  The pronounced dependence of the nonlinear parameters of both dielectric and
semiconductor materials on the wavelength, and the nonlinear interaction
between the ultra-short laser pulse and the material requires precise control
of the wavelength of the pulse, in addition to the precise control of the pulse
energy, pulse duration and focusing optics. This becomes particularly important
for fine sub-wavelength single pulse sub-surface processing. Based on two
different numerical models and taking Si as example material, we investigate
the spatio-temporal behavior of a pulse propagating through the material while
covering a broad range of parameters. The wavelength-dependence of material
processing depends on the different contributions of two- and tree-photon
absorption in combination with the Kerr effect which results in a particularly
sharp nonlinear peak at ~2100 nm. We could show that in silicon this makes
processing preferable close to this wavelength. The impact of the nonlinear
nonparaxial propagation effects on spatio-temporal beam structure is also
investigated. It could be shown that with increasing wavelength and large
focusing angles the aberrations at the focal spot can be reduced, and thereby
cleaner and more precise processing can be achieved. Finally, we could show
that the optimum energy transfer from the pulse to the material is within a
narrow window of pulse durations between 600 to 900 fs.
