Laser beam self-symmetrization in air in the multifilamentation regime

We show experimental and numerical evidence of spontaneous self-symmetrization of focused laser beams experiencing multi-filamentation in air. The symmetrization effect is observed as the multiple filaments generated prior to focus approach the focal volume. This phenomenon is attributed to the nonlinear interactions amongst the different parts of the beam mediated by the optical Kerr effect, which leads to a symmetric redistribution of the wave vectors even when the beam consists of a bundle of many filaments.Comment: 9 pages, 7 figure

Standoff detection of an electric field by bidirectional nitrogen lasing

International audienceWe report on standoff detection of dc electric field by bidirectional cavity-free lasing emission of neutral nitrogen molecules excited by intense circularly polarized femtosecond laser pulses. We observed that both the backward and forward 337.4 nm coherent lasing emission present a monotonous dependence on the strength of a remotely applied dc field up to ∼ 1 kV/cm field strength. Moreover, this method shows a dependence on the polarity of the external dc field, providing a sensitive method for remote characterization of the electric field amplitude and direction. We attribute the underlying mechanism of lasing signal modulation to the electric field induced electron acceleration and deceleration, which results in a variation of the kinetic energy of the free electrons and a modulation of the population inversion responsible for the nitrogen molecules’ lasing.The polarity-sensitive detection anisotropy is interpreted by the symmetry breaking of the electron motion in the plane perpendicular to the laser propagation due to the injection of a weak second harmonic laser field produced in the quarter-wave plate for a circularly polarized pump laser. Numerical simulations based on the two-dimensional time-dependent Schrödinger equation for electron kinetic energy support our interpretation.This study provides a proof-of-principle method for standoff detection of electric fields based on nitrogen lasing, which can be potentially useful for atmospheric and metrological applications

Hypothetical observed fraction of people infected with malaria (parasite rate) within and near malaria foci.

<p>Transmission only occurs within focal areas (top panel). Observed parasite rate shown in bottom panel with varying distances travelled. As humans move further and/or more frequently from within focal areas, they export parasites, causing larger extra-focal extents to exhibit nonzero parasite rate (dotted line; bottom panel).</p