  We present a novel approach for tailoring the laser induced surface
topography upon femtosecond-fs pulsed laser irradiation. The method employs
spatially controlled double fs laser pulses to actively regulate the
hydrodynamic microfluidic motion of the melted layer that gives rise to the
structures formation. The pulse train used, in particular, consists of a
previously unexplored spatiotemporal intensity combination including one pulse
with Gaussian and another with periodically modulated intensity distribution
created by Direct Laser Interference Patterning (DLIP). The interpulse delay is
appropriately chosen to reveal the contribution of the microfluidic melt flow,
while it is found that the sequence of the Gaussian and DLIP pulses remarkably
influences the surface profile attained. Results also demonstrate that both the
spatial intensity of the double pulse and the effective number of pulses per
irradiation spot can further be modulated to control the formation of complex
surface morphologies. The underlying physical processes behind the complex
patterns generation were interpreted in terms of a multiscale model combining
electrodynamic excitation with melt hydrodynamics. We believe that this work
can constitute a significant step forward towards producing laser induced
surface structures on demand by tailoring the melt microfluidic phenomena.
