Optical self-interference bubble produced in femtosecond double-pulse laser ablation for ultrathin film deposition

The surface-ionized air channel on plasma during its expansion critically influences the properties of the shockwave and its interaction with ambient air. In this study, a self-interference bubble induced by double-pulse ablation on a copper surface was observed using a time-resolved shadowgraph imaging technique. This bubble is formed due to stronger local air ionization, which results from the interference of the first reflected pulse with the second incident pulse. We found that the intensity of the two-interference pulse was strongly influenced by the laser fluence and the transmissivity of the materials, thus a smaller bubble was generated with lower laser fluence or higher transparency materials. Meanwhile, the self-interference bubble vanished, as the increasing numerical aperture of the convergent optical lens attenuates the subsequent focusing Rayleigh length. Furthermore, the radial expansion of the bubble was typically a planar (one-dimensional) propagation with half the velocity of light, which is consistent with the evolution of air plasma and shockwaves under the increasing probe delay. Ultrathin copper film with good surface quality was obtained by femtosecond double laser-induced backward transfer, and the geometric parameters of the surface morphology can be adjusted by changing the double pulse delay time. Ultrafast transient absorption (TA) spectroscopy results of the copper film elucidated that the weaker thermal electron transport causes a slower initial thermal diffusion. With the increase of electron temperature and pump–probe delay time, the optical response is dominated by the thermal electron transport process instead of the joint effect of the electron–lattice coupling and thermal electron transport. The electrical thermal transport and electron − lattice coupling properties of copper film and are simultaneously calculated with the thermal conduction and optical response models. These results encourage the further progress of ultrafast double-pulse laser ablation for regulating and controlling material ablation morphology to acquire excellent capabilities.