Formation of nanocavities in the surface layer of an aluminum target irradiated by a femtosecond laser pulse

Abstract

176Femtosecond (τ L ~ 100 fs) laser pulses incident on a metallic target are absorbed by conduction electrons in the skin layer with a depth of 10-20 nm. Owing to heat conduction, the energy absorbed by the electrons propagates into the target in the form of an electron heat wave [1][2][3][4][5]. Electronic energy is simultaneously transferred to the lattice through electron-phonon heat transfer. As a result, a heated layer with thickness d T ≈ 120 nm is formed in aluminum within the elec tron-ion relaxation time t eq = 2-3 ps. The lattice is melted when the energy flux of the femtosecond laser pulse exceeds the melting threshold for the given material. For aluminum, the absorbed energy at the melting threshold is estimated at ~15 mJ/cm 2 .The pressure in the heated layer increases strongly owing to the supersonic heat conduction for which the acoustic time t s = d T /c s ≈ 20 ps is much longer than the heating time t eq [1, 2, 5]. Acoustic decay of the heated and pressurized layer (d'Alembert's solution) and sub sequent reflection of acoustic waves at the vacuum interface are accompanied by generation of thermo mechanical tensile stresses [1,2,[5][6][7], where the amplitude of the tensile stresses increases with the energy density of the laser pulse F. Such motion results in deformation of the surface layer d T . When F exceeds the ablation threshold F abl , spallation (thermome chanical ablation) of part of the melted layer occurs as a result of the tensile stress exceeding the tensile strength of the liquid metal, and a crater forms on the surface. Our experiments and calculations indicate that, when the energy flux is slightly higher than the ablation threshold, the heated layer first expands to a certain value and then returns back with some residual deformation (see, e.g., Fig. 1).In the described experiments, the deformation of the surface of the target in the heated region was stud ied using femtosecond interference microscopy [8]. To analyze the structure of the surface layer after irradia tion near the ablation threshold, transmission electron microscopy was used.To heat and probe the surface of the target, 100 fs pulses generated by a femtosecond chromium-for sterite laser system were used. The surface of the target was heated by pulses having a fundamental wavelength of 1240 nm at an angle of incidence of 45°. The target was probed by the 620 nm second harmonic pulses with the measured time delay with respect to the heat ing pulse. The spatial distribution of the energy density in the focal spot had the Gaussian formIt has been revealed experimentally that nanocavities remain inside a surface layer of aluminum after action of a femtosecond laser pulse. This result is in agreement with numerical simulation. A detailed picture of melting, formation of expansion and compression waves, and bubble nucleation in the stretched melt has been reconstructed through atomistic simulation. It has been shown that the bubbles do not fully collapse but remain as frozen disk shaped nanocavities upon recrystallization ...

Generation of subsurface voids and a nanocrystalline surface layer in femtosecond laser irradiation of a single-crystal Ag target

“…The effect of swelling is not unique to Ag and has also been observed for femtosecond laser irradiation of Al targets [3,62,63], where the presence of sub-surface voids was directly confirmed in transmission electron microscopy (TEM) and scanning electron microscopy (SEM)…”

“…imaging of cross-sections of the swollen areas of the target [3,63]. The smaller thermal conductivity and larger melting depth at fluences close to the spallation threshold in Al, as compared to Ag, reduce the extent of swelling to about a hundred of nanometers and lead to a partial collapse of the voids prior to solidification, as reflected in the flattening of the void shapes parallel to the surface of the target observed in the TEM and SEM images.…”

Two-temperature hydrodynamics of laser-generated ultrashort shock waves in elasto-plastic solids

“…Mach number of shift velocity of a point of liquidus decreases from ≈ 5 to ≈ 0.02 at this interval. Much latter at temporal scale 0.1-1 ns the recrystallization solidifies molten layer if aluminum film is significantly thicker than a thermal depth d T ≈ 100 nm [18]. Transonic transition lasts from 4 to 6 ps.…”

Thin 10–100 nm film in contact with substrate: Dynamics after femtosecond laser irradiation

“…Ultrashort laser pulse may induce the interesting combinations of thermal and hydrodynamic phenomena including foaming and freezing of molten metals and semiconductors [1][2][3] formation of chaotic surface nanostructures and mesoporous layers [1][2][3], and superelastic shocks [4,5]. Appearance of negative pressures within the frontal surface layer heated by a laser has a key importance for understanding of frontal nucleation, foaming, and spallation often called ablation (mass removal) in laser community.…”