Riccardo Bruzzese

Emission of prompt electrons during excimer laser ablation of aluminum targets

We report on prompt emission of fast electrons occurring during xenon–fluoride (351 nm) laser ablation of aluminum targets in the nanosecond regime. We have measured both the kinetic energy distribution and the energy-integrated time-of-flight distribution of these electrons. Experimental data evidence that the energetic electrons are produced during the laser pulse as a consequence of two-photon processes, and that space-charge effects influence the photoemitted electron kinetic energy, leading to prompt electrons kinetic energy distributions extending up to ≈15 eV.

Harmonic generation in gases by use of Bessel–Gauss laser beams

We have experimentally investigated the use of diffraction-free beams to generate low-order harmonics in gas jets. Bessel–Gauss beams have been generated from a Ti:sapphire, 100-fs laser by use of annular apertures and then by focusing of the ring-type–produced radiation. The conversion efficiencies for these beams for the third- and fifth-order harmonic generation in xenon have been measured and compared with those of pure Gaussian beams of the same energy content. We have thus found that, by matching the gas medium length to the range of the Bessel–Gauss beams, conversion efficiencies considerably higher than those of Gaussian beams can be obtained. The experimental results have also been discussed and interpreted in terms of a simple numerical model that emphasizes the effects of phase matching in the two different geometries.

Plume composition control in double pulse ultrafast laser ablation of metals

Ultrafast laser ablation of a metallic target induced by a pair of identical laser pulses temporally delayed from ≈1 to 2000 ps was studied by optical emission spectroscopy, imaging, and ion probe. Our experimental results demonstrate that plume excitation/ionization enhancement or nanoparticles reduction is achieved by properly delaying the two pulses. This possibility of controlling plume composition via an efficient coupling of the energy of the second pulse to the various ablation components produced by the first pulse is of particular interest in the process of material deposition and film growth.

Direct Femtosecond Laser Surface Structuring with Optical Vortex Beams Generated by a q-plate

Creation of patterns and structures on surfaces at the micro- and nano-scale is a field of growing interest. Direct femtosecond laser surface structuring with a Gaussian-like beam intensity profile has already distinguished itself as a versatile method to fabricate surface structures on metals and semiconductors. Here we present an approach for direct femtosecond laser surface structuring based on optical vortex beams with different spatial distributions of the state of polarization, which are easily generated by means of a q-plate. The different states of an optical vortex beam carrying an orbital angular momentum ℓu2009=u2009±1 are used to demonstrate the fabrication of various regular surface patterns on silicon. The spatial features of the regular rippled and grooved surface structures are correlated with the state of polarization of the optical vortex beam. Moreover, scattered surface wave theory approach is used to rationalize the dependence of the surface structures on the local state of the laser beam characteristics (polarization and fluence). The present approach can be further extended to fabricate even more complex and unconventional surface structures by exploiting the possibilities offered by femtosecond optical vector fields.

High fluence laser ablation of aluminum targets: Time-of-flight mass analysis of plasmas produced at wavelengths 532 and 355 nm

We report on Time-of-Flight Mass Spectrometry (TOFMS) analysis of plasmas produced in laser ablation of Al targets. We used both the second (532 nm) and third (355 nm) harmonic of a Nd: YAG laser system, carrying out the investigation in a regime of relatively high laser fluence (up to 70 J/cm2), where the production of ionized species in the plume is maximized. We present TOF mass spectra of ions in the laser-produced plasma, and a detailed analysis of the relative abundance of different charged species as a function of the laser fluence. The presence of single, doubly and triply ionized Al atoms has been observed and the fluence threshold for their production is reported. We also studied the total ion and electron yield at different laser fluences, its saturation above specific energy densities, and singly ionized cluster-ions produced in the laser plasma.

Kinetic energy distribution of ions in the laser ablation of copper targets

Abstract In this work we report on a study of the plasmas produced in laser ablation of copper targets by XeF (λ=351 nm) excimer laser irradiation. The kinetic energy of positive ions was measured by energy resolved time of flight mass spectrometry. Average kinetic energies of the Cu+ ions of the order of 1–30 eV were observed, as a function of the laser pulse fluence. The experimental kinetic energy distributions show a double-peak structure. The first located at very low kinetic energy (∼1 eV) can be related to a thermoionic component, whereas the second is strongly dependent on laser fluence.

Multidiagnostic analysis of ion dynamics in ultrafast laser ablation of metals over a large fluence range

The dynamics of ions in ultrafast laser ablation of metals is studied over fluences ranging from the ablation threshold up to ≈75u2009J/cm2 by means of three well-established diagnostic techniques. Langmuir probe, Faraday cup, and spectrally resolved intensified charge coupled device imaging simultaneously monitored the ions produced during ultrafast laser ablation of a pure copper target with 800u2009nm, ≈50 fs, Ti: Sapphire laser pulses. The fluence dependence of ion yield is analyzed, resulting in the observance of three different regimes. The specific ion yield shows a maximum at about 4–5u2009J/cm2, followed by a gradual reduction and a transition to a high-fluence regime above ≈50u2009J/cm2. The fluence dependence of the copper ions angular distribution is also analyzed, observing a gradual increase in forward-peaking of Cu ions for fluences up to ≈10u2009J/cm2. A broader ion component is observed at larger angles for fluences larger than ≈10u2009J/cm2. Finally, an experimental characterization of the ionic angular distrib...

Laser produced plasmas in high fluence ablation of metallic surfaces probed by time-of-flight mass spectrometry

Abstract We report on TOF mass spectrometric analysis of plasmas produced by laser ablation on Al targets. We obtained mass spectra of ionized species in the UV ({ce:inline-formula}λ = 355nm{/ce:inline-formula}) and in the visible ({ce:inline-formula}λ = 532nm{/ce:inline-formula}) and at high laser fluences (1–80 J cm−2). The total ion yield, the relative abundance of different charged species and the laser fluence threshold for the appearance of specific ions as a function of energy density were measured.

Ultrashort laser ablation of bulk copper targets: Dynamics and size distribution of the generated nanoparticles

We address the role of laser pulse fluence on expansion dynamics and size distribution of the nanoparticles produced by irradiating a metallic target with an ultrashort laser pulse in a vacuum, an issue for which contrasting indications are present in the literature. To this end, we have carried out a combined theoretical and experimental analysis of laser ablation of a bulk copper target with ≈50 fs, 800u2009nm pulses, in an interval of laser fluencies going from few to several times the ablation threshold. On one side, molecular dynamics simulations, with two-temperature model, describe the decomposition of the material through the analysis of the evolution of thermodynamic trajectories in the material phase diagram, and allow estimating the size distribution of the generated nano-aggregates. On the other side, atomic force microscopy of less than one layer nanoparticles deposits on witness plates, and fast imaging of the nanoparticles broadband optical emission provide the corresponding experimental charac...

Fast ion generation in femtosecond laser ablation of a metallic target at moderate laser intensity

The generation of ions during laser ablation of a metallic target (copper) with ≈50 fs Ti:Sa laser pulses of moderate intensity (≈1014 W cm−2) is studied by simultaneous fast-imaging and ion-probe techniques. The spatiotemporal distribution of excited ions and neutrals in the laser-produced plasma plume is analyzed by exploiting appropriate band-pass filters in the imaging set-up, while the ion flux angular distribution is characterized by angle-resolved ion probe measurements. An interesting feature of our results is the generation of a fast ion population separated from the neutral component of the atomic plasma plume and characterized by sub-keV kinetic energies, which is interpreted in the frame of a simple model of ambipolar diffusion.