Filamentation Nonlinear Optics Of Intense Femtosecond Laser Pulses In Air

The nonlinear filamentation of the intense femtosecond laser pulses in air isinvestigated. Long plasma filaments are formed with a length from tens up to a fewhundred meters. Five methods are developed to diagnose the formation and thestructure of plasma filaments, including imaging of the cross-section, fluorescencedetection, resistivity measurement, acoustic diagnostics, and shadowgraphy. Thediagnostics of plasma filaments allows us to choose appropriate detecting methodsand provide further insight into the dynamic evolution of the plasma filaments in air.Based on these diagnostics, we find that the propagation of the filaments shows verycomplicated process includingthe evolution from a single filament into two and threeand even more distinct filaments periodically, and the merging of multiple filamentsinto two filaments. Using fluorescence measurement and acoustic diagnosticssimultaneously, multiple refocusing of laser pulses is also observed. Strong thirdharmonicemissionis observed withamaximum conversionefficiencyof about 0.3%.The divergence angle of the TH emission in the conical emission of the plasmafilamentsisalsoinvestigated,whichisingoodagreementwithcalculations.From the point of view of applications, the filaments distribution is optimizedusing a pinhole with different diameters. It is shown that the number, pattern,propagation distance, and spatial stability of filaments can be controlled by avariable-aperture pinhole along the laser beam. Two schemes are performed toprolong the lifetime of the plasma filaments. Asucceeding fs laser pulse and a sub-nslaser pulse are injected into the plasma filaments after a delay time of 10 nsrespectively. It is foundthat thelifetimecanbeprolongedtotheorderof microsecondwhen the sub-ns laser pulse is introduced, which is much better than that with the succeeding fs laser pulse. Calculations show that the lifetime can be extended tomicroseconds by using another laser pulse for detachment. As for the long distancepropagation of the laser pulses, the filamentation process and the surpercontinuum(SC) emission are closely dependent on the initial negative chirp and the divergenceangle of the laser beam. The"rainbow-like"SC generation is observed when a pulsewithanappropriatenegativechirp.Mostofenergydepositedinthebackgroundservesas an energyreservoir. We have shown that an energyreservoir over 10 times the sizeof the filament core (mm size) is necessary to feed a single filament undisturbedpropagation. The characteristics of the multiple filaments formed by prefocused andfreely propagating fs laser pulses are investigated and compared. It is shown that thediameter, intensity, electron density, length, supercontinuum generation, stability, andinteraction for the two cases can be quite different. Our experiments also demonstratesimultaneous triggering and guiding of large gap discharges in air by laser filaments,which opens promising perspectives for lightning control. Besides, sophora leavesfrom several areas in Beijing have been analyzed by femtosecond laser-inducedbreakdownspectroscopy(femto-LIBS).