  Direct laser Interference Patterning (DLIP) with ultrashort laser pulses
(ULP) represents a precise and fast technique to produce tailored periodic
sub-micrometer structures on various materials. In this work, an experimental
and theoretical approach is presented to investigate the previously unexplored
fundamental mechanisms for the formation of unprecedented laser-induced
topographies on stainless steel following proper combinations of DLIP with ULP.
DLIP is aimed to determine the initial conditions of the laser-matter
interaction by defining an ablated region while double ULP are used to control
the reorganisation of the self-assembled laser induced sub-micrometer sized
structures by exploiting the interplay of different absorption and excitation
levels coupled with the melt hydrodynamics induced by the first of the double
pulses. A multiscale physical model is presented to correlate the interference
period, polarization orientation and number of incident pulses with the induced
morphologies. Special emphasis is given to electron excitation, relaxation
processes and hydrodynamical effects that are crucial to the production of
complex morphologies. Results are expected to derive new knowledge of
laser-matter interaction in combined DLIP and ULP conditions and enable
enhanced fabrication capabilities of complex hierarchical sub-micrometer sized
structures for a variety of applications.
