Propagation Of Femtosecond Laser Radiation Research Articles

Postfilament-Induced Two-Photon Fluorescence of Dyed Liquid Aerosol Enhanced by Structured Femtosecond Laser Pulse

Laser-induced fluorescence spectroscopy (LIFS) is actively used for remote sensing of atmospheric aerosols and is currently one of the most sensitive and selective techniques for determining small concentrations of substances inside particles. The use of high-power femtosecond laser sources for LIFS-based remote sensing of aerosols contributes to the development of new-generation fluorescence atmospheric lidars since it makes it possible to overcome the energy threshold for the nonlinear-optical effects of multiphoton absorption in particles and receive the emission signal at long distances in the atmosphere. Our study is aimed at the development and experimental demonstration of the technique of nonlinear laser-induced fluorescence spectroscopy (NLIFS) based on the remote excitation of aerosol fluorescent emission stimulated by a spatially structured high-power femtosecond laser pulse. Importantly, for the first time to our knowledge, we demonstrate the advances in using stochastically structured plasma-free intense light channels (postfilaments) specially formed by the propagation of femtosecond laser radiation through a turbulent air layer to improve NLIFS efficiency. A multiple increase in the received signal of two-photon-excited fluorescence of polydisperse-dyed aqueous aerosols by the structured postfilaments is reported.

Experimental study of the filamentation phenomenon and measurement of filaments parameters

The article experimentally investigated the phenomenon of filamentation in the propagation of femtosecond laser radiation in transparent solid media. The experimental scheme for measuring the parameters of filaments using spherical and cylindrical optics was developed and assembled. A series of experiments on the spatial ordering of filaments was carried out. The sample was structured in the filamentation mode with a predetermined distribution of laser radiation intensity. The spatial redistribution of laser radiation passing through the formed structures in the quartz glass sample is also investigated.

Formation of channels with changed refractive index at the filamentation of femtosecond laser radiation in quartz glass

The paper considers the phenomenon of filamentation during the propagation of femtosecond laser radiation in transparent solid media as a tool for structuring quartz glasses, contributing to the formation of channels with a modified refractive index. This approach allows changing the intensity distribution in the transverse profile of the laser beam as it passes through the sample. The spatial intensity distribution is determined by the geometry of the formed channels. The use of high-intensity femtosecond laser radiation contributes to the formation of intensity distribution channels.

Study of the Filamentation Phenomenon of Femtosecond Laser Radiation

The filamentation phenomenon during propagation of femtosecond laser radiation in transparent solid media is experimentally studied. An experimental scheme for measuring the filament parameters using spherical and cylindrical optics is developed and assembled. The sample is structured in the filamentation mode, which makes it possible to obtain a specified distribution of laser radiation intensity.

Kinetic equation in the self-consistent field approximation for the number density of filaments forming in the propagation of femtosecond laser radiation

The general form of the kinetic equation for the filament number density with allowance for effects of their creation and decay is formulated—a nonlinear diffusion equation. The equation includes phenomenological parameters obtained by direct numerical simulation of the propagation of a high-power femtosecond laser pulse based on the stationary nonlinear Schrobinger equation for a number of limiting special cases.

Investigation of the Elementary Physical Processes in Plasma of Filamentation and Optical Breakdown Regions Accompanying the Propagation of the Femtosecond Laser Pulse with Wavelength of 950 nm in Air at Atmospheric Pressure

For the first time comparative investigations are performed of the emission spectra of pure air breakdown plasma initiated by a high-voltage pulse nanosecond discharge and nanosecond and femtosecond optical pulses. The plasma initiated in air by femtosecond laser pulses differs significantly from the plasma initiated by the nanosecond high-voltage discharge and optical breakdown. The propagation of femtosecond laser radiation in pure nitrogen in the filamentation region is accompanied by stimulated emission on transitions of the molecular nitrogen ion at a wavelength of 427.8 nm. The energy of the laser electromagnetic field is accumulated in the field of charged particles and is mainly dissipated in the processes of dissipative recombination. It is demonstrated that the emission spectra of air in the range 0.6–1 µm coincide almost completely for all excitation types, and in the range 0.2–0.6 µm, these spectra have the characteristic features caused by the parameters of high-voltage and optical breakdown plasma.