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Actively mode-locked electron-beam-sustained-discharge CO laser producing a train of ∼5–15 ns (FWHM) spikes following with repetition rate 10 MHz for both single-line and multiline mode of operation in the mid-IR range of ∼5 μm was experimentally studied. Total laser pulse duration was ∼0.5 ms for both mode-locked and free-running laser. Specific output energy in multiline CO laser mode of operation was up to 20 Jl −1 Amagat −1 and the laser efficiency up to 3.5%. The active mode-locking was achieved for single-line CO laser mode of operation in spectral range 5.2–5.3 μm. This sort of radiation can be used for pumping an optical parametric amplifier for optical stochastic cooling in relativistic heavy ion collider, for laser ablation, and for studying vibrational and rotational relaxation of CO and NO molecules.

In the present work it is demonstrated that a wideband CO laser operating at fundamental and overtone vibrational transitions is a promising source of laser radiation for remote laser sensing of the atmosphere. A compact slab RF-discharge CO laser has been designed. The optimal operating conditions are determined for this CO laser. In experiments, the maximum average output laser power was ∼12 W for lasing efficiency of ∼14%. Under fixed experimental conditions, stable lasing (with fluctuations of the output laser characteristics ≤5%) was observed during more than one hour, which suggests that we first excited the sealed-off mode of cryogenic slab RF-discharge CO laser. The frequency-selective mode was first obtained for this laser. Wavelengths for sensing of nitrous oxide, nitrogen dioxide, methane, formaldehyde, and some other gases on near-ground propagation paths are determined. Our experiments and calculations confirm that this CO laser is promising for laser gas analysis.

The optogalvanic and optoacoustic methods are used to determine the mechanisms of the nonlinear absorption of intense ultraviolet femtosecond laser pulses in gaseous argon. An increase in the nonlinearity of the absorption process from 3 to 4 with an increase in the laser radiation intensity in the range 0.03–12 TW/cm2 is revealed. The possible physical mechanisms of this phenomenon are discussed.

It is shown that the addition of molecular oxygen to the working mixture of a pulsed electron-beam sustained-discharge (EBSD) laser CO amplifier leads to the increase in the small-signal gain (SSG) at the fundamental vibrational v+1→v transitions in a CO molecule for v=6–13, which is explained by the increase in the population of vibrational levels. In this case, the temporal parameters of the dynamics decrease at all probed transitions. Variations observed in the SSG dynamics are explained by analysing the kinetic processes of vibrational—vibrational exchange between molecules. The study of lasing parameters of a pulsed CO laser showed that the threshold energy input decreased with increasing the oxygen content in the working mixture. It is found that the CO laser efficiency at fundamental transitions increases with the addition of oxygen, the maximum efficiency being achieved at lower energy inputs. It is shown that the pulsed EBSD CO laser can operate on the air working mixture both at the fundamental transitions and the first vibrational overtone transitions v+2→v.

The first stage of the petawatt excimer laser project started at the P.N. Lebedev Physical Institute, implements a development of multiterawatt hybrid GARPUN-MTW laser facility for generation of ultra-high intensity subpicosecond ultraviolet (UV) laser pulses. Under this project, a multi-stage e-beam-pumped 100-J, 100-ns GARPUN KrF laser was upgraded with a femtosecond Ti:Sapphire front-end, to produce combined subpicosecond/nanosecond laser pulses with variable time delay. Attractive possibility to amplify simultaneously short and long pulses in the same large-scale KrF amplifiers is analyzed with regard to the fast-ignition, inertial confinement fusion problem. Detailed description of hybrid laser system is presented with synchronized KrF and Ti:Sapphire master oscillators. Based on gain and absorption measurements at GARPUN amplifier and numerical simulations with a quasi-stationary code, we are predicting that 1.6 J can be obtained in a short pulse at hybrid GARPUN-MTW Ti:Sapphire/KrF laser facility, combined with several tens of joules in nanosecond pulse. Amplified spontaneous emission, which is responsible for the pre-pulse formation on a target, was also investigated: its acceptable level can be provided by properly choosing staged gain or loading the amplifiers by quasi-steady laser radiation. Fluorescence and transient absorption spectra of Ar/Kr/F2 mixtures conventionally used in KrF amplifiers were recorded to find out the possibility for femtosecond pulse amplification at the broadband Kr2F (42Γ → 1,2 2Γ) transition, which benefits in 100 times higher saturation energy density than for KrF (B → X) transition.

The method of multifrequency laser probing is developed which can be used for diagnostics of the temperature and population of vibrational levels in gas mixtures containing CO molecules in excited vibrational states. The method is tested by studying the dynamics of the gas temperature and population of vibrational levels of the CO molecule in gas mixtures excited by a pulsed discharge. It is shown that the method provides the reduction of the gas temperature measurement error down to 3%. It is found that the population of lower vibrational levels in the CO–O2 mixture can exceed the population of levels in CO–He and CO–N2 laser mixtures by several times.

Small-signal gain (SSG) dynamics G(t) in the active medium of a pulsed laser amplifier operating on the v+1→vP(J) vibrational–rotational transitions of the CO molecule, including high (v > 15) vibrational transitions, is studied experimentally. It is demonstrated that as the vibrational number increases from 7 to 31, G changes with time slower, while Gmax decreases in this case by three times. It is found that at a fixed value of v the rate of the SSG rise increases with increasing the rotational number J > 6. It is shown that in oxygen-containing gas mixtures (CO:O2 = 1:19) the value of Gmax at low vibrational levels (for v < 13) can substantially exceed Gmax in mixtures containing nitrogen (CO:N2 = 1:19) instead of oxygen. It is found that the efficiency (47%) of a CO laser on mixtures with a high concentration of oxygen considerably exceeds the efficiency (30%) of a CO laser operating on a nitrogen-containing mixture.

Results are presented from experimental and theoretical studies of the production of singlet delta oxygen in a pulsed electron-beam-sustained discharge ignited in a large (∼18-1) volume at a total gas mixture pressure of up to 210 Torr. The measured yield of singlet oxygen reaches 10.5%. It is found that varying the reduced electric field from ∼2 to ∼11 kV/(cm atm) slightly affects singlet oxygen production. It is shown experimentally that an increase in the gas mixture pressure or the specific input energy reduces the duration of singlet oxygen luminescence. The calculated time evolution of the singlet oxygen concentration is compared with experimental results.

Multiline laser probing was applied for the diagnostics of the active medium of a wide-aperture carbon monoxide laser amplifier. The temporal behavior of the small-signal gain in the active medium of a pulsed electron-beam sustained discharge CO laser amplifier was studied with a specially designed probe CO laser in a wide range of ro-vibrational spectral lines (from V = 5 up to V = 31). The results were analyzed in order to reconstruct the temporal behavior of gas temperature and vibrational populations in CO-containing gas mixtures, CO:He and CO:N2, and oxygen-rich CO: O2 mixtures.

Lasing parameters of a rf discharge-pumped CO laser in a subsonic gas flow with subsequent cooling of the laser medium in a supersonic flow are studied. The output power of the laser achieved 2.1 kW for the efficiency up to 21% in the spectral range 4.9–5.7 μm corresponding to the V+1→V fundamental vibrational transitions in the CO molecule. The laser output power at the V+2→V first vibrational overtone transitions (2.6–2.7 μm) was 50 W. Possible ways of improving the laser design for generating radiation at higher overtone transitions of the CO molecule are discussed.

The electrical and spectroscopic characteristics of electron-beam-sustained discharge (EBSD) in oxygen and oxygen-containing gas mixtures are studied experimentally under gas pressures up to 100 Torr in a large excitation volume ({approx}18 L). It is shown that the EBSD in pure oxygen and its mixtures with inert gases is unstable and is characterised by a small specific energy contribution. The addition of small amounts ({approx}1%-10%) of carbon monoxide or hydrogen to oxygen or its mixtures with inert gases considerably improves the stability of the discharge, while the specific energy contribution W increases by more then an order of magnitude, achieving {approx}6.5 kJ L{sup -1} atm{sup -1} per molecular component of the gas mixture. A part of the energy supplied to the EBSD is spent to excite vibrational levels of molecular additives. This was demonstrated experimentally by the initiation of a CO laser based on the O{sub 2} : Ar : CO = 1 : 1 : 0.1 mixture. Experimental results on spectroscopy of the excited electronic states O{sub 2}(a{sup 1{Delta}}{sub g}) and O{sub 2}(b{sup 1{Sigma}}{sub g}{sup +}), of oxygen formed in the EBSD are presented. A technique was worked out for measuring the concentration of singlet oxygen in the O{sub 2}(a{supmore » 1{Delta}}{sub g}) state in the afterglow of the pulsed EBSD by comparing with the radiation intensity of singlet oxygen of a given concentration produced in a chemical generator. Preliminary measurements of the singlet-oxygen yield in the EBSD show that its value {approx}3% for W {approx} 1.0 kJ L{sup -1} atm{sup -1} is in agreement with the theoretical estimate. Theoretical calculations performed for W {approx} 6.5 kJ L{sup -1} atm{sup -1} at a fixed temperature show that the singlet-oxygen yield may be {approx}20%, which is higher than the value required to achieve the lasing threshold in an oxygen-iodine laser at room temperature. (laser applications and other topics in quantum electronics)« less

The possibility of obtaining a high specific input energy in an electron-beam sustained discharge ignited in oxygen gas mixtures O2 :A r :C O (or H2) at the total gas pressures of 10–100 Torr was experimentally demonstrated. The specific input energy per molecular component exceeded ∼ 6k J l −1 atm −1 (150 kJ mol −1 ) as a small amount of carbon monoxide was added into a gas mixture of oxygen and argon. It was theoretically demonstrated that one might expect to obtain a singlet delta oxygen yield of 25% exceeding its threshold value needed for an oxygen–iodine laser operation at room temperature, when maintaining a non-self-sustained discharge in oxygen gas mixtures with molecular additives CO, H2 or D2. The efficiency of singlet delta oxygen production can be as high as 40%.

The filamentation of focused beams at wavelength of 800 and 248 nm in air is studied experimentally and numerically. The results indicate that relatively tight focusing can lead to the coalescence of individual regions of high fluence and high plasma density that result from multiple refocusing, whereas in the case of weak focusing such regions are separated in the pulse propagation direction. The lower multiphoton ionization order in the case of UV radiation leads to a stronger effect of geometric focusing on filament formation. We show the possibility to control the parameters of femtosecond laser plasma filaments by introducing astigmatism in laser beam wavefront. Strong astigmatism can lead to the splitting of the channel into two separate regions. We demonstrate that the self-phase modulation in the thin passthrough dielectric plate decreases the distance to the filament start in air and increases the length of plasma channel.

The gain dynamics in the active medium of an overtone CO laser operating on the transitions between high vibrational levels is studied experimentally and theoretically. The gain dynamics is measured on the vibration — rotation transitions of the CO molecule [the P(12) spectral line in the vibrational bands from 20 → 18 to 36 → 34] in CO — He and CO — N2 laser mixtures. The maximum small-signal gain was 0.43 m-1. The theoretical model of the CO laser was significantly improved. It was shown that multiquantum and asymmetric VV-exchange processes should be included in the analysis of the population dynamics of high vibrational levels of the CO molecule.

Multiphoton ionization mechanisms and ionization rates of atmospheric air and constituent gases are studied at the 248-nm KrF laser wavelength within a laser pulse intensity range of 108–1013 W/cm2 using both long 25-ns and short 160-fs pulses. We have experimentally shown that it is the photoionization of water vapor naturally contained in atmospheric air that acts as the dominant process of air ionization. (2 + 1) Resonance-Enhanced Multiphoton Ionization (REMPI) occurs through 2-photon resonant excitation of water molecules, which results in a quadratic dependence of electron density on laser intensity at lower laser intensities of 108–1010 W/cm2 in the long pulse and in a cubic dependence at higher intensities of 1010–1013 W/cm2 in the short pulse. Direct 3-photon ionization and (3 + 1) REMPI take place in pure O2 and N2, respectively, and their contributions to air ionization are in the ratio of 5:3. The total ionization rate of O2 and N2 in atmospheric air is about an order of magnitude less than th...

A technique is presented to create uninterrupted long ultraviolet filaments in air using appropriately structured transmission mesh. The mesh with different cell sizes was inserted into 10-cm parallel beam of 0.2-J, 248-nm, and 870-fs pulse propagating along ~100-m corridor. Transverse positions of multiple filaments formed by the optimum size cells were reproducible within at least 15 m along the propagation path. 3D+time simulations confirmed uninterrupted plasma channels with fixed positions in the transverse space similar to the experiment. Unoptimized cell size resulted in filaments shifting towards the cell center and destruction of uninterrupted filaments.

Silicon based micro- and nanophotonics yielding plenty of applications from black silicon for photovoltaics to all-dielectric nanoantennas for sensing and nonlinear optics requires advanced methods of fabrication. Here we provide a detailed investigation of femtosecond laser fabrication of silicon based miro- and nanostructures formed via self-organization processes revealing a strong influence of nonlinear optical feedback during their growth under multipulse irradiation. Our results are supported by the experimental study of the effects of laser wavelength, fluence, exposure time, and ambient gas pressure. We demonstrate that both sub-µm and µm-scale structures have similar principles of formation, which paves the way to controllable and low-cost fabrication of advanced optoelectronics devices with multilevel patterning of functional elements.

The influence of a multi-line CO laser focusing on spectral characteristics of broadband sum-frequency generation in ZnGeP2 nonlinear crystal was experimentally and numerically studied. Maximal frequency conversion was experimentally observed under a tight focusing laser beam of a multi-line CO laser rather than a single-line one. The tight focusing resulted in a broadening sum-frequency generation spectrum and an increasing total, i.e. integrated over the spectrum, frequency conversion efficiency. These effects were due to the increasing phase-matching bandwidth and angular dispersion. The maximal conversion efficiency of the multi-line CO laser was numerically demonstrated to take place at a focal length of 0.4 times that required for the single-line one.

Experiments have been performed at hybrid Ti:sapphire/KrF laser facility GARPUN-MTW to develop a novel technique to create a hollow-core sliding-mode plasma-filament waveguide for directed transfer of microwave radiation. Efficient multiphoton air ionization was produced by a train of picosecond 1-TW UV pulses at 248 nm wavelength, or by amplitude-modulated 100 ns pulse combining a short-pulse train with a free-running 1-GW pulse, which detached electrons off O2− ions. Multiple filamentation of UV laser radiation in air was observed, and filamentation theory based on resonance-enhanced ionization was developed to explain the experimental results.

An influence of pass-through optics on femtosecond laser pulse filamentation in ambient air is analyzed for the first time both experimentally and numerically. Propagation of high-power femtosecond laser pulse through solid optical elements introduces spatiotemporal phase modulation due to the Kerr effect. This modulation may have a strong ef-fect on the pulse filamentation in air. We demonstrated that the phase modulation obtained in the thin pass-through dielectric plate reduces the distance to the filament onset and increases the plasma channel length, 4 pages, 4 figures

We have demonstrated for the first time that an array of nanoantennas on an aluminum surface, fabricated using a double-pulse femtosecond laser irradiation scheme, results in a 28-fold enhancement of electron photoemission yield, driven by a third femtosecond laser pulse. The numerical electrodynamic modeling indicates that the electron emission is increased owing to instant local electromagnetic field enhancement near the nanoantenna, contributed by both the tip’s “lightning rod” effect and the focusing effect of the pit as a microreflector in parabolic nanoantenna.

Filamentation of focused femtosecond laser pulse in the vicinity of the geometric focus is studied both theoretically and experimentally. Keywords—femtosecond pulse; geometric focus, plasma channel; filamentation; nonlinear optics High power femtosecond laser pulse propagates through the transparent medium with pulse energy localizati on accompanied with thin plasma channels (1). The phenomenon of filamentation occurs owing to the dynamic balanc e between two major physical effects: Kerr self-focusing and plasma defocusing of the beam. The paper (2, 3) demonstrat ed that main part energy contained in background reservoir during the propagation of femtosecond laser pulses in air. The filamentation process was terminated immediately when filament passed through a pinhole (3). It was shown in (4) that for collimated and tightly focused laser beam the f ilament parameters (radius of the filament, laser pulse int ensity, electron density in plasma channel) are strongly di fferent. But there are no data about background reservoir behavi or in the vicinity of the geometric focus with propagation of tightly focused laser beam. In this paper the some features of self- focusing and plasma channel formation with femtosecond laser pulse filamentation in focused laser beam at the area closed to the geometric focus are studied and discu ssed both experimentally and theoretically. Femtosecond laser pulse (740 nm, 100 fs) focused by concave dielectric mirrors (for the experiment desc ribed below the focal length was 50 cm). In the vicinity of the geometric focus a pinhole (diameter was about 300 μm) was placed. Linear plasma density was measured by hemisphere electrodes. When plasma channel formed by laser pulse was appeared between the electrodes the recharge curren t of the capacitor was measured by oscilloscope. The current was proportional to linear plasma density in the plasma channel. The laser pulse fluence distribution was visualized along the pulse propagation in filamentation area by a CCD-ca mera also.

Studies of the vibration-vibration (VV) exchange in CO molecules excited up to vibration quantum numbers v = 20 have been performed both theoretically and experimentally. A new kinetic model which takes into account the multi-quantum VV exchange in the temperature range T = 100-300 K is described for the first time. A description is given of the experimental methodology allowing for studies of the effects of the relaxation of the vibrational distribution after a sudden disturbance. The disturbance of the vibrational distribution is produced by a Q-switched short pulse of single line radiation in fundamental band. The relaxation is studied by measuring the laser pulse energy of the second pulse initiated by resonator Q-switching produced with a variable time delay relative to the first pulse. A set of kinetic rate constants accepted in the model for various gas temperatures, vibration level numbers and number of exchanged vibration quanta from 1 to 4 is presented. The good agreement between the experimental data and the results of the advanced theory is the first direct evidence in support of the multi-quantum exchange model.

Resonant absorption of first-overtone (FO) CO laser radiation by atmospheric water vapor and various organic and nonorganic pollutants is discussed. Suitable ro-vibrational lines are selected for their potential use in traditional spectroscopic schemes of atmospheric pollution diagnostics. Nonlinear atmospheric absorption of intense multiline FO CO laser radiation is simulated and laser-induced changes in absorption spectra are demonstrated. Absorption coefficients of a few substances on several FO CO ro-vibrational lines are experimentally measured. A comparative analysis of spectral characteristics of an FO CO laser and an HF (DF) laser demonstrates that the potential of FO CO laser applications for spectroscopic detection of various molecular trace gases is greater than that of HF and DF lasers.

Characteristics of a pulsed, multifrequency electroionisation CO laser operating on the first vibrational overtone of the CO molecule are studied experimentally and theoretically. It is shown experimentally that a pulsed, multifrequency overtone CO laser is an efficient source of coherent radiation in the 2.5–4.1-μm region whose specific output reaches 50 J l-1 amagat-1 and the electro-optical efficiency amounts to 11 %. A theoretical analysis based on experimental data shows the feasibility of obtaining overtone emission with efficiency as high as ~20 %. The alternation of the intensity of vibrational bands in the overtone CO laser is discussed. This effect is caused by the cascade mechanism of formation of the emission spectrum. The comparison of experimental data with calculations showed that they agree well with respect to spectral and energy characteristics of the overtone CO laser. Moreover, it revealed the necessity of refinement of the kinetic model of the active medium of the CO laser for obtaining an adequate description of time parameters of a laser pulse.

Output characteristics of a single-pulse e-beam controlled discharge laser operating on the first-overtone (FO) transitions (/spl Delta//spl nu/=2) of CO molecules have been studied both experimentally and theoretically. Various sets of dielectric mirrors with high reflectivity in the range of the overtone spectrum have been used for the laser resonator. Multiwavelength lasing has been obtained in the wide spectral range of 2.5-4.1 /spl mu/m on vibrational transitions from 6/spl rarr/4 up to 37/spl rarr/35. A maximum output efficiency of 11%, the highest ever published for a FO CO laser, has been experimentally obtained for the broad-band multiline FO CO laser. Maximum specific output energy is 50 J/1 Amagat. FO CO lasing was observed within a predischarge gas temperature range of 100-220 K. Spectral characteristics of the overtone laser operating on a selected set of vibrational bands have been analyzed theoretically. Theoretical calculations based on the experimental data predict that multiline FO CO laser efficiency can be increased up to 20%. Experiments with a single-pulse FO CO laser using a diffraction grating as a frequency-selective element demonstrated the feasibility of developing a frequency-tunable single-line FO CO laser. Tunable FO CO lasing on wavelengths from 2.7 up to 4.2 /spl mu/m corresponding to vibrational transitions from 13/spl rarr/11 up to 38/spl rarr/ 36 was obtained. The maximum specific output energy came up to 2.8 J/1 Amagat, with single-line output efficiency being up to 0.6%.

Tunable single-line first-overtone (FO) CO lasing on wavelengths from 2.7 up to 4.2 μm corresponding to overtone vibrational transitions from 13→11 up to 38→36 on 413 ro-vibrational lines was experimentally obtained. A parametric study of energetic and spectral characteristics of the single-line FO CO laser was carried out. Energy distribution over ro-vibrational lines was measured. The maximum specific output energy (SOE) came up to ∼3 J/l Amagat, with single-line output efficiency being up to 0.6%. For the first time, a multi-quantum theoretical model was used to describe the tunable single-line FO CO laser. This multi-quantum approach demonstrated better agreement between theoretical calculations and observed experimental data for laser output as a function of vibrational quantum numbers.

The effect of `spectrum alternation', i.e. the alternation of strong and weak vibrational bands, in a pulsed first-overtone CO laser spectrum is experimentally observed for the first time. Theoretically calculated spectral distribution also indicates the intensity alternation, which is determined by two sets of cascade transitions operating independently. An influence of laser mixture composition and gas density on the `spectrum alternation' is discussed.

The temporal dynamics and efficiency of phase-conjugate reflection in the course of intracavity degenerate four-wave mixing of radiation from a pulsed frequency-selective electron-beam-sustained CO laser was investigated experimentally and theoretically. The energy efficiency of the phase-conjugate reflection in the experiments reached 1.5 - 2.5% for a CO laser emitting as a result of one vibrorotational transition, diminishing on expansion of the emission spectrum of the laser. Comparison of the experimental and calculated data indicates the dominant role of the resonance amplitude phase-conjugation mechanism in the active medium of a CO laser. (nonlinear optical phenomena)

A compact sealed-off repetitively pulsed radio frequency discharge slab overtone CO laser was studied and developed. Maximum average output power was 0.3 W at a laser efficiency of 0.5%. The laser spectrum consisted of 80 vibrational-rotational spectral lines within the wavelength range of 2.5-4.0 microns.

Mechanisms of non-linear absorption and ionization of molecular nitrogen gas by UV femtosecond laser pulses were studied using photogalvanic and photoacoustic techniques. The effect of the intermediate Rydberg resonance, its dynamic Stark perturbation and ponderomotive upshift of the first ionization potential of nitrogen molecules by the intense laser pulses has been revealed by observing an increase of a power slope of ion yield from three to four at increasing laser intensity.

Output efficiency as high as 11% and specific output energy up to ∼50 J/l Amagat were experimentally obtained for a cryogenically cooled multiline pulsed first-overtone (FO) CO laser for the first time. The longest generated wavelength was 4.1 μm, corresponding to the 37→35 vibrational transition. FO CO lasing was observed within an initial gas temperature interval of 100–220 K. Theoretical calculations based on the experimental data predict that the FO CO laser efficiency can be increased up to 20%. High efficiency and high specific output energy obtained in the experiment places the FO CO laser amongst other high-power lasers.

Spectral properties of radiation of a pulsed electron-beam controlled discharge laser operating on the first-overtone transitions (Δv=2) of CO molecules have been studied both experimentally and theoretically. Various sets of dielectric mirrors with high reflectivity in the wide range of overtone spectrum have been used for the laser resonator. Multiwavelength lasing has been obtained in the wide spectral range of 2.5–4.0 μm. Efficiency of the laser operating on few vibrational transitions within a relatively narrow spectral range comes up to 5% at entirely suppressed fundamental band (Δv=1) lasing. Spectral characteristics of the overtone laser operating on a selected set of vibrational transitions have been analyzed theoretically. A comparison of the experimental and theoretical data has been made.

Phase conjugation process at intracavity degenerate four wave mixing of a long pulse CO2 laser radiation in its active medium has been studied experimentally. Dependencies of energetic and temporal characteristics of the phase conjugation signal upon specific energy stored into electric discharge, Q-factor of laser resonator, laser mixture composition and pressure, interaction angle, optical delay between probe and co-propagating waves, intensity ratio for these waves J3/J1, a degree of linear polarization, and laser spectrum are analyzed. Reflectivity of the phase conjugation signal is shown to increase with specific input energy rise and a decrease of the ratio J3/J1, being up to 7.5%. An increase of nitrogen content and a decrease of helium content in the laser mixture results in reflectivity rise and peculiar laser pulse profile with two maximums being due to amplitude and phase nonlinearities of the active medium. Specific input energy rise and an increase of Q-factor leads to a decrease of response time for the phase conjugation signal. Nonlinear properties of degenerate four wave mixing on amplitude gain and phase thermal diffraction gratings in the active medium of a long pulse CO2 laser are analyzed theoretically. An interconnection of nonlinear parameters of the amplifying media with the temporal behavior of phase conjugation reflectivity is determined. A comparison of the theoretical calculations and the experimental data has been done.

Parametric study of a first-overtone (FO) CO laser was carried out both experimentally and theoretically. A special optical filter was used for suppression of fundamental band lasing. The theoretical model is in satisfactory agreement with the experiment for a laser operating on a He-containing gas mixture. The maximum FO CO laser efficiency of 5.5% and specific output energy (SOE) of ∼20 J/l Amagat were obtained for a laser resonator with rather high useless optical losses, FO CO lasing taking place from 13→11 up to 30→28 vibrational transitions ( λ =2.7–3.6 μm). Important influence of intracavity water vapor on the output characteristics of FO CO laser and its spectrum, in particular, is discussed. Both theoretical and experimental results indicate that the FO CO laser efficiency up to 15–20% and SOE up to 50–80 J/l Amagat can be obtained.

We have demonstrated for the first time that an array of nanoantennas (central nanotips inside sub-micron pits) on an aluminum surface, fabricated using a specific double-pulse femtosecond laser irradiation scheme, results in a 28-fold enhancement of the non-linear (three-photon) electron photoemission yield, driven by a third intense IR femtosecond laser pulse. The supporting numerical electrodynamic modeling indicates that the electron emission is increased not owing to a larger effective aluminum surface, but due to instant local electromagnetic field enhancement near the nanoantenna, contributed by both the tip's lightning rod effect and the focusing effect of the pit as a microreflector and annular edge as a plasmonic lens., 4 pages, 3 figures

A review of studies fulfilled at the Lebedev Institute in collaboration with the Moscow State University and Institute of Atmospheric Optics in Tomsk (Siberia) on influence of various characteristics of ultrashort laser pulse on plasma channels formed under its filamentation is presented. Filamentation of high-power laser pulses with wavefront controlled by a deformable mirror, with cross-sections spatially formed by various diaphragms and with different wavelengths was experimentally and numerically studied. An application of plasma channels formed due to filamentation of ultrashort laser pulse including a train of such pulses for triggering and guiding electric discharge is discussed.

Electric breakdown and non-self-sustained electric discharge were triggered and guided by a train of ultrashort sub-TW ultraviolet (UV) pulses overlapped with a long free-running UV pulse of a hybrid Ti:Sapphire-KrF laser facility. Photocurrent sustained by this train is two orders of magnitude higher, and electric breakdown distance is twice longer than those for the discharge triggered by the long UV pulse only. UV filaments of ~100 m length were observed when transporting the laser radiation over the long distance.

An influence of plasma channel created by a filament of focused UV or IR femtosecond laser pulse ({\lambda}=248 nm or 740 nm) on characteristics of other plasma channel formed by a femtosecond pulse at the same wavelength following the first one with varied nanosecond time delay was experimentally studied. A dependence of optical transparency of the first channel and plasma density of the second channel on the time delay was demonstrated to be quite different for such a double UV and IR femtosecond pulses., Comment: 5 pages, 3 figures

An effective suppression of multiple filamentation of the sub-TW peak power supercritical laser beam in xenon gas was demonstrated in direct amplification of subpicosecond UV pulses at Ti:sapphire/KrF laser facility GARPUN-MTW. A large negative nonlinear refractive index due to a two-photon resonance of KrF laser radiation with Xe state ensured Kerr self-defocusing of a few hundred filaments with a local peak intensity of ~0.2 TW cm−2, 200-fold higher than the average one over the beam cross section, and thus homogenized the laser beam. UV filaments in Xe produced a narrow-angle monochromatic coherent cone emission at 828 nm wavelength due to stimulated hyper-Raman scattering and amplified spontaneous emission at the transition .

The fusion of several coherent 800 nm femtosecond filaments is induced experimentally and numerically by transmitting a beam through a mask with circular apertures followed by the focusing lens. The far-field image of the four-filament fusion region reveals bright on-axis maximum and differs drastically from the diffraction pattern of a low energy beam propagating through the mask in the linear regime. In 3D+time numerical simulations with the carrier wave resolved we show a factor-of-5 saturable growth in the peak plasma density with successive increase in the number of mask openings. An overall spectral blueshift of the fundamental and the third harmonics follows the plasma density increase. The simulated far-field on-axis emission agrees with the experiment and serves as the indication of nonlinear interaction in the fusion region.

An experimental and theoretical study of the post-filamentation stage of focused high-power Ti:Sa-laser pulses in air is presented. For the first time to our knowledge, the angular and spatial characteristics of specific spatially localized light structures, the ionization-free post-filament channels (PFCs), formed inside the laser beam in the post-filamentation region are systematically quantified under different external focusing and energy of initial pulse. We show that PFC angular divergence tends to decrease with the increase of the laser pulse energy and beam focal distance. These findings are discussed in the framework of the Bessel–Gauss-like beam formation in a course of pulse filamentation stage.

Non-self-sustained electric discharge and electric breakdown were triggered and guided by a train of ultrashort sub-TW ultraviolet (UV) pulses overlapped with a long free-running UV pulse of a hybrid Ti:Sapphire - KrF laser facility. Photocurrent sustained by this train is two orders of magnitude higher, and electric breakdown distance is twice longer than those for the discharge triggered by the long UV pulse only. When transporting the laser radiation over the long distance, UV filaments of ~ 100 m length were observed.

Two femtosecond laser nanograting fabrication mechanisms, including new ones recently found in our laboratory and related to sub-and near-threshold surface nanostructuring, are illustrated using experimental data for diverse solids nanostructured by 744-nm femtosecond laser pulses.

Non-self-sustained electric discharge and electric breakdown were triggered and guided by a train of ultrashort sub-TW ultraviolet (UV) pulses overlapped with a long free-running UV pulse of a hybrid Ti:Sapphire-KrF laser facility. Photocurrent sustained by this train is two orders of magnitude higher, and electric breakdown distance is twice longer than those for the discharge triggered by the long UV pulse only. When transporting the laser radiation over the long distance, UV filaments of ∼100 m length were observed.