Power Density Of Laser Radiation Research Articles

Formation of the composition and topography of surface layers of Cu50Ni50 foils under laser irradiation

The influence of the laser radiation power density on the changes in the composition and mechanical properties of surface layers of Cu50Ni50 foils has been investigated using X-ray photoelectron spectroscopy, scanning probe microscopy, X-ray diffraction, and microhardness measurements. It has been found that, after laser irradiation, the redistribution of elements occurs in the surface layer with a thickness of ∼30 nm on the irradiated side of the foil. It has been revealed that there are microdistortions in the crystal lattice of the alloy, microdeformations of grains, and variations in the microhardness of the irradiated surface. The mechanisms explaining the observed changes in the foils after laser irradiation have been proposed.

Effects of 830 and 670 nm Laser on Viability of Random Skin Flap in Rats

This study aimed to investigate the effect of 830 and 670 nm diode laser on the viability of random skin flaps in rats. Low-level laser therapy (LLLT) has been reported to be successful in stimulating the formation of new blood vessels and reducing the inflammatory process after injury. However, the efficiency of such treatment remains uncertain, and there is also some controversy regarding the efficacy of different wavelengths currently on the market. Thirty Wistar rats were used and divided into three groups, with 10 rats in each. A random skin flap was raised on the dorsum of each animal. Group 1 was the control group, group 2 received 830 nm laser radiations, and group 3 was submitted to 670 nm laser radiation (power density=0.5 mW/cm(2)). The animals underwent laser therapy with 36 J/cm(2) energy density (total energy=2.52 J and 72 sec per session) immediately after surgery and on the 4 subsequent days. The application site of laser radiation was one point at 2.5 cm from the flap's cranial base. The percentage of skin flap necrosis area was calculated on the 7th postoperative day using the paper template method. A skin sample was collected immediately after to determine the vascular endothelial growth factor (VEGF) expression and the epidermal cell proliferation index (KiD67). Statistically significant differences were found among the percentages of necrosis, with higher values observed in group 1 compared with groups 2 and 3. No statistically significant differences were found among these groups using the paper template method. Group 3 presented the highest mean number of blood vessels expressing VEGF and of cells in the proliferative phase when compared with groups 1 and 2. LLLT was effective in increasing random skin flap viability in rats. The 670 nm laser presented more satisfactory results than the 830 nm laser.

Shapes of laser radiation pulses modified by nonlinear scattering in aqueous suspension of carbon nanotubes

An improved scheme of z-scanning was used to study the parameters of nanosecond 1064-nm laser radiation pulses scattered at right angle under the conditions of optical limiting in an aqueous suspension of purified carbon nanotubes (CNTs). CNTs were synthesized by the electric-arc evaporation of graphite. It is established that the amplitude, shape, duration, and temporal position of the peak of scattered light pulses significantly depend on the laser radiation power density. The results agree with the mechanism of thermoinduced nonlinear scattering that is operative during the optical limiting of laser pulses in CNT suspensions.

Diagnostics of plasma plume produced by laser ablation using ICCD imaging and transient electrical probe technique

The dynamics of transient plasmas generated by high-fluence nanosecond laser ablation has been investigated by means of fast ICCD imaging and electrical probes in transient regime. Measurements have been carried out on plasmas produced in vacuum (5× 10−6 Torr residual pressure) by a pulsed Excimer laser (20 ns, XeCl, λ= 308 nm) irradiating stainless steel targets. Two plasma expansion velocities were estimated, one from probe measurements and another from recorded ICCD images. Electron plasma temperature (0.1 − 0.3 eV) and density (1 − 5×1011 cm−3) were measured by electrical probes for a laser radiation power density of 6×108 W/cm2.

A model of pulsed target evaporation and ion generation in laser plasma

Processes in laser plasma generated under conditions of a real temporal variation in the laser radiation power density in the interval of average values within 2 × 109–2 × 1010 W/cm2 have been studied. A layer model of the pulsed evaporation of a target is employed with the corresponding step-by-step description of the process of ion generation. It is shown that plasma is formed in an expanding vapor. At the stage of optical breakdown, the electron temperature grows much faster than the ion temperature. During the entire period of irradiation of the target, the rate of ion generation is significantly greater than the rate of recombination. The kinetics of ion formation at the laser radiation pulse front is considered.

Influence of wavelength, power density and exposure time of laser radiation on chondrocyte cultures – An in-vitro investigation

Abstract The influence of laser radiation on human osteoarthrotically changed chondrocytes was investigated using various wavelengths, power density and dependence on the exposure time in order to confirm the positive results obtained in an animal experiment. It was manifested that, if there was a specific parameter constellation (2 W; 16 W/cm 2 ; 60 s; 120 J), an enhanced matrix synthesis (cartilage material from 36 patients) could be achieved. The proof succeeded by applying the radioisotope marking method ( 3 H-proline). Interestingly enough it turned out that the application of too high a power density but constant energy density resulted in a reduced matrix synthesis rate (reduction of 28%). In this study, it was demonstrated for the first time that laser radiation may have a positive influence on human cartilage thus contributing to future in-vivo application.

Metal-diamond-metal planar structures for off-angle UV beam positioning with high lateral resolution

A position sensitive detector (PSD) for deep-UV excimer laser has been realised and characterised. CVD diamond was chosen as the active material for its high cohesive energy that ensures good reliability under the high power density of excimer laser radiation. Besides, the high gap-energy of diamond ensures blindness to visible radiation and a high sensitivity to the UV radiation used in the last generation of photolithographic tools. The device is based on a voltage division scheme that makes possible to obtain the sensitivity to the beam position while reducing the sensitivity to its intensity. Two contact structures are compared, and the performances of these detectors are evaluated in term of non-linearity, position measurement error and position sensitivity.

Increasing the power density and quality of laser radiation

The results of studies on the improvement of the laser beam quality initiated by N.G. Basov and performed at the Laboratory of Quantum Radiophyisics (FIAN) are briefly discussed.

CO 2 laser-induced structure changes in lead zirconate titanate Pb(Zr 0.58Ti 0.42)O 3 sol–gel films

Based on absorption spectra of lanthanum modified lead zirconate titanate Pb 0.9La 0.1(Zr 0.65Ti 0.35)O 3 (PLZT10/65/35) ferroelectric ceramics an idea of local and selective treatment of lead zirconite titanate Pb(Zr X Ti 1− X )O 3 (PZT) sol–gel film in the multi-layer structure by CO 2 laser radiation was formulated. The possibility to obtain PZT perovskite film on the SiO 2/Si substrate by irradiation with CO 2 laser was shown experimentally. X-ray studies revealed the PZT perovskite structure ratio growth at the expenses of pirochlore structure with an increased power density of laser radiation.

Characteristics of the spectra of a plasma near the surface of solid and liquid targets exprosed to a laser pulse of complex form

The results of investigation of the dynamics of the emission spectra of a plasma generated at the surface of liquid and solid targets by a laser pulse of a complex time form are presented. It is shown that the shock wave arising as a result of breakdown on the surface of the solid and developing to heights of the order of 4 mm is a laser-supported detonation shock wave. In the case of solid targets, we recorded additional laser plasma light pulses coming after the main pulse. Two additional light pulses have been recorded for the emission line AlI 396.1 nm. In the case of the liquid, we have not detected additional pulses for the investigated emission lines of the elements at the laser radiation power densities used.

Efficiency of fractal structure formation during laser evaporation

The formation of fractals during laser evaporation of various media (metals and dielectrics) by 10-ms Nd laser pulses is studied experimentally as a function of the laser radiation power density and external pressure. It is established that fractals are formed during the action of a laser pulse in the bulk of a plasma plume. It is found that finely dispersed phase forms under a certain critical pressure a bound shell (macrofractal) in peripheral layers of the plume. The presence of the shell confining the plasma expansion leads to a threshold variation of the characteristics of the optical discharge.

Indicatrices of the Radiation Intensity of Laser Plasma Formations in the Air

We have investigated the indicatrices of the visible and IR radiation of laser plasma formed under irradiation of cadmium, indium, and silicon in the air by radiation from a monopulsed neodymium laser with a power density at the irradiation spot of up to 12 GW/cm2 on the first harmonic and up to 4 GW/cm2 on the second harmonic. It is shown that the radiant intensity indicatrices have a prolate form depending on the target material, the spectral range of observation, and the power density of the acting laser radiation. The radiant intensity of laser plasma in the 0.3–4.2 μm range is approximately proportional to the laser radiation power density and depends on the target material.

Analysis of characteristic X-ray generation induced by laser plasma electrons accelerated by an electric field

The results of experiments devoted to the study of spectral, spatial, and time characteristics of a spectrally bright point x-ray source based on a vacuum diode with a laser-plasma cathode and a titanium needle anode with a photon energy approximately equal to 4.5 keV are presented. The experimental results revealed a considerable difference between the electron emission from laser plasma in a strong electric field and the explosive electron emission and demonstrated the effectiveness of laser plasma as an electron source. The optimization of the laser radiation power density, the accelerating voltage, and the interelectrode spacing made it possible to create a point x-ray source whose spectral brightness exceeds available sources in the class of small-size pulse x-ray instruments (tubes with explosive cathodes). It has been proved experimentally that the maximum contrast of the characteristic lines of the anode material is attained in the case of an optimal choice of accelerating voltage. The x-ray source has the following parameters: (1) spectral brightness of the K-lines of titanium of the order of 1021 photons/cm2 s sr keV; (2) emitting region size of 250 mm; and (3) laser pulse duration less than 20 ns.

Two photon absorption in semi-insulating gallium arsenide photoconductive switch irradiated by a picosecond infrared laser

We study gallium arsenide (GaAs) high voltage photoconductive switches triggered by a 30 ps neodymium: yttrium aluminium garnet (Nd:YAG) laser, i.e. using photons of energy smaller than the band gap. We measure optical absorption at Brewster incidence under optical pulse excitation and determine the extrinsic one photon and intrinsic two photon absorption coefficients. Analyzing the photoconductive resistance under different power densities of laser radiation, and using the absorption data, we demonstrate that only about 20% of the photons absorbed by the extrinsic process are converted into free electrons. We conclude that high efficiencies can only be obtained by using two photon absorption, which is feasible with ultrafast lasers and focussed beams.

Interferometric diagnostic of picosecond laser ablation in air

Investigations of a spatial density distribution at the focus of an intense picosecond Nd:YAG laser pulse ( λ=1064 nm) near a metallic target in air have been performed by interferometry and absorption photography with high temporal and spatial resolution (50 ps, <1 μm). An effect of microchanneling and an intense shock-wave formation in laser-produced plasma were observed. Estimations show that the current density in the microchannel is of the order of 10 10 A/cm 2, followed by the generation of a self-magnetic field in the narrow channel having a magnitude of 7 MG. When the power density of laser radiation was reduced to a value of the order of 9×10 9 W/cm 2 a significant change in electron density distribution has been estimated: a shock wave with a strong boundary front vanished, an electron density failed under the value of the order of 10 18 cm −3.

Ponderomotive action of light in the problem of multiple scattering of light in a randomly inhomogeneous medium

The time correlation function of light reflected diffusely from a semi-infinite randomly inhomogeneous medium is calculated with allowance for the acceleration of the scatterers in the field of the laser beam incident on the medium. An analytical expression is found for the characteristic coherence time due to the ponderomotive action of light. It is shown that even with laser radiation power densities of the order of 1–10 W laser-acceleration effects substantially alter the character of the time autocorrelation function of the scattered light and must be taken into account in theoretical calculations.

Shaping dynamics and structure of large-scale surface plasma formations under single-pulse laser action on various materials

It is established that, in the case of laser action on materials in large illuminated spots in atmospheric-pressure air for a laser radiation power density q≤2 MW/cm2 (λ=1.315 μm), an evaporative regime with plane scattering of the laser-produced erosion plasma is realized while, for q≈5–17 MW/cm2, the plasma front is transferred to the air, leading to plasma screening of the target and shaping of a subsonic radiation wave in the air. As the duration of the laser pulses increases (τ∼160 μsec), in spite of the large illuminated spots (S∼150 cm2) the evaporative regime of the laser action with plane plasma scattering goes over into a regime with jet outlow and formation of a quasistationary shock wave.

Dynamics of erosion plasma formation and formation of a plasma flame with a condensed disperse phase when quasi-continuous laser radiation acts on bismuth

The dynamics of near-surface plasma formation and the formation of an erosion plasma flame was investigated for quasi-continuous laser radiation (λ=1.06 μm, q=0.1–10 MW/cm2, τ∼1.5 msec) acting on bismuth targets in air. The absence of low-threshold plasma formation at q<2 MW/cm2 was established and explained. Instabilities in the plasma formation were revealed and the range of laser radiation power densities (2≤q≤4 MW/cm2) at which these instabilities take place was determined. At q≥5.4 MW/cm2, brightening of the flame was noted. The dependence of the luminescence temperature of the flame on the laser radiation power density q was determined.

Influence of the laser beam parameters on the depth and efficiency of the melting of metals by pulsed laser radiation

An experimental investigation was made of the geometric parameters of the melting zone and of some kinetic characteristics of the growth of a vapour—gas channel as a function of the spatial characteristics of a laser beam and of the profile of the radiation pulses. The depth of melting increased considerably, other conditions being equal, on reduction in the caustic angle of the laser beam incident on the irradiated surface. A new physical effect was discovered experimentally: the rate of growth of the vapour-gas channel increased when the length of this channel increased. This was the result of an increase in the laser radiation power density at the bottom of a channel during its growth. A molten zone depth of 6.5 mm was achieved for the first time for a pulse energy of 17.0 J.

Feasibility of constructing a cw copper vapour-flow laser

The feasibility of cw lasing as a result of transitions from a resonant to a metastable level in the copper atom is demonstrated for the first time. A theoretical model is proposed for the description of relaxation of a plasma formed in a plasma accelerator. It is shown that at small distances from the accelerate edge the laser radiation power density can be ~5 W cm-2.