Transformation of spectral parameters of a femtosecond laser pulse propagating in the atmosphere and in pure molecular nitrogen in the filamentation mode is experimentally studied; its causes are analyzed. The spectral broadening of pumping radiation in a wide range, from 350 to 1100 nm, is shown to be due to the combined action of Stokes stimulated Raman scattering at rotational transitions of nitrogen molecules in air, coherent anti-Stokes scattering, and cascade parametric four-wave mixing. The study of these processes is to make it possible to create highly efficient white light sources for atmospheric spectroscopy and remote sensing.

The paper presents the results of studies of argon-nitrogen plasma of a non-self-sustained low-pressure glow discharge with a hollow cathode by optical emission spectrometry. The plasma was generated in a mixture of Ar-N-2 gases with an argon percentage from 0 to 100% at a total pressure of 1 Pa. The discharge current and voltage were maintained constant and amounted to 18 A and 165 V, respectively. A significant increase in the amount of Ar+ (up to 30%) at a nitrogen content of 10-25% was shown, which is associated with a shift in the reaction of charge exchange of nitrogen with argon towards the generation of argon ions. It was found that at a low partial pressure of nitrogen (approximate to 10%), a sharp increase in the content of atomic nitrogen is observed which is probably due to the dissociation of nitrogen molecules upon collisions with an excited argon atom.

We report the results of a high temporal resolution study on the formation of superradiance (SR) in plasma on the first negative system of molecular nitrogen ions in plasma of an air filament. The filament is formed by a femtosecond laser pulse with a wavelength of λ = 950 nm. For the first time, the actual SR pulse duration at λ = 428 nm is measured, which constitutes 1.15 ps (FWHM). According to measurements of a spectral width equal to 0.26 nm, this pulse duration is close to transform-limited (1 ps). It is shown that there is a delay by 6 ps in the SR pulse maximum relative to the pump.

The results of spectral-temporal studies of the filament plasma emission in air with a time resolution of 2 ps were presented. The filament was created by laser radiation with pulse duration of 60 fs and wavelength of 940 nm. It was shown that a spectral composition of the filament plasma depended on the numerical aperture (NA). At NA > 0.035, the atomic and ion lines of oxygen and nitrogen dominate in the spectrum on the background of an intense continuous continuum in the visible region. Molecular lines of the second positive system of nitrogen and the first negative system of molecular ions appear in the spectrum for NA < 0.03. In case of NA < 0.01, the spectrum consists of only the molecular and ion lines of N-2 and N-2(+). The emission intensity maxima of these lines delay relative to the laser pulse. The atomic lines begin to glow per (80-100) ps after the laser pulse. The characteristic time of the luminescent lines decay at 1/e level for ions is 18 ps, for molecules is 27 ps, and for atomic lines is 20 ns. Rapid decrease in the intensity of molecular lines emission is due to the deactivation of the particles upper states by electrons. The formation of N-2(+)(B-2 Sigma(+)(u)) excited states is most likely through their N-2(+)(X-2 Sigma(+)(g)) ground state in the process of multiphoton absorption of pumping radiation by nitrogen molecules and subsequent their excitation by electron impact.

The experimental results of studying the temporal and spectral characteristics of air lasing of a first negative system of molecular nitrogen ions in laser plasma are presented here. Pumping was carried out by femtosecond laser pulse with a wavelength of 950 nm. By using streak camera, the pulse duration of lasing at lambda = 428 nm (1.15 ps, FWHM) was measured. It was close to transform-limited (1 ps). It is shown that there is a delay (5.6 ps) of lasing pulse maximum with respect to the pumping. Lasing has higher directionality and shorter duration compared with spontaneous emission.

We report the results of the spectral and temporal investigations of air plasma glow with a temporal resolution of similar to 2 ps. The plasma was produced by 950-nm laser pulses 60 fs in duration. It is shown that the spectrum obtained for a numerical aperture NA > 0.1 is dominated by atomic and ionic lines of oxygen and nitrogen against the background of a high-intensity continuous spectrum in the visible region. For NA < 0.07, the lines of the second positive system of molecular nitrogen N-2 and the first negative system of N-2(+) molecular ions make their appearance in the spectrum. For NA < 0.03, the spectrum consists only of the molecular lines of N-2 and N-2(+). The emission intensity peaks of these lines lag behind the laser pulse by 10-12 ps, but the atomic lines begin to shine 80-100 ps after the instant of irradiation. The characteristic time of line glow decay to a 1/e level amounts to 18 ps for N-2(+), to 27 ps for N-2, and to 20 ns for atomic lines.

Results on quasi-resonant optical pumping of Yb vapors by KrF*-laser radiation (λ p = 248 nm) are presented. The radiation was converted to the visible range to be available for study. Strong lines of amplified spontaneous emission are detected. These lines are due to transitions between the groups of even (6 s 5 d 1 D 2 , 6 s 6 d 3 D 1, 2, 3 ) and odd Yb levels (6 s 6 p 1 P 1 0 , 6 s 6 p 3 P 0,1,2 0 ). Coherent radiation is detected at Yb resonance lines with λ = 398.8 and 555.6 nm, which are caused, respectively, by the 6 s 6 p 1 P 1 0 → 6 s 6 s 1 S 0 and 6 s 6 p 3 P 1 0 → 6 s 6 s 1 S 0 transitions in the Yb atom. A sequence of processes that lead to the formation of inversion at the Yb atomic transitions is qualitatively examined.

The effect of astigmatism on laser air filamentation was studied numerically and experimentally. Limited supply of energy to the filament is shown to be the main feature of aberrational focusing, which facilitated the organization of quasi-solitons in the post-filament region. Quasi-solitons are realized due to the balance between linear diffraction and Kerr nonlinearity in the presence of background environment around zones with increased intensity. A highly directional SC of visible light is formed outside the visible filament zone in the resulting spatial quasi-solitons.

The results of formation conditions studies of a highly directional supercontinuum (SC) in a visible spectrum range obtained upon aberration spherical-mirror focusing of a radiation pulse with a wavelength of 940 nm, duration of 70 fs, and energy of 8–15 mJ are presented. It is shown that after visible filament there are two directed white light beams diverging relative to each other at an angle of 1.40 . Formation every light beam occurs through a gradual conversion of the spectral composition from long wavelength to short wavelength (to 350 nm) in a spatially stable structure similar to a soliton with a transverse dimension ≤ 300 μm. The nature of the appearance these beams is due to formation of two zones with higher intensity before meridional plate owing to the distortion of the wavefront of the laser beam in conditions of the astigmatism and the Kerr effect. In result two minima in the phase distribution located outside the beam axis are realized, which lead to the appearance of two off-axis areas with higher radiation intensity and as a consequence of this the formation of two highly directional laser beams.

Results of the experimental study of population inversion in the resonant electronic transition \({B^3}{\pi _g} - {A^3}\sum _u^ + \) of nitrogen ions by optical pumping of atmospheric air and pure nitrogen by a femtosecond laser pulse at a wavelength of 950 nm are presented. It is shown that the inversion results from selective population of the \(N_2^ + \left( {{B^2}\sum _u^ + ,v' = 0} \right)\) excited state during multiphoton excitation of the autoionization state of the nitrogen molecule with an energy of 18.7 eV. Seed photons for superradiance at transitions of molecular nitrogen ions are photons of the axial supercontinuum that occurs in a filament at the corresponding wavelengths. The superradiance mode is implemented at the wavelength λ = 358.4 nm referred to the transition of the CN molecule.

Special features of lasing in the most widespread molecular and atomic gases excited by a nanosecond transverse discharge are investigated. It is shown that the maximum of the lasing band on the C3Пu – B3Пg transition of the 0–0 vibrational molecular nitrogen band depends on the composition and pressure of the employed gas mixture and can be tuned from 0.2 to 0.3 nm. Simultaneous lasing on nitrogen molecules and ions at the wavelengths λ = 337.1 and 427.8 nm, respectively, is possible at a certain nitrogen content in the He/N2 mixture. Wherein, regions of lasing at different wavelengths are spatially separated in the output beam cross section. To obtain maximal energy of laser radiation in argon at λ = 912.3 nm, high He pressures (4 atm) and maximal charge voltages (25 kV) are required. The most probable reason for the limited lifetime of a CO2 laser is the accumulation of CN molecules in the mixture.

This paper presents the results of experimental studies on the formation conditions of a highly directional supercontinuum (SC) produced via filamentation of a femtosecond laser pulse in air. The laser beam had an energy of 8–15mJ, radiation wavelength of 940nm and pulse duration of 70 fs. A spherical mirror, with or without aberrations, was used to focus the laser beam. It is shown that a SC forms behind a visible filament through a step-by-step conversion of the spectral composition from long wavelength to short wavelength (to 350nm). The radiation is most stable when it is generated in the presence of aberrations in the wave front of the laser beam. On the track section 35–135cm from the filament, radiation propagates in the form of a spatially stable structure similar to a soliton with a transverse dimension ≤300µm. In this case, the SC significantly differs from the conical off-axis emission that occurs in the aberration-free filament, in that it displays a divergence close to the diffraction limit, linear polarisation and a shorter-range wing of the spectrum. The infrared component of the SC has a duration 2.8-times shorter than the pulse duration of the initial laser beam.

The experimental results of the inversion population mechanisms study in the resonant electronic transition B3Πg–A3Σu+ of nitrogen ions by optically pumped of air and pure nitrogen by femtosecond laser pulse at a wavelength of 950 nm are presented. It is shown that the inversion results from selective settling of N2+(B2Σu+, v' =0) excited state by multiphoton excitation of the autoionization states of the nitrogen molecule with energy of 18.7 eV. Seed photon for superradiance at transitions of molecular nitrogen ions are photons the axial supercontinuum occurring in the filament on the respective wavelengths. The mode of the superradiance at a wavelength λ = 358.4 nm referred to the transition of the CN molecules was realized.

The association of a ratio of blood porphyrin fraction concentrations in pregnant women with hyperandrogenism (HA) with the osmoresistance of red blood cells and the microviscosity of their membranes was studied. A cohort examination was made in parallel groups of 64 pregnant women aged 20 to 30 years at 28-36 weeks gestation. The levels of dehydroepiandrosterone sulfate (DHEAS) and testosterone were measured. External cardiotocography, ultrasonography, and Doppler study were conducted. The blood concentrations of endogenous protoporphyrin (PP) and coproporphyrin (CP) were estimated by the spectral fluorescence technique. Erythrocytic membrane microviscosity was determined from the degree of pyrene eximeration, by measuring the fluorescence spectra; erythrocytic osmoresistance was ascertained by the procedure developed by N. L. Vasilevskaya. The pregnant women with placental insufficiency in the presence of HA were found to have an altered ratio of the concentrations of erythrocytic porphyrin fractions, which enhanced the microviscosity and resistance of red blood cell membranes, causing worse microcirculation. There was an inverse correlation between the level of DHEAS and the ratio of the concentrations of endogenous PP to CP. It is concluded that the development of placental insufficiency in pregnant women with HA is attended by the decreased ratio of PP to CP along with the higher microviscous characteristics of erythrocytic membranes and the lower amplitude of their osmoresistance, by increasing the lower osmoresistance range.

A new device for drug monitoring was developed and tested. The principle of this device is based on detection of spectral characteristics of studied objects – electromagnetic radiation on their irradiation with pulse and continuous sources of laser and non-laser radiation and X-ray and electron beams. The device provides real-time monitoring of molecular and element structure of medical products using absorption (reflection), fluorescence, and Raman scattering spectroscopy. No preliminary sample preparation is required.

An experimental study of stimulated Raman conversion of high-coherent XeCl laser radiation in H-2 has been carried out. About 50 vibrational-rotational components were realized with a circularly polarized laser beam pump. High spatial and temporal coherence of conversed radiation was obtained. Quantum efficiency of conversion in first vibrational Stokes was achieved 95%.

An optogalvanic effect in a pulsed copper-vapor laser is discovered and investigated. It is found that the generalized deexcitation rate constant of the resonance level to the ionization state of the active medium is <σv>∼ (3±1)·10−7 cm3·s−1. An optothermal effect caused by the hysteresis of the optogalvanic phenomenon is found experimentally. The feasibility of measuring the stepped ionization constants of resonance and metastable states for a number of metal atoms in a pulse-discharge plasma is discussed.