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Author: Husakou, Anton × Type: Report ×

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    Tailoring THz radiation by controlling tunnel photoionization events in gases
    (Berlin : Weierstraß-Institut für Angewandte Analysis und Stochastik, 2011) Babushkin, Ihar; Skupin, Stefan; Husakou, Anton; Köhler, Christian; Cabrera-Granado, Eduardo; Bergé, Luc; Herrmann, Joachimj
    Applications ranging from nonlinear terahertz spectroscopy to remote sensing require broadband and intense THz radiation which can be generated by focusing two-color laser pulses into a gas. In this setup, THz radiation originates from the buildup of the electron density in sharp steps of attosecond duration due to tunnel ionization, and subsequent acceleration of free electrons in the laser field. We show that the spectral shape of the THz pulses generated by this mechanism is determined by superposition of contributions from individual ionization events. This provides a straightforward analogy with linear diffraction theory, where the ionization events play the role of slits in a grating. This analogy offers simple explanations for recent experimental observations and opens new avenues for THz pulse shaping based on temporal control of the ionization events. We illustrate this novel technique by tailoring the spectral width and position of the resulting radiation using multi-color pump pulses.
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    Symmetry breaking and strong persistent plasma currents via resonant destabilization of atoms
    (Berlin : Weierstraß-Institut für Angewandte Analysis und Stochastik, 2017) Brée, Carsten; Hofmann, Michael; Babushkin, Ihar; Demircan, Ayhan; Morgner, Uwe; Kosareva, Olga G.; Savelev, Andrei B.; Husakou, Anton; Ivanov, Misha
    The ionization rate of an atom in a strong optical field can be resonantly enhanced by the presence of long-living atomic levels (so called Freeman resonances). This process is most prominent in the multiphoton ionization regime meaning that ionization event takes many optical cycles. Nevertheless, here we show that these resonances can lead to fast subcycle-scale plasma buildup at the resonant values of the intensity in the pump pulse. The fast buildup can break the cycletocycle symmetry of the ionization process, resulting in generation of persistent macroscopic plasma currents which remain after the end of the pulse. This, in turn, gives rise to a broadband radiation of unusual spectral structure forming a comb from terahertz (THz) to visible. This radiation contains fingerprints of the attosecond electronic dynamics in Rydberg states during ionization.