concepts. However, it is crucial to have a better understanding of field evolution and electron dynamics during laser-matter interactions before the employment of such exotic targets. This paper focuses on the magnetic field generation in nano-forest targets consisting of parallel nanorods grown on plane surfaces. A general scaling law for the self-generated quasi-static magnetic field amplitude is given and it is shown that amplitudes up to 1 MT field are achievable with current technology. Analytical results are supported by three-dimensional particle-in-cell simulations. Non-parallel arrangements of nanorods has also been considered which result in the generation of donut-shaped azimuthal magnetic fields in a larger volume.
- ItemAttosecond control of spin polarization in electron–ion recollision driven by intense tailored fields([Bad Honnef] : Dt. Physikalische Ges., 2017-07-07) Ayuso, David; Jiménez-Galán, Alvaro; Morales, Felipe; Ivanov, Misha; Smirnova, OlgaTunnel ionization of noble gas atoms driven by a strong circularly polarized laser field in combination with a counter-rotating second harmonic generates spin-polarized electrons correlated to the spin-polarized ionic core. Crucially, such two-color field can bring the spin-polarized electrons back to the parent ion, enabling the scattering of the spin-polarized electron on the spin-polarized parent ion. Here we show how one can control the degree of spin polarization as a function of electron energy and recollision time by tuning the laser parameters, such as the relative intensities of the counter-rotating fields. The attosecond precision of the control over the degree of spin polarization opens the door for attosecond control and spectroscopy of spin-resolved dynamics.
- ItemStrong-field assisted extreme-ultraviolet lasing in atoms and molecules([Bad Honnef] : Dt. Physikalische Ges., 2017-07-10) Bredtmann, Timm; Patchkovskii, Serguei; Ivanov, Misha YuUsing ab-initio simulations, we demonstrate amplification of extreme-ultraviolet (XUV) radiation during transient absorption in a high-harmonic generation type process using the example of the hydrogen atom. The strong IR driving field rapidly depletes the initial ground state while populating excited electronic states through frustrated tunnelling, thereby creating a population inversion. Concomitant XUV lasing is demonstrated by explicit inclusion of the XUV seed in our simulations, allowing a thorough analysis in terms of this transient absorption setup. Possibilities for increasing this gain, e.g. through preexcitation of excited states, change of the atomic gain medium or through multi-center effects in molecules, are demonstrated. Our findings should lead to a reinterpretation of recent experiments.
- ItemJitter-correction for IR/UV-XUV pump-probe experiments at the FLASH free-electron laser([Bad Honnef] : Dt. Physikalische Ges., 2017-04-10) Savelyev, Evgeny; Boll, Rebecca; Bomme, Cédric; Schirmel, Nora; Redlin, Harald; Erk, Benjamin; Düsterer, Stefan; Müller, Erland; Höppner, Hauke; Toleikis, Sven; Müller, Jost; Kristin Czwalinna, Marie; Treusch, Rolf; Kierspel, Thomas; Mullins, Terence; Trippel, Sebastian; Wiese, Joss; Küpper, Jochen; Brauβe, Felix; Krecinic, Faruk; Rouzée, Arnaud; Rudawski, Piotr; Johnsson, Per; Amini, Kasra; Lauer, Alexandra; Burt, Michael; Brouard, Mark; Christensen, Lauge; Thøgersen, Jan; Stapelfeldt, Henrik; Berrah, Nora; Müller, Maria; Ulmer, Anatoli; Techert, Simone; Rudenko, Artem; Rolles, DanielIn pump-probe experiments employing a free-electron laser (FEL) in combination with a synchronized optical femtosecond laser, the arrival-time jitter between the FEL pulse and the optical laser pulse often severely limits the temporal resolution that can be achieved. Here, we present a pump-probe experiment on the UV-induced dissociation of 2,6-difluoroiodobenzene (C6H3F2I) molecules performed at the FLASH FEL that takes advantage of recent upgrades of the FLASH timing and synchronization system to obtain high-quality data that are not limited by the FEL arrival-time jitter. We discuss in detail the necessary data analysis steps and describe the origin of the time-dependent effects in the yields and kinetic energies of the fragment ions that we observe in the experiment.
- ItemPassive and hybrid mode locking in multi-section terahertz quantum cascade lasers([Bad Honnef] : Dt. Physikalische Ges., 2018-05-24) Tzenov, P.; Babushkin, I.; Arkhipov, R.; Arkhipov, M.; Rosanov, N.; Morgner, U.; Jirauschek, C.It is believed that passive mode locking is virtually impossible in quantum cascade lasers (QCLs) because of too fast carrier relaxation time. Here, we revisit this possibility and theoretically show that stable mode locking and pulse durations in the few cycle regime at terahertz (THz) frequencies are possible in suitably engineered bound-to-continuum QCLs. We achieve this by utilizing a multi-section cavity geometry with alternating gain and absorber sections. The critical ingredients are the very strong coupling of the absorber to both field and environment as well as a fast absorber carrier recovery dynamics. Under these conditions, even if the gain relaxation time is several times faster than the cavity round trip time, generation of few-cycle pulses is feasible. We investigate three different approaches for ultrashort pulse generation via THz quantum cascade lasers, namely passive, hybrid and colliding pulse mode locking.