Direct visualization of the complete evolution of femtosecond laser-induced surface structural dynamics of metals

Despite extensive studies of femtosecond laser-material interactions, even the simplest morphological responses following femtosecond pulse irradiation have not been fully resolved. Past studies have revealed only partial dynamics. Here we develop a zero-background and high-contrast scattered-light-based optical imaging technique through which we capture, for the first time, the complete temporal and spatial evolution of the femtosecond laser-induced morphological surface structural dynamics of metals from start to finish, that is, from the initial transient surface fluctuations, through melting and ablation, to the end of resolidification. We find that transient surface structures first appear at a delay time on the order of 100 ps, which is attributed to ablation driven by pressure relaxation in the surface layer. The formation dynamics of the surface structures at different length scales are individually resolved, and the sequence of their appearance changes with laser fluence is found. Cooling and complete resolidification, observed here for the first time, are shown to occur more slowly than previously predicted by two orders of magnitude. We examine and identify the mechanisms driving each of these dynamic steps. The visualization and control of morphological surface structural dynamics not only are of fundamental importance for understanding femtosecond laser-induced material responses but also pave the way for the design of new material functionalities through surface structuring. An ultrafast optical imaging technique has been used to thoroughly characterize the interaction between a femtosecond laser beam and a metal. While an impressive array of techniques have been used to explore what happens when a femtosecond laser beams interacts with a metal surface, none has succeeded in fully characterizing the process, and so many questions remain. Now, researchers at the University of Rochester in the USA led by Chunlei Guo have developed an optical imaging technique that analyzes scattered rather than reflected light. This technique allowed them to capture the complete laser-induced dynamics of a metal surface in both space and time. The imaging technique revealed that transient surface structures appear after about 100 picoseconds and that cooling and resolidification of molten metal occur two orders of magnitude slower than previously predicted.