  In-volume ultrafast laser direct writing of silicon is generally limited by
strong nonlinear propagation effects preventing the initiation of
modifications. By employing a triple-optimization procedure in the spectral,
temporal and spatial domains, we demonstrate that modifications can be
repeatably produced inside silicon. Our approach relies on irradiation at
$\approx 2$-$\mu$m wavelength with temporally-distorted femtosecond pulses.
These pulses are focused in a way that spherical aberrations of different
origins counterbalance, as predicted by point spread function analyses and in
good agreement with nonlinear propagation simulations. We also establish the
laws governing modification growth on a pulse-to-pulse basis, which allows us
to demonstrate transverse inscription inside silicon with various line
morphologies depending on the irradiation conditions. We finally show that the
production of single-pulse repeatable modifications is a necessary condition
for reliable transverse inscription inside silicon.
