Coulomb effects in semiconductor quantum dots

A theoretical analysis of the linear and third-order nonlinear optical properties that includes the one- and two-electron-hole-pair (EHP) states was carried out numerically. The results consistently show that Coulomb effects are important even for the smallest quantum dots. The binding energy of the energetically lowest two-EHP state (loosely referred to as a biexciton), which is a measure of the strength of the Coulomb interaction, is found to increase with decreasing dot radius. It is predicted that energetically higher two-pair states in quantum dots should be quite easily observable as increasing probe absorption on the high-energy side of the one-EHP resonances. Calculations were performed for intrinsic semiconductor quantum dots as well as for quantum dots with trapped charges or impurities. All the investigations show that the predicted features are quite universal and even enhanced in systems with charged impurities or traps. Experiments on quantum dots in a glass matrix clearly show the predicted induced absorption in addition to the bleaching of the inhomogeneously broadened one-EHP resonances.