Like other dark objects, dark excitons proved easier to spot when the researchers were able to effectively shine light on them in the right direction. The transition between states in “bright” excitons has a dipole moment associated with it in the plane of the 2D material surface, which is easier to probe with standard optical techniques. A typical approach is photoluminescence where the light scattered from a laser bears the spectral signature of the excitons. However the dark exciton transition dipole moment points out of the plane of the 2D surface, so the researchers developed a set up that allowed them to enhance the electromagnetic field in this direction.

Markus B. Raschke and his colleagues brought a gold tip close to 2D MoS2 on a gold surface. Much like a lightning rod, the tip gave rise to localised field enhancements aligned with the it. The electrons in gold are also prone to slosh around collectively as “plasmons” in response to incident light, which also enhances the local electromagnetic field.

The researchers were able to control the distance between tip and surface to within 0.2 nm using atomic force microscope technology. At distances of a few nanometres from the surface, the tip can couple with the non-propagating electromagnetic near-field and transform it into a far-field mode. With this set up they obtained clear photoluminescence signals from dark exciton states.

The energy difference between bright and dark exciton states is less than 50 meV. To avoid thermal excitation into the bright exciton channel previous efforts to probe dark excitons have required cryogenic conditions. However, with the tip the researchers enhanced the quantum yield of this dark exciton signature by around 6 x 105 at room temperature.

Full details in Nature Nanotechnology.