  We present the generation of high order harmonics in crystalline silicon
subjected to intense near-infrared 30fs laser pulse. The harmonic spectrum
extends from the near infrared to the extreme ultraviolet spectral region.
Depending on the pulsed laser intensity, we distinguish two regimes of harmonic
generation: (i) perturbative regime: electron-hole pairs born during each
half-cycle of the laser pulse via multiphoton and tunnel transitions are
accelerated in the laser electric field and gain kinetic energy; the
electron-hole pairs then recombine in the ground state by emitting a single
high-energy photon. The resultant high harmonic spectrum consists of sharp
peaks at odd harmonic orders. (ii) non-perturbative regime: the intensity of
the harmonics increases, their spectral width broadens and the position of
harmonics shifts to shorter wavelengths. The blueshift of high harmonics in
silicon are independent on the harmonic order which may be helpful in the
design of continuously tunable XUV sources.
