Transition dipole moment of InAs/InGaAs quantum dots from experiments on ultralow-threshold laser diodes

Semiconductor ultralow-threshold InAs quantum-dot lasers are investigated operating at 1230–1250 nm at room temperature (laser threshold range is of 16–83 A/cm² for ground-state emission). The dependence of gain on current is derived from measurements of the threshold current as a function of the cavity length. The ground-state gain appears at very low current: the inversion threshold of ∼13 A/cm² is a record low value. Analysis of these data for diodes of different molecular beam epitaxial-grown wafers leads to a squared dipole moment of the transition of $∼9.2×10−57 C2 m2$ that corresponds to the length of elementary dipole of ∼0.6 nm.