The paper demonstrates results of our experimental study of THz emission from the plasma of a single-color laser filament for two laser wavelengths of 740 and 940 nm. Particular attention is paid to the study of a frequency-angular distribution of the THz emission within 0.1 – 3 THz spectral range. It is observed that different spectral components of THz radiation have different angular distribution, and an increase in the laser beam numerical aperture leads to a growth of the THz emission divergence, especially in its low-frequency range. The study revealed a significant effect of a laser pulse initial wavelength on the THz emission characteristics. Transformation of frequency-angular THz emission spectrum produced by a single-color (740 nm) laser filament plasma under an external electrostatic field of various strength is also experimentally studied. If there is no 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, THz emission frequency being within of 0.3 - 0.5 THz. Higher frequency emission of ~1 THz fills the whole cone with the emission maximum along the laser filament axis. Furthermore, the angular distribution for the low-frequency THz emission depends significantly on the laser pulse energy in contrast to the case of no electric field. Namely, the laser pulse energy rise results in a decrease of the propagation angles for low-frequency THz emission and disappearance of the local minimum in the angular distribution on the propagation axis.
While propagating in air in the filamentation regime a high-power ultrashort laser pulse experiences the self-organizing into spatially localized light channels, which represent parts of laser beam with the highest intensity and have angular divergence reduced in comparison with the beam as a whole. We present the experimental results on the main characteristics of post-filamentation channels formed by the filamentation of Ti:Sapphire-laser pulses (744 nm, 90 fs) in air. We found that these post-filamentation channels are characterized by a number of features, namely, broader spectral composition with pronounced red shift against the initial pulse spectrum, strong nonlinear phase modulation, and reduced pulse duration. We showed that the increase of initial pulse energy from 0.5 to 2 mJ does not affect post-filamentation channel energy (about 0.4 mJ) though leads to significant change in its spectrum. Thereby, relatively high intensity (more than 0.1 TW/cm2), low angular divergence (fractions of mrad) and wide spectral range of post-filamentation channel help to obtain an ultra supercontinual pulse spectrum by means of post-filamentation channel recurrent filamentation in a medium with high optical nonlinearity (e.g., solid dielectric). The obtained results can be useful in solving the practical task of high-intensive post-filamentation light channels application in laser pulse energy long-range delivery.
Emission of THz radiation from a single-color ultraviolet (248 nm) and infrared (744 nm) filament in air is studied experimentally and compared at similar pulse durations, focusing conditions and excess of peak pulse power over the critical power for self-focusing. An angular distribution of the terahertz emission for both ultraviolet and infrared pump is conical with the closed cone angle. In contrast, the terahertz radiation energy and spectrum differ significantly. The energy of terahertz emission from ultraviolet filament is 1-2 orders of magnitude lower than the terahertz yield from the infrared filament. The terahertz spectrum of ultraviolet filament is shifted to the low-frequency range and narrower as compared to the spectrum of terahertz emission from infrared filament. We explain qualitatively the difference in terahertz yields and spectra by lower intensity and plasma density in the ultraviolet filament. Similar behavior of THz spectra is observed when changing the IR filament parameters.
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