Dual-state lasing and the case against the phonon bottleneck

Quantum Dot lasers exhibit the novel phenomenon of dual state lasing where population inversion can be achieved on two optical transitions within the dots. In principle this might occur if a phonon bottleneck exists to impede relaxation of carriers from the higher energy state. Here we present an alternative explanation whereby different lasing modes compete for carriers and are spatially separable. Evidence comes from a comparison of electrical and optical measurements made on the devices. The evolution of a particular lasing mode depends on diffusion of carriers between dots and we show how, using an equivalent circuit model, this is consistent with our measurements.