Transition from plasma- to Kerr-driven laser filamentation
  While filaments are generally interpreted as a dynamic balance between Kerr focusing and plasma defocusing, the role of the higher-order Kerr effect (HOKE) is actively debated as a potentially dominant defocusing contribution to filament stabilization. In a pump-probe experiment supported by numerical simulations, we demonstrate the transition between two distinct filamentation regimes at 800\,nm. For long pulses (1.2 ps), the plasma substantially contributes to filamentation, while this contribution vanishes for short pulses (70 fs). These results confirm the occurrence, in adequate conditions, of filamentation driven by the HOKE rather than by plasma. 
Pierre B\'ejot, Edouard Hertz, J\'er\^ome Kasparian, Bruno Lavorel,   Jean-Pierre Wolf, Olivier Faucher
physics.optics