We experimentally demonstrate that terahertz emission yield from single-color filament plasma can be increased by more than 3 times by ordinary laser pulse chirping at constant energy. The enhancement is spectrally selective, for each terahertz frequency there is an optimal laser pulse duration providing maximal terahertz emission. This provides a prospective tool for tailoring both spectrum and energy of the terahertz emission from a single-color filament.

We study the angular distributions of various spectral components of terahertz radiation generated in an air plasma of a single-color filament with a laser wavelength of 930 nm. It is experimentally shown that at a 20-fold excess over the critical power for self-focusing the distribution of terahertz radiation at all studied frequencies is unimodal. The obtained pattern differs significantly from the angular distribution of terahertz radiation observed in the case of filamentation of pulses with a shorter wavelength.

We experimentally carried out frequency conversion of a chirped and stretched up to 200 ps near-IR 90 fs Ti:Sapphire laser pulse into the mid-IR through difference frequency generation in either GaSe or LiGaS 2 crystal. The crystal was pumped by laser radiation that passed through a BaWO 4 crystal, where Stokes frequency-shifted radiation (signal wave) was generated through SRS. The generated ∼60 nJ mid-IR pulse had a central wavelength of 11.1 μ m and FWHM spectral width of about 0.2 μ m, which made it possible to achieve a pulse duration of ∼0.9 ps after its compression. The mid-IR pulse generation efficiency was ∼10 −3 % at SRS efficiency of 3%. To the best of our knowledge, this all solid-state scheme combining Raman shifting and second-order nonlinear crystals achieves the frequency down-conversion of femtosecond near-IR laser pulses to ∼11 μ m radiation for the first time.

This paper presents results of experimental study of the stochastic and regularized filamentation of terawatt femtosecond radiation on a distances 1–40 m in the atmosphere. The robust array of femtosecond filaments were observed if the initial beam was modulated with four hole amplitude mask and each sub beam contains peak power in the certain range. The distance of a stable spatial structure formation was found to be much less than the value deduced from the Marburger formula. It was shown that the distance of filament array formation is determined by the laser radiation diffraction at each hole in the amplitude mask.

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≲1THz to the conical one at ν>8THz 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 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.

Transformation of the frequency-angular spectrum of THz emissions produced by a single-color laser filament plasma under an external electrostatic field of various strength is experimentally studied. While there is no any static electric field, THz emission is predominantly generated in the low-frequency spectral range around 0.1 THz and propagates within a hollow cone. When the electric field is applied, the transition from the hollow cone to a filled one is observed with the field strength rise and the THz emission frequency within the range of 0.3–0.5 THz. Higher frequency emissions of ∼1 THz fills the whole cone with the emission maximum along the laser filament axis.

At the frequencies from 0.1 to 1 THz, we measured the angular distributions of terahertz (THz) emission from DC-biased femtosecond filament. The external electric field (DC bias) was increased from 0 to 3.3 kV/cm and provided continuous transition from forward conical emission, corresponding to the unbiased single-color filament, to on-axis emission, corresponding to the DC-biased one. We decomposed the measured far-field THz distributions into the quadrupole and dipole contributions, the latter being increased with increasing biasing field. The superposition of quadrupole and dipole local sources was integrated numerically over the plasma channel length and fit to the experimentally obtained angular distributions. The transition from the conical to the on-axis emission occured at the external field of (3.2 ± 0.8) kV/cm in the range of frequencies studied.

The experimental results of energy deposition into the medium measurements and THz radiation measurements in case of two-color filamentation and filamentation in the presence of external electric field are presented. We considered different focusing conditions (NA=0.00218..0.00014) with the size of the beam equals to 8 mm Two-color radiation was made by placing BBO crystal into the converging beam.

We traced experimentally transition from a single air filament to the superfilament under action of powerful loosely focused (NA ∼ 0.0021) femtosecond beam. Two regimes were exploited with multifilament formation by artificial amplitude or intrinsic amplitude/phase front modulation of the beam having 10–60 critical powers P cr. Transverse spatial structure and energy density in the filament were studied using wideband acoustic detection and beam mode imaging single shot techniques at different distances along the optical path. We showed that with intrinsic front modulation a single extremely long ionized channel is formed provided peak power P of the initial beam does not exceed 20P cr. Its volumetric energy density is ∼1.5–3 times higher than in the single filament, while linear energy density is almost 10 times higher. Artificial amplitude modulation leads to formation of either a single long filament or two closely spaced filaments at the same initial conditions. Maximal volumetric energy density was the same in both cases and slightly less than without this modulation. A few closely spaced filaments are generated at higher peak powers P with volumetric and linear energy densities experiencing fast nonlinear increase with P. Highest linear energy density achieved was 600 μJ cm−1, i.e. almost 100 times higher than that of the single filament with increase in energy 10 times only. The volumetric energy density also increases by a factor of 10 to ∼800 mJ cm−3 proving huge increase in intensity and electron density that is characteristic feature of the superfilamentation. These findings were supported by the numerical simulations based on the Forward Maxwell equation with resolved driver of the field that showed superfilament splitting and confirmed energy densities estimated from the experimental data.

A new method for the in situ acoustic measurement of the beam pointing stability (BPS) of powerful pulsed lasers is tested. A broadband (similar to 6 MHz) piezoelectric transducer placed a few millimeters from the laser spark produces an electric pulse. We show that variation in time of the position of this pulse can be used to assess the BPS down to 1 piad in a few hundred laser shots. The estimated value coincides well with the BPS estimated using standard measurement in the far field.

A wideband piezoelectric transducer was used to receive signals from different filaments inside a multiple filament. A technique for calculating parameters of filaments in each laser pulse has been proposed. For a regularized superfilament created using of the amplitude mask a growth of absorbed linear energy density has been observed by more than an order compared to a single filament.

Comparative experimental data on filamentation of a powerful femtosecond laser beams with amplitude or phase front modulation is presented. We show that phase discontinuities and zero intensity lines prevented filament merging and superfilament formation.