Space and time characterization of laser-induced plasmas for applications in chemical analysis and thin film deposition

After decades of development, laser ablation has become an important technique for a large number of applications such as thin film deposition, nanoparticle synthesis, micromachining, chemical analysis, etc. Experimental and theoretical studies have been conducted to understand the physical mechanisms of the laser ablation processes and their dependence on the laser wavelength, pulse duration, ambient gas and target material.  The present dissertation describes and investigates the relative importance of the physical mechanisms influencing the characteristics of aluminum laser-induced plasmas. The general scope of this research encompasses a thorough study of the interplay between the plasma plume dynamics and the ambient gas in which they expand. This is achieved by imaging and analyzing the temporal and spatial evolution the plume in terms of spectral intensity, electron density and excitation temperature within various environments extending from vacuum (10‾7 Torr) to atmospheric pressure (760 Torr), in an inert gas like Ar and He, as well as in a chemically active gas like N2.  Our results show that the plasma emission intensity generally differs with the nature of the ambient gas and it is strongly affected by its pressure. In addition, for a given time delay after the laser pulse, both electron density and plasma temperature increase with the ambient gas pressure, which is attributed to plasma confinement. Moreover, the highest electron density is observed close to the target surface, where the laser is focused and it decreases by moving away (radially and axially) from this position. In contrast with the significant axial variation of plasma temperature, there is no large variation in the radial direction. Furthermore, argon was found to produce the highest plasma density and temperature, and helium the lowest, while nitrogen yields intermediate values. This is mainly due to their physical and chemical properties such as the mass, the excitation and ionization levels, the thermal conductivity and the chemical reactivity. The expansion of the plasma plume is studied by time- and space-resolved imaging. The results show that the ambient gas does not appreciably affect plume dynamics as long as the gas pressure remains below 20 Torr and the time delay below 200 ns. However, for pressures higher than 20 Torr, the effect of the ambient gas becomes important and the shorter plasma plume length corresponds to the highest gas mass species and the lowest thermal conductivity. These results are confirmed by Time-Of-Flight (TOF) measurements of Al+ line emitted at 281.6 nm. Moreover, the velocity of aluminum ions is well defined at the earliest time and close to the target surface. However, at later times, the ions travel through the plume and become thermalized through collisions with plasma species and with surrounding ambient gas.

Keyphrases: Ablation of aluminum, Laser-induced plasmas, Plasma plume dynamics, Ablation in different ambient gas, Optical emission spectroscopy, Time of flight, Electron density, Excitation temperature, Boltzmann plot