Optical spectra of doubly charged quantum dot molecules in electric and magnetic fields

We present an experimental and theoretical study of the photoluminescence spectra of individual doubly charged quantum dot molecules. The quantum dot molecules consist of two vertically stacked InAs self-assembled quantum dots in a GaAs Schottky diode structure. We study two cases: (1) the two dots are charged with two electrons coherently coupled through electron tunneling and (2) the two dots are charged with two holes and coherently coupled through hole tunneling. The optically excited states consist of the two charges along with one or two additional electron-hole pairs, i. e., a doubly charged exciton and biexciton. We determine the spin states and the corresponding spectral fine structure and show how this fine structure depends on vertical electric and magnetic fields. We find that the results are in large part qualitatively similar for the two cases. However, when magnetic fields are applied, we find a strong g factor resonance and evidence of a bonding/antibonding reversal for the hole-tunneling case only. We discuss the implications for quantum information processing using spins confined in proximate dots.