The pulses at 744 nm with the duration 90 fs, energy 6 mJ and weakly divergent wavefront propagated for more than 100 m and generated a filament followed by unprecedently long high intensity (≥ 1 TW/cm 2 ) light channel. Over a 20 m long sub-section of this channel the pulse energy is transferred continuously to the infrared wing, forming spectral humps that extend up to the wavelength of 850 nm. From 3D+time carrierresolved simulations of 100 m pulse propagation, we show that spectral humps indicate the formation of a train of fs pulses appearing at a predictable position in the propagation path.
Multiphoton ionization mechanisms and ionization rates of atmospheric air and constituent gases are studied at the 248-nm KrF laser wavelength within a laser pulse intensity range of 108–1013 W/cm2 using both long 25-ns and short 160-fs pulses. We have experimentally shown that it is the photoionization of water vapor naturally contained in atmospheric air that acts as the dominant process of air ionization. (2 + 1) Resonance-Enhanced Multiphoton Ionization (REMPI) occurs through 2-photon resonant excitation of water molecules, which results in a quadratic dependence of electron density on laser intensity at lower laser intensities of 108–1010 W/cm2 in the long pulse and in a cubic dependence at higher intensities of 1010–1013 W/cm2 in the short pulse. Direct 3-photon ionization and (3 + 1) REMPI take place in pure O2 and N2, respectively, and their contributions to air ionization are in the ratio of 5:3. The total ionization rate of O2 and N2 in atmospheric air is about an order of magnitude less than that of water vapor. Relevant ionization coefficients (effective multiphoton ionization cross sections) have been measured and that for the H2O molecule is more than 2–3 orders of magnitude larger than the others.
The fusion of several coherent 800 nm femtosecond filaments is induced experimentally and numerically by transmitting a beam through a mask with circular apertures followed by the focusing lens. The far-field image of the four-filament fusion region reveals bright on-axis maximum and differs drastically from the diffraction pattern of a low energy beam propagating through the mask in the linear regime. In 3D+time numerical simulations with the carrier wave resolved we show a factor-of-5 saturable growth in the peak plasma density with successive increase in the number of mask openings. An overall spectral blueshift of the fundamental and the third harmonics follows the plasma density increase. The simulated far-field on-axis emission agrees with the experiment and serves as the indication of nonlinear interaction in the fusion region.
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