  A new type of artificial molecule is proposed, which consists of coupled
defect atoms in photonic crystals, named as photonic molecule. Within the major
band gap, the photonic molecule confines the resonant modes that are closely
analogous to the ground states of molecular orbitals. By employing the
variational theory, the constraint determining the resonant coupling is
formulated, that is consistent with the results of both the scattering method
and the group analysis. In addition, a new type of photonic waveguide is
proposed that manipulates the mechanism of photon hopping between photonic
molecules and offers a new optical feature of twin waveguiding bandwidths.
