IBM claims ultra-fast optical communication with carbon chips

IBM Corp. researchers have demonstrated that graphene, a layer of crystalline carbon only one atom thick, acts as a receiver of optical signals. In the claiming the world's first graphene photodetector, scientists at IBM's Thomas J. Watson Research Center predicted it could operate at up to terahertz frequencies. The 40-GHz graphene photodetector demonstration matched IBM's earlier demonstration of a nanotube optical emitter, the other component needed for bi-directional carbon chip-based optical transceivers.

Graphene is formed as a single atomic layer of carbon atoms, which unlike metals have no bandgap. Optical materials ordinarily require a bandgap so that photons can be absorbed (received by a detector) or emitted (sent by a transmitter). An optical transceiver needs both types of devices: a detector and an emitter.

IBM previously solved the carbon chip emitter problem by injecting electrons and holes into opposite ends of a light-emitting nanotube. It now claims to have solved the carbon chip receiver problem by using nanoscale p-n junctions formed by the electric fields surrounding metal contacts in a graphene field-effect transistor.

According to IBM Fellow Phaedon Avouris, who oversees its carbon-based materials activities, graphene can form the basis of a very fast photodetector with unique properties, despite the fact that graphene doesn't have a bandgap. As he explains, when light strikes graphene, it generates electron-hole pairs that ordinarily just recombine, but the fields generated near electrodes separate the electron-hole pairs to generate a current.

Avouris said that graphene has several unique qualities, such as indifference to wavelength. This differs from most optical materials, which work well only at particular wavelengths. IBM claims other advantages including zero source–drain bias, low dark current, high efficiency (up to 16 percent), and speeds approaching the terahertz range.

IBM said its current instrumentation limited its measurements to 40 GHz. A commercial device made using an expensive material such as palladium for the electrodes would probably extend the upper limit to about 600 GHz.