Shine light of just the right frequency on a metal film and it’s possible to generate electromagnetic waves that travel along its surface. These waves, called surface plasmon polaritons (SPPs), are more localized and have a shorter wavelength than the light that excited them. Such attributes could be used to make ultracompact plasmonic circuits, in which the waves shuttle information around an optical chip much smaller than those in use today. Now, in Physical Review Letters, Padmnabh Rai, of the French National Center for Scientific Research (CNRS) in Dijon, and colleagues report the first example of a plasmonic circuit that packs two essential pieces, the light source and a waveguide for the plasmons, into a single device.
Typically, a laser and some optical elements are needed to couple light into a metal to create SPPs. In their devices, Rai et al. replaced these bulky components with a carbon nanotube field-effect transistor (a device widely used in nanoelectronic circuits in which a voltage “gate” controls the nanotube’s resistance) and electrically connected it to a metal strip (the waveguide) in a T-shaped configuration. The authors then ramped up a voltage across the nanotube until it emitted a narrow band of light—a process called electroluminescence. At the same time, a camera showed that light was also being emitted from the edges of the metal strip, evidence that the nanotube had excited SPPs. One interesting finding was that SPP intensity increased in devices where the carbon nanotube was more aligned with the length of the strip, suggesting that future designs could be optimized to produce plasmons more efficiently. – Jessica Thomas