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In this paper, we measure broadband terahertz spectra from a single 744-nm filament produced in tight, medium, and loose geometrical focusing conditions. Terahertz spectra are measured using interferometer coupled to a helium-cooled bolometer avoiding any cutoff frequencies. Based on THz spectrum maximum position and spectral width, we estimate electron-heavy particle (neutrals and ions) collision rate. Numerical simulations are performed using the state-of-the-art model of unidirectional femtosecond pulse propagation and convergence/divergence of optical radiation at large angles.

The paper demonstrates results of our experimental study of THz emission from the plasma of a single-color laser filament for two laser wavelengths of 740 and 940 nm. Particular attention is paid to the study of a frequency-angular distribution of the THz emission within 0.1 – 3 THz spectral range. It is observed that different spectral components of THz radiation have different angular distribution, and an increase in the laser beam numerical aperture leads to a growth of the THz emission divergence, especially in its low-frequency range. The study revealed a significant effect of a laser pulse initial wavelength on the THz emission characteristics. Transformation of frequency-angular THz emission spectrum produced by a single-color (740 nm) laser filament plasma under an external electrostatic field of various strength is also experimentally studied. If there is no static electric field, THz emission is predominantly generated in the low-frequency spectral range around 0.1 THz and propagates within a hollow cone. When the electric field is applied, the transition from the hollow cone to a filled one is observed with the field strength rise, THz emission frequency being within of 0.3 - 0.5 THz. Higher frequency emission of ~1 THz fills the whole cone with the emission maximum along the laser filament axis. Furthermore, the angular distribution for the low-frequency THz emission depends significantly on the laser pulse energy in contrast to the case of no electric field. Namely, the laser pulse energy rise results in a decrease of the propagation angles for low-frequency THz emission and disappearance of the local minimum in the angular distribution on the propagation axis.

Propagation of a tightly focused high-power ultrashort laser pulse in an optical medium is usually substantially influenced by the medium optical nonlinearity that can noticeably affect the laser pulse parameters around the nonlinear focus and lead to unavoidable and often undesirable spatial distortions of the focal waist. We present the results of our experimental study and numerical simulations on a femtosecond Ti:Sapphire laser pulse propagation in air under different spatial focusing. We concentrated our study on spectral-angular and spatial pulse transformations under different focusing regimes - from linear to nonlinear one, when pulse filamentation occurs. For the first time to the best of our knowledge, we found the laser pulse numerical apertures range – namely, from NA = 2·10-3 to 5 10-3(for the laser pulse energy of 1 mJ), where the laser pulse distortions both in frequency-angular spectrum and pulse spatial shape are minimal. By means of the numerical simulations, we found the threshold pulse energy and peak power in a wide range of focusing conditions, within which a transition between the linear and strongly nonlinear laser pulse focusing in air takes place. This energy limit is shown to decrease with pulse numerical aperture enhancement. Our findings identify the laser pulse numerical apertures and energes adequate for getting a maximum laser intensity with a good beam quality around the focal point suitable for various laser micropatterning and micromachining technologies.

Frequency-angular distributions for THz emission in the range of 0.1–3 THz generated in a single-color laser filament plasma both under an electrostatic field and without one are experimentally studied for what we believe is the first time. The angular distribution for various spectral components of this THz emission is demonstrated to differ significantly. The maximal propagation angles for these components increase under the electric field. The angular distribution for the low-frequency THz emission depends significantly on the laser pulse energy in contrast to the case of no electric field. An increase in the laser pulse energy leads to a decrease of the propagation angles for low-frequency THz emission and to disappearance of the local minimum in the angular distribution on the propagation axis.

A design of high-efficient solid-state source of sub-picosecond ~10-μm wavelength laser pulse is presented. It is based on difference frequency generation in LiGaS 2 nonlinear crystal of ~0.5 μm wavelength laser pulse and its Raman-shifted pulse produced in BaWO 4 crystal. Numerical efficiency of the laser system was estimated to be ~1-2% at experimentally demonstrated efficiency of the Raman shifter ~20%.

Spectra of THz radiation generated in a single-color filament are experimentally studied. It is shown that the spectra of THz emission depend on the wavelength of laser radiation and the focusing conditions and do not depend on the pulse energy.

We for the first time theoretically predict and experimentally demonstrate that selective, low-threshold excitation and interference between short-wavelength plasmons on silicon surfaces photoexcited by infrared femtosecond laser pulses give rise to appearance of permanent sub-diffraction surface gratings with their periods down to 100 nm, dramatically differing from near-wavelength surface gratings succeeding the common “laser photon-surface polariton” interference. Such selectivity was provided in this work by in situ white-light (supercontinuum) femtosecond source generated in a multi-filamentation regime in a thin water layer atop the silicon surface, supporting high-efficiency excitation and dynamic tracking of the transiently tunable plasmon resonance on silicon surface for its easy, robust and ultimate surface nanopatterning.

Emission of THz radiation from a single-color ultraviolet (248 nm) and infrared (744 nm) filament in air is studied experimentally and compared at similar pulse durations, focusing conditions and excess of peak pulse power over the critical power for self-focusing. An angular distribution of the terahertz emission for both ultraviolet and infrared pump is conical with the closed cone angle. In contrast, the terahertz radiation energy and spectrum differ significantly. The energy of terahertz emission from ultraviolet filament is 1-2 orders of magnitude lower than the terahertz yield from the infrared filament. The terahertz spectrum of ultraviolet filament is shifted to the low-frequency range and narrower as compared to the spectrum of terahertz emission from infrared filament. We explain qualitatively the difference in terahertz yields and spectra by lower intensity and plasma density in the ultraviolet filament. Similar behavior of THz spectra is observed when changing the IR filament parameters.

Nonlinear frequency conversion of broadband mid-IR radiation of different molecular gas lasers by various nonlinear crystals converting their multi-line emission into broadband radiation on sum and difference frequencies within ∼ 1.7 – 17 micron is discussed.

We experimentally compare spectrum conversion of an ultrashort laser pulse under its filamentation in various gases. The laser pulse propagation through both atomic and molecular gases is observed to result in a separated local maximum formation in the long-wavelength spectral wing. This redshifted hump is more noticeable and does have a higher amplitude under the laser pulse filamentation just in molecular gases as compared to atomic ones. Therefore, we claim that the laser pulse self-phase modulation plays a crucial role in the formation of such a hump, while the Raman scattering in molecular gases enhances this process efficiency.

Two new methods for the formation of nanotip arrays under the action of single or double femtosecond laser pulses on a surface of bulk aluminum are presented. It is shown that a key role in the formation of nanotips is played by the excitation of surface electromagnetic waves and their mutual interference, and by their interference with the exciting electromagnetic field of the laser pulse.

Electric collector investigations of the singleand multi-shot femtosecond laser ablation of optical-quality surfaces of different materials, including aluminum, copper, titanium, silicon, and graphite, show that the emission of erosion plasma is significantly lower than the energy density of laser ablation of these materials and replaces the dominant electron emission at lower energy densities. I–V characteristics and cumulative dependences of the collector signal are studied in the emission mode. The observed dependences of the electron and plasma emission signals on the laser pulse energy density are discussed.

The influence of plasma channel length on an angular terahertz (THz) radiation distribution is experimentally studied for the channel formed under filamentation of an ultrashort laser pulse. It is shown that the angular distribution of the THz emission depends only on laser intensity in the filament and plasma density of the plasma channel and does not depend on the plasma channel length. A qualitative explanation of the THz emission screening by the filament plasma channel is proposed.

Super-broadband mid-IR laser systems are under development in the Gas Lasers Lab of the Lebedev Physical Institute. These systems consist of different sorts of molecular gas lasers and various nonlinear crystals converting multi-line emission of those lasers into broadband radiation on sum and difference frequencies. As frequency converters, both main mid-IR nonlinear crystals such as ZnGeP2, GaSe, AgGaSe2 and new ones such as BaGa2GeSe6, PbIn6Te10 are applied. As radiation sources, repetitively-pulsed Q-switched lasers pumped by both longitudinal DC discharge and transverse slab RF discharge emitting microsecond pulses, and also actively mode-locked e-beam sustained discharge lasers emitting nanosecond pulses are applied. It was experimentally demonstrated for the developed laser systems to emit broadband radiation on hundreds spectral lines covering mid-IR range from ~1.7 to ~17 micron due to sum and difference frequency generation including third harmonic generation.

Ultrafast intense photoexcitation of a silicon surface is complementarily studied experimentally and theoretically, with its prompt optical dielectric function obtained by means of time-resolved optical reflection microscopy and the underlying electron-hole plasma dynamics modeled numerically, using a quantum kinetic approach. The corresponding transient surface plasmon-polariton (SPP) dispersion curves of the photo-excited material were simulated as a function of the electron-hole plasma density, using the derived optical dielectric function model, and directly mapped at several laser photon energies, measuring spatial periods of the corresponding SPP-mediated surface relief nanogratings. The unusual spectral dynamics of the surface plasmon resonance, initially increasing with the increase in the electron-hole plasma density but damped at high interband absorption losses induced by the high-density electron-hole plasma through instantaneous bandgap renormalization, was envisioned through the multi-color mapping.

We report the results of laboratory experiments and numerical calculations of the spatial position and structure of a plasma channel produced in air by high-power focused femtosecond laser radiation with wavelengths of 740 and 248 nm as a result of its self-focusing and filamentation. A comparative analysis of the physical patterns of filamentation of IR and UV laser beams with variations in the beam focal length, transverse size and power is performed. It is found that a plasma channel beyond the linear focal waist of the laser beam is formed differently for two different spectral ranges.

Emission of erosive plasma has been observed during electric probe and optical emission spectral measurements of plumes produced by single-shot femtosecond laser ablation of optical-quality surfaces of various materials—copper, titanium, and silicon—at laser fluences well below the corresponding thermal ablation thresholds, replacing presumably electron emission at lower fluences. The onset of erosive plasma correlates on the fluence scale with saturation of dependences of self-reflectivity of the pumping femtosecond laser pulses, reflecting the “freezing” of electron dynamics (variation of electron density or temperature) during the pumping pulses, despite the monotonically increasing laser fluences.

The critical power of self-focusing in the air for ultrashort UV laser pulses stretched due to dispersion from 90 up to 730 fs was experimentally measured. It was shown that the pulse duration enhancement due to its propagation in condensed media within above mentioned time-range leads to almost linear decrease of self-focusing critical power. It was also observed that when the pulse peak power exceeds the critical one, the maximum of linear plasma distribution along the UV laser filament does not shifted in the direction opposite to the laser pulse propagation as in the case of IR laser filament, but remains at the geometrical focus., 4 pages, 2 figures

We have demonstrated experimentally and numerically the possibility of controlling parameters of plasma channels formed during filamentation of a femtosecond laser pulse by introducing astigmatism in the laser beam wavefront. It is found that weak astigmatism increases the length of the plasma channel in comparison with the case of aberration-free focusing and that strong astigmatism can cause splitting of the plasma channel into two channels located one after another on the filament axis.

Terahertz (THz) emission from a single-color ultraviolet (UV) (248 nm) filament in air is studied experimentally and compared to that of an infrared (IR) (744 nm) filament for similar pulse durations, focusing conditions, and excess of pulse power over the critical power for self-focusing. An angular distribution of the THz emission for both UV and IR filaments is conical with the close cone angle. In contrast, the THz radiation energy and spectrum differ significantly. The energy of THz emission from the UV filament is 1–2 orders of magnitude lower than the THz yield from the IR filament. The THz spectrum of the UV filament is shifted to the low-frequency range and is narrower compared to the spectrum of THz emission from the IR filament. We explain qualitatively the difference in THz yields and spectra by lower intensity and plasma density in the UV filament.

Relief ripples with sub-diffraction periods (≈λlas/3, λlas/4) were produced on a aluminum surface immersed in water and irradiated in a multi-filamentation regime by focused 744 nm femtosecond laser pulses with highly supercritical, multi-GW peak powers. For the VUV (8.5 eV) surface plasmon resonance on the wet aluminum surface, such small-scale surface nanogratings can be produced by high – second and third – optical harmonics, coming to the surface from the optical filaments in the water layer. Then, the sub-diffraction surface ripples may appear through interference of their transverse electric fields with the longitudinal electric fields of their counterparts, scattered on the surface roughness and appeared as the corresponding high-energy, high-wavenumber surface polaritons.

We have developed a new method for manipulating nanotopology and chemical composition of titanium surfaces by a scanning femtosecond laser beam with an asymmetrical spatial fluence distribution. Using of a common Gaussian laser beam leads to formation of combined nano- and micro-scale surface topologies owing to superimposition of different laser beam parts during the surface scanning. In contrast, beam shaping results in presumable surface structuring at specific laser fluences yielding in “clean” surface structures. In a simple case, cutting of a half beam perpendicular to the scanning direction enables to eliminate surface nanotopology, leaving less oxidized, cleaner and smoother microspikes, just by changing the scanning direction. Optimal experimental conditions were revealed to obtain in air the topologically and chemically different surface structures using the asymmetrical femtosecond laser beam and the opposite scanning directions.

Nonlinear propagation and focusing within a metallic ring of intense (near or higher than metal ablation threshold) surface plasmon-polaritons excited by a femtosecond laser pulse was experimentally and theoretically studied. Fundamental possibility of local fluence increasing at focus of such plasmonic lens (the ring) by two orders of magnitude was shown. Formation of surface nanostructures as a result of the surface plasmon-polaritons focusing was experimentally observed.

A surface electromagnetic wave has been excited on an atomically smooth silicon surface by an intense infrared femtosecond laser pulse as a result of its self-diffraction on a microscale short-lived optical inhomogeneity of the excitation region rather than on the roughness of its surface relief. This wave has been visualized in the form of the pattern of its interference with the same incident infrared ultrashort pulse, which corresponds to the instantaneous surface dielectric constant grating (reflection), as well as the resulting surface relief grating, using time-resolved far-field optical reflection microscopy.

Nanostructured near-IR antireflective layer was produced on a GaAs slab surface by direct femtosecond laser fabrication of a surface diffraction grating. The single nanostructured layer on the GaAs slab reduces its total reflection at the wavelength λ ≈ 2.5 μm by 42 %, in agreement with the second-order approximation of the effective medium theory, with negligible increase of its absorbance.

A laser system emitting within the IR wavelength range from 12 to 16.6 micron is discussed. The hybrid laser system consists of molecular gas lasers with a frequency conversion in a nonlinear crystal. One gas laser is a carbon monoxide laser operating in multi-line or single-line mode. Another one is a carbon dioxide laser operating in multi-line mode. These lasers operate in a Q-switched mode. Spectrum formation at difference frequency generation of CO and CO2 lasers in AgGaSe2 crystal is studied experimentally and theoretically.

The results of theoretical and experimental investigations of the influence of air pressure on the dimensional and power parameters of preliminary focused GW femtosecond laser radiation are presented. A nonlinear region of the focus of a laser beam is examined. It is established that in rarefied air the achievement of higher values of laser radiation intensity in the focal waist is possible due to a decrease in the plasma blocking action.

This paper examines the propagation of focused femtosecond gigawatt laser pulses in air under normal and reduced pressure in the case of Kerr self-focusing and photoionisation of the medium. The influence of gas density on the beam dimensions and power and the electron density in the plasma column in the nonlinear focus zone of the laser beam has been studied experimentally and by numerical simulation. It has been shown that, in rarefied air, the radiation-induced reduction in the rate of plasma formation diminishes the blocking effect of the plasma on the growth of the beam intensity in the case of tight focusing. This allows higher power densities of ultrashort laser pulses to be reached in the focal waist region in comparison with beam self-focusing under atmospheric pressure.

Color marking of rough or smooth metal (Al, Cu, Ti) and semiconductor (Si) surfaces was realized via femtosecond laser fabrication of periodic surface nanorelief, representing one-dimensional diffraction gratings. Bright colors of the surface nanorelief, especially for longer electromagnetic wavelengths, were provided during marking through pre-determined variation of the laser incidence angle and the resulting change of the diffraction grating period. This coloration technique was demonstrated for the case of silicon and various metals to mark surfaces in any individual color with a controllable brightness level and almost without their accompanying chemical surface modification.

The prepared samples were characterized by reflection and transmission spectra of broadband visible-range radiation, and the volume fraction of substances introduced into pores of artificial opals is estimated. It is found that the introduction of solid-state nanoparticles with negative real part of the permittivity into opal pores results in a decrease in the effective refractive index and a short-wavelength shift in the spectral position of the stop band in artificial opal.

One-dimensional quasi-periodic nanogratings with spacings in the range from 160 to 600 nm are written on a dry or wet titanium surface exposed to linearly polarized femtosecond IR and UV laser pulses with different surface energy densities. The topological properties of the obtained surface nanostructures are studied by scanning electron microscopy. Despite the observation of many harmonics of the one-dimensional surface relief in its Fourier spectra, a weak decreasing dependence of the first-harmonic wavenumber (nanograting spacing) on the laser fluence is found. Studies of the instantaneous optical characteristics of the material during laser irradiation by measuring the reflection of laser pump pulses and their simulation based on the Drude model taking into account the dominant interband absorption allowed us to estimate the length of the excited surface electromagnetic (plasmon-polariton) wave for different excitation conditions. This wavelength is quantitatively consistent with the corresponding nanograting spacings of the first harmonic of the relief of the dry and wet titanium surfaces. It is shown that the dependence of the first-harmonic nanograting spacing on the laser fluence is determined by a change in the instantaneous optical characteristics of the material and the saturation of the interband absorption along with the increasing role of intraband transitions. Three new methods are proposed for writing separate subwave surface nanogratings or their sets by femtosecond laser pulses using the near-threshold nanostructuring, the forced adjustment of the optical characteristics of the material or selecting the spectral range of laser radiation, and also by selecting an adjacent dielectric.

Optical breakdown and the formation of hot subcritical plasma in the laser plume during nanosecond laser ablation of a graphite target are studied by contact and contactless methods of ultrasound diagnostics. It is shown that optical breakdown of vapors during plasma formation is associated with threshold near-critical explosive boiling of an overheated material melt; further heating of subcritical plasma is described by known dimensional relations.

One-dimensional periodic nanostructures have been produced on the surface of an aluminum specimen using femtosecond laser pulses at wavelengths of 744 and 248 nm. The nanostructurization of the specimen has been conducted in water and in air in the preablation regime. We investigate the dependence that the surface topology has on the parameters of laser radiation (wavelength, fluence, and number of pulses), as well as on the medium in contact with the specimen surface. A calculation of the optical characteristics of aluminum as they depend on the electron temperature is performed that is good at describing the dependence that the reflection of the p-polarized infrared femtosecond pulses of pumping has on the fluence. Using these optical characteristics of the photoexcited aluminum within the interferential model, periods of the aluminum surface nanogratings are estimated which are in good agreement with the periods measured experimentally.

Gradual evolution of self-induced silicon surface topology from one-dimensional ridge-like to two-dimensional spike-like nanogratings and then to isotropic sets of micro-columns was observed by evenly increasing IR and UV femtosecond laser irradiation dose. This topological evolution exhibits clear indications of consequent melting and vaporization processes being set up during the prolonged laser irradiation. Monotonously decreasing cumulative IR and UV femtosecond laser-nanostructuring thresholds may indicate an increase of optical absorbance of the laser-nanostructured silicon surfaces versus the increasing laser dose, consistent with the consequent onset of the abovementioned thermal modification processes.

The influence of focusing conditions (numerical apertures from 0.004 to 0.06) on absolute energetic characteristics of third harmonic generation (THG) in air was experimentally studied for pumping 1R (744 nm wavelength) femtosecond laser pulses. THG was observed both for sub-critical and super-critical laser pulses in the linear and non-linear propagation modes, respectively. The maximum THG efficiency of 1.6 × 10−3 was obtained in our experiments at the tight focusing conditions and the sub-critical pulse powers.

Peculiarities of the self-focusing and filamentation of high-power femtosecond laser pulses in air have been experimentally and theoretically studied under conditions of broad variation of the beam focusing parameter. The influence of the numerical aperture (NA) of the initial radiation focusing on the main characteristics of laser-induced plasma columns (characteristic transverse size, length, and concentration of free electrons) is considered. It is established that, for a rigid (NA > 0.05) initial laser beam focusing, the transverse size of the plasma channel ceases to decrease at a level of Rpl ≈ 2–4 μm as a result of strong refraction of radiation on the plasma formed at the focal waist, which prevents further contraction of the laser beam due to its focusing and self-focusing.

Point-by-point multi-shot femtosecond laser writing of micro-scale linear damage tracks and symbolic logo information was performed inside single-crystal natural diamonds in transversal writing geometry at variable basic operational parameters (numerical aperture of focusing optics, depth of focusing inside samples and peak laser pulse powers). The filamentary character of femtosecond laser writing of buried damage tracks in this material at the supercritical peak laser powers was revealed.

The propagation of focused femtosecond laser pulses with supercritical peak powers in air has been investigated by the methods of optical visualization, profilometry, and calorimetry. Laser pulses with supercritical powers create a bundle of submillimeter filaments with a diameter of about 5 µm ahead of the lens focus; the maximum number of filaments in the beam cross section and their length increase linearly and sublinearly, respectively, with the radiation peak power. The optical visualization and calorimetry indicate that the plasma channels of filaments are optical contrast (a plasma density of 1018–1019 cm−3), ensuring the refraction of laser radiation incident on them.

A compact capacitive transverse RF-discharge-pumped slab CO laser with cryogenically cooled electrodes, which operates both in the cw and repetitively pulsed regimes, is fabricated. The laser operation is studied in the free running multifrequency regime at the vibrational – rotational transitions of the fundamental (V + 1 → V) vibrational bands of the CO molecule in the spectral region from 5.1 to 5.4 μm. Optimal operation conditions (gas mixture composition and pressure, RF pump parameters) are determined. It is shown that only gas mixtures with a high content of oxygen (up to 20% with respect to the concentration of CO molecules) can be used as an active medium of this laser. It is demonstrated that repetitively pulsed pumping is more efficient compared to cw pumping. In this case, quasi-cw lasing regime can be obtained. The maximum average output power of ~12 W was obtained for this laser operating on fundamental bands and its efficiency achieved ~14 %. The frequency-selective operation regime of the slab RF-discharge-pumped CO laser was realised at ~ 100 laser lines in the spectral region from 5.0 to 6.5 μm with the average output power of up to several tens of milliwatts in each line. Lasing at the transitions of the first vibrational overtone (V + 2 → V) of the CO molecule is obtained in the spectral region from 2.5 to 3.9 μm. The average output power of the overtone laser achieved 0.3 W. All the results were obtained without the forced gas mixture exchange in the discharge chamber. Under fixed experimental conditions, repetitively pulsed lasing (with fluctuations of the output characteristics no more than ±10 %) was stable for more than an hour.

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 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.

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.

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.

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.

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