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.

The linear-to-elliptical transformation of a $400\mathrm{nm}$ femtosecond-probe pulse in the birefringent filament in argon of an $800\mathrm{nm}$ linearly polarized femtosecond-pump pulse is studied numerically and experimentally. The rotation of the probe elliptical polarization is the largest in the high-intensity filament core. With propagation, the rotated radiation diffracts outward by the pump-produced plasma. The transmission of the analyzer crossing the probe’s polarization is maximum at the pump–probe angle of $45°$ and gives equal values for each pair of angles symmetrically situated at both sides of the maximum.

Efficient direct electron acceleration in the plasma channel with injection through the breaking of plasma waves generated by parametric instabilities was demonstrated experimentally and reproduced in the 2D3V PIC simulations. The electron bunch was produced using the specific plasma profile containing arbitrary sharp, ∼0.5 λ , gradient at the vicinity of 0.1–0.5 critical density and a long tail of a tenuous preplasma. Such a preplasma profile was formed by an additional nanosecond laser pulse with intensity of 5 × 10 ¹² W cm ⁻² . In the case of optimal preplasma parameters femtosecond laser pulse with an intensity of 5 × 10 ¹⁸ W cm ⁻² and an energy of 50 mJ generates a collimated electron bunch having divergence of 50 mrad, exponential spectrum with the slope of ∼2 MeV and charge of tens of pC. The charge was confirmed measuring neutron yield from Be(g, n) photonuclear reaction with threshold of 1.7 MeV. By the contrast, a ring-like electron beam with divergency of 300 mrad and significantly lower charge is generated if the prepulse intensity drops to 5 × 10 ¹¹ W cm ⁻² . The 2D PIC simulations confirmed beamed electron’s acceleration in the plasma channel (so-called direct laser acceleration). This channel is formed in a long tail of teneous preplasma by the laser pulse specularly reflected from the arbitrary sharp gradient. The ring-like electron beam was attributed to the longer gradient case enlarging divergence of the reflected laser beam, preventing channel’s formation and electron acceleration by the so-called vacuum laser acceleration, or VLA. We also showed that injected electrons appeared from the wave breaking of plasma waves of hybrid SRS-TPD instability for the both gradients. Electrons received an initial momentum from this breaking to be effectively injected into the plasma channel.