We compare transverse structure evolution and energy deposition into the medium within focused multifilament arrays created using two different types of diffraction optical elements (DOEs): TEM₁₁ phase plate and a Dammann grating. We show that the employment of the Dammann grating provides a robust way to create regular multifilament arrays, which is far less dependent on laser beam quality than one using the phase plate.

In the experiment, the laser pulse (744 nm, 0.5 mJ, 90 fs) focused into the air gap between the plane electrodes biased by a 10 kV/cm field (DC-biased filament) produced terahertz (THz) radiation. At the selected frequencies of ν = 0.3 , 0.5, 1 THz, a wide flat-top angular distribution was measured by a bolometer rotating in the plane of the electrodes. The simulations based on the unidirectional pulse propagation equation with fine 0.01 THz resolution and 3 PHz frequency domain showed the transition of the THz directional diagram from the flat-top at ν ≲ 1 T H z to the conical one at ν > 8 T H z due to the destructive interference of THz waves from the ionization front propagating with the superluminal velocity. Refraction on the plasma is not the major factor in ring formation.

We study the angular distribution of different spectral components of the terahertz emission from a single-color laser filament plasma. The opening angle of a terahertz cone is experimentally demonstrated to be proportional to the inverse square root of both plasma channel length and terahertz frequency in the non-linear focusing mode, whereas in the case of linear focusing this dependence breaks down. We also experimentally show that any conclusions of terahertz radiation spectral composition require the angle range from which it is collected to be specified.

High-intensity (∼1 TW/cm${}^2$ and higher) region formed in the propagation of ∼60 GW, 90 fs Ti:Sapphire laser pulse on a ∼100 m path in air spans for several tens of meters and includes a plasma filament and a postfilament light channel. The intensity in this extended region is high enough to generate an infrared supercontinuum wing and to initiate laser-induced discharge in the gap between the electrodes. In the experiment and simulations, we delay the high-intensity region along the propagation direction by inserting metal-wire meshes with square cells at the laser system output. We identify the presence of a high-intensity region from the clean-spatial-mode distributions, appearance of the infrared supercontinuum wing, and occurrence of the laser-induced discharge. In the case of free propagation (without any meshes), the onset of the high-intensity zone is at 40–52 m from the laser system output with ∼30 m extension. Insertion of the mesh with 3 mm cells delays the beginning of the high-intensity region to 49–68 m with the same ∼30 m extension. A decrease in the cell size to 1 mm leads to both delay and shrinking of the high-intensity zone to 71–73 m and 6 m, respectively. Three-dimensional simulations in space confirm the mesh-induced delay of the high-intensity zone as the cell size decreases.

At the selected frequencies from 0.3 to 10 THz we measured the two-dimensional (2D) distributions of fluence and polarization of terahertz (THz) emission from a single-color femtosecond filament. At the majority of frequencies studied, the THz beam has a donut-like shape with azimuthal modulations and radial polarization. At the maximal modulation, THz beam takes the form of the two lobes and polarization of the THz field degenerates into orthogonal to the laser pulse polarization direction. Violation of the radially polarized donut beam shape is due to destructive interference of THz waves driven by light pressure directed along the laser beam propagation axis and ponderomotive force parallel to the laser polarization.