Monday, May 9, 2011

Revolution in Communication: Berkeley Scientists Create World’s Smallest, Fastest Optical Modulator Using Graphene

The wonder material Graphene continues to amaze. A research team, headed by Xiang Zhang, a UC Berkeley engineering professor, has built an ultra-small optical device that can control the switching on and off of light pulses, using Graphene. This extraordinary device is guaranteed to revolutionise communication, both in terms of speed and how we do it, in the very near future.
Graphene is a one-atom thick sheet of carbon atoms arranged in a hexagonal pattern – the so called sp2hybridized structure. (Read more about Graphene here). Graphene can be switched on and off extremely fast and this is the property exploited here. According to an externally applied voltage, Graphene can modulate pulses of light by letting some go through and restricting some others.

An artist's impression of Graphene

But why Graphene?

The modulator will not only be fast, it will be very compact. It only takes a 25 micron (that’s roughly 400 times thinner than a human hair) a side square of Graphene to make this modulator. Graphene also has the added advantage of supporting a huge bandwidth of optical frequencies. Any frequency of light ranging across the optical range up to ultra-violet and down to even infra-red can be effectively modulated. Graphene is also quite cheap to produce, especially with improved techniques like Chemical Vapor Deposition (CVD). It can be easily integrated with other materials, like Silicon, without bothering much about contamination. Graphene has the tremendous advantage of being highly conducting in pure form. It needs no doping, unlike Silicon. Further, its conductivity remains constant down to very low temperatures, like a few Kelvin.
A disk of Graphene
A disk of Graphene (coated with an oxide)

How the modulation happens: Interplay between electrons and light

UC Berkeley researchers were attracted by the behavior of electrons and light within Graphene, especially their interaction. Here’s the watered down version of the technicalities: The electrons that matter are called valence electrons. They lie near the top of a so-called valence band, having energy equal to that of the Fermi level (the level till which the electrons are filled – here the top of the valence band). Electrons can jump from just below the Fermi level to above the level by absorbing light. The Fermi level can, however, be varied by an external voltage. Given enough negative voltage, electrons can be drawn out of the bands completely or packed tight (Technically, external voltage alters the Fermi level). In both these cases, light cannot be absorbed by the electrons (either because they are absent or because they have no place to jump to). The material is then transparent to light. Researchers hit upon the ‘Goldilocks voltage range’ in which Graphene is opaque and used it to turn the modulator off. Due to the high mobility of electrons in Graphene, a square voltage pulse with Gigahertz frequency easily dumps Graphene in and out of the opaque state, effectively modulating the transmission of light pulses. This is the first time light has been modified and guided at such small scales. Generally, light requires bulky mirrors or photonic crystals.
The Schematic for Graphene modulatoes
Schematic for Graphene Modulators. The honeycomb layer is the Graphene which sits on top of the Silicon waveguide. Pulses of light are sent through the waveguide and according to the voltage applied (the square pulses on the left), light is either transmitted or absorbed. (Courtesy: UC Berkeley)
The team put a Graphene layer on top of a Silicon waveguide (images above and below). They were able to achieve 1GHz modulation speed. Theoretically, 500 GHz is possible, so the 1 GHz figure will definitely be revised.

An image showing the actual fabrication (Courtesy: UC Berkeley)

The Future … is Here

In the near future, Zhang says, “Instead of broadband, we will have ‘extremeband’”, because the bandwidth offered will be huge, up to 10 nanometers (above 1000 Terahertz).
So there is another glimpse of the future, courtesy Graphene: It’s super-fast, super-small, cheap and offers huge bandwidth. Hope you download a lot of HD movies in 3D on your phone in future – it’ll, after all, take just a few seconds.

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