R. Bruzzese

Characterization of laser-ablation plasmas

We discuss the generation of high-density and high-temperature plasmas by focusing high peak power laser radiation onto a solid target. Emphasis will be put on the process of laser ablation and on its basic, physical mechanisms. A survey will be given of the main experimental techniques, namely optical emission and absorption spectroscopy, mass spectrometry, time-of-flight and charge collection measurements, devised to characterize laser-produced plasmas. The fundamental theoretical and numerical approaches developed to analyse laser-target interaction, plasma formation, as well as its expansion will also be reviewed, and their predictions compared with the experimental findings. Although the main emphasis of the review will be on metal target ablation, reference and comparison to results on multicomponent targets will also be frequently given.

Generation of silicon nanoparticles via femtosecond laser ablation in vacuum

We demonstrate that femtosecond laser ablation of silicon targets in vacuum is a viable route to the generation and deposition of nanoparticles with radii of ≈5–10 nm. The nanoparticles dynamics during expansion has been analyzed through their structureless continuum optical emission, while atoms and ions, also present in the plume, have been identified by their characteristic emission lines. Atomic force microscopy analysis of the material deposited at room temperature has allowed the characterization of the nanoparticles size distribution. Taking into account the emissivity of small particles we show that the continuum emission is a blackbody-like radiation from the nanoparticles. Our results suggest that nanoclusters are generated as a result of relaxation processes of the extreme material state reached by the irradiated target surface, in agreement with recently published theoretical studies.

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...

Pulsed laser deposition of SrTiO3/LaGaO3 and SrTiO3/LaAlO3: Plasma plume effects

Pulsed laser deposition of SrTiO3/LaGaO3 and SrTiO3/LaAlO3 interfaces has been analyzed with a focus on the kinetic energy of the ablated species. LaGaO3 and LaAlO3 plasma plumes were studied by fast photography and space-resolved optical emission spectroscopy. Reflection high energy electron diffraction was performed proving a layer-by-layer growth up to 10-1 mbar oxygen pressure. The role of the energetic plasma plume on the two-dimensional growth and the presence of interfacial defects at different oxygen growth pressure has been discussed in view of the conducting properties developing at such polar/non-polar interfaces.

Propagation of a femtosecond pulsed laser ablation plume into a background atmosphere

We investigate the effects of ambient gas on the expansion dynamics of laser plume produced during femtosecond laser ablation of a metallic target. We studied experimentally plume propagation for ambient air pressure ranging from 10−6 to 50 mbar, observin

Pulsed laser ablation of complex oxides: The role of congruent ablation and preferential scattering for the film stoichiometry

By combining structural and chemical thin film analysis with detailed plume diagnostics and modeling of the laser plume dynamics, we are able to elucidate the different physical mechanisms determining the stoichiometry of the complex oxides model material SrTiO3 during pulsed laser deposition. Deviations between thin film and target stoichiometry are basically a result of two effects, namely, incongruent ablation and preferential scattering of lighter ablated species during their motion towards the substrate in the O2 background gas. On the one hand, a progressive preferential ablation of the Ti species with increasing laser fluence leads to a regime of Ti-rich thin film growth at larger fluences. On the other hand, in the low laser fluence regime, a more effective scattering of the lighter Ti plume species results in Sr rich films.

Ultrashort laser ablation of solid matter in vacuum: a comparison between the picosecond and femtosecond regimes

We have investigated the main features of ultrashort laser ablation of solid matter by using laser pulses of different durations, ranging from fs to ps timescales, and wavelengths, in the visible-near infrared spectral range. The analysis has been carried out on Si and Ni in terms of the two main characteristics of the ablation process: vacuum expansion of the ablated material and generation of nanoparticles of the target material. Fast photography and optical emission spectroscopy were used to characterize the plume expansion dynamics, while atomic force microscopy analysis of less than one layer deposits was employed to analyse the size distribution of the produced nanoparticles. Our analysis indicates that the properties characterizing the plume expansion in vacuum as well as the size distribution of the nanoparticles produced with laser pulses in the range of 100 fs–1 ps are almost independent of the specific material properties and laser pulse characteristics, thus representing general features of the process in these conditions.

Thermal and nonthermal ion emission during high-fluence femtosecond laser ablation of metallic targets

We have investigated the emission of positive ions from metallic targets irradiated with intense, ultrashort laser pulses (≈120 fs) at 780 nm, in both S and P polarized states. The measured energy spectra show the presence of a nonthermal, high-energy (several keV) ion component accompanying low-energy ions (tens of eV) produced by a thermal mechanism. The yield of the high-energy component shows a strong dependence on both laser fluence and light polarization. For the low-energy component a higher ablation efficiency was observed for P polarization, and ascribed to a more effective absorption mechanism active during the laser–target interaction.

Dynamics of the plumes produced by ultrafast laser ablation of metals

We have analyzed ultrafast laser ablation of a metallic target (Nickel) in high vacuum addressing both expansion dynamics of the various plume components (ionic and nanoparticle) and basic properties of the ultrafast laser ablation process. While the ion temporal profile and ion angular distribution were analyzed by means of Langmuir ion probe technique, the angular distribution of the nanoparticulate component was characterized by measuring the thickness map of deposition on a transparent substrate. The amount of ablated material per pulse was found by applying scanning white light interferometry to craters produced on a stationary target. We have also compared the angular distribution of both the ionic and nanoparticle components with the Anisimov model. While the agreement for the ion angular distribution is very good at any laser fluence (from ablation threshold up to ≈1 J/cm2), some discrepancies of nanoparticle plume angular distribution at fluencies above ≈0.4 J/cm2 are interpreted in terms of the ...