  We investigate photoinization, energy deposition, plasma formation and the
ultrafast optical breakdown in crystalline silicon irradiated by intense
near-infrared laser pulses with pulse duration $\tau \le $ 100 fs. The
occurrence of high-intensity breakdown was established by the sudden increase
of the absorbed laser energy inside the bulk, which corresponds to threshold
energy fluence $\Phi_{th} > $ 1 J/cm$^2$. The optical breakdown is accompanied
by severe spectral broadening of the transmitted pulse. For the studied
irradiation conditions, we find that the threshold fluence increases linearly
with the increase of the pulse duration, while the corresponding laser
intensity threshold decreases. The effect of the high plasma density on the
stability of diamond lattice is also examined. For near threshold fluences,
when about 5 \% of valence electrons are promoted into the conduction band, the
Si-Si bonds are softened and large Fermi degeneracy pressure arises (with
pressure up to 100 kbar). The mechanical instability of the diamond lattice
suggests that the large number of electron-hole pairs leads directly to
ultrafast melting of the crystal structure.
