LAUSR

Abstract We found a fast hydrodynamic process in air double pulse Laser Induced Plasma (LIP) during the first 50 ns of plasma life. This process leads to strong inhomogeneity in the refractive index of air LIP, disturbance in the Gouy phase shift, and an increase in Third Harmonic Generation (THG). The efficiency of THG strongly depends on the confocal parameter of the focusing beam. The decrease in the focus of the pumping lens, despite the increase in the pump intensity, leads to a decrease in TH intensity. This can be attributed to the change from weak to tight focusing conditions in THG.