LASING IN LOWER-DIMENSIONAL STRUCTURES FORMED BY CLEAVED EDGE OVERGROWTH

We have used the molecular beam growth technique, which we call cleave d edge overgrowth, to fabricate highly efficient lasers that operate i n the 1D quantum limit. The active region of our laser consists of ato mically precise quantum wires that form at the T-shaped intersections of 7 nm wide GaAs quantum wells grown along the [001] and, after an in situ cleave, along the [110] crystal axis. The origin of the quantum mechanical bound state is the relaxation of quantum well confinement a t this intersection, which leads to an expansion of the electron and h ole wavefunction into the larger available volume at the T-junction. T he high degree of structural perfection achievable in this way allows the observation of stimulated optical emission from the lowest exciton state in optically pumped devices. The interpretation that the observ ed quantum wire response is due to exciton recombination is based on t he near spectral constancy of the emission over almost three orders of magnitude in excitation power from low-power luminescence to a single -mode lasing line. The implied absence of bandgap renormalization effe cts suggests that the Mott density is never reached and indicates inte resting new behaviour of excitons in 1D. In contrast, the quantum well photoluminescence peak indeed shifts to lower energies consistent wit h the notion that the 2D excitons ionize and a free electron-hole plas ma forms.