  We investigate the particle-hole pair excitations of dipolar molecules in
optical lattice, which can be described with an extended Bose-Hubbard model.
For strong enough dipole-dipole interaction, the particle-hole pair excitations
can form bound states in one and two dimensions. With decreasing dipole-dipole
interaction, the energies of the bound states increase and merge into the
particle-hole continuous spectrum gradually. The existence regions, the energy
spectra and the wave functions of the bound states are carefully studied and
the symmetries of the bound states are analyzed with group theory. For a given
dipole-dipole interaction, the number of bound states varies in momentum space
and a number distribution of the bound states is illustrated. We also discuss
how to observe these bound states in future experiments.
