We compare detailed spatio-temporal propagation simulations with experiments on generation of terahertz (THz) pulses in single filaments with two-color optical excitation, which elucidates the mechanism whereby positively chirped optical pulses lead to the maximum THz yield. The long-wavelength components in the leading edge of the positively chirped 2ω-pulse preserve the overlap with the ω-pulse. The THz yield is extremely sensitive to the temporal overlap: for a 35-fs ω-pulse, the yield decreases by an order of magnitude upon a ω–2ω pulse shift of only 5 fs.

A phenomenological model of the terahertz radiation of the plasma channel of a femtosecond filament has been elaborated that satisfactorily describes the experimental results of the detection of low-frequency radiation in air. The angular distributions of the terahertz radiation intensity in the absence and presence of an external electrostatic field have been obtained. The dependence of the divergence angle of the terahertz radiation on the filament parameters has been determined.

Filamentation of focused UV and IR femtosecond laser pulses and plasma channel formation governed by variable wavefront distortions was experimentally and numerically studied. A deformable mirror was used to control the plasma channel length by introducing a spherical aberration into the initial transverse spatial distribution of a femtosecond laser pulse. An at least double increase of the plasma channel length was observed with increasing deformation of the mirror. Numerical calculations show that the hat-like phase shape of the aberration ensures that the energy of the initial laser pulse remains confined for a longer distance within the limited transverse size of the filament.

We experimentally observed laser-induced remote high-voltage discharge triggering between two needle electrodes with half-a-cm spacing. The discharge was initiated by a 744-nm, 90-fs, 6-mJ laser pulse undergoing filamentation in air. For the direct voltage below the self-breakdown threshold, triggering of air-gap discharge was synchronized with a 10-Hz laser repetition rate and occurred between 40 and 80 m of the propagation path. No discharge guiding was observed. The experimentally registered and simulated remote triggering probability was above 80% in the range of 45–60 m from laser output and about 50% in the range of 60–80 m. The probability decreases as the postfilament hot spot diverges with a simultaneous increase in stochastic laser beam wandering.

We measured the key characteristics of the backward terahertz (THz) emission from a two-color femtosecond laser spark in air: waveform, spectrum, and energy. Through the controlled reflection of the forward THz pulse, we observed the backward and forward ones in the same waveform and hence directly evidenced the origin of the backward THz radiation from the plasma spark. The spectrum of the backward THz radiation is shifted to the low-frequency range as compared with the forward one. Measurements of the energy of the backward and forward THz pulses for different pump pulse energies provide their ratio to be ∼5.5%, which quantitatively agrees with the results of our numerical simulations. These results can be useful for the noninvasive monitoring of forward THz emission parameters by the backward one.