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Metals exposed to ultrafast laser irradiation close to ablative regimes show often a submicron-scale (near 0.5 μm) periodic organization of the surface as ripples. Using two classes of metallic materials (transition and noble), we have determined that the ripples amplitude is strongly correlated to the material transport properties, namely electron-phonon relaxation strength, electronic diffusion, and to the energy band characteristics of the electronic laser excitation. This particularly depends on the topology of the electronic structure, including d-band effects on electronic excitation. Comparing the effects of electron-phonon nonequilibrium lifetimes for the different metals under similar irradiation conditions, we indicate how the electron-phonon coupling strength affects the electronic thermal diffusion, the speed of phase transformation and impacts on the ripples contrast. The highest contrast is observed for ruthenium, where the electron-phonon coupling is the strongest, followed by tungsten, nickel, and copper, the latter with the least visible contrast. The dependence of surface patterns contrast with fluence is linked to the dependence of the relaxation characteristics with the electronic temperature. [ABSTRACT FROM AUTHOR]

The characterization of diamondlike carbon (DLC) films is a challenging subject, considering the diversity of carbon-based nanostructures depending on the deposition process. We propose to combine multiwavelength (MW) Raman spectroscopy and electron energy-loss spectroscopy (EELS) to probe the structural disorder and the carbon hybridizations of DLC films deposited by pulsed laser ablation performed either with a nanosecond laser (film labeled ns-DLC), either with a femtosecond laser (film labeled fs-DLC). Such deposition methods allow to reach a rather high carbon sp3 hybridization but with some significant differences in terms of structural disorder and carbonaceous chain configurations. MW Raman investigations, both in the UV and visible range, is a popular and nondestructive way to probe the structural disorder and the carbon hybridizations. EELS allows the determination of the carbon plasmon energy in the low-loss energy region of the spectra, as well as the fine structure of the ionization threshold in the high-loss energy region. The paper shows that the combination of MW Raman and EELS is a powerful way to elucidate the nanostructure of DLC films. Complementary nanoindentation investigations allow to correlate the analytical results with the mechanical properties of the films. The ns-DLC film presents a stronger sp3-bonded C character (74%-85%) with a significant content of sp2 chains, whereas the fs-DLC contains less sp3 bonds (35%-50%) with a significant content of sp2-bonded C rings. The ns-DLC film exhibits a higher proportion of disordered sp2 C mainly in the form of chains. Comparatively, the fs-DLC exhibits a predominance of more ordered sp2 C structures in the form of graphitic aggregates whose size has been estimated near three aromatic rings. The film characteristics are in agreement with their mechanical properties. We also propose a correlation between the nanostructure and composition of the films with the deposition mechanisms. The difference in kinetic energy distribution in the plasma plume, together with an absence of interaction between the plasma plume and the femtosecond laser, are responsible for the observed differences in sp3 C content and sp2 C configuration ranging between a predominance of more ordered sp2 rings in the fs-DLC film and a predominance of sp2 chains in the ns-DLC film. These results are consistent with the mechanisms of subplantation occurring during DLC deposition [ABSTRACT FROM AUTHOR]

The laser ablation characteristics of yttria-stabilized zirconia (YSZ) have been investigated as a function of the target microstructure and dopant level for different nanosecond- [ArF, KrF, and XeCl excimers; Nd:YAG (yttrium aluminum garnet) (fourth harmonic)] and femtosecond-laser sources [Ti:sapphire (fundamental and third harmonic)]. Particle ejection, which compromises the quality of coatings prepared by pulsed laser deposition (PLD), was analyzed in detail. Nanosecond-laser pulses cause a severe thermomechanical surface cracking and exfoliation of micron-sized fragments on a microsecond to millisecond time scale in the case of 8–9.5 mol % Y2O3-doped, fully stabilized zirconia (8YSZ and 9.5YSZ) targets. As a consequence of the intrinsic material brittleness, fully stabilized YSZ coatings deposited by PLD contained particles for all tested conditions. Lower doped partially stabilized zirconia (3YSZ) exhibits a superior fracture toughness attributed to a laser-induced partial transition to the monoclinic phase, detected by Raman spectroscopy, which enables the deposition of particle-free dense thin films by conventional PLD using nanosecond-UV laser radiation at moderate fluences of 1.2–1.5 J/cm2. The ablation dynamics of ultrashort laser pulses differ fundamentally from the nanosecond regime as evidenced, e.g., by time-resolved shadowgraphy and light scattering experiments. Femtosecond pulses prevent the exfoliation of micron-sized fragments but result invariably in a pronounced ejection of submicron particles. The resulting PLD coatings are porous and reveal a large surface roughness as they consist of an agglomeration of nanoparticles. Femtosecond-NIR pulses provide a factor of 2.5–10 higher material removal rates compared to nanosecond- and femtosecond-UV pulses. The ablation metrics, i.e., threshold fluence and effective absorptivity, mainly depend on the laser wavelength while the pulse duration, target microstructure, and dopant level are of minor importance. Evidence is presented that incubation effects play a significant role in nanosecond- and femtosecond-laser ablations of YSZ enabling material removal at comparatively low fluences for sub-bandgap photon energies. [ABSTRACT FROM AUTHOR]

Provides information on a study of a three-dimensional Monte Carlo simulation of the laser induced plasma plume transport in vacuum. Methodology used to conduct the study; Gas flow characteristics observed in pulsed laser ablation; Indepth look at the model; Results of study.

T.E. Itina has questioned the reliability of using a three-dimensional Monte Carlo simulation to model the expansion of a pulsed-laser induced plume, claiming that results were not valid and that parameters were set incorrectly in the program code. However, some confusion was made between some basic points of the direct simulation Monte Carlo (DSMC) method and the technique of random trajectories that was employed. The correct description of collisions by the proposed approach is verified by the good agreement with analytical results and with those obtained from other experiments.