“Compared with silicon-based optical modulators, this double-layer graphene device has separate electrical and optical control modules,” team member Ming Liu told nanotechweb.org. “This is a first and allows us to optimize both the electrical and optical design separately and avoid the trade-off between speed and optical losses in the device.”

Graphene consists of a single, flat sheet of carbon arranged in a honeycombed lattice. Since the material was first created in 2004, its unique electronic and mechanical properties have amazed researchers, who have been eyeing it up for a host of device applications such as ultrafast transistors. Graphene also has attractive optical properties over a range of wavelengths in the electromagnetic spectrum from the visible to mid-infrared. It also absorbs more than 2% of the light falling on it in the UV and visible ranges, which is surprising as the material is only one atom thick.

Graphene active layers

In their work, the researchers, led by Xiang Zhang, exploited the unique optical properties of graphene, and in particular, the fact that the material becomes transparent when it is charged. The researchers used two graphene layers to form an anode/cathode pair and made the material transparent by driving voltage in between the two. The two layers are the active region in the finished optical modulator.

The design is similar to a conventional forward/reverse-biased silicon modulator, say Zhang and colleagues, in which the doped silicon is replaced by graphene. It therefore does not suffer from the insertion losses that plague doped silicon waveguides.

The new modulator has the smallest footprint of any device of its kind as it can reach a modulation depth of 6.5 dB with a footprint of merely 16 µm2. It also has the potential to be the fastest device in its category and operate at the broadest bandwidth. What is more, it is easy to fabricate and is cheap to make.

Telecommunications and on-chip data communication

The device may find use in two broad application areas, says Liu. The first is in long-haul telecommunications where the graphene modulator would work as one the key elements in FTTX architecture. Such systems incorporate optical modulators in the last kilometre or so of the telecommunications fibre.

The second potential market might be in on-chip data communication. “Modern computer structures are increasingly facing severe difficulties such as high energy consumption and data jam at I/O interfaces since multicore CPUs are widely being used these days,” explained Liu. “Optical communication is believed to be a solution to such problems. Our prototype graphene modulator with its low power consumption, high-speed, broad bandwidth and the fact that it is stable over a range of temperatures make it a promising candidate for such an application.”

Encouraged by its preliminary results, the team is now working hard to improve the speed of its device to 50 GHz.

The modulator is detailed in Nano Letters.