Graphene makes light work of optical signals

Ability to convert light to electrical signals efficiently holds potential for high-speed computing.

Graphene is already revered for its remarkable strength and electrical conductivity — properties that have sent researchers scrambling to use it in applications from tennis rackets to flexible electronics.

Now the one-atom-thick lattice of carbon has added another string to its bow. Three research groups have independently shown that graphene can efficiently convert infrared light into electrical signals, as part of devices known as photodetectors. As fast and accurate translators of optical data, graphene photodetectors could speed up computers and significantly cut their power consumption. The devices, each with a slightly different architecture, are reported in Nature Photonics^1–3.

This performance already rivals that of existing photodetectors. “We’re seeing graphene getting to a point where it can compete with today’s technologies,” says Dirk Englund, a physicist at the Massachusetts Institute of Technology in Cambridge who developed one of the graphene photodetectors¹. “That’s an important new step.”

Optical fibres already use light to carry data over long distances. Inside a computer, however, electrons must still lumber through copper tracks to ferry data bits within a chip and also from chip to chip. Not only are conventional electronic circuits slower than fibre optics, but they also waste a lot of energy in overcoming the resistance of the copper.

To integrate chips with optical fibres, electrical engineers need minuscule light sensors that can convert multiple wavelengths of light back into pulses of electricity. But conventional photodetectors, which use the semiconductor germanium, convert a limited range of wavelengths.

Closing the gap

That is where graphene triumphs. Germanium can detect only photons that have enough energy to push electrons across an energy barrier called the band gap, enabling electrical charge to move freely though the semiconductor. But “graphene can detect any wavelength because it has no band gap”, says Thomas Mueller, a physicist at the Vienna University of Technology, who has also created a graphene photodetector². Furthermore, graphene is cheaper than germanium, and easier to incorporate into a silicon chip.

But the first graphene photodetectors, made in 2009⁴, were extremely inefficient, because the material let most of the light through. The three groups overcame this problem by funnelling the light along a silicon waveguide that runs parallel to a graphene sheet (see image at top), and increases light interaction without reducing speed.

The latest devices generate 50–100 times more current than the earlier detectors from the same amount of light. Although that is still ten times less than germanium photodetectors, “the gap is closing very, very quickly”, says Englund.

Electrons can also move through graphene at high speed, making the material ideal for handling vast quantities of data. “Here, these devices are probably better than existing detectors,” says Englund. His team’s photodetector could handle 12 gigabits of data per second — “a pretty standard number for high-speed optoelectronic devices” — and is likely to surpass that.

A third group of researchers, at the Chinese University of Hong Kong in Shatin, created³ a graphene photodetector that could detect longer wavelength of light, in the mid-infrared part of the spectrum. Moreover, it could do so at room temperature, while conventional detectors that pick up these wavelengths need to be cooled with liquid nitrogen. That could make it particularly useful in applications that measure molecules’ responses to infrared light to identify particular chemicals in the environment, or in medical samples, for example.

The main barrier to commercializing these devices is the difficulty of manufacturing high-quality graphene in bulk, says Englund. All three groups made their graphene by hand, peeling the carbon sheets off a larger sample. But researchers are working on promising alternative production technologies, such as chemical vapour deposition, that could make scaling-up easier, says Ming Liu, an electrical engineer at the University of California, Berkeley, who has co-authored a News and Views article to accompany the papers in Nature Photonics.

Superconductivity to meet humanity's greatest challenges

The stage is now set for superconductivity to branch out and meet some of the biggest challenges facing humanity today.

This is according to a topical review `Superconductivity and the environment: a Roadmap', published today, 16 September, in IOP Publishing's journal Superconductor Science and Technology, which explains how superconducting technologies can move out of laboratories and hospitals and address wider issues such as water purification, earthquake monitoring and the reduction of greenhouse gases.

Lance Cooley, a guest editor of the article who is based at the Fermi National Accelerator Laboratory, said: "Superconductivity has been meeting some great challenges over the past 50 years. The Large Hadron Collider, mankind's largest machine, would not exist were it not for superconductivity."

"There are many uses of superconductors in other big science projects, laboratory devices, and MRI systems. Now, as the roadmap outlines, new materials and technologies enable researchers and entrepreneurs to be more versatile and apply superconductivity in other ways that contribute to our everyday lives, such as innovations to benefit our environment."

By utilising superconducting quantum interference devices (SQUIDs) – very sensitive contraptions that can measure extremely small changes in magnetic fields – one section explains how unexploded weapons, otherwise known as unexploded ordnances (UXOs), can be detected and safely recovered.

Thousands of UXOs are still discovered each year around Europe, especially in areas that were heavily bombed during the Second World War. They can be very unstable and still pose a major threat; however, the sheer scale and complexity of the terrain that needs to be surveyed makes detecting them very complicated.

A section by Pascal Febvre, from the University of Savoie, explains how a complete network of SQUIDs dotted around the globe could also aid the detection of solar bursts which send magnetic particles hurtling towards Earth, potentially wreaking havoc with our communication systems.

A similar network of SQUIDs could also help detect the specific magnetic signature of Earthquakes before they strike.

One area already progressing with the help of superconducting technology is high-speed rail travel. Magnetically levitating (Maglev) trains, whereby the carriage is levitated by magnets and has no contact with the track, have already been deployed in Germany, China, Japan and Brazil.

These countries are now looking to develop high temperature superconducting maglev trains which use liquid nitrogen instead of liquid helium to cool the tracks. This is expected to simplify the cooling process, reduce operational costs, offer more stable levitation and allow lighter carriages to be used, according to Motoaki Terai from the Central Japan Railway Company.

Kyeongdal Choi and Woo Seok Kim, from Korea Polytechnic University, explain how high temperature superconducting technologies can be used to effectively store power from wind and solar plants, as the weather dictates how much power can be generated at any one time, unlike non-renewable sources such as coal and oil which have a constant output.

Superconducting cables could also carry an electrical current with no resistance across large distances from the wind and solar power plants to cities and towns. According to Steven Eckroad, from the Electric Power Research Institute, and Adela Marian, from the Institute for Advanced Sustainability Studies, advances in cryogenics, the development of low-cost wires and ac-to-dc current converters will make this technology cost-effective and environmentally friendly.

Professor Shigehiro Nishijima of Osaka University points out the increasing need for clean water for domestic purposes and describes the possibility of using high field magnetic separation systems based on superconducting magnets for this purpose.

Magnetic jet shows how stars begin their final transformation

Astronomers have for the first time found a jet of high-energy particles from a dying star. The discovery, by a team including Chalmers scientists, is a crucial step in explaining how some of the most beautiful objects in space are formed – and what happens when stars like the sun reach the end of their lives.

​​At the end of their lives, stars like the sun transform into some of the most beautiful objects in space: amazing symmetric clouds of gas called planetary nebulae. But how planetary nebulae get their strange shapes has long been a mystery to astronomers.

Chalmers University of Technology scientists have together with colleagues from Germany and Australia discovered what could be the key to the answer: a high-speed, magnetic jet from a dying star.

Using the CSIRO Australia Telescope Compact Array, an array of six 22-metre radio telescopes in New South Wales, Australia, they studied a star at the end of its life. The star, known as IRAS 15445−5449, is in the process of becoming a planetary nebula, and lies 23000 light years away in the southern constellation Triangulum Australe (the Southern Triangle).

“In our data we found the clear signature of a narrow and extremely energetic jet of a type which has never been seen before in an old, sun-like star”, says Andrés Pérez Sánchez, graduate student in astronomy at Bonn University, who led the study.

The strength of the radio waves of different frequencies from the star match the expected signature for a jet of high-energy particles which are, thanks to strong magnetic fields, accelerated up to speeds close to the speed of light.

Similar jets have been seen in many other types of astronomical object, from newborn stars to supermassive black holes.

“What we’re seeing is a powerful jet of particles spiralling through a strong magnetic field”, says Wouter Vlemmings, astronomer at Onsala Space Observatory, Chalmers. “Its brightness indicates that it’s in the process of creating a symmetric nebula around the star.”

Right now the star is going through a short but dramatic phase in its development, the scientists believe.

“The radio signal from the jet varies in a way that means that it may only last a few decades. Over the course of just a few hundred years the jet can determine how the nebula will look when it finally gets lit up by the star”, says team member Jessica Chapman, astronomer at CSIRO in Sydney, Australia.

The scientists don’t yet know enough, though, to say whether our sun will create a jet when it dies.

”The star may have an unseen companion – another star or large planet – that helps create the jet.  With the help of other front-line radio telescopes, like ALMA, and future facilities like the Square Kilometre Array (SKA), we’ll be able to find out just which stars create jets like this one, and how they do it”, says Andrés Pérez Sánchez.

Text: Robert Cumming

Caption, top picture: Two older objects, the Calabash nebula (a proto-planetary nebula) and M 2-9 (a young planetary nebula) show how IRAS 15445-5449 (left panel) may evolve in the future. The white bar indicates 0.5 light year.

Credits: E. Lagadec/ESO/A. Pérez Sánchez; NASA/ESA & Valentin Bujarrabal; B. Balick, V. Icke, G. Mellema and NASA/ESA

More about planetary nebulae

Seen in a small telescope, some planetary nebulae look like planets, hence the name. They are made of gas ejected from stars with similar mass to the sun at the end of their lives, glowing thanks to instense radiation from the star’s tiny but hot remaining core. The sun will become a red giant in a few billion years’ time, though at present it’s not clear whether it will then form a planetary nebula.