Femtosecond and picosecond laser microablation: ablation efficiency and laser microplasma expansion

Laser ablation efficiency and plasma plume expansion were studied using the interaction of Ti-Al2O3 laser pulses (wavelength 800 nm; energy 20 mu J; m ode TEM00; waist diameter 11 mu m; pulse durations 70 fs, 150 fs, 0.4 ps, 0 .8 ps, 2 ps, and 10 ps) with copper in air. A moderate laser pulse energy o f 20 mu J was used to eliminate the sharply focused femtosecond laser beam disturbance caused by its nonlinear interaction with air. The craters forme d at the surfaces were measured with 0.1 mu m longitudinal and 0.5 mu m tra nsverse resolution. Laser plasma expansion was measured by an ICCD camera w ith 3 mu m spatial and 1 ns temporal resolution. These measurements were pe rformed in a time delay range of 0-50 ns. The laser pulse duration range used in our study was of particular interest as it corresponded to the characteristic time for electron-phonon interact ions in solids (of the order of one picosecond). Thus we could study the di fferent regimes of laser ablation without (fs pulses) and with (ps pulses) laser beam/plasma plume interaction. Laser ablation efficiencies, crater pr ofiles, plasma plume shapes at different time delays, and rates of plasma e xpansion in both longitudinal and transverse directions to the laser beam w ere obtained for all the laser pulse durations mentioned above. The experim ental results of our investigation on laser ablation with short laser pulse s were analysed from the point of view of different theoretical models of l aser beam interaction with plasma and metallic surfaces.