Plasma plume induced during laser ablation of graphite

The plasma plume induced during ArF laser ablation of a graphite target is studied. Velocities of the plasma expansion front are determined by the optical time of flight method. Mass center velocities of the emitting atoms and ions are constant and amount to 1.7×10⁴ and 3.8×10⁴ m s⁻¹, respectively. Higher velocities of ions result probably from their acceleration in electrostatic field created by electron emission prior to ion emission. The emission spectroscopy of the plasma plume is used to determine the electron densities and temperatures at various distances from the target. The electron density is determined from the Stark broadening of the Ca II and Ca I lines. It reaches a maximum of ∼9.5×10²³ m⁻³ 30 ns from the beginning of the laser pulse at the distance of 1.2 mm from the target and next decreases to ∼1.2×10²² m⁻³ at the distance of 7.6 mm from the target. The electron temperature is determined from the ratio of intensities of ionic and atomic lines. Close to the target the electron temperature of ∼30 kK is found but it decreases quickly to 11.5 kK 4 mm from the target.     

Early stage optical emission in nanosecond laser ablation

This paper describes the results of an experimental and theoretical investigation of the physical origin of the visible continuum emission usually observed in the early stages of nanosecond laser ablation of solid materials. It has been suggested, but not confirmed that the continuum is due to radiative recombination and bremsstrahlung emission. Time and space-resolved emission spectroscopy with an absolutely calibrated spectrometer was used to study the spectral emission in laser ablation of zinc in vacuum at 4.1 J?cm^?2 using a 8 ns, 1064 nm laser pulse. By modelling the spectral emission with a spectral synthesis code, it has been shown that the continuum emission is primarily due to bound-bound transitions between strongly Stark broadened energy levels. Similarly, it can be concluded that the optical absorption is primarily due to bound-bound transitions.     

Monitoring nanoparticle formation during laser ablation of graphite in an atmospheric-pressure ambient

Excimer laser ablation of highly oriented pyrolytic graphite (HOPG) was performed at atmospheric pressure in an N₂ and in an air ambient. During the ablation, nanoparticles condensed from the material ejecta, and their size distribution was monitored in the gas phase by a Differential Mobility Analyzer (DMA) in combination with a Condensation Particle Counter (CPC). Size distributions obtained at different laser repetition rates revealed that the interaction between subsequent laser pulses and formed particles became significant above 15 Hz. This interaction resulted in laser heating, leading to considerable evaporation and a decrease in the size of the particles. X-ray photoelectron spectroscopy revealed that approximately 8% nitrogen was incorporated into the CN_x particles generated in the N₂ ambient, and that the nitrogen was mostly bonded to sp³-hybridized carbon. Monodisperse particles were also deposited and were analyzed by means of Raman spectroscopy to monitor size-induced effects. PACS 81.07.-b; 61.46.+w; 79.70.+q     

Features of plasma plume evolution and material removal efficiency during femtosecond laser ablation of nickel in high vacuum

We present an experimental characterization describing the characteristics features of the plasma plume dynamics and material removal efficiency during ultrashort, visible (527 nm, ≈300 fs) laser ablation of nickel in high vacuum. The spatio-temporal structure and expansion dynamics of the laser ablation plasma plume are investigated by using both time-gated fast imaging and optical emission spectroscopy. The spatio-temporal evolution of the ablation plume exhibits a layered structure which changes with the laser pulse fluence F. At low laser fluences (F<0.5 J/cm²) the plume consists of two main populations: fast Ni atoms and slower Ni nanoparticles, with average velocities of ≈10⁴ m/s for the atomic state and ≈10² m/s for the condensed state. At larger fluences (F>0.5 J/cm²), a third component of much faster atoms is observed to precede the main atomic plume component. These atoms can be ascribed to the recombination of faster ions with electrons in the early stages of the plume evolution. A particularly interesting feature of our analysis is that the study of the ablation efficiency as a function of the laser fluence indicates the existence of an optimal fluence range (a maximum) for nanoparticles generation, and an increase of atomization at larger fluences. PACS 52.50.-b; 52.38.Mf; 79.20.Ds; 81.07.-b     

Time-resolved plasma diagnostics and mass removal during single-pulse laser ablation

Laser ablation processes occurring over several orders of magnitude in time were investigated by using time-resolved spectroscopy, shadowgraphs and interferograms. A picosecond ablation plasma was measured with an electron density on the order of 10²⁰ cm^-3 originating from the breakdown of air. The longitudinal expansion of this plasma was suppressed due to the development of a strong space-charge field. At post-pulse times, the lateral (radial) expansion of the plasma was found to follow the relation, r~t^1/2, consistent with the expansion from an instantaneous line source of energy. The electron number density and temperature were deduced by measuring spectroscopic emission-line broadening during the early phase (30-300 ns) of a mass (atomic/ionic) plasma. These properties were measured as a function of the delay time and irradiance. Possible mechanisms such as inverse bremsstrahlung and self-regulation were used to describe the data before an explosion threshold of 20 GW/cm². The laser self-focusing and critical temperature are discussed to explain dramatic changes in these properties after the irradiance threshold. On the microsecond time scale, the surface explodes and large (>7m) particles are ejected. Mass removed from single-crystal silicon by high power (10⁹-10¹¹ W/cm²) single-pulse laser ablation is studied by measuring the crater morphology. Time-resolved shadowgraph images show that the rapid increase in the crater depth at the threshold corresponds to large-size droplets leaving the surface. This rapid growth of the crater volume is attributed to explosive boiling.     

Estimation of the absorptivity of a constructive material at unknown losses in ablation plume during laser ablation

Computation of the processes of laser heating of carbon silicon carbide composite material (CSCCM) samples in air (to temperatures above 2000°C for 1 s) by IR laser radiation with a wavelength of 1.3 μm and intensity of 3 kWcm⁻² in the presence of screening ablation plume have been carried out using the KARAT code. A comparison of the simulation results with the experimental dependences of spatial and temporal fields of sample temperatures made it possible to determine the absorptivity of thematerial, energy loss in the ablation plume, and, correspondingly, its influence on the heating and ablation of the material under study.     

Shadowgraphic imaging of material removal during laser drilling with a long pulse excimer laser

After the development of a novel XeCl excimer laser with a nearly diffraction-limited beam and 175 ns pulse length, research was done on different industrial applications of this laser. Hole drilling, one of these applications, was studied extensively. A better understanding of the drilling process is necessary to optimise the drilling efficiency and to control the quality of the holes. A shadowgraphic imaging technique was used for studying the removal of material from the hole and the absorption of the laser beam by this removed material. Images were made at successive times both during and after the laser pulse.In drilling of thin foils, it was shown that the material was ejected mainly after the laser pulse. A comparison of different materials showed that the drilling process should be optimised for each material independently. Furthermore, the plume was found to be not fully transparent for processing materials with a strong absorption line at or near the laser wavelength. The correlation between material and drilling speed suggests improved energy transfer and improved melt ejection for the materials with this absorption. PACS 42.62.Cf; 52.38.Dx; 52.38.Mf     

Dynamics of ultrashort pulsed laser radiation induced non-thermal ablation of graphite

We report on the dependence of a laser radiation induced ablation process of graphite on the applied pulse duration of ultrashort pulsed laser radiation smaller than 4 ps. The emerging so-called non-thermal ablation process of graphite has been confirmed to be capable to physically separate ultrathin graphitic layers from the surface of pristine graphite bulk crystal. This allows the deposition of ablated graphitic flakes on a substrate in the vicinity of the target. The observed ablation threshold determined at different pulse durations shows a modulation, which we ascribe to lattice motions along the c axis that are theoretically predicted to induce the non-thermal ablation process. In a simple approach, the ablation threshold can be described as a function of the energy penetration depth and the absorption of the applied ultrashort pulsed laser radiation. Based on the analysis of the pulse duration dependence of those two determining factors and the assumption of an invariant ablation process, we are able to reproduce the pulse duration dependence of the ablation threshold. Furthermore, the observed pulse duration dependences confirm the assumption of a fast material specific response of graphite target subsequent to optical excitation within the first 2 ps.     

Preparation of polyynes by laser ablation of graphite in aqueous media

Polyynes were prepared by liquid-phase laser ablation of a graphite target at 1064 nm and identified by analyzing UV absorption spectra in deionized water and various aqueous solutions. We observed that major UV absorption peaks coincide with the electronic transitions corresponding to linear hydrogen-capped polyynes (C_nH₂: n = 6, 8, 10). The peak intensities increased when polyynes were produced by irradiating the target immersed in acidic media, while those were relatively weak in basic media. This leads us to conclude that OH⁻ or H⁺ ions play a certain role in the formation of polyynes.     

Target-plane deposition of diamond-like carbon in pulsed laser ablation of graphite

In pulsed Nd:YAG laser ablation of highly oriented pyrolytic graphite (HOPG) at 10⁻⁶ Torr, diamond-like carbon (DLC) are deposited at laser wavelengths of 1064, 532, and 355 nm on substrates placed in the target-plane. These target-plane samples are found to contain varying sp³ content and composed of nanostructures of 40-200 nm in size depending on the laser wavelength and laser fluence. The material and origin of sp³ in the target-plane samples is closely correlated to that in the laser-modified HOPG surface layer, and hardly from the backward deposition of ablated carbon plume. The surface morphology of the target-plane samples shows the columnar growth and with a tendency for agglomeration between nanograins, in particular for long laser wavelength at 1064 nm. It is also proposed that DLC formation mechanism at the laser-ablated HOPG is possibly via the laser-induced subsurface melting and resolidification.     

Dynamics of the ejected material in ultra-short laser ablation of metals

A molecular dynamics model is applied to study the formation and the early stages of ejection of material in ultra-short laser ablation of metals in vacuum. Simulations of the ablation process for iron at a pulse duration of 0.1 ps and at different laser fluences are performed. Different features of the ejection mechanism are observed below, near, and above the ablation threshold. The last is estimated as approximately 0.1 J/cm². The structure of the ablated material is found to depend on the applied laser fluence. The expanded plume consists mainly of large clusters at fluences near to the threshold. With the increase of the laser fluence the presence of the large clusters decreases. Clear spatial segregation of species with different sizes is observed in the direction normal to the surface several tens of picoseconds after the laser pulse onset. The angular distribution of the ejected material is estimated for different regimes of material removal. Above the ablation threshold the distribution is forward peaking. PACS 79.20.Ds; 52.38.Mf; 02.70.Ns; 81.05.Bx     

Model and computer simulation of nanosecond laser material ablation

A computer model to simulate the evolution of parameters describing laser ablation processes was developed. The absorbed laser energy, the heat diffusion, the phase transformations and the shielding effect of the ablated material were taken into account. The temporal development of the ablated volume, pore depth and extension of the melt zone were calculated for single pulses of 500, 100, 20, 5 and 1 ns. Simulations were performed for pulse energies of 50 J and spot diameters of 10 m. From temporal evolution curves of the ablated volumes, the stoppage of the ablation process was evidenced before the end of the processing pulse. Comments with respect to optimal pulse duration (in the ns regime) are also formulated. PACS 81.40.Wx     

Pulsed laser photodeposition of a-Se nanofilms by ArF laser

Pulsed laser photodeposition from amorphous selenium aqueous colloid solutions using ArF laser radiation at a wavelength of λ = 193 nm has been investigated. Nanometer thick layers were obtained on UV transparent silica substrates in contact with the solution for various photodeposition parameters. Amorphous Se layers, 20 nm thick, were obtained typically by 40 laser pulses of 30 ns duration with a fluence of 50 mJ/cm². Deposition thresholds for depositing 1 nm thick layers were as low as 5 pulses. The deposited nanometer thin surface morphology was analyzed by Evanescent Field Optical Microscopy, Scanning Electron Microscopy and Atomic Force Microscopy. The nanometer thicknesses were evaluated by utilizing the differential evanescent light pattern emanating from the substrates.     

Analysis of the sizes and number of microparticles in the ablation plume,which attenuates laser radiation during ablation of a constructive material

The effect of laser radiation with a wavelength of 1.3 μm, power of 25 kW, pulse width of 1 s, and irradiated spot area of 9 cm² on carbon silicon carbide composite material (CSCCM) is analyzed. The formation of an ablation plume (which consists of vapor of irradiated material, burning products, drops and microparticles of various chemical composition and size) above the irradiated surface leads to a significant loss of laser energy. The fractions of the scattered and absorbed laser radiation in the plume are determined, the size and mass distributions of microparticles in the plume are estimated, their concentration is calculated, and the microparticle escape velocities from the irradiated CSCCM surface are evaluated.     

Interface kinetics during pulsed laser ablation

This work investigates evaporation kinetics -- the relation between the surface temperature and pressure during excimer laser ablation. Nickel targets are ablated by excimer laser pulses in a laser fluence range between 1 and 6 J/cm², with the upper limit exceeding the threshold of phase explosion (5 J/cm²). The surface pressure is determined with a polyvinylidene fluoride (PVDF) piezoelectric transducer. When phase explosion occurs, the surface temperature is known to be near the thermodynamic critical temperature, therefore, by measuring the surface pressure, the surface temperature-pressure relation is determined at the threshold fluence of phase explosion. The surface temperature and the threshold fluence of phase explosion are also estimated from the measured velocity of the vapor plume and gas dynamics calculations. It is shown that, during excimer laser ablation, the temperature and pressure relation deviates significantly from the equilibrium kinetic relation.     

Computer simulation of graphite target ablation under the action of a nanosecond laser pulse

A quasi-gasdynamic approach is used for computer simulation of plasma expansion from a graphite plate subjected to a nanosecond laser pulse. A one-component plasma consisting of carbon molecules alone is considered. This simplifies the experimental conditions used previously to study the dynamics of the gas resulting from evaporation. The results of computer experiment conducted for different initial temperatures and pressures of the plasma are in good qualitative agreement with the real experimental data including in the time instant the density of the expanding gas reaches a maximum. It is shown that high-density clusters are likely to appear in front of the main plasma flux. The results of the computer simulation are compared with the Singh approximation of pressure, velocity, and density of the gas flow. It is concluded that this approximation is valid only within a short (compared with the entire expansion length) plasma expansion interval existing during the initial spread for t = 4 × 10^?9 s.     

Nano-graphene structures deposited by N-IR pulsed laser ablation of graphite on Si

Thin nano-structured carbon films have been deposited in vacuum by pulsed laser ablation, from a rotating polycrystalline graphite target, on Si 〈1 0 0〉 substrates, kept at temperatures ranging from RT to 800 °C. The laser ablation was performed by a Nd:YAG laser, operating in the near IR (λ = 1064 nm).X-ray diffraction analysis, performed at grazing incidence angle, both in-plane (ip-gid) and out-of-plane (op-gid), has shown the growth of oriented nano-sized graphene particles, characterised by high inter-planar stacking distance (d_? ∼ 0.39 nm), compared to graphite. The film structure and texturing are strongly related both to laser wavelength and substrate temperature: the low energy associated to the IR laser radiation (1.17 eV) generates activated carbon species of large dimensions that, also at low T (∼400 °C), easy evolve toward more stable sp² aromatic bonds, in the plume direction. Increasing temperature the nano-structure formation increases, causing a further aggregation of aromatic planes, voids formation, and a related density (by X-ray reflectivity) drop to very low values. SEM and STM show for these samples a strongly increased macroscopic roughness. The whole process, mainly at higher temperatures, is characterised by a fast kinetic mode, far from equilibrium and without any structural or spatial rearrangement.     

Simulation of material removal efficiency with ultrashort laser pulses

Understanding physical processes accompanying ablation is necessary for the optimal use of ultrashort laser pulse (USLP) material processing. We describe the implementation of self-consistent electromagnetic propagation-energy absorption in our numerical models. We evaluate absorption as a function of the pulse duration, energy, angle of incidence and polarization. We formulate a measure of material removal and use it to estimate the effect of energy, pulselength and prepulses on material removal.     

Time progression of ultrashort laser ablation in a transparent material

Laser ablation on the ultra-short-pulses regime (femtosecond), has an impact on materials processing and micromachining in a quite profound ways. The investigation of the time progression of the laser ablation process within the material can produce much information and it is not conventionally used. The implementation of such study is the main aim of this paper. The temporal dependence of the diameter and depth of micro-drilling in the sample was verified, beyond the simple understanding of the mechanism for plasma generated during the ablation. From the monitoring of the time progression of the ablation, the time dependence for the velocity of ablation had been determined. In most cases, fume attenuation of the incoming laser beam and fume escape paths, produce dominant influence the characteristics of the region ablated. The method here employed is simple and can be carried out in real time without interruption of the process. In the implemented method light scattered from an auxiliary source allow visualization and record of the ablated geometry as it progress.