Pulse-periodic Laser Radiation Research Articles

Heat accumulation and phase transition induction in a VO2 thin film by a femtosecond pulse-periodic radiation.

The kinetics of optical switching due to the insulator-metal phase transition in a VO2 thin film is studied experimentally at different laser pulse repetition frequencies (PRFs) in the NIR range and compared with temperature kinetics obtained through the thermal conductance calculations. Two switching processes have been found with characteristic times <2 ms and <15 ms depending on the PRF; the former is explained by the accumulation of metallic domains remaining after a single-pulse phase transition, and the latter is referred to the heat accumulation in the film. Consequently, the dynamics of the microscopic domains is leading in the initiation of phase transition under pulse-periodic conditions compared to the macroscopic heat transfer. The reverse transition at the radiation turn-off depends on the PRF with a time coefficient of 17.5 µs/kHz and is determined by the metallic domains' decay in the film. The results are important for understanding the nature of the insulator-metal transition in thin films of VO2 as well as using them in all-optical switches of pulse-periodic laser radiation.

Saturated Emission States in Fluorescent Nanostructured Media: The Role of Competition between the Stimulated Emission and Radiation Losses in the Local Emitters of Fluorescence.

A fundamental limitation of the spectral response of laser-pumped fluorescent nanostructured media was considered in terms of a probabilistic model establishing the relationship between the enhancement factor of a spectral quality and characteristic propagation and amplification scales of pump light and fluorescence in the medium. It was shown that the minimum spectral width of fluorescent response of the pumped medium is limited by competition between the stimulated emission and radiation losses in microscopic fluorescence emitters associated with the speckles randomly modulating the pumping light field. Theoretical results were compared to the experimental data on the spectral properties of the fluorescent response of laser-pumped nanostructured “anatase nanoparticles—dye solutions” systems with various structural and optical properties. Rhodamine 6G and 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) were applied as fluorescent components in the examined systems with various scatter volume fractions, which were pumped by pulse-periodic laser radiation with various intensities at 532 nm. Comparison showed a fair agreement between the theoretical and experimental results.

Thermal Action of the Nanoparticle Heated by Pulse-Periodic Laser Radiation on a Biotissue

The problem of thermal interaction between the gold nanoparticle heated by pulse-periodic laser radiation and the biotissue in which the nanoparticle is placed is numerically solved. A linear dependence between the heating time of the medium and the ratio of the time between laser radiation pulses to the time of the pulse for particles equally spaced in the biotissue is obtained. From the results of numerical calculations of biotissue heating by the single particle heated by periodic laser radiation the distance between the particles necessary for effective thermal damage of the biotissue is estimated.

Second Harmonic Generation in Microstructured Barium Titanate

We investigate the second harmonic generation under femtosecond pulse-periodic laser radiation in barium titanate in the form of ceramics, in pores of a globular photonic crystal, and in a water colloidal suspension. We measure the dependence of the second harmonic radiation intensity on the incident laser power. Excitation of the second harmonic was carried out by powerful (108 W) pulses of a solid-state Yb:KGW laser (wavelength, 1,026 nm) operating at 200 kHz. We estimate the efficiency of the second harmonic generation in various microstructured phases of barium titanate and show that the threshold of plasma formation in a suspension of barium titanate microparticles in water is substantially higher than in ceramics and in the ferroelectric photonic crystal. The second-harmonicgeneration power can be significantly increased in a water suspension of barium titanate microparticles.

Xenon plasma sustained by pulse-periodic laser radiation

The possibility of sustaining a quasi-stationary pulse-periodic optical discharge (POD) in xenon at a pressure of p = 10–20 bar in a focused 1.07-μm Yb3+ laser beam with a pulse repetition rate of frep ⩾ 2 kHz, pulse duration of τ ⩾ 200 μs, and power of P = 200–300 W has been demonstrated. In the plasma development phase, the POD pulse brightness is generally several times higher than the stationary brightness of a continuous optical discharge at the same laser power, which indicates a higher plasma temperature in the POD regime. Upon termination of the laser pulse, plasma recombines and is then reinitiated in the next pulse. The initial absorption of laser radiation in successive POD pulses is provided by 5p56s excited states of xenon atoms. This kind of discharge can be applied in plasma-based high-brightness broadband light sources.

Thermal self-action of high-power continuous and pulse-periodic CO2 laser radiation in air: II. Laboratory experiments

Results of laboratory experiments on nonlinear propagation of high-power pulse-periodic and continuous CO2 laser radiation in a gas medium are presented. The experiments were carried out in a cell filled with a mixture of air and carbon dioxide in different partial concentrations (∼1–4%) under strong absorption and thermal blooming of laser radiation. The experimental conditions simulated the atmospheric propagation of high-power laser radiation along a kilometer-long path. Sharply focused laser beams were used; the regions of heat release along the beam channel were recorded by the shadow imaging technique. We found that, despite increased intensity, the focal waist of a laser beam is characterized by reduced heat release as compared to that in the pre- and postfocal beam regions. The saturation effect of the resonance absorption coefficient of CO2 at high pulse peak intensity (blooming effect) is considered as the most probable physical cause of the above feature. For continuous radiation, this effect is much weaker.

Thermal self-action of high-power continuous and pulse-periodic CO2 laser radiation in air: I. Numerical simulation of propagation along an atmospheric path

A problem of transportation of high-power laser radiation energy at a wavelength of 10.6 μm in the atmosphere is theoretically considered in terms of its thermal and wind nonlinearities. The power characteristics of continuous and pulse-periodic radiation are studied during propagation along a surface kilometer-long atmospheric path with sharp focusing onto a receiver. Key factors most affecting high-power radiation propagation are found. It is ascertained that an undesirable effect of thermal defocusing can be reduced using a train of low-power short pulses with a quite high repetition rate due to the effect of light-induced “blooming” of the gas medium. The questions of scaling optical paths from the viewpoint of their equivalence in manifestation of thermal nonlinearity are discussed.

Thermomechanical effect of pulse-periodic laser radiation on cartilaginous and eye tissues

This paper is devoted to theoretical and experimental studies into the thermomechanical action of laser radiation on biological tissues. The thermal stresses and strains developing in biological tissues under the effect of pulse-periodic laser radiation are theoretically modeled for a wide range of laser pulse durations. The models constructed allow one to calculate the magnitude of pressures developing in cartilaginous and eye tissues exposed to laser radiation and predict the evolution of cavitation phenomena occurring therein. The calculation results agree well with experimental data on the growth of pressure and deformations, as well as the dynamics of formation of gas bubbles, in the laser-affected tissues. Experiments on the effect of laser radiation on the trabecular region of the eye in minipigs demonstrated that there existed optimal laser irradiation regimens causing a substantial increase in the hydraulic permeability of the radiation-exposed tissue, which can be used to develop a novel glaucoma treatment method.

SUPER LONG CONDUCTIVE CHANNEL PRODUCTION BY IMPULSAR

The attempts to create a super long conductive channels were taken in order to study the upper atmosphere and to settle special tasks, related to solar energy transmission, accumulated by orbital solar stations. There upon the program of creation of ”Impulsar” represents a great interest, as this program in a combination with high-voltage high repetition rate electrical source can be useful to solve the above mentioned problems. The principle of conductive channel production can be shortly described as follows. The ”Impulsar”- laser jet engine vehicle - propulsion take place under the influence of powerful high repetition rate pulse-periodic laser radiation. In the experiments the CO2- laser and solid state Nd: YAG laser systems had been used. Active impulse appears thanks to air breakdown (＜ 30 km) or to the breakdown of ablated material on the board (＞30km), placed in the vicinity of the focusing mirror-acceptor of the breakdown waves. With each pulse of powerful laser the device rises up, leaving a bright and dense trace of products with high degree of ionization and metallization by conductive nano-particles due to ablation. Conductive dust plasma properties investigation in our experiments was produced by two very effective approaches: high power laser controlled ablation and by explosion of wire. Experimental and theoretical results of conductive canal modeling will be presented. The estimations show that with already experimentally demonstrated figures of specific thrust impulse the lower layers of the Ionosphere can be reached in several ten seconds that is enough to keep the high level of channel conductivity and stability with the help of high repetition rate high voltage generator. Some possible applications for new technology are highlighted.

Effect of pulse-periodic CO2 laser radiation on the laminar flame of a propane-air mixture

To date, an acute ecological problem is toxic gas exhausts into the atmosphere. One approach to solving it is to control the process of combustion by various electromagnetic fields, in particular laser irradiation of the combustion zone. The present work reports the results of an experimental study of the effect of pulse-periodic CO2 laser radiation on the combustion of a propane-air mixture outflowing into the atmosphere.

Simulation of the process of sintering of spherical powders under the effect of pulsed laser radiation

Numerical simulation is performed of the process of sintering of a charge of two layers of spherical particles under the effect of pulsed and pulse-periodic laser radiation of wavelength λ = 1.06 µm and duration of 1–5 ms.

Ultraviolet Luminescence of Microorganisms and DNA Excited by Nanosecond Laser Pulses

Studies of the luminescence of Bacillus thuringiensis spores and DNA of chicken‐blood erythrocytes are presented. Luminescence was excited by pulse-periodic ultraviolet laser radiation with duration of a separate pulse of 20 ns. The decay kinetics of luminescence and changes in the shape of the luminescence spectrum are considered. Noticeable changes in the photoluminescence spectra with delay in the registration system are detected. For a pulse‐periodic mode of excitation, exciton–exciton quenching of luminescence in bacterial and DNA suspensions is observed. Studies of the influence of the geometrical shape of the cell on the photoluminescence spectra are performed.

The effect of pulse-periodic laser radiation on polyvinylchloride

Changes in the conductivity, permittivity, and dielectric loss tangent are registered for ten polyvinylchloride-based organic dielectrics (polymers), caused by the exposure for 3–5 s to the unpolarized radiation of a pulsed copper-vapor laser with a wavelength of 510.6 nm. The duration of light pulses is 20 ns, and their repetition frequency, 8.5 kHz.

Conversion of pulse-periodic CO2 laser radiation to the second harmonic in AgGaSe2 crystals

An investigation was made of pulse-periodic (quasi-cw) conversion, to the second harmonic, of CO2 laser radiation with an energy up to 1.9 mJ per pulse and a peak power of 10 kW. The external conversion efficiency (in terms of the peak power) was 1%. Local heating of a crystal was avoided by moving it transversely across the laser beam. The average pump (first-harmonic) power was 18 W and the second-harmonic power was 22 mW when the pulse repetition frequency was 12 kHz. There was no significant reduction in the conversion efficiency or significant damage to the crystal surface after operation for several hours.

Two-photon-excited luminescence of biologically active solid-state structures

A study was made of the characteristics of two-photon-excited luminescence of a number of pharmaceutical and biologically active solid-state materials: aspirin, sulfadimethoxine, Streptocid (sulfanilamide), pig insulin, and lysozyme. This luminescence was excited by pulse-periodic laser radiation. The characteristics of the luminescence spectra were compared with the familiar spectra of two-photon-excited luminescence of L-tryptophan. The results were used to determine the characteristics of molecular excitons in the investigated organic crystals and to estimate the concentration of tryptophan in solid-state proteins.

Polymerisation of acrylic oligomers photoinitiated by XeCl laser radiation

Polymerisation of OKM-2 and TGM-3 films was photoinitiated by pulse-periodic XeCl laser radiation (λ = 308 nm). The radiation-dose dependences of the mass and thickness of hardened films, and of the degree of conversion of double bonds were determined for polymerisation both in air and in an inert gas. A study was made of the influence of the energy density of laser pulses on the polymer yield. The changes in the transmission spectra of the irradiated films were studied. The energy expenditure in the hardening of the films depended strongly on the energy density of the laser pulses. The parameters of the polymerisation kinetics were estimated.

Pulse-periodic laser etching of diamond-like carbon coatings

The rates of etching of α-C:H films by excimer laser radiation were determined in air and in vacuum. Two ablation mechanisms—chemical and physical—dominated in different energy density ranges. An analysis was made of the processes influencing the occurrence of chemical etching and limiting its rate. A comparison was made of the energy losses in the treatment of α-C:H films by cw and pulse-periodic laser radiation.

Efficient surface-erosion plasma formation in air due to the action of pulse-periodic laser radiation

A study was made of the interaction of a series of periodic laser (λ = 1.06 μm) pulses with a number of materials (aluminum, copper, graphite, ebonite) in air at laser radiation power densities q = 107–109 W/cm2 and repetition frequencies f≤50 kHz. The radiation was concentrated in spots of ~ 10 − 2 cm2 area. Efficient formation of plasma as a result of laser erosion (q > 2 × 108 W/cm2, f≥5 kHz) was observed. A screening layer of an air plasma created by the first pulse of the series was expelled from the interaction zone and this was followed by erosion plasma formation under conditions of slight screening of the target during the action of the subsequent laser pulses.

Interaction of pulse-periodic XeCl laser radiation with metals

An experimental investigation and calculations were made of the interaction of pulse-periodic XeCl laser radiation of intensity 106–108 W/cm2 with metals. The main energy loss channels associated with evaporation of the metal, its thermal radiation, and convective cooling of the surrounding gas were determined. An optimal regime was shown to exist for heating metal samples which was a function of the laser radiation intensity and was determined for each of the investigated metals by its thermophysical and optical properties.

Characteristics of the evolution of a plasma formed by cw and pulse-periodic CO2laser radiation in various gases

An investigation was made of the interaction between high-power cw and pulse-periodic CO2 laser radiation and a low-threshold optical breakdown plasma near a metal surface. Characteristics of the breakdown plasma were studied as a function of the experimental conditions. A qualitative analysis was made of the results using a simple one-dimensional model for laser combustion waves.