Graphene optical modulators could speed communications

Compiled by Photonics Spectra staff

A new optical device that uses graphene to switch a light on and off could soon break digital communication speed limits.

The switching ability is the fundamental characteristic of a network modulator, which controls the speed at which data packets are transmitted. The faster the data pulses can be sent out, the greater the volume of information that can be transmitted.

Researchers at the University of California tuned the graphene electrically to absorb light in wavelengths used in data communications. The graphene-based modulators could soon allow customers to stream full-length, high-definition 3-D movies onto a smart phone in a matter of seconds.

The researchers found that the energy of the electrons – or Fermi level – in the material can be easily altered, depending upon the voltage applied to it. The Graphene’s Fermi level, in turn, determines whether the light is absorbed or not. Electrons can be drawn out of graphene when a sufficient negative voltage is applied, making it no longer possible to absorb photons. The light is switched on” because the graphene becomes totally transparent as the photons pass through.

Graphene is also transparent at certain positive voltages because the electrons become packed so tightly that they cannot absorb photons. The scientists found a sweet spot in the middle, however, where there is just enough voltage applied so the electrons can prevent the photons from passing, effectively turning the light “off.”

To test this, the team layered graphene on top of a silicon waveguide to fabricate optical modulators. They achieved a modulation speed of 1 GHz but found that the speed theoretically could reach as high as 500 GHz for a single modulator.

Graphene-based modulators could overcome the space barrier of optical devices, the researchers believe. They successfully shrunk a graphene-based optical modulator down to a relatively tiny 25 square microns, a size roughly 400 times smaller than a human hair. The footprint of a typical commercial modulator can be as large as a few square millimeters.

Even at such a small size, graphene packs a punch in bandwidth capability. It can absorb a broad spectrum of light, ranging over thousands of nanometers, from the ultraviolet to the infrared. This allows it to carry more data than current state-of-the-art modulators, which operate at a bandwidth of up to 10 nm.

Their findings appeared online May 8, 2011, in Nature (doi: 10.1038/nature10067).