  Rabi oscillations in two-level Dirac systems have been shown to alter the
frequency content of the system's nonlinear response. In particular, when
considering Rabi oscillations in a quantum model beyond the semiclassical
Boltzmann theory, even harmonics may be generated despite the centrosymmetric
nature of these systems. This effect magnifies with increasing excitation
intensity. In this work, we extend the Rabi theory to a three-level Dirac
system arising from a line-centered-square optical lattice. In this case, the
Dirac cones are bisected at the Dirac point by a flat band that persists
throughout the Brillioun zone. Due to the presence of this flat band, we expect
a significant enhancement of the coupling between Dirac states, resulting in a
large increase of the Rabi effects and the associated nonlinearities, leading
to continuum generation of terahertz radiation.
