We have analyzed ultrafast laser ablation of a metallic target ͑Nickel͒ in high vacuum addressing both expansion dynamics of the various plume components ͑ionic and nanoparticle͒ 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 Ϸ1 J/ cm 2 ͒, some discrepancies of nanoparticle plume angular distribution at fluencies above Ϸ0.4 J / cm 2 are interpreted in terms of the influence of the pressure exerted by the nascent atomic plasma plume on the initial hydrodynamic evolution of the nanoparticle component. Finally, analyses of the fluence threshold and maximum ablation depth were also carried out, and compared to predictions of theoretical models. Our results indicate that the absorbed energy is spread over a length comparable with the electron diffusion depth L c ͑Ϸ30 nm͒ of Ni on the timescale of electron-phonon equilibration and that a logarithmic dependence is well-suited for the description of the variation in the ablation depth on laser fluence in the investigated range.
We have studied ultrafast laser ablation of nickel using a pair of identical Ϸ250 fs 527 nm laser pulses separated by Ϸ1 to Ϸ1000 ps. Scanning white light interferometry was used to measure the ablated volume, and an ion probe was used to measure the angular distribution of the ablation plasma plume and the total ion emission. As the delay of the second pulse increased from Ϸ10 to 100 ps the ablated volume decreased by more than a factor of 2; indeed it falls to a value below the single pulse case. Conversely, it is found that the ion yield is sharply increased in this delay regime. It seems that both these features can be explained by the interaction of the second laser pulse with the ablated material produced by the first pulse.
A new method for pulsed laser deposition of plasmonic silver nanoparticle (NP) films in flowing gas at atmospheric pressure is described. The ablation was done using an excimer laser at 248 nm. Fast optical imaging shows that the ablation plume is captured by the flowing gas, and is expected to form a NP aerosol, which is carried 5-20 mm to the substrate. The dependence of the deposition rate on laser fluence, gas flow velocity, and target-substrate distance was investigated using electron microscopy and absorption spectroscopy of the deposited films. The NP films were annealed in argon and hydrogen at 400 °C, and in air for temperatures in the range 200 °C-900 °C, leading to strong enhancement, and narrowing of the surface plasmon resonance. The films were used for surface enhanced Raman spectroscopy of a 10 molar solution of Rhodamine 6G; films annealed in air at 400 °C were five times more sensitive than the as-deposited films.
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