Ultrafast Moving-Spot Microscopy: Birth and Growth of Laser-Induced Periodic Surface Structures

Laser-induced periodic surface structures (LIPSS) are a universal phenomenon observed in all classes of solid materials, giving rise to a variety of self-assembled subwavelength structures with different symmetries. These promising features have opened new opportunities for laser structuring of materials in a wide range of applications, including plasmonics, nanophotonics, nanoelectronics, sensing, and even mechanics. However, there is an ongoing debate about the formation mechanism of LIPSS, and the current picture stems mainly from the combined effort of theoretical modeling and experimental studies of the final structures produced. Here we demonstrate femtosecond-resolved imaging of the formation process of such structures produced by ultrashort laser pulses in silicon. The particular type of LIPSS studied are well-aligned amorphous crystalline fringes generated in dynamic processing conditions, whose period can be tuned and which can be extended over large areas. Using a moving-spot, multiple-pulse irradiation approach we are able to spatially and temporally resolve the birth and growth of individual fringes. We demonstrate that the formation process is initiated by free electron generation leading to nonthermal melting, liquid phase overheating, and rapid solidification into the amorphous phase.