Coupling of bonding and antibonding electron orbitals in double quantum dots by spin-orbit interaction

We perform a systematic exact diagonalization study of spin-orbit coupling effects for stationary few-electron states confined in quasi-two-dimensional double quantum dots. We describe the spin-orbit-interaction induced coupling between bonding and antibonding orbitals and its consequences for magneto-optical absorption spectrum. The spin-orbit coupling for odd electron numbers (one, three) opens avoided crossings between low energy excited levels of opposite spin orientation and opposite spatial parity. For two electrons the spin-orbit coupling allows for low-energy optical transitions that are otherwise forbidden by spin and parity selection rules. We demonstrate that the energies of optical transitions can be significantly increased by an in-plane electric field but only for odd electron numbers. Occupation of single-electron orbitals and effects of spin-orbit coupling on electron distribution between the dots are also discussed.