A new mechanism of nonlinear absorption of intense femtosecond laser radiation in air in the intensity range I = 10(11)-10(12) W/cm(2) when the ionization is not important yet is experimentally observed and investigated. This absorption is much greater than for nanosecond pulses. A model of the nonlinear absorption based on the rotational excitation of molecules by linearly polarized ultrashort pulses through the interaction of an induced dipole moment with an electric field is developed. The observed nonlinear absorption of intense femtosecond laser radiation can play an important role in the process of propagation of such radiation in the atmosphere.

Results of investigations into the spectral and power characteristics of two-photon fluorescence of a liquid drop with the Rhodamine 6G dye irradiated by femtosecond laser pulses are presented. It is demonstrated that stimulated emission of two types is realized in drops with the concentrated dye solution, namely, superfluorescence within the drop volume and lasing on whispering gallery modes.Creation of laser systems capable of generating femtosecond light pulses with high intensity sufficient for realization of multiphoton effects in the process of radiation interaction with substance in the middle 80s stimulated the formation of a number of new directions of modern physics and technology. Among these are new methods of laser control over physical and chemical processes based on the selective multiphoton absorption of femtosecond radiation by substance molecules, including finely dispersed substance (femtochemistry) [1, 2]. Creation of highly informative optical communication channels in the atmosphere also posed the problem of studying the multiphoton interaction of femtosecond radiation with substance, in particular, with disperse media incorporating the atmospheric aerosol. Upgrading systems of optical information transfer and processing aimed at increasing their operation speed and compactness of optical elements makes urgent a study of femtosecond radiation interaction with such objects as microfibers and dielectric microresonators that can serve as miniature emitters or frequency converters [3,4].It should be emphasized that the above-mentioned objects -disperse media -exposed to femtosecond radiation are specific physical systems. Their specifics are that spherical particles have focusing properties and form high-Q microresonators in which morphological resonances or modes of whispering gallery are realized [5]. This results in a sharp increase in the intensity of acting optical fields in the particle volume and, as a consequence, in reduced power thresholds of nonlinear optical processes proceeding in particles. In spite of the fact that optical processes in spherical particles and drops have been investigated already during a quarter of the century, only few works study the interaction of multiphoton femtosecond radiation with the aerosol and liquid-drop substance.The present paper is aimed at studying fluorescence of drops with an organic dye excited due to the multiphoton absorption of femtosecond laser pulses.A titanium-sapphire laser generating pulses at the wavelength λ = 0.8 µm with duration t p = 80 fs and 9 ns and energy per pulse <17 mJ served as a source of femtosecond radiation [6]. The width of the radiation spectrum for nano-and femtosecond pulses at half-maximum was ~25 nm. The intensity distribution over the beam cross-section was close to Gaussian. The femtosecond pulse energy was controlled by registration of radiation in the reference channel with the help of a calibrated photodiode.Unfocused femtosecond radiation was directed onto a drop with the Rhodamin 6G dye in ethanol. The drop ...

This paper presents the results of experimental investigations into the interaction of laser pulses of femtosecond duration with aerosol having various composition (aqueous aerosol, dye solution with embedded nanoparticles), as well as with individual particles in the case of two-photon induced stimulated fluorescence. The acoustic method was employed to study the decay ofthe energy ofthe filament generated at different focusing, as well as the transmittance of the filament energy as it passed through the aerosol layer.

The propagation of high-power laser radiation (HPLR) in an atmosphere is accompanied by radiation of acoustic waves.The characteristics of optoacoustical signals depend on factor of absorption HPLR and its power parameters. It is necessary to take into account absorption and attenuation of acoustic waves arising in the channel HPLR, in tasks of restoration of power parameters HPLR on optoacoustical signals. The deforming influence of an atmosphere on amplitude-frequency a spectrum registered on distance up to 1 km of optoacoustical signals is reduced to the account of classical absorption, of turbulent and preearthly attenuation. In work the questions of optimum program processing registered in experimental measurements of optoacoustical signals generated by the channel HPLR in an atmosphere are considered. The developed algorithm is realized in the created software <, allowing in real time on the basis of the meteorological data to compensate deforming influence of an atmosphere and spreading surface on amplitude-frequency the characteristics of optoacoustical signals of a sound range of frequencies