Molecular quantum wake-induced pulse shaping and extension of femtosecond air filaments

The filamentation of femtosecond laser pulses in solids, liquids, and gases, accompanied by plasma generation, is rich in nonlinear physics and applications [1]. The recent experimental demonstration that quantum molecular rotational revivals in the atmosphere [2] can have a dominant effect on filament propagation has accompanied a resurgence of interest in filamentation and applications [3]. The rotational revivals propagate behind a filamenting pump pulse like a wake, and this wake can steer, trap, or destroy an intense injected probe pulse. The molecular rotational response is sufficiently fast that it dominates the propagation of single ∼100-fs pulses filamenting in the atmosphere [4,5]. In this paper, we demonstrate that a molecular quantum wake can shape a filamenting probe pulse while significantly extending the nonlinear propagation distance and plasma generation. It does so by disrupting the usual interplay between nonlinear focusing and plasma defocusing responsible for extended filament propagation. In a single-pulse filament, the radially confined high intensity region (typically <100 μm in diameter) is not really akin to the intensity confinement in a glass [6] or plasma [7] optical fiber where there is little transverse energy exchange with zones outside the confinement region. In a typical single-pulse filament, the high intensity region is sustained by simultaneous incoming and outgoing energy exchange with a wider co-propagating “reservoir” [8] whose transverse extent is roughly defined by the spatial envelope of the original beam. Here, we show that, by applying an intense probe pulse to the co-propagating rotational wake induced by the pump pulse, the wake is shaped and is enhanced so as to self-consistently support an enhanced filament in which quantum molecular lensing dominates both Kerr focusing and plasma defocusing. Remarkably, the wake shaping and filament extension are sensitive to pump-probe delay on a 10-fs time scale. Theory and simulations agree well with experiments and provide significant insight. Prior work by other groups explored the cross-phase modulation and guiding of weak probe pulses in molecular alignment revivals generated in short nitrogen filaments [9]. In addition, simulations have shown filament extension by probe pulses sampling prefixed molecular alignment produced by a nonfilamenting pump [10]. Extension of a probe filament in the wake of a filamenting pump has been qualitatively inferred from plasma fluorescence measurements [11]. However, pulse shaping and filament plasma extension have never been directly measured, nor has a self-consistent two-pulse filamentation simulation been performed. II. DIRECT MEASUREMENT OF FILAMENT PLASMA DENSITY AND FILAMENTING PULSE ELECTRIC FIELD