Laser-ablated Particles Research Articles

Monte Carlo simulation of the laser-induced plasma-plume expansion under vacuum and with a background gas

The plasma-plume created above a copper target irradiated by a pulsed KrF laser beam (wavelength=248 nm, pulse duration=20 ns, fluence=17 J/cm 2) is investigated by means of a Monte Carlo simulation. The plume expansion under both vacuum and background gas is followed in time by simulating particle motion and collisions in the gas phase. The expansion under vacuum of laser-ablated particles is dominated by the laser energy absorption by the evaporated particles during the laser pulse. A comparison between simulated time-of-flight curves and experimental curves obtained by spectroscopic time-of-flight measurements has shown that about 6% of the incoming laser energy was contributing to the expansion process through the time-delayed recombination of this energy into kinetic energy. The plasma-plume expansion under a residual argon pressure greater than 50 Pa has been found to be very much affected by collisions between laser-ablated and ambient gas particles. The compression of the ambient gas particles by the ejected particles in the leading edge of the plume (snowplow effect) is clearly observed.

Laser-induced Plasma X-ray Source and Its Applications. Time Resolved Measurement of Laser-Ablated Particles by LAPXAS(Laser Plasma Soft X-ray Absorption Spectroscopy).

The time-and spatially-resolved properties of laser ablated carbon, boron and silicon particles were measured by LAPXAS (Laser Plasma Soft X-ray Absorption Spectroscopy). The maximum speed of positively charged ions is higher than those of neutral atoms and negatively charged ions. The spatial distributions of the laser-ablated particles in the localized rare gas environment were measured. In helium gas environment, by the helium cloud generated on the top of ablation plume depressed the ablation plume. There is no formation of silicon clusters till 15μs after laser ablation in the argon gas environment.

Explosive phase transformation in excimer laser ablation

This work investigated phase change mechanisms during excimer laser ablation of nickel specimens. Time-resolved measurements were carried out to determine optical properties and the velocity of the laser-ablated plume, the ablation rate per pulse, light scattering from the laser-ablated particles and the size of the laser-ablated particles, in the laser fluence range between 2.5 J cm −2 and 10.5 J cm −2 (or 100 MW cm −2 and 400 MW cm −2 for a laser pulse of 26 ns). It was found that normal surface evaporation occurred when the laser fluence was below 5.2 J cm −2. At a laser fluence of about 5.2 J cm −2 or higher, the temperature at the target surface approached the critical point. The surface experienced an explosive-type vaporization process, ejecting large size droplets from the molten pool. Further increase of the laser fluence up to 9.0 J cm −2 did not significantly change the surface temperature and the velocity and transmission of the laser-ablated plume. Explosive phase transformation was determined to be the main material removal mechanism when the laser fluence was higher than 5.2 J cm −2.

Control over the lateral distribution of hydrogen content in the pulsed-laser deposited diamond-like carbon films

We propose a method of controllable variation of the hydrogen content along the lateral direction of the pulsed-laser deposited DLC films. Hydrogen incorporates into the film by the adsorption of atmosphere hydrocarbon molecules on the growing surface in a vacuum chamber. The variation of hydrogen content is provided by use of the angular distribution of a carbon mass flow in a laser plume and by means of cleaning off the adsorbed hydrocarbons with help of impinging by laser-ablated carbon particles under oblique angles to the substrate. Deposition of DLC films at different angles of incidence of laser-ablated carbon flow reveals the decrease of the hydrogen content at the periphery of the film from 22 down to <3 at% as the angle changes from normal to the oblique one, while hydrogen content in the centre of the film remains nearly constant at a level of 10 at%.

Selection of kinetic energy of laser-ablated particles and its application for deposition of Au, Pt and Ag thin films

Controlling the kinetic energy of deposition species is an effective way to improve a thin film's physical properties. An apparatus that can select the kinetic energy of laser-ablated particles was developed. It was demonstrated that the energy of the laser-ablated particles was selected by chopping with the slit of a rotating disk. The energy profile of each ablated particle is illustrated, and the relationships between the selected velocity region and the selection conditions, such as disk rotation speed and delay time, are demonstrated. This apparatus was applied to the deposition of Au, Pt and Ag thin films. This process was also useful to improve surface morphology because the number of droplets on these films drastically decreased.

Light Emission from Nanometer-Sized Silicon Particles Fabricated by the Laser Ablation Method

Luminescent nanometer-sized silicon particles are fabricated by laser ablation of a silicon target in helium gas. The formation of products is found to be governed by the dynamics of the laser-ablated silicon particles in the gas. The products deposited on silicon substrates exhibit light emission with a peak at 1.6 eV in the air, while 2.1 eV after annealing for 20 min at 800°C in oxygen gas. Applying the laser ablation technique, we demonstrate a novel technique for fabricating silicon particles embedded in a silicon dioxide film. The particles also exhibit visible light emission.

Time-resolved detection of laser-ablated particles based on intensity decrease of cw probe laser beam

A prototype system was constructed for the detection of laser-ablated particles. The second harmonic of a picosecond Nd:YAG laser was used for the ablation of a polycrystalline copper target in vacuum. The typical laser intensity was 10 11 W cm −2. A probe HeNe laser beam which passed through the ablation plume was scattered by the ablated particles. The intensity of the probe beam was measured as a function of the elapsed time from the ablation laser pulse, and its dip was observed to exhibit a Maxwell-Boltzmann-like distribution. The delay time of the dip depended linearly on the distance between the target and the probe beam. The velocity of the expansion at the peak of the dip was derived as8.3 × 10 3cm s −1, and this reflected the expansion velocity of the plume. When helium, neon, argon, and nitrogen gases up to 40 kPa were introduced, the expansion velocity decreased and a long tail of the dip was observed.

Sputtering Phenomenon Induced by Laser-Ablated Particles

We have investigated the dynamic behavior of laser-ablated Si particles by means of time-resolved X-ray absorption spectroscopy. Laser-ablated Si particles are found to induce sputtering of an Al plate which is located near the Si target. Moreover, it is suggested that Si cluster formation is enhanced by collisions between laser-ablated Si particles and Si particles backscattered by a Si wafer located near the Si target. We tentatively assign unidentified X-ray absorption peaks appearing near 120 eV to small Si clusters.

Dynamics of silicon plume generated by laser ablation and its chemical reaction

Chemical reaction of laser-ablated silicon particles with oxygen gas diluted with a helium buffer gas is found by means of soft X-ray absorption spectroscopy. In order to make clear the dynamics of silicon laser plume and the mechanism of the chemical reaction, we measured time-resolved spatial distribution of light emission from the laser-ablated particles as well as time- and space-resolved emission spectra.

INITIAL PROCESS OF CLUSTER FORMATION OF LASER-ABLATED Si PARTICLES

We have performed x-ray absorption measurements of laser-ablated Si particles in vacuum and in Hegas environment. It is found that the spatial distributions of laser-ablated particles are very different between the two cases. This suggests that a higher-density region of helium gas is formed at the top of the plume of the ablated particles, and free expansion of particles is restrained by this helium cloud. Moreover, the spectrum obtained at 260 ns indicates that it takes more than 260 ns to form Si clusters. It is also demonstrated that Al particles can be sputtered in vacuum by laser-ablated Si particles if an Al plate is placed in front of the plume.

DYNAMIC OBSERVATION OF LASER-ABLATED CARBON PARTICLES USING TIME-RESOLVED X-RAY ABSORPTION SPECTROSCOPY

Time- and space-resolved characteristics of laser-ablated carbon particles were studied by x-ray absorption spectroscopy. Absorption peaks from the 1s→2p transition of neutral carbon atom ( C 0), C −, C +, and C 2+ ions were observed, An absorption peak from 1s→π* and a broad absorption maximum from the 1s→σ* transition of small clusters were also observed. Neutral carbon, C −, C +, and small carbon clusters were emitted from the sample long after laser irradiation. Spatial distributions of laser-ablated carbon particles in the localized helium-gas environment were measured. In the helium-gas environment, the ablation plume was depressed by the helium cloud generated on the top of the ablation plume.

Time-and-Space Resolved X-Ray Absorption Spectroscopy of Laser-Ablated Si Particles

We have performed time-and-space resolved X-ray absorption spectroscopy with a time resolution of 10 ns to study laser-ablated Si particles in a time scale ranging from 0 ns to 120 ns. Neutral and charged particles produced by laser ablation are observed through X-ray absorption spectra. Assignments of transitions from 2s and 2p initial states to higher Rydberg states of Si atom and ions are achieved, and we experimentally determine the LI \\kern-1ptI,I \\kern-1ptI \\kern-1ptI absorption edges of neutral Si atom ( Si0) and Si ions such as Si+, Si2+, Si3+ and Si4+. The main ablated particles are found to be Si atom and Si ions in the initial stage of 0 ns to 120 ns. The relative amounts depend strongly on times and laser energy densities. We find that the spatial distributions of particles produced by laser ablation are changed with supersonic helium gas bombardment, but no cluster formation takes place. This suggests that a higher-density region of helium gas is formed at the top of the plume of ablated particles, and free expansion of particles is restrained by this helium cloud, and that it takes more than 120 ns to form Si clusters.

Laser-Ablated Particles From Porous Silicon

AbstractIn order to search for the possibility to synthesize materials with new functions by laser deposition using source materials consisting of characterized microstructures, the porous silicon was ablated in ultrahigh vacuum with ultraviolet lasers of fluence as low as a few Jem”2. The ablated particles were analyzed by the time of flight method and the main species detected were atoms and small molecules such as Si, SÍ2, and SiO. The kinetics of the laser ablation of porous silicon in comparison with crystal silicon was also studied.

Electric field-induced growth of linear fibre-like structures in a medium of laser-ablated micrometre-sized particles

Electric field-induced growth of linear fibre-like structures in a medium of laser-ablated micrometre-sized particles

Modification of the plasma edge properties during divertor plate biasing experiments on TdeV

A laser-ablated fast particle diagnostic has been used for the measurement of n e (r) from the wall to 50 mm inside the TdeV separatrix. Divertor plate biasing has been found to modify substantially the n e ( r ) profile. The e-folding length at the separatrix, ν n , has been evaluated as a function of the biasing potential, and the results suggest a modification of the radial transport as a function of the radial electric field. The “LaBombard model” using different diffusivities ( D ÷ ) and mobilities (μ ÷ ) for ions and electrons cannot reproduce the dependence of ν n as a function of biasing. A simple model using the same D ÷ and μ ÷ for electrons and ions has been developed which gives a reasonable fit to the experimental measurements for D ÷ =3.2 m 2 /s and μ ÷ =0.039 m 2 /s V. Also, the particle confinement time τ p has been estimated with the help of the model, suggesting that negative biasing of the divertor plates increases τ p .

Use of laser-ablated fast particle beams for the measurement of n e and T e profiles in the TEXTOR boundary layer

We have developed a source of suprathermal atoms consisting of a laser system, which ablates small portions of coating materials from the rear side of a glass substrate. The coating can be either a single species or a multilayer of several-thousand-angstrom thickness. Neutral particle beams produced by this method can have energies in the range of about 10 eV and penetrate into the plasma up to line densities of about nel=1×1013 cm−2. For the determination of the electron temperature we have chosen two elements, the ionization and excitation cross sections of which differ in the range up to 100 eV. For TEXTOR a two-component target with a C-LiF coating is used, which delivers a Li and C beam with about 5- and 15-eV energy, respectively, ablated by a ruby laser with 1-J energy. The beam attenuation is determined from the emitted light by using a camera with a Si-diode array system for spatial recording. The different attenuations and emission characteristics then allow the determination of radial electron density and temperature profiles over the spatial range from the liner into a few centimeters beyond the limiter.