  The fabrication of laser-induced periodic surface structures (LIPSS) over
extended areas at high processing speeds requires the use of high repetition
rate femtosecond lasers. It is known that industrially relevant materials such
as steel experience heat accumulation when irradiated at repetition rates above
some hundreds of kHz, and significant debris redeposition can take place.
However, there are few studies on how the laser repetition rate influences both
the debris deposition and the final LIPSS morphology. In this work, we present
a study of fs laser-induced fabrication of low spatial frequency LIPSS (LSFL),
with pulse repetition rates ranging from 10 kHz to 2 MHz on commercially
available steel. The morphology of the laser-structured areas as well as the
redeposited debris was characterized by scanning electron microscopy (SEM) and
{\mu}-Raman spectroscopy. To identify repetition rate ranges where heat
accumulation is present during the irradiations, we developed a simple heat
accumulation model that solves the heat equation in 1 dimension implementing a
Forward differencing in Time and Central differencing in Space (FTCS) scheme.
Contact angle measurements with water demonstrated the influence of heat
accumulation and debris on the functional wetting behavior. The findings are
directly relevant for the processing of metals using high repetition rate
femtosecond lasers, enabling the identification of optimum conditions in terms
of desired morphology, functionality, and throughput.
