Synopsis: Guided by voltage
In a solid, an exciton is a bound state between an excited electron and the positively charged hole it leaves behind. So-called indirect excitons, which can exist in materials and artificial structures in which the electrons and holes can each be confined in different layers, have particularly long recombination times. Like electrons, these long-lived particles could provide a means to transport information, but because they are neutral overall, they are harder to move electrically.
Writing in Physical Review Letters, Alexander Winbow at the University of California, San Diego, and colleagues demonstrate they can transport excitons using alternating (ac) voltages that couple to the exciton’s large dipole moments. The team grew a “coupled quantum well,” in which they could produce indirect excitons optically, on a gallium-arsenide surface. By applying an ac voltage to an electrode grid that overlays the device, they were able to create a wavelike potential for the excitons, which then slide across the sample as though on a conveyer belt. Sample regions with a higher density of excitons luminesce, allowing the team to track the location of the excitons.
The device transports excitons in a manner that is similar to the way electrons are guided in charge-coupled devices (CCDs), and could lead to new applications of exciton electronics, as well as answer more fundamental questions about the delocalization-localization transition of excitons. – Daniel Ucko