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- ItemPlasma induced pulse breaking in filamentary self-compression(Berlin : Weierstraß-Institut für Angewandte Analysis und Stochastik, 2009) Brée, Carsten; Demircan, Ayhan; Skupin, Stefan; Berg´e, Luc; Steinmeyer, GünterA plasma induced temporal break-up in filamentary propagation has recently been identified as one of the key events in the temporal self-compression of femtosecond laser pulses. An analysis of the Nonlinear Schrödinger Equation coupled to a noninstantaneous plasma response yields a set of stationary states. This analysis clearly indicates that the emergence of double-hump, characteristically asymmetric temporal on-axis intensity profiles in regimes where plasma defocusing saturates the optical collapse caused by Kerr self-focusing is an inherent property of the underlying dynamical model.
- ItemDirectionality of THz emission from photoinduced gas plasmas(Berlin : Weierstraß-Institut für Angewandte Analysis und Stochastik, 2011) Köhler, Christian; Cabrera-Granado, Eduardo; Babushkin, Ihar; Bergé, Luc; Herrmann, Joachim; Skupin, StefanForward and backward THz emission by ionizing two-color laser pulses in gas is investigated by means of a simple semi-analytical model based on Jefimenko's equation and rigorous Maxwell simulations in one and two dimensions. We find the emission in backward direction having a much smaller spectral bandwidth than in forward direction and explain this by interference effects. Forward THz radiation is generated predominantly at the ionization front and is thus almost not affected by the opacity of the plasma, in excellent agreement with results obtained from a unidirectional pulse propagation model.
- ItemUltrafast spatio-temporal dynamics of terahertz generation by ionizing two-color femtosecond pulses in gases(Berlin : Weierstraß-Institut für Angewandte Analysis und Stochastik, 2010) Babushkin, Ihar; Kuehn, Wihelm; Köhler, Christian; Skupin, Stefan; Bergé, Luc; Reimann, Klaus; Woerner, Michael; Herrmann, Joachim; Elsaesser, ThomasWe present a combined theoretical and experimental study of spatio-temporal propagation effects in terahertz (THz) generation in gases using two-color ionizing laser pulses. The observed strong broadening of the THz spectra with increasing gas pressure reveals the prominent role of spatio-temporal reshaping and of a plasma-induced blue-shift of the pump pulses in the generation process. Results obtained from (3+1)-dimensional simulations are in good agreement with experimental findings and clarify the mechanisms responsible for THz emission.
- ItemTailoring 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, JoachimjApplications 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.
- ItemCascaded self-compression of femtosecond pulses in filaments(Berlin : Weierstraß-Institut für Angewandte Analysis und Stochastik, 2010) Brée, Carsten; Bethge, Jens; Skupin, Stefan; Demircan, Ayhan; Steinmeyer, GünterHighly nonlinear wave propagation scenarios hold the potential to serve for energy concentration or pulse duration reduction of the input wave form, provided that a small range of input parameters be maintained. In particular when phenomena like rogue-wave formation or few-cycle optical pulses generation come into play, it becomes increasingly difficult to maintain control of the waveforms. Here we suggest an alternative approach towards the control of waveforms in a highly nonlinear system. Cascading pulse self-compression cycles at reduced nonlinearity limits the increase of input parameter sensitivity while still enabling an enhanced compression effect. This cascaded method is illustrated by experiments and in numerical simulations of the Nonlinear Schrödinger Equation, simulating the propagation of short optical pulses in a self-generated plasma.
- ItemFilamentary pulse self-compression : the impact of the cell windows(Berlin : Weierstraß-Institut für Angewandte Analysis und Stochastik, 2011) Brée, Carsten; Demircan, Ayhan; Bethge, Jens; Nibbering, Erik T.J.; Skupin, Stefan; Bergé, Luc; Steinmeyer, GünterSelf-compression of multi-millijoule laser pulses during filamentary propagation is usually explained by the interplay of self-focusing and defocusing effects, causing a substantial concentration of energy on the axis of the propagating optical pulse. Recently, it has been argued that cell windows may play a decisive role in the self-compression mechanism. As such windows have to be used for media other than air their presence is often unavoidable, yet they present a sudden non-adiabatic change in dispersion and nonlinearity that should lead to a destruction of the temporal and spatial integrity of the light bullets generated in the self-compression mechanism. We now experimentally prove that there is in fact a self-healing mechanism that helps to overcome the potentially destructive consequences of the cell windows. We show in two carefully conducted experiments that the cell window position decisively influences activation or inhibition of the self-healing mechanism. A comparison with a windowless cell shows that presence of this mechanism is an important prerequisite for the exploitation of self-compression effects in windowed cells filled with inert gases.
- ItemGeneration of terahertz radiation from ionizing two-color laser pulses in Ar filled metallic hollow waveguides(Berlin : Weierstraß-Institut für Angewandte Analysis und Stochastik, 2010) Babuškin, Ihar; Skupin, Stefan; Herrmann, JoachimThe generation of THz radiation from ionizing two-color femtosecond pulses propagating in metallic hollow waveguides filled with Ar is numerically studied. We observe a strong reshaping of the low-frequency part of the spectrum. Namely, after several millimeters of propagation the spectrum is extended from hundreds of GHz up to 150 THz. For longer propagation distances, nearly single-cycle near-infrared pulses with wavelengths around 4.5 μm are obtained by appropriate spectral filtering, with an efficiency of up to 0.25 %.