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 ...
The filamentation dynamics of 15 GW femtosecond laser pulses at 800 and 400 nm wavelengths upon their tight focusing in air is studied experimentally. The spatial position and extent of plasma channel formed within the filamentation zone as a function of laser pulse power are investigated. The processing of the experimental data according to the Marburger formula by J. H. Marburger [Prog. Quantum. Electron. 4, 35 (1975)] and the dispersion relations for air cubic nonlinearity has allowed to estimate the effective value of the Kerr-driven air refractive index on 400 nm as 5.36 × 10−19 cm2/W with a 5% error.
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.
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