  To clarify a key role of orbital degrees of freedom in the response of the
many-body electron state of correlated electrons to an external field, we
investigate the real-time dynamics in an $e_{\rm g}$-orbital Hubbard model
under applied electric fields by exploiting numerical techniques such as a
time-dependent density-matrix renormalization group (DMRG) method. The ground
state without applying an electric field is found to be an
antiferromagnetic/ferro-orbital state. After we switch on an oscillating
electric field, it gives rise to the creation of holon-doublon pairs when the
frequency of the applied electric field exceeds a charge excitation gap. We
argue that a pair-hopping process yields a dynamical deformation of the orbital
configuration.
