  A hybrid 2D theoretical model is presented to describe thermoplastic
deformation effects on silicon surfaces induced by single and multiple
ultrashort pulsed laser irradiation in submelting conditions. An approximation
of the Boltzmann transport equation is adopted to describe the laser
irradiation process. The evolution of the induced deformation field is
described initially by adopting the differential equations of dynamic
thermoelasticity while the onset of plastic yielding is described by the von
Mise's stress. Details of the resulting picometre sized crater, produced by
irradiation with a single pulse, are then discussed as a function of the
imposed conditions and thresholds for the onset of plasticity are computed.
Irradiation with multiple pulses leads to ripple formation of nanometre size
that originates from the interference of the incident and a surface scattered
wave. It is suggested that ultrafast laser induced surface modification in
semiconductors is feasible in submelting conditions, and it may act as a
precursor of the incubation effects observed at multiple pulse irradiation of
materials surfaces.
