Electrical and optical investigations of plasma plumes generated by femtosecond laser ablation of various metals

Full text: Plasma plumes generated by femtosecond laser ablation on several metals (W, In, Te, Mn, Ni, Cu, Al) were studied by optical and electrical means. The aim of these experiments has been to investigate the link between the plasma parameters (electron temperature, particle density, plasma potential, expansion velocity) and the physical properties of the targets (electrical and thermal conductivities, melting, boiling points etc.). The experiments were performed at various background pressures (10^-2 ÷ 10^-5 Torr), target biases and probe-target axial distances. The probe current was analyzed in terms of a shifted Maxwell-Boltzmann distribution function. Similar temporal behaviors were found for both positive and negative polarities. Consequently, we hypothesize that unified distribution functions should exist for both types of charged particles. This was further confirmed by common oscillation frequencies evidenced in both polarities. Plasma temperatures ("hot" and "cold" particles) and average charge states were deduced from the model. Moreover, by applying various probe biasing voltages and collecting the plasma current, the I-V characteristics were reconstructed at different evolution times, in order to study the temporal evolution of plasma proprieties. Two different behaviors of laser generated plasma plumes were evidenced, with an important effect of the applied target bias at short evolution times. The optical investigations were focused on studying the dynamics of the expanding plume as well as of the excited plasma particles. Background pressure was proved to be the major factor on the expanding velocity of the global plume. The target bias only influenced the expansion of plume produced on materials with low atomic mass. ICCD imaging revealed the presence of two or three plasma structures, depending on the background pressure, result which is in good agreement with the Langmuir probe investigations. (author)