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 μJ; mode TEM00; waist diameter 11 μm; 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 of 20 μJ was used to eliminate the sharply focused femtosecond laser beam disturbance caused by its nonlinear interaction with air. The craters formed at the surfaces were measured with 0.1 μm longitudinal and 0.5 μm transverse resolution. Laser plasma expansion was measured by an ICCD camera with 3 μm spatial and 1 ns temporal resolution. These measurements were performed 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 interactions in solids (of the order of one picosecond). Thus we could study the different regimes of laser ablation without (fs pulses) and with (ps pulses) laser beam/plasma plume interaction. Laser ablation efficiencies, crater profiles, plasma plume shapes at different time delays, and rates of plasma expansion in both longitudinal and transverse directions to the laser beam were obtained for all the laser pulse durations mentioned above. The experimental results of our investigation on laser ablation with short laser pulses were analysed from the point of view of different theoretical models of laser beam interaction with plasma and metallic surfaces.