Directional ion emission from thin films under femtosecond laser irradiation

Abstract

Thin films of nickel have been irradiated using femtosecond laser pulses in vacuum. Subsequent emission of plasma ions is diagnosed using an ion probe. Angular distributions of the emitted ions are presented for a range of target film thicknesses. Data are compared to the Anisimov model of plasma expansion [S. I. Anisimov, D. Bauerle, and B. S. Luk’yanchuk, Phys. Rev. B 48, 12076 (1993)]. The tendency of the ions to be ejected at small angles to the normal of the target surface is explained in terms of the initial conditions of the plume. Results are explained in terms of the initial shape and adiabatic index of the plasma.

Dynamics of the plumes produced by ultrafast laser ablation of metals

“…33 Instead, larger values of the aspect ratio ͑from Ϸ6 to 10͒ were reported when irradiating Ni thin films with thicknesses from 2 to 50 nm with the same laser pulses at a peak fluence of 1.9 J / cm 2 , and an adiabatic index ␥ Ϸ 1.27 was estimated for the Ni ion plume. 19 Moreover, a progressive reduction in the aspect ratio was observed as the film thickness increases, thus suggesting that our experimental aspect ratio should correspond to a value Z 0 of the initial plume larger than 50 nm, which is consistent with the earlier estimate of a 50-200 nm lower limit for Z 0 based on the propagation of a rarefaction wave through the layer which is vaporized.…”

“…One of these is the adiabatic isentropic expansion model of Anisimov et al, 15,16 which has been extensively applied to PLD with ns pulse duration to describe both the laser ablation plume propagation and the angular profile of the deposition rate. [17][18][19] These studies indicate that the Anisimov model provides a rather good description of how the ablated material expands from a small, hot, dense vapor cloud at the end of the ns laser pulse to the time when the ablated material has traveled several centimeters, or more, from the target. More recently, the Anisimov model was also shown to describe fairly well the angular distribution of the ionised plume produced by ULA of metallic thin films or bulk targets.…”

“…2 This would result in an extremely narrow angular width of the plume in the inertial expansion stage, which was not observed experimentally. 19,20 Indeed, it has been observed that two distinct plumes are produced in the ULA process, a fastermoving atomic plume and a slower NP one. [10][11][12] The validity of the Anisimov model should be tested for both components, which is the main aim of the present work.…”

“…The presence of a faster peak in ion probe signals has been previously observed in various studies of ULA of metallic targets, and assigned to low-Z contamination of the target surface. 19,28,29 Optical emission spectroscopy analysis of ULA of a Cu target has shown the presence of hydrogen emission line due to impurities, which can be reduced by appropriate target conditioning in multishot ablation experiments. 30 To see if this is the origin of the fast signal in our experiment the following tests were carried out.…”

“…More recently, the Anisimov model was also shown to describe fairly well the angular distribution of the ionised plume produced by ULA of metallic thin films or bulk targets. 19,20 In contrast, the application of the Anisimov model to describe the expansion of the NP plume in ULA has not yet been explored.…”