Credit: © 2008 APS

Quantum wells — potential wells that confine electrons to two-dimensions — are widely used to fabricate lasers and photodiodes, and are created in ultrathin layers of compound semiconductors with different energy gaps. Intriguingly, although the wavelength of light emitted from a quantum-well structure is controlled by varying the thickness of the quantum well, the electrons confined there do not cause changes in the wavelength of any light that may be incident onto the semiconductor. Now, Guillaume Schull and colleagues1 at Christian-Albrechts University of Kiel report the first observation of two-dimensional electrons, confined in a layer of gold, affecting the wavelength of plasmonic light emitted from the tip of a scanning tunnelling microscope.

The experiments were conducted under ultrahigh vacuum and at low temperatures. Tungsten tips, covered with gold, were lowered to the surface of a gold crystal, and the light emitted at the tunnelling junction was monitored by a grating spectrometer and liquid-nitrogen-cooled CCD detector. The researchers observed that the emission wavelength depended on the location on the gold surface of the scanning tunnelling microscope tip, and that the emission spectra was in effect an image of the electrons confined in the gold surface.

Schull and co-workers suggest the results are an important step towards the development of single-photon sources through the local excitation of one-dimensional quantum structures.