METAMORPHOSIS OF A QUANTUM-WIRE INTO QUANTUM DOTS

Bound states of electron-hole pairs (excitons) in semiconductors posse ss desirable properties-such as an enhanced oscillator strength for ra diative recombination-that hold promise for the next generation of opt ical devices, However, at typical device operating conditions (room te mperature and moderate charge densities), excitons dissociate to form an electron-hole plasma, Dissociation may be prevented by confining ex citons to lower dimensions, where their binding energy is expected to increase significantly(1). But such confinement may in turn influence the dynamical properties of the excitons. Here we report spatially res olved photoluminescence images of excitons confined. to an isolated ga llium arsenide quantum wire. As the temperature of the structure is lo wered, we observe a striking transition from broad and fairly continuo us photoluminescence to an intense set of emission peaks which are bot h energetically sharp and spatially localized, Such behaviour indicate s that, at sufficiently low temperatures, the quantum wire acts like a sparse set of quantum dots. Furthermore, at the site of an isolated q uantum dot, we observe an unusual decrease in the relaxation rate of e xcitons, such that they radiate (via recombination) ham higher energy states before relaxing to their ground state, We argue that this is th e manifestation of an exciton relaxation 'bottleneck', the existence o f which could pose problems for the development of optical devices bas ed on quantum dots.