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

Generation of high quality tunable bessel beams using liquid immersion axicon,” Opt

International audienceBy immersing a conventional glass axicon in index-matching liquid, we generated high quality, tunable, quasi-non-diffracting Bessel beams. The aberrations resulting from the roundness of the axicon tip are minimized when a large base angle is used in liquid-immersion. This configuration also allows coarse and fine tunability through changing the liquid and adjusting the temperature, respectively. Our experimental results match very well with calculated intensity profiles. We succeeded to generate two-meter long plasma channels in air by focusing femtosecond laser pulses with the liquid-immersion axicon

High-energy ultrashort laser pulse compression in hollow planar waveguides

We demonstrate compression of high energy ultrashort laser pulses by nonlinear propagation inside gas-filled planar hollow waveguides. We adjust the input beam size along the non-guiding dimension of the planar waveguide to restrain the intensity below photoionization, while the relatively long range guided propagation yields significant selfphase modulation (SPM) and spectral broadening. We compare the compression in different noble gases and obtain 13.6 fs duration with output pulse energy of 8.1 millijoule (mJ) in argon, and 11.5 fs duration with 7.6 mJ energy in krypton. The broadened spectra at the output of the waveguide are uniform over more than 70 % of the total pulse energy. Shorter duration could be obtained at the expense of introduction of spatial structure in the beam (and eventual formation of filaments) resulting from small-scale self focusing in the non-guided direction. OCIS codes: 320.5520, 320.7110 The advent of high energy laser pulses with durations of few optical cycles provided scientists with very high electric fields, sufficient to suppress the Coulomb potential in atoms, accelerate electrons up to keV energies and produce coherent intense UV and XUV radiation with duration

Fluorescence and lasing of neutral nitrogen molecules inside femtosecond laser filaments in air: mechanism and applications

International audienceHigh power femtosecond laser pulses launched in air undergo nonlinear filamentary propagation, featuring a bright and thin plasma channel in air with its length much longer than the Rayleigh length of the laser beam. During this nonlinear propagation process, the laser pulses experience rich and complex spatial and temporal transformations. With its applications ranging from supercontinuum generation, laser pulse compression, remote sensing to triggering of lightning, the underlying physical mechanism of filamentation has been intensively studies. In this review, we will focus on the fluorescence and cavity-free lasing effect of the plasma filaments in air. The different mechanisms underlying the fluorescence of the excited neutral nitrogen molecules will be throughtly examined and it is concluded that the electron collision excitation is the dominant channel for the formation of the excited nitrogen molecules. The recently discovered “air lasing” effect, a cavity-free bidirectional lasing emission emitted by the filaments, will be introduced and its main properties will be emphasized. The applications of the fluorescence and lasing effect of the neutral nitrogen molecules will be introduced, with two examples on spectroscopy and detection of electric field. Finally, we discuss the quenching effect of the lasing effect in atmosphere and the mechanisms responsible will be analyzed. An outlook for the achievement of backward lasing in air will be briefly presented

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