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.

Spectral broadening of 0.3 ps 515 nm laser pulse in a highly Raman-active B a W O 4 crystal and fused silica demonstrates significantly different behavior with the incident pulse energy. While the broadening in fused silica is fairly symmetric with respect to the pump laser pulse wavelength, the Stokes wing broadening in the B a W O 4 crystal is 2 times wider than that of anti-Stokes wing, the former demonstrating a step-like increase with the pulse energy. To the best of our knowledge, the obtained data are the first clear evidences of the following facts: (i) stimulated Raman scattering with sufficiently high efficiency of conversion to Stokes components slows down spectral broadening induced by self-phase modulation, and (ii) the mechanism of Kerr nonlinearity, which is responsible for self-phase modulation in B a W O 4 , is of orientational nature. The nonlinear refraction coefficient and its decay time following from our experiments with B a W O 4 were estimated as n 2 ≈ 6.4. 10 − 15 c m 2 / W and τ N L ≈ 0.35 p s .

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For the first time, with the use of I-scan and z-scan techniques, the five-photon absorption cross-section in a CaF 2 sample has been measured to be σ 5 = 5 × 10 −148 cm 10 s 4 at 473 nm. The applicability of the technique and the contribution to the measured absorption of an avalanche and tunnel ionization are discussed.

The influence of plasma channel length on an angular terahertz (THz) radiation distribution is experimentally studied for the channel formed under filamentation of an ultrashort laser pulse. It is shown that the angular distribution of the THz emission depends only on laser intensity in the filament and plasma density of the plasma channel and does not depend on the plasma channel length. A qualitative explanation of the THz emission screening by the filament plasma channel is proposed.