Nikolay N. Nedialkov

Femtosecond laser ablation of nickel in vacuum

We present an experimental characterization and a theoretical analysis of ultrashort laser ablation of a nickel target, which highlights the more general and peculiar features of femtosecond (fs) laser ablation of metals. The study has been carried out by using visible (527 nm) laser pulses of ≈ 300 fs duration. The vacuum expansion dynamics of the ablated species has been investigated by using fast photography and optical emission spectroscopy, while the fs laser pulse–metal interaction has been studied theoretically by means of molecular dynamics simulations. Special attention has been given to the study of the dependence of ablation depth on laser fluence, which has been carried out by comparing the SEM analysis of micro-holes drilled into the nickel samples with the predictions of the theoretical model. The main outcomes of our investigation, which are very satisfactorily reproduced and accounted for by the theoretical model, are (i) the nonlinear dependence of the ablation yield on the laser fluence, and its reliance to the electron heat diffusion, in the process of redistribution of the absorbed energy, (ii) the splitting of the material blow-off into two main classes of species, atoms and nanoparticles, characterized by different expansion dynamics, and (iii) the different degrees of heating induced by the laser pulse at different depths into the material, which causes the simultaneous occurrence of various ablation mechanisms, eventually leading to atoms and nanoparticles ejection.

Experimental and theoretical investigations of femtosecond laser ablation of aluminum in vacuum

We used time-gated optical emission spectroscopy to investigate the characteristics of aluminum plumes and their vacuum expansion after femtosecond laser ablation at different fluences. The prominent feature is the presence of two main classes of species in the plume: very fast Al atoms and ions preceding the plume bulk essentially constituted of much slower Al nanoparticles expanding with a ten times smaller average velocity. Atomic force microscopy of deposited Al nanoparticles evidenced an average size of about 10nm with a pretty narrow size distribution. These results and the peculiar feature of nanoparticle formation during femtosecond laser irradiation of matter were very satisfactorily interpreted and reproduced by molecular-dynamics simulation of the process. Finally, the analysis of the dependence on laser fluence of the ablation process showed an initial logarithmic increase of ablation yield, up to about 500mJ∕cm2, followed by a sudden and very steep increase at higher fluences. According to ou...

Laser drilling of silicon nitride and alumina ceramics: A numerical and experimental study

Laser drilling of silicon nitride (Si3N4) and alumina (Al2O3) ceramics targets was studied theoretically and experimentally. A one-dimensional model based on the heat-transfer equation was developed in order to describe the process. It includes a change of the type of the equivalent heat source during the laser pulse. The drilling of Si3N4 and Al2O3 ceramic slabs was performed by using a TEM00 Q-switched 10 ns pulse Nd:YAG laser. When the absorption of the plasma formed during the drilling process was taken into account, the theoretical results obtained agreed well with the experimental ones.

An analysis of the dependence on photon energy of the process of nanoparticle generation by femtosecond laser ablation in a vacuum

The dependence on laser wavelength of the process of nanoparticle generation by ultrashort laser ablation of solid matter in a vacuum has been investigated both experimentally and theoretically. The study has been carried out for a Ni target by using laser pulses of ≈300 fs duration at two different laser wavelengths: in the visible (λ = 527 nm) and ultraviolet (λ = 263 nm), respectively. The size distribution of the nanoparticles, which is quite broad in the case of visible light, becomes significantly narrower and slightly shifts towards smaller sizes for ultraviolet light. Molecular dynamics simulations confirm the dependence of the process on the laser wavelength by showing that the laser photon energy affects the material relaxation and, thus, the nanoparticle generation process. This, in turn, indicates that the photon energy can be used as an effective parameter to control the nanoparticle size distribution in femtosecond laser ablation of solid matter.

Ablation of metals by ultrashort laser pulses

Ablation of Fe, Al, Ni, and Cu by laser pulses at durations of 0.1, 1, and 5 ps is investigated experimentally. The laser fluence used vaires from below the ablation threshold up to 100 J/cm2. The ablation rate depends on the laser pulse duration at laser fluences above several J/cm2 as the shorter pulse produces higher ablation rate. A change of the ablation regime with the laser fluence increase is also observed. The presence of molten material is clearly expressed at fluences above 10 J/cm2 for all pulse durations used. These effects can be referred to the contribution of the electron heat diffusion in the distribution of the absorbed energy. The traces of solidified molten material suggest for realizations of melt ejection mechanism of ablation.

Laser treatment of the 38HMJ steel surface in a liquid nitrogen environment

Samples of the 38HMJ steel kept in a liquid nitrogen bath (LN2) at 77 K are treated by a 1 kW CO2 laser beam of intensity of about 1.3x105W/cm2 on the sample surface. For thermal processing by means of the 10,6 micrometers radiation the transparency window of LN2 below 0.3 eV is utilised. The irradiation effects are compared with results obtained for the reference samples of the same material under standard gas shielding conditions. Measurements of microhardness HV 0.1 indicated on reduction of the heat affected zone for the LN2 case. A similar effect is observed for higher sample velocities and a value of about 700 HV is obtained in a 30 micrometers surface layer. Surface analysis by means of Auger electron spectroscopy confirms the local nitrogen element of the laser treated areas depending on the processing conditions.

INFLUENCE OF THE PROCESSING PARAMETERS ON THE ULTRASHORT LASER ABLATION OF METALS

Laser ablation of Al, Ni, Fe, and Cu was investigated for pulse durations of 0.1, 1 and 5 ps at fluences up to 100 J.cm-2. The ablation depth per pulse was measured and the ablation thresholds were estimated. The increase of the pulse duration results in a decrease of the ablation rate. The presence of a molten phase is clearly expressed at fluences above 10 J.cm-2. Molecular Dynamics simulation was used to model the laser ablation process in the case of Fe, Al and Ni.

Deep-hole drilling in Fe by ultrashort laser pulses: molecular dynamics simulation study

Some basic features of ultrashort laser drilling of deep holes in Fe are investigated using molecular dynamics simulation model. The process is simulated for laser pulse duration of 0.1 ps at aspect ratio of the hole (hole depth/hole width) higher than 1. The interaction of the ablated material with the holes wall affects its shape. The number of re-deposited, reflected, and secondary ejected particles from the walls is estimated as a function of the laser fluence. Significant secondary ejection of material from the walls is observed at fluences above 5 J/cm2.

Laser drilling of AIN ceramics using nanosecond pulses

Laser drilling of aluminum nitride (AIN) ceramics by 6 ns SHG (λ=532 nm) Nd:YAG laser pulses is studied. The drilling rate, quality of the holes, and the effects related to plasma formation are investigated. Numerical model based on the heat-transfer equation is developed to describe the drilling process. Different mechanisms of material decomposition are found to be realized, which depend on the laser fluence. They determine the quality of the holes produced. Below 5 GW/cm2 the material ejection is realized by decomposition of ceramics into solid or liquid Al and gaseous N2. The letter blows away culsters and flakes from the irradiated area and the holes drilled have irregular shape. At higher laser intensities, the decomposition into gaseous Al and N2 or direct evaporation of the ceramics results in good hole quality. A saturation of the ablation rate as a function of the laser intensity is observed above 15 GW/cm2. The calculated depths of the holes drilled are in agreement with the experimental data when absorption of the laser radiation from the plasma plume is taken into account.

Ultrashort laser ablation of iron: molecular dynamics simulation

Laser ablation of iron by femtosecond and picosecond pulses is investigated using molecular dynamics (MD). The electron heat conduction is taken into account in the simulations. Several stages of the ejection of the particles is determined. The compression and the subsequent relaxation of the pressure into the bulk lead to removal of large amount of the material. These processes are also accompanied with the formation of voids in the material. Two different regimes of the ablation are observed depending on the laser fluence. The estimated threshold of ablation is in good agreement with the experimental data found in the literature. The dynamics of the shock wave and the temperature distribution in the material are also obtained.