Evaluation of the role of beam homogeneity on the mechanical coupling of laser-ablation-generated impulse

The material emitted from a target surface during laser ablation generates a net thrust (propulsion) in the opposite direction. The momentum generation efficiency of this laser-driven propulsion is given by the mechanical coupling coefficient (m). In this work, we considered nanosecond UV laser ablation of the aluminum 6061 alloy to study the m behavior with different irradiating conditions. This is done by systematically changing fluence, uniform/nonuniform intensity, and incident angle of the laser beam. In particular, we found that when dealing with nonuniform laser intensity, characterizing m exclusively in terms of fluence is not fully satisfactory because the energy distribution over the irradiated area plays a key role in the way material is removed—interplay between vaporization and phase explosion—and thrust is generated.