  Exciton levels and fine-structure splitting in laterally-coupled quantum dot
molecules are studied. The electron and hole tunneling energies as well as the
direct Coulomb interaction are essential for the exciton levels. It is found
that fine-structure splitting of the two-lowest exciton levels is contributed
from the intra- and inter-dot exchange interactions, both of which are largely
influenced by the symmetry and tunnel-coupling between the two dots. As the
inter-dot separation is reduced, fine-structure splitting of the exciton ground
state is largely increased while those of the excited states are decreased.
Moreover, the dependence of the fine-structure splitting in quantum dot
molecules on the Coulomb correlation is clearly clarified.
