  Solid-state materials have recently emerged as a new stage of strong-field
physics and attosecond science. The mechanism of the electron dynamics driven
by an ultrashort intense laser pulse is under intensive discussion. Here we
theoretically discuss momentum-space strong-field electron dynamics in graphene
and crystalline dielectrics and semiconductors. First, within massless Dirac
fermion and tight-binding models for graphene, we rigorously derive intraband
displacement and interband transition, which form the basis for understanding
solid-state strong-field physics including high-harmonic generation (HHG).
Then, based on the time-dependent Schr\"odinger equation for a one-dimensional
model crystal, we introduce a simple, multiband, momentum-space three-step
model that incorporates intraband displacement, interband tunneling, and
recombination with a valence band hole. We also analyze how the model is
modified by electron-hole interaction. Finally, actual three-dimensional
materials are investigated. We present a time-dependent density-matrix method
whose results for HHG are compared with experimental measurement results.
Moreover, we describe the dynamical Franz-Keldysh effect in femtosecond time
resolution, i.e., the time-dependent modulation of a dielectric function under
an intense laser field, using a real-time time-dependent density functional
theory.
