Excitons Form Luminescent Rings

Two independent teams have reported observing photoluminescent rings from indirect excitons in coupled quantum wells. Excitons are boson particles similar to a hydrogen atom, except with a charge hole instead of a proton. Indirect excitons are created when the electron and hole pair divides between two coupled wells. This separation leads to a longer lifetime for the particle -- up to 100 ns.

The groups created the excitons by directing an approximately 630-nm laser beam into the well structures. The particles spread out from this spot in a dark state and then began to luminesce up to hundreds of microns away, creating a visible ring around the laser spot.

It is not known how or why the excitons migrate or luminesce in such a way, although both teams report that excitation density has a direct effect upon the distance that the ring forms from the laser spot.

A team from the Lawrence Berkeley National Laboratory in California, the Russian Academy of Sciences in Chernogolovka and the University of California at Santa Barbara and at Berkeley, demonstrated this phenomenon in GaAs/AlGaAs quantum wells. Scientists from the University of Pittsburgh and from Bell Labs in Murray Hill, N.J., used InGaAs quantum wells. Both groups reported their results in the Aug. 15 issue of Nature.