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The results of a study of pulse duration of superradiance on molecular nitrogen ions in air under different conditions of focusing a femtosecond laser pulse with a wavelength of 950 nm are presented. The active-medium parameters are analyzed and a mechanism is proposed for formation of a picosecond superradiance pulse at a femtosecond seed-pulse duration.

Evolution of the spectral characteristics of frequency-modulated radiation with pulse duration of 250 ps, energy of 2 mJ, and central wavelength λ = 475 nm in XeF(C-A) amplifier of THL-100 laser system is studied by numerical simulation methods. The laws of changing the spectra and spectral energy density of laser radiation have been investigated. The maximum spectral energy density of output radiation in the positive and negative chirp modes reaches 0.92 and 0.91 J∙nm–1, respectively, for a VUV amplifier pump radiation energy of 270 J. It has been shown that in the amplification saturation mode, the radiation wavelength with maximum spectral energy density increases from 475 to 477.8 nm for positive chirp and decreases from 475 to 472.5 nm for negative chirp.

Results of studies of the electric-discharge KrF laser with a specific pulse energy of ~6 J/L and an internal energy efficiency of more than 4% are presented. The formation of instabilities of various types in the volume discharge from an initial inhomogeneity of the electric field in the discharge gap and the partial composition of the gas mixture are shown. The possibility of stable discharge burning during several half-periods of the pump current is demonstrated.

The conditions of broadening the second harmonic spectrum in KDP upon pumping by a negatively chirped pulse of fundamental frequency with central wavelength of 950 nm are analyzed numerically. It is shown that broadening of the spectrum (K = 1.4) is mainly limited by difference in group velocities of radiation pulse between first and second harmonics. The article is based on materials of the report at the first All-Russian scientific conference with international participation "YENISEI PHOTONICS – 2020"

The possibility of generating high power terahertz radiation in a nonlinear crystal of zinc germanium phosphide (ZnGeP2) with pumping in a short-wavelength absorption shoulder of 0.67–2.5 μm is studied in detail. High power femtosecond Ti:Sapphire laser system operating at wavelengths of 800 and 950 nm are considered as a pump source. A peak terahertz radiation power of ~1 kW is obtained.

Experimental studies of the physical processes and parameters of radiation and plasma in the region of filamentation upon the axisymmetric and aberrational propagation of pump laser radiation with femtosecond pulse duration in air are performed. Conditions for the formation of the highly directional radiation of a white light supercontinuum are described.

Radiation spectrum broadening upon the generation of the second harmonic in nonlinear crystal is studied. Spectrum broadening is ensured in one or two nonlinear KDP crystals detuned by the phase matching angle or with chirped pulses of pump radiation. Possibility of the spectrum width doubling of the second harmonic is shown.

The results of investigation of the possibility of reducing the duration of the second-harmonic radiation pulse generated by the starting complex of an THL-100 laser system by conversion of a pre-chirped fundamental frequency pulse are presented. A front-end compressor of the complex is used to obtain a fundamental harmonic pulse chirp at a central wavelength of 950 nm with duration of 70 fs. It is shown that a small excess of the group velocity dispersion (±3200–4000 fs2) of fundamental frequency radiation increases the width of the second-harmonic radiation spectrum by a factor of 1.5–2. A compensation for the excess dispersion of radiation pulses with broadened spectrum makes it possible to reduce the second-harmonic radiation pulse duration approximately proportionally to its spectral broadening and to obtain a minimal pulse duration of 35 fs.

This paper presents the results of research on parameters of the THL-100 hybrid laser system of the visible spectrum with a view to modernizing it, in order to achieve the boundary parameters required for the pumping of non-linear crystals and production of the unprecedented THz radiation powers. The laser system consists of the Start-480M Ti:Sa starting complex and photodissociation XeF(C-A) amplifier with a 24 cm aperture. After modernization, the starting complex provides the spectrally limited radiation pulse of 60 fs at the first harmonic (950 nm) at energy of 25 mJ and 50 fs at the second harmonic (475 nm) at energy of 10 mJ. In the case of the positively chirped pulse with duration of 50 ps, the energy at the second harmonic is 3 mJ. When amplifying the negatively chirped radiation pulse in the XeF(C-A) amplifier at the wavelength of 475 nm with duration of 1.8 ps, the power of 40 TW is reached, and the maximum energy is 2 J. When amplifying the positively chirped pulse with duration of 50 ps, the highest energy of 3 J is achieved.

Results of investigations into the development of effective N2-lasers pumped by a longitudinal discharge with specific pump powers exceeding 3 MW/cm3 are presented. It is shown that the main reason that limits the duration and energy of the output beam is unstable burning of the diffusion discharge in these modes of laser operation. Methods of forming the diffusion discharge are proposed that allow the lasing pulse duration to be increased from 5 to 20 ns.

We present the thorough studies of dielectric properties of BiB 3 O 6 (BIBO) crystal in the sub-THz range. We observe a large birefringence Δn = nZ −nX = 1.5 and the values of absorption coefficients of all three axes to be less than 0.5 cm−1. The difference from visible range in angle ϕ between the axes z and X is found to be more than 6°. The simulated phase-matching curves show the optimal value of the angle θ to be around 25.5°±1° for an efficient millimeter-wave generation under the pump of 1064 nm laser radiation.

The optical properties and hardness of GaSe, GaSe:Al, GaSe:S:Al, and GaSe:Al:O crystals are studied to create effective terahertz radiation generators. Doped crystals differ from pure GaSe in high mechanical and optical properties at a optimum doping level. Oxygen doping allows wide ranging changes in birefringence in the terahertz range.

We report the results of investigations aimed at raising the peak output power of a THL-100 visible-range hybrid femtosecond laser system based on a XeF(C − A) photodissociation amplifier. The increase in power was achieved due to the shortening of the pulse duration at the system output by broadening the spectral profile of the second harmonic from 5.3 to 8 nm, which takes place in the transformation of a negatively chirped pulse of the fundamental frequency radiation in a nonlinear KDP crystal. After the KDP the radiation pulse was stretched to 1.8 ps in a prism stretcher and amplified in the XeF(C − A) amplifier. The energy obtained at the amplifier output was 1.2 J. After compression of the amplified output pulse, its duration was equal to 29.4 fs, which means that a peak output power of 40 TW was reached at the output of the laser system.

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.

Methods for increasing the energy characteristics of the THL-100 hybrid laser system are investigated by numerical modeling. The influence of the input laser radiation energy and system of mirrors on the energy and maximal laser radiation intensity in the amplifier is investigated. The systems of amplifier mirrors in which the number of passes of the active medium changed from Nm = 27 to 39 are considered. It is shown that at Nm =27, an increase in the pump energy of the amplifier from 270 to 400 J allows the output radiation energy to be increased by a factor of 2.6 (to 8.34 J) with a maximal radiation intensity in the amplifier no more than 7 GW·cm–2. Pulse compression to an initial duration of 50 fs will allow the average output power of the THL-100 system to be increased up to 160 TW.

Conditions for THz generation in the plasma of a filament produced by ultrashort laser pulses in air under aberrations in a one- and two-color optical scheme are observed. Possibilities are discussed of controlling the THz radiation pattern by application of an electric field and introduction of aberrations in the laser beam.

Results of investigation of conversion of radiation with the wavelength λ = 950 nm and pulse duration of 100 ps into the second harmonic are reported. It is demonstrated that the second harmonic pulse duration is half the pulse duration at the fundamental frequency. The converted radiation linewidth remains unchanged. The beam quality factor М2 increases from 1.16 to 1.56. The maximal conversion efficiency is 12.2%.

Results are presented from studying the parameters of second harmonic radiation under conditions of the transformation of a positively and negatively chirped pulse of fundamental frequency. It is shown that when the duration of a bandwidth-limited pulse (50 fs) is increased to 630 fs and beyond, the distribution of second harmonic intensity becomes close to Gaussian. The efficiency of second harmonic generation with duration of 2.5 ps falls from 38.69 to 4%, and the quality of the second harmonic improves until it becomes equal to М2 = 1.3. Substantial modulation of the amplitude and the narrowing of the second harmonic spectrum are observed upon negative chirping of the fundamental frequency.

The possibility of recompressing a positively chirped pulse with a duration of 50 ps to a femtosecond under conditions of the chirping of the fundamental harmonic is examined. The characteristics of a compressor based on diffraction gratings are presented, along with the procedure for adjusting it and the parameters of laser radiation. It is shown how a pulse of second harmonic radiation can be compressed to a duration of 75 fs.

The results are presented from experimental studies of the emission of NO radicals in a filament of femtosecond radiation produced in a N2/NO2 gas medium with total pressures of up to 1 atm. It is shown that in the recorded emission spectrum of the filament, emission transitions A2Σ+ (ν' = 0–2) – X2Π (ν'' = 0–5) of NO molecules are observed at concentrations of NO2 gas of up to 10 ppb−1. It is shown that the intensity of emission of NO A–X molecules grows along with the nitrogen pressure, due to the intermolecular nonradiative transfer of energy from the metastable $${{{\text{A}}}^{{\text{3}}}}\Sigma _{u}^{ + }$$ level of a N2 molecule to the electron A2Σ+ level of a NO molecule.

A study is performed of the generation of ultrashort THz pulses in Ti, Ni, and TiNi plates, compared to Au, Ag, Cu plates. The possibility of using them as photosensitive materials in bolometers is considered. Such sources can be used in atmosphere monitoring systems.

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 Σ u + ) excited states is most likely through their N 2 + ( X 2 Σ g + ) ground state in the process of multiphoton absorption of pumping radiation by nitrogen molecules and subsequent their excitation by electron impact.

We present the thorough studies of dielectric properties of BiB3O6 (BIBO) crystal in the subterahertz range. We observe a large birefringence Δn = nZ −nX = 1.5 and the values of absorption coefficients of all three axes to be less than 0.5 cm−1 at the frequency of 0.3 THz. The difference from visible range in angle ϕ between the dielectric axis z and crystallophysical axis X is found to be more than 6°. The simulated phase-matching curves in the xz plane of the crystal show the optimal value of the angle θ to be around 25.5°±1° for an efficient millimeter-wave generation under the pump of 1064 nm laser radiation.

THL-100 hybrid laser system is described and last results are presented. 1.2 J output energy and 29.4 fs pulse duration were obtained.

The results of experimental studies on the creation of a powerful source of narrowly directed THz radiation suitable for use in remote sensing systems are presented. A GaSe crystal is used as a radiation source when pumped by radiation pulses of femtosecond laser system with a wavelength of 950 nm.

THz wave generation was investigated in nonlinear ZnGeP2 crystals with the fs pumping at 800 nm and 950 nm. Transmission and absorption spectra were analyzed. Refractive indices of the ZnGeP2 in the THz range were measured and approximated in the form of Sellmeier equations. Phase-matching conditions were calculated. Calculated conditions are well correlated with experimental data.

We report the results of the spectral and temporal investigations of air plasma glow with a temporal resolution of ~ 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 N2 and the first negative system of molecular ions make their appearance in the spectrum. For NA < 0.03, the spectrum consists only of the molecular lines of N2 and . 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 , to 27 ps for N2, and to 20 ns for atomic lines.The bibliography includes 18 references.

We present the thorough studies of optical properties of BiB3O6 (BIBO) crystal in the millimeter-wave (subterahertz) range. We observe a large birefringence Δn = nZ -nX = 1.5 and the values of absorption coefficients of all three axes to be less than 0.5 cm−1 at the frequency of 0.3 THz. The difference from visible range in angle ϕ between the dielectric axis z and crystallophysical axis X is found to be more than 6°. The simulated phase-matching curves in the xz plane of the crystal show the optimal value of the angle θ to be around 25.5°±1° for an efficient millimeter-wave generation under the pump of 1064 nm laser radiation.

Due to their high optical damage threshold, borate crystals can be used for the efficient nonlinear down-conversion of terawatt laser radiation into the terahertz (THz) frequency range of the electromagnetic spectrum. In this work, we carried out a thorough study of the terahertz optical properties of the lithium tetraborate crystal (Li2B4O7, LB4) at 295 and 77 K. Approximating the terahertz refractive index in the form of Sellmeier’s equations, we assessed the possibility of converting the radiation of widespread high-power laser sources with wavelengths of 1064 and 800 nm, as well as their second and third harmonics, into the THz range. It was found that four out of eight types of three-wave mixing processes are possible. The conditions for collinear phase matching were fulfilled only for the o − e → o type of interaction, while cooling the crystal to 77 K did not practically affect the phase-matching curves. However, a noticeable increase of birefringence in the THz range with cooling (from 0.12 to 0.16) led to an increase in the coherence length for o − o → e and e − e → e types of interaction, which are potentially attractive for the down-conversion of ultrashort laser pulses.

Results of investigation of the parameters of the THL-100 hybrid laser system of visible range developed at the IHCE SB RAS in collaboration with the LPI RAS are presented. The laser system consists of the Ti:Sa Start-480M starting complex and the photodissociation XeF(C-A) amplifier with aperture of 24 cm. The starting complex provides 50-fs spectrally limited radiation pulses with energy of the second harmonic (475 nm) up to 20 mJ and 50-ps positively chirped radiation pulses with energy up to 5 mJ. The active medium of the XeF(CA) amplifier is created in the XeF2/N2 mixture upon exposure to VUV radiation of xenon excited by a highpower electron beam. It is demonstrated that record power of 14 TW is attained by amplification of negatively chirped radiation pulse with duration of 1.6 ps in the XeF(C-A) amplifier, and maximum energy of ~3.2 J is realized by amplification of positively chirped radiation pulse.

The results of studies on the development of laser sources in the UV and IR spectral ranges for the lidar systems operating on the basis of a method of laser fragmentation with subsequent laser-induced fluorescence (LF/LIF) are presented. It is shown that LF/LIF is an effective method for the remote detection of harmful and dangerous substances including nitrogen oxides in the atmosphere. The formation of a high-quality sounding UV radiation in excimer lasers and its transportation along the atmospheric path is studied. The causes are revealed that lead to the formation of noise arising in the atmosphere and limiting the sensitivity of the applied method.

Results from model studies of the possibility of creating a source of high-power picosecond pulses of terahertz radiation via optical rectification and phase-matched down conversion of femtosecond pulses of Ti:Sapphire lasers in a nonlinear LBO crystal are presented. Modified Sellmeier equations are used in calculations. It is shown that the lengths of coherence for the generation of THz radiation at frequencies higher than 0.5 THz are more than 0.5 mm, allowing the technologically simple production of periodic structures. The maximum length of coherence is achieved for s–s type interactions in the XY plane. Phase-matched s–f type down conversion is possible only in the XY plane, including noncritical spectral matching conditions. Maximum efficiency can be expected for three-wave interactions with the polarization of interacting waves parallel to Z axis.

The results from the generation and amplification of positively chirped pulses of 0.05 ns duration at a central wavelength of 475 nm in a THL-100 laser system are presented. It is found that the initial complex allows the generation of such radiation pulses with Gaussian intensity profiles and energies of up to 7 mJ. When such pulses with energies of 2–4 mJ are amplified in an XeF(C-A) amplifier, it is possible to achieve a saturation mode and obtain at the output laser beams with energies of up to 3.2 J. The possibility of compressing subnanosecond pulses at a wavelength of 475 nm in a compressor based on diffraction gratings is demonstrated.

The conditions for filamentation of femtosecond pulse laser radiation when focusing in air are studied experimentally and theoretically. A good agreement between experimental and calculated results is shown if neglecting the filament plasma. It is shown that the Kerr nonlinearity plays a fundamental role in the generation, existence, and cessation of a filament at a small numerical aperture (NA ≤ 2.15 × 10–3). The Kerr effect first leads to the beam self-focusing and generation of a filament, and at the final stage, to radiation defocusing and a sharp decrease in its axial intensity due to the beam wavefront distortions. In the case of aberration focusing, a spatial quasi-soliton is formed after a visible filament due to the balance between Kerr self-focusing and diffraction spreading. The quasi-soliton is a source of the directional white supercontinuum.

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 8–15 mJ, radiation wavelength of 940 nm 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 350 nm). 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 35–135 cm from the filament, radiation propagates in the form of a spatially stable structure similar to a soliton with a transverse dimension ≤300 µm. 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.

Comparative model analysis of the possibilities of down-conversion into the terahertz (THz) frequency range of radiation of all-solid-state femtosecond Ti:Sapphire laser systems and hybrid systems based on the same type lasers and equipped with amplifiers on XeF(C-A) and XeCl(C-A) molecules of excimer gases with harmonic generators on nonlinear LBO crystals is performed. It is established that when the crystal is cooled to a temperature of 80 K, the coefficient of optical losses in the frequency range from 0.2 to 1.5 THz linearly decreases in the direction of the X axis by 5–6 times down to 0.1–0.2 cm–1. It is shown that the use of hybrid laser systems as pumping sources will allow the power of generated THz pulses to be increased from 3.5 to 20 times.

We present experimental study results of THz range radiation generation in a laser plasma. Comparison of one and two-color optical pumping schemes is presented. Dependence of THz radiation energy from the electrical field application point to filament is shown. The maximum generation efficiency is observed when an electric field is applied to geometrical focus point (laser filament is formed before geometrical focus point). The mechanisms of this effect are discussed.

Optical properties of β-BBO nonlinear crystal in the THz range are studied. Frequency conversion of the femtosecond Ti:Sapphire laser complex operating at 950 nm into 0.25 – 0.8 THz range at room temperature is presented in detail.

Absorption and refractive indices spectra for nonlinear potassium titanyl arsenate (KTA) crystal are measured by THz time-domain spectrometer (THz-TDS) at 12 discrete temperatures in the range of 5 – 300 K with the accuracy of ±2 K and presented.

We report the results of experimental and theoretical investigations on the influence of spatial and energy parameters of input radiation with a pulse duration of on output characteristics of a XeF(C – A) amplifier in a visible-range, multi-terawatt THL-100 laser system. Dynamics of the energy density radial distribution for laser radiation passing through the amplifier is studied. Results of numerical simulation are presented for amplification of laser beams with Gaussian and super-Gaussian radial energy density distributions. It is shown that the laser energy of obtained experimentally is not the limiting value. According to calculations, the output energy of the amplifier with such mirror configuration may reach , which in the case of a pulse compressed down to corresponds to the radiation power of .

Amplification of conically diverging laser beams in the XeF(C-A) amplifier of the THL-100 hybrid laser system operating in the visible range (λ = 475 nm) is investigated experimentally and theoretically. Energies of 2 and 2.5 J are obtained experimentally and theoretically at the output from the XeF(C-A) amplifier for the chirped start pulse with energy of 0.8 mJ and duration of 2 ps. Patterns of changes of energy and spatial and temporal distribution of the amplified laser beam intensity are investigated theoretically. The maximal calculated energy density and the radiation pulse intensity are Wmax = 100 mJ/cm2 and Imax = 64 GW/cm2, respectively.

The results of theoretical and experimental studies of the second harmonic generation in a Ti:Sapphire femtosecond complex are described. The complex includes a femtosecond pulse generator, a stretcher, a regenerative amplifier, two multipass amplifiers, a compressor, and a second-harmonic generator. The effects of the radiation intensity, beam spatial profile, and the level of the noise component of the first harmonic at a wavelength of 950 nm on the homogeneity of the second harmonic are studied. It is theoretically shown that a good homogeneity of the second harmonic intensity is observed in the absence of the noise component in the beam at the fundamental wavelength. When making the amplitude inhomogeneities in the first harmonic even greater, inhomogeneities in the second harmonic appear. It is experimentally shown that an increase in the noise component in a pumping beam due to the use of a spatial filter allows strong suppression of small-scale inhomogeneities in the second harmonic.

In this paper, we give a review of some most powerful pulsed systems developed at the Institute of High Current Electronics (HCEI), Siberian Branch, Russian Academy of Sciences, and describe latest achievements of the teams dealing with these installations. Besides the presented high-power systems, HCEI performs numerous investigations using much less powerful generators. For instance, last year much attention was paying to the research and development of the intense low-energy (