  Femtosecond laser surface processing (FLSP) is an emerging fabrication
technique to efficiently control the surface morphology of many types of
materials including metals. However, the theoretical understanding of the FLSP
formation dynamics is not a trivial task, since it involves the interaction of
various physical processes (electromagnetic, thermal, fluid dynamics) and
remains relatively unexplored. In this work, we tackle this problem and present
rigorous theoretical results relevant to low-fluence FLSP that accurately match
the outcomes of an experimental campaign focused on the formation dynamics of
laser induced periodic surface structures (LIPSS) on stainless steel. More
specifically, the topology and maximum depth of LIPSS trenches are
theoretically and experimentally investigated as a function of the number of
laser pulses. Moreover, precise LIPSS morphology measurements are performed
using atomic force microscopy (AFM). The proposed comprehensive simulation
study is based on two-temperature model (TTM) non-equilibrium thermal
simulations coupled with fluid dynamic computations to capture the melting
metal phase occurring during FLSP. Our rigorous simulation results are found to
be in excellent agreement with the AFM measurements. The presented theoretical
framework to model FLSP under low-fluence femtosecond laser pulses will be
beneficial to various emerging applications of LIPSS on metallic surfaces, such
as cooling high-powered laser diodes and controlling the thermal emission or
absorption of metals.
