  Nonlinear optical phenomena in silicon such as self-focusing and multi-photon
absorption are strongly dependent on the wavelength, energy and duration of the
exciting pulse. Thus, a pronounced wavelength dependence of the sub-surface
modifications with ultra-short pulsed lasers exists, especially for wavelengths
> 2 $\mu$m. This wavelength dependence is investigated for wavelengths in the
range of 1950-2400 nm, at a pulse duration between 0.5-10 ps and the pulse
energy varying from 1 $\mu$J to 1 mJ. Numerical and experimental analyses have
been performed on both the surface and sub-surface of Si wafers processed with
fibre-based lasers built in-house that operate in this wavelength range. The
results have been compared to the literature data at 1550 nm. The analysis
carried out has shown that due to a dip in the nonlinear absorption spectrum
and a peak in the spectrum of the third-order non-linearity, the wavelengths
between 2000 - 2200 nm are more favourable for creating sub-surface
modifications in silicon. This is the case even though those wavelengths do not
allow as tight a focusing as those at 1550 nm in the linear regime. This
problem is compensated by an increased self-focusing due to the nonlinear
Kerr-effect around 2100 nm at high light intensities, characteristic for
ultra-short pulses.
