Conduction band population in graphene in ultrashort strong laser field: case of massive Dirac particles
The Dirac-like quasiparticles in honeycomb graphene lattice are taken to possess a non-zero effective mass. The charge carriers involve to interact with a femtosecond strong laser pulse. Due to the scattering time of electrons in graphene ($τ\approx 10-100 fs$), the one femtosecond optical pulse is used to have coherence effect, and consequently, it is realized to use the time-dependent Schr$\ddot{o}$dinger equation for coupling electron with strong electromagnetic field. Generalized wavevector of relativistic electrons interacting with electric field of laser pulse leads to obtain a time-dependent electric dipole matrix element. Using the coupled differential equations of a two-state system of graphene, the density of probability of population transition between valence and conduction bands of gapped graphene is calculated. In particular, the effect of bandgap energy on dipole matrix elements in the Dirac points, and also on conduction band population is investigated. The irreversible electron dynamics is achieved when the optical pulse end. Increasing the energy gap of graphene results in a decreasing the maximum conduction band population.