  Irradiation of solids with ultrashort pulses and laser processing in the
mid-Infrared (mid-IR) spectral region is a yet predominantly unexplored field
with a large potential for a wide range of applications. In this work, laser
driven physical phenomena associated with processes following irradiation of
fused silica (SiO2) with ultrashort laser pulses in the mid-IR region are
investigated in detail. A multiscale modelling approach is performed that
correlates conditions for formation of perpendicular or parallel to the laser
polarisation low spatial frequency periodic surface structures for low and high
intensity mid-IR pulses (not previously explored in dielectrics at those
wavelengths), respectively. Results demonstrate a remarkable domination of
tunneling effects in the photoionisation rate and a strong influence of impact
ionisation for long laser wavelengths. The methodology presented in this work
is aimed to shed light on the fundamental mechanisms in a previously unexplored
spectral area and allow a systematic novel surface engineering with strong
mid-IR fields for advanced industrial laser applications.
