Tunnel coupling in an ensemble of vertically aligned quantum dots at room temperature

We report unambiguous observation of the formation of mixed electronic states in an ensemble of self-assembled vertically aligned quantum dots at room temperature. Three closely spaced layers containing stacked In(Ga)As/GaAs quantum dots are placed in the active region of a two-section semiconductor device, and investigations of the quantum-dot optical properties at different applied electric fields are carried out by means of differential-absorption spectroscopy. A simple semianalytical model, which describes absorption of two layers of coupled quantum dots with an account of the size dispersion, is developed. A comparison between our experimental and theoretical results allows clear attribution of the observed low-photon-energy field-dependent spectral features to the four mixed optical transitions due to the two upper quantum-dot layers. Interpretation of the experimental results reveals an anticrossing of spatially direct and indirect transitions characterized by the energy splitting of approximately 30 meV.