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.
Analysis of experimental data obtained in study of cryogenic CO laser with glow discharge and cryogenic slab CO laser with RF discharge showed that oxygen concentration in laser mixture is much less than in initial gas mixture. With low oxygen content, ozone plays role of supplier of electrons in cryogenic discharge. Since ionization potential of ozone is somewhat higher than that of oxygen, and ozone quickly condenses on cold walls of cryogenic discharge, conversion of O2 to O3 is accelerated. In addition, in plasma-chemical reactions, ozone regenerates carbon monoxide after dissociation. Acceleration of ozone generation in cryogenic barrier discharge was experimentally demonstrated.
Experimental and theoretical study of the post-filamentation stage of focused high-power Ti:Sa laser pulses in air is presented. Angular divergence of the laser beam, as well as angular and spatial characteristics of specific spatially localized light structures, the post-filament channels (PFCs), under different initial focusing conditions and laser beam energy are investigated. We show that PFC angular divergence is always less than that of the whole laser beam and tends to decrease with laser pulse energy increase and beam focal length elongation.
Remote monitoring of water pollution, namely thin films of oil or oil products on water surface, can be carried out by laser fluorimetry. The pollutants fluorescence during its interaction with ultrashort UV laser pulses was experimentally studied in this paper. The laser pulses power was considered in a wide range of values including the filamentation regime. We compared fluorescence stimulated by femtosecond UV laser pulses with two central wavelengths (248 and 372 nm) for detection of crude oil and the following oil products: oil VM-5, oil 5W-40 and solvent WhiteSpirit. It was shown that shorter UV wavelengths are more suitable for fluorescence excitation. The spatial resolution of the fluorescence localization was no worse than 30 cm. We discuss techniques of high intensity emission delivery to the remote target as post-filamentation channels and multifilamentation beam propagation regime as well experimentally and numerically.
The filamentation of focused beams at wavelength of 800 and 248 nm in air is studied experimentally and numerically. The results indicate that relatively tight focusing can lead to the coalescence of individual regions of high fluence and high plasma density that result from multiple refocusing, whereas in the case of weak focusing such regions are separated in the pulse propagation direction. The lower multiphoton ionization order in the case of UV radiation leads to a stronger effect of geometric focusing on filament formation. We show the possibility to control the parameters of femtosecond laser plasma filaments by introducing astigmatism in laser beam wavefront. Strong astigmatism can lead to the splitting of the channel into two separate regions. We demonstrate that the self-phase modulation in the thin passthrough dielectric plate decreases the distance to the filament start in air and increases the length of plasma channel.