Broken symmetry in laterally coupled InAs/GaAs quantum dots molecule

Laterally coupled quantum dot (QD) structures (artificial molecules) are investigated using one layer composed of InAs islands grown by molecular beam epitaxy. We have elaborated, at different InAs growth rates, a series of samples. Those elaborated at lower InAs growth rate present a weak and inhomogeneous QD density. The atomic force microscopy images show that the QDs are gathered by pair having a variable interdot separation. This is what enables us to study the lateral coupling. At 10 K, microphotoluminescence (micro-PL) measurements (realized on a design of 0.2 mu m diameters) present either one, two, or four peaks. To understand the origin of these peaks, we have calculated the excitonic transitions of a system of two identical laterally coupled QDs according to the distance that separates them. The results of this model do not explain PL spectra. Then, we have considered a system of two QDs having, slightly different sizes (asymmetric system). We have shown that when the interdot distance is ranging between 20 and 28 nm, the presence of four optically active excitonic transitions is in good agreement with the experience. For interdot distances lower than 18 nm, the calculation shows that the four excitonic states become a mixture of equiprobable states of two completely decoupled QDs. Two among these states are symmetric and then optically bright, while the two others are asymmetric and optically dark. In this case, the result is comparable to the one obtained with a system having identical QDs. (C) 2008 American Institute of Physics. [DOI: 10.1063/1.2968238]