  We theoretically investigate the role of intra-band transitions in
laser-induced carrier-generation for different photon energy regimes: (i)
strongly off-resonant, (ii) multi-photon resonant, and (iii) resonant
conditions. Based on the analysis for the strongly off-resonant and
multi-photon resonant cases, we find that intra-band transitions strongly
enhance photo-carrier generation in both multi-photon absorption and tunneling
excitation regimes, and thus, they are indispensable for describing the
nonlinear photo-carrier generation processes. Furthermore, we find that
intra-band transitions enhance photo-carrier generation even in the resonant
condition, opening additional multi-photon excitation channels once the laser
irradiation becomes sufficiently strong. The above findings suggest a potential
for efficient control of photo-carrier generation via multi-color laser pulses
through optimization of the contributions from intra-band transitions.
