Excitation dependence of lateral beam width in a quantum-dot laser

We investigate the influence of the quantum-dot active material on the lateral field distribution in an edge-emitting geometry. We use the semiconductor Bloch-equations to describe the optical transitions between the localized dot states in a coupled quantum-dot/quantum-well system that acts as the active medium. Highly excited quantum-dots experience energy renormalizations due to the surrounding carriers, which in turn affect the gain and refractive index spectra. The shifts introduced by the renormalization of the dot transition energies change the position of the peak gain and also the refractive index spectrum. In particular, around the energetically higher dot resonance the refractive index increases for increasing carrier densities, indicating a negative linewidth enhancement factor. To investigate the consequences of this density dependence, we used the quantum-dot gain spectra as input for a wave-optical laser model. Our results suggest the possibility of a negative antiguiding factor in quantum-dot lasers due to many-body Coulomb effects. The implication is a wider beam profile that stays wider than in lasers with comparable quantum-well active media