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Premièring a premier event

By Dens Milne

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Courtesy Daresbury Laboratory

Yesterday I was at the Cockcroft Institute at the Daresbury Laboratory for the 1st Vacuum Symposium UK. This included a one-day technical programme covering a broad range of talks on vacuum science and technology. The event followed the 9th Residual Gas Analyser Users Meeting held the day before, which boasted its own day-long seminar programme organized by the RGA Users Group.

I joined more than100 delegates enjoying presentations on the many and varied applications of vacuum — covering everything from the challenges of pumping the enormous vacuum chamber at the Joint European Torus (JET) fusion experiment to the intricacies of applying coatings in the production of large-area glazing products (that’s windows to you and me).

The event (co-sponsored by the Institute of Physics’ Vacuum Group, the British Vacuum Council and the Science and Technology Facilities Council) was free to attend thanks to the support of the 26 hi-tech companies showing their wares at the associated trade exhibition. Attendees could also sign up for two training seminars taking place in parallel to the main talks or peruse the mini poster session at the back of the main auditorium highlighting research work taking place at the University of Liverpool, Nottingham University and Université de Provence. A further international feel was added by keynote speaker Dr Manfred Leisch of Graz University of Technology in Austria, who talked about his surface-science research on stainless steel, one of the most commonly used construction materials for vacuum chambers and components.

It is hoped that this will be just the first in a series of annual symposia. Judging by the reactions of both attendees and exhibitors, it looks set to become the premier event in the UK vacuum calendar.

Showcasing the applications of physics

By Matin Durrani

Most readers of this blog first got interested in physics for a variety of reasons — be it an inspiring teacher, a good popular-science book, or just a deeply held desire to get to the bottom of something really quite hard.

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Sadly, not everyone has the same passion for physics as physicists themselves. People are, of course, perfectly happy to reap the benefits of physics — be it finding their way in the car using a GPS sat-nav system, downloading the latest movies over the optical fibres of the Internet, or getting treated with an MRI scanner when they’re ill.

But that does not mean non-physicists want to know anything about physics. Even worse, many people aren’t even aware of what physics can do.

Now, though, my colleagues at the Institute of Physics have published an excellent report that outlines, at a very simple level, how physics has contributed to 10 different technological developments.

Entitled Physics for an Advanced World, the glossy full-colour report can be downloaded for free here

Launched at the House of Commons earlier this week, it has 10 case studies showcasing the the social and economic benefits of physics — each with great photos, accessible text and a useful timeline. Other applications in addition to those mentioned above include holography, lasers and, of course, the Web itself.

Without which you would not be reading this blog.

New optical clock breaks accuracy record

A new optical clock that is twice as accurate as any other has been unveiled by physicists in the US. The clock, which is based on a single aluminium ion, could remain accurate to within one second over 3.7 billion years. The previous record was held by a clock with one mercury ion, which was good to one second in 1.7 billion years.

The new optical clock also differs from most other designs in that it uses a quantum-information protocol. It could therefore be used to further boost the performance of such clocks and ultimately lead to them replacing atomic clocks as our official timekeepers.

Since the first atomic clock was unveiled in 1955, the technology has been used in national time standards around the world. Tiny atomic clocks, which use microwaves to probe atomic transitions, are also used as timepieces on Global Positioning System (GPS) satellites. The best atomic clocks have an accuracy of about 5 × 10–15, but this has been surpassed recently by optical clocks, which use light rather than microwaves and can be more than 100 times as accurate.

Stability is a must

An optical clock has three main components. The first is a highly stable “reference” frequency provided by a narrow optical absorption line in an atom or ion. The second element is a feedback system that “locks” the output of a laser (called the local oscillator) to the reference frequency. The third component provides a very precise measurement of the frequency of the laser – usually with a “femtosecond comb”.

The most important component, however, is the atom or ion itself – which must absorb light over an extremely narrow and extremely stable frequency range. To ensure this, the atom or ion is isolated in a vacuum chamber and cooled to near absolute zero using laser beams – whereby light is absorbed and reemitted by the ion in such a way as to reduce its kinetic energy.

Aluminium ions are relatively easy to isolate from their surroundings – particularly from the effects of black-body radiation from the surrounding chamber. Unfortunately, aluminium ions are very hard to cool with laser light because they re-emit the absorbed radiation at a much lower frequency. This also makes it tricky to design a feedback mechanism to keep the laser locked on to the reference frequency.

James Chin-wen Chou, David Wineland and colleagues at the National Institute of Standards and Technology (NIST) in Colorado have got around this problem by pairing the aluminium with a second ion of magnesium, which is easy to cool. The team begins by trapping the magnesium ion using electric fields and then cooling it using a laser. They then introduce the aluminium ion into the chamber, which interacts with the magnesium via electrical forces. These interactions allow the team to chill the aluminium ion using “sympathetic cooling”.

Quantum logic measurement

The local-oscillator laser is then fired at the aluminium ion and the efficiency at which the ion absorbs the light is determined by a technique called quantum logic spectroscopy (QLS). QLS is possible because the ions are just a few microns apart and therefore they behave as a quantum system – the result being that the absorption efficiency of the aluminium ion can be determined from light emitted from the magnesium ion. The greater the absorption efficiency, the closer the match between the aluminium ion’s reference frequency and the local-oscillator laser – and this is used in a feedback loop to tune the laser to the ion.

The team first applied QLS to an optical clock in 2008, when it created a similar device using aluminium and beryllium (Al-Be) ions that had an uncertainty of about 2.3 × 10–17. By using magnesium and a new type of ion trap, the team has improved by this nearly a factor of three to 8.6 × 10–18. This shatters the previous record for precision – 1.9 × 10–17 – which was held by NIST’s mercury ion clock.

The team was also able to compare the output of the new aluminium-magnesium clock with the older Al-Be clock to understand the differences between the two devices. Such comparisons are important if several optical clocks are deployed around the world – as is the case with standard atomic clocks.

‘Very impressive result’

Patrick Gill of the UK’s National Physical Laboratory (NPL) is impressed by the new clock. “QLS is complicated, but the aluminium clock does it very well,” he says. “It’s a very good advertisement of the capability of the technique.”

NISTS’s Chou told physicsworld.com that the team hopes to reduce the frequency uncertainties in the device by developing new types of traps. The physicists are also working on new schemes to stabilize the frequency of the laser.

Despite their success, Chou believes that we are still a long way from adopting a specific optical clock as an international time standard. Indeed, other teams are looking at clocks based on ytterbium and indium ions.

“It is not quite clear which one will be most suitable as the next time standard”, says Chou. “People all around the world would need to be able to construct such devices and agree on the frequencies,” he adds. “Optical clocks are thus still in their early stage toward being standards.”

The work is will be described in Physical Review Letters and a preprint is also available at arXiv: 0911.4527v2.

Zonal swishing at the Earth’s core

Zonal flows are ubiquitous in nature. Familiar examples include the jet stream winds that circulate the globe or the alternating currents in the world’s oceans such as the Gulf Stream. Now, researchers in Japan say that zonal flows could also exist at the centre of the Earth where the planet’s magnetic field is generated.

Most geophysicists agree that the main component of the Earth’s field – which defines the magnetic poles – is a dipole generated by the convection of molten iron deep within the planet’s interior. Researchers can only use indirect methods, however, to infer the fine details of the geodynamo, which may provide clues as to why the Earth’s magnetic poles have flipped every million years or so throughout Earth history.

In search of these finer details, Akira Kageyama at Kobe University and colleagues have modelled the geodynamo to build a more detailed picture of convection in the Earth’s outer core. Their simulation quickly established a secondary flow pattern, consisting of inner sheet-like radial plumes, surrounded by westward cylindrical zonal flow.

This work was carried out using the Earth Simulator supercomputer, based in Japan, which offered the spatial resolution to determine these secondary effects. Kageyama and his team also confirmed, using a numerical model, that this dual-convection structure can co-exist with the dominant convection that generates the north and south poles.

Kageyama told physicsworld.com that this kind of zonal flow has not been seen in geodynamo models before and that it could provide a “hint” about the mechanism of polar reversal. The last time the north and south poles flipped was 780 000 years ago so, statistically at least, we could be heading for another reversal soon. Geologists speak of the poles “flipping” in a very short time, but in reality the full process still takes around 10 000 years.

This research is published in Nature.

Nuclear physics gets a stepping stone to the ‘island of stability’

Researchers in Europe have made a key breakthrough in fundamental nuclear physics by making the first direct mass measurement of an element heavier than uranium. They say that their new technique will provide experimentalists with much better understanding of the superheavy elements, and it may even provide a stepping stone to the fabled “island of stability” – a hypothesized group of superheavy elements with much longer half-lives than uranium.

The element in question is nobelium (No), defined by its 102 protons, which the researchers produce by firing isotopes of calcium at a lead target. By fusion, this process produces one isotope every second of either 252No, 253No or 254No.

A major challenge that faced the researchers was to find a way of slowing down the energetic young nobelium ions to get them under control for their weigh-in. This was achieved by guiding them through a cloud of neutral helium atoms, which forced the sprightly ions to shed much of their energy through a series of collisions.

The weigh-in

At the weigh-in itself, nobelium isotopes were led into a vacuum chamber encasing powerful magnetic fields, called a “Penning trap”. Once inside the trap, the charged isotopes got picked up in the magnetic field lines and their masses were determined by the frequency at which they circulated. “The trick is to catch the particles in a trap that provides a clean environment,” says Michael Block, one of the researchers at the GSI heavy-ion research lab in Germany.

By comparing their results with the latest theoretical predictions, Block and his colleagues declare that their results are accurate to a precision of 0.05 parts per million and confirm previous results from indirect measurements. Before this research, the only way to gauge the mass of elements heavier than uranium was to study their decay products and “back track” to the parent isotope by a series of calculations.

“It is indeed a step change to make direct mass measurements for elements heavier than uranium,” says Philip Walker, a nuclear physicist at the University of Surrey. Walker believes that improved accuracy and reduced likelihood of systematic errors are the two main advantages.

Catch them while you can

The main challenge in the study of trans-uranium elements is to catch them while you can, as the isotopes are hard to produce and tend to have very short half-lives. The reason for their fleeting existence is that the large number of protons and neutrons crammed into their nuclei tends to leave them highly unstable and prone to fission.

Theoreticians predict, however, that as even heavier elements are discovered, we should eventually reach an “island of stability” around proton number 120 and neutron number 184. According to the theory, this geological feature in the sea of heavy isotopes would exist because certain “magic numbers” lead to stronger nuclear binding between protons and neutrons, the effect being related to the shells they occupy in the nucleus.

Block and his team intend to further improve their technique to determine the masses of heavier elements. “We are still a long way from reaching this island of stability, but it will be important to build a detailed picture of the radioactive elements along the way,” he told physicsworld.com.

This research was published in Nature.

A new view of our volatile star

A mission to study the Sun in greater detail than ever before has been launched today from Cape Canaveral, Florida. NASA‘s $850m Solar Dynamics Laboratory (SDO) will use a suite of extremely sensitive instruments to make a wide range of measurements of the Sun – charting solar disturbances from their origins deep inside the core of the star to their appearance on the surface and their rapid ejection into space.

The SDO is the first – and will eventually be the largest – mission to emerge from NASA’s Living With a Star programme, which was launched in 2001 to gain a better understanding of how solar storms affect life on Earth. In the worst case, they could lead to geosynchronous satellites being knocked out by high-energy particles entering the atmosphere or to power supplies on the Earth’s surface being severely damaged, debilitating telecommunications, navigation based on Global Positioning System satellites, banking and transport networks.

The SDO will initially operate for five years with the possibility to run for a further five years after that. “It will observe the Sun faster, deeper and in greater detail than any previous observatory,” says astrophysicist Madhulika Guhathakurta, the mission’s programme scientist.

Solar strength

The cause of the Sun’s volatility lies in its magnetic field, which results from the continual churning of the charged plasma surrounding its core. Because the solar equator rotates more quickly than the poles, the magnetic field becomes twisted and entangled, producing regions of dense field lines that extend outwards from the centre of the Sun and which restrict the upward convection of heat. This leads to the dark patches on the solar surface known as sunspots that are seen to vary in number and position over an 11-year cycle.

It will observe the Sun faster, deeper and in greater detail than any previous observatory astrophysicist Madhulika Guhathakurta, mission programme scientist

The last solar cycle apparently lasted for 12.5 years, therefore delaying the build up of the current cycle, which started with a sunspot observed on 4 January 2008. This cycle is now picking up with the peak in activity expected in 2013, so the SDO is well timed to observe the build-up of this activity.

The disruptive phenomena associated with sunspot activity are believed to be caused by the huge amounts of energy released when the entangled fields reach breaking point and suddenly disentangle. These phenomena include solar flares – explosions in the Sun’s atmosphere that accelerate charged particles and emit broadband electromagnetic radiation – as well as coronal mass ejections – enormous expulsions of plasma threaded with magnetic fields that push against the charged particles of the solar wind and create a shock wave that batters the protective bubble provided by the Earth’s magnetic field.

Eric Priest, a solar physicist at the University of St Andrews in the UK, describes the SDO as “brilliantly conceived and designed” and says that it will lead to “a major advance in our understanding” of the Sun’s variability.

Peering beneath the solar skin

The SDO has been placed in an inclined geosynchronous orbit – in other words, it rotates around the Earth exactly once a day but does so at a finite angle to the equator. This orbit has two big advantages: the Sun can be almost continuously observed (apart from a few weeks a year when the satellite will be eclipsed by the Earth) and the spacecraft will have nearly continuous contact with a single, dedicated ground station, allowing fast, real-time data transmission. However, the high altitude required for such an orbit has meant a relatively expensive launch and protection against the very effects of space weather that the spacecraft is designed to study.

To carry out its studies the SDO will use three instruments. The Atmospheric Imaging Assembly consists of an array of four telescopes that will produce a continuous stream of high-definition images of the surface and atmosphere of the Sun in 10 wavelength bands. This high resolution in both time and wavelength means that the SDO will be far better placed to observe individual solar events with short duration and a narrow spectrum than were previous observatories. As such, says Guhathakurta, it may explain why the atmospheric corona, at 1–2 million degrees Kelvin is so much hotter than the surface of the Sun, which is just a few thousand Kelvin. (One theory is that the corona is maintained at such extremely high temperatures by mini solar flares.)

Solar surveillance
Solar surveillance

Moving beyond the visible, solar scientists will use the Extreme Ultraviolet Variability Experiment to study sunlight between 0.1 and 105 nm. Unlike total solar irradiance, which typically varies by just a fraction of a per cent over a solar cycle, emissions at extreme ultraviolet wavelengths can fluctuate by a factor of 1000 or more within the space of a few seconds. This radiation is largely responsible for heating and therefore ionizing the Earth’s upper atmosphere, which can alter and sometimes severely disturb radio communications.

The new observatory will not just be capable of observing the visible and ultraviolet output of the Sun, however. Using the Helioseismic and Magnetic Imager (HMI) it will also be able to study what is happening inside the Sun, and in particular probe the origin of the solar magnetic field, by virtue of the sound waves and magnetic fields on the solar surface. The speed of these sound waves is determined by what the waves encounter as they travel through the Sun, and this speed can be calculated by measuring the Doppler shift of spectral lines within the sunlight as measured across the solar disk. Likewise, the polarization of these same spectral lines reveals the magnetic field over the visible face of the Sun.

[The SDO] will be a major advance in our understanding of the Sun’s variability Eric Priest, a solar physicist at the University of St Andrews, UK

According to Guhathakurta, the HMI will allow researchers to work out what is happening on the far side of the Sun and identify regions of strong magnetic field that could migrate to the front of the Sun and cause geomagnetic storms. Because such migration can take up to two weeks, these data could allow significantly longer-term forecasts of space weather than the two or three days currently provided by NASA’s STEREO mission.

Bernhard Fleck, project scientist of the currently operating Solar Heliospheric Observatory (SOHO), which is a joint project of NASA and the European Space Agency, believes that the SDO is likely to yield “very significant advances” in solar science. He likens the difference between the imaging capabilities of the SDO and SOHO to the gulf between state-of-the-art single-lens reflex (SLR) digital cameras and the first devices to be incorporated into mobile phones. But he points out that predicting space weather requires understanding all the elements in a long chain that links processes inside the Sun to those on the solar surface and outwards into the solar system.

Achieving this, he says, will involve combining the results from the SDO with those of the other existing solar observatories and with those of the other missions that will make up Living With a Star and which are due to take off over the coming decade. These missions include radiation belt storm probes, due for launch in 2012 and designed to study the plasma processes responsible for geomagnetic storms. Solar Probe Plus, on the other hand, will make measurements within the Sun’s corona.

Warm white dwarfs could reveal ‘inelastic’ dark matter

Direct observations of dark matter – the substance thought to account for 80% of matter in the universe – are sketchy, at best. Some experiments have found what seem like dark-matter signals, while others looking within the same parameter range have found nothing. Yet there is a hypothetical candidate for dark matter, known as “inelastic” dark matter, that could reconcile such results – and now two teams of physicists have proposed new ways to see if it exists.

The story of inelastic dark matter begins over a kilometre beneath Gran Sasso mountain in Italy, which is home to the underground DAMA experiment. Here, a bank of detectors watches out for the flash of light that is expected when a dark-matter particle strikes a nucleus within the experiment. Although such collisions are very rare, in theory there should be more flashes in summer, when the Earth is orbiting against the prevailing “wind” of dark matter in our galaxy.

The DAMA team claimed the first such signature in 2000, and over the subsequent decade has steadily increased its precision. In its most recent analysis (available at arXiv:1002.1028), the researchers claim evidence for dark-matter particles at a statistical significance of 8.9 sigma – or, to put it another way, about as precise as one can hope to expect.

No matching signals

These results have failed to convince many physicists, however. Part of the problem is that there could be some other factor that modulates with the seasons, thereby imitating the much sought-after dark-matter signal. But more troubling is that several other direct-detection experiments – CDMS in the US or ZEPLIN in the UK, for instance – have found no matching signals using different detectors.

Regardless of the details of estimating the amount of dark matter, it’s quite an interesting scenario for looking for evidence of inelastic dark matter Matthew McCullough, University of Oxford

To get round this, some physicists have proposed different types of dark matter that would interact only with DAMA’s detectors. One of these, inelastic dark matter, was put forward in 2001 by David Smith at the University of California at Berkeley and Neal Weiner at the University of Washington. Unlike normal, “elastic” dark matter, inelastic dark matter would get heavier when it hits an atomic nucleus of sufficient mass. So because sodium iodide has the most massive nuclei, only DAMA would be able to give inelastic dark matter the mass jump to enable it to scatter and produce the tell-tale flashes.

Now, two independent groups have tried to find ways to spot evidence for inelastic dark matter somewhere far outside the lab – namely in old white dwarf stars. White dwarves are the dim, compact forms that the vast majority of stars adopt in their latter years, and as they get older they cool down. The oldest and coolest ones tend to reside in the centres of galaxies, or in regions of high dark-matter density.

Warming old white dwarfs

The two groups – one comprising Dan Hooper and colleagues at Fermilab in the US; and the other being Matthew McCullough at the University of Oxford and Malcolm Fairbairn at King’s College London in the UK – have calculated how dark matter should interact with an old white dwarf’s atomic nuclei as it is accelerated inwards by the star’s huge gravitational forces. According to their calculations, inelastic dark matter should interact much more strongly than elastic dark matter, which means that much more of it would end up being absorbed by the star. And this added mass would in turn mean that the star’s temperature is held a little higher, at around 7000 K as opposed to 3000 K.

It may be that the true importance of these works will come after we know what the dark matter is, which is a truly exciting prospect Neal Weiner, New York University

“If you looked at [a dense dark matter region] and, instead of seeing very cold white dwarves, you saw white dwarves at nothing below, say, six or seven thousand Kelvin, then you might say, huh, maybe these white dwarves are cooling but they’ve hit this bottom floor where they can’t cool any further, because inelastic dark matter is heating them up,” explains Hooper. “And that would be evidence of the [inelastic dark matter] picture.”

Both groups suggest a search for unusually warm white dwarves as a means to find evidence for inelastic dark matter, although there are differences between their studies. Whereas McCullough and Fairbairn have focused on close regions of tightly bound stars known as globular clusters, Hooper’s group believes that there are higher densities of dark matter in galactic centres, which would make the search for the white dwarves easier. Still, as McCullough point out, the issue isn’t settled. “Regardless of the details of estimating the amount of dark matter, it’s quite an interesting scenario for looking for evidence of inelastic dark matter,” he adds.

Weiner – now at New York University – agrees with Hooper’s group that galactic centres present the most promising region to test the hypothesis. However, he also thinks that observations of old white dwarves could instead be used to map distributions of dark matter once direct-detection experiments have found the true nature of dark-matter particles. “Rather than being tests of theories of dark matter, [white dwarves] may become excellent probes of the structure of the galaxy’s dark matter halo,” he says. “It may be that the true importance of these works will come after we know what the dark matter is, which is a truly exciting prospect.”

Preprints of the studies can be found at arXiv: 1001.2737 and arXiv:1002.0005.

Astronomer Royal tackles space, politics and scientific advice

The Royal Society – perhaps the world’s oldest and most prestigious scientific society – is celebrating its 350th anniversary this year. It was founded in 1650 by a group of 12 natural philosophers, including Robert Boyle, best known for his law describing how the pressure of a gas rises as it is compressed at constant temperature. Over the years, the society has had plenty of links with physics – past presidents include Isaac Newton, J J Thomson, Lord Kelvin and Ernest Rutherford and the current president is the Cambridge University astrophysicist and cosmologist Martin Rees.

Speaking in this exclusive video interview with physicsworld.com, Rees explains why he thinks the Royal Society still has an essential role to play in the modern world. For Rees, the society’s strengths lie in its ability to promote and disseminate science – and in the increasing amount of scientific advice it offers to politicians on topics like energy and climate change.

In a wide-ranging discussion, Rees also welcomes President Obama’s decision not to return astronauts to the Moon.

“Given the financial constraints, if I were an American taxpayer I would entirely support it,” he says. “I think it is very important we pursue science in space [but] the case for sending people into space is getting weaker all the time with every advance in robotics and miniaturization. I still believe in the long run that there is a role for people in space, but that’s just for an adventure – not for any practical purpose.”

As for what are the most exciting developments in astronomy, Rees cites the search for Earth-like extrasolar planets, the study of the cosmic microwave background by the Planck satellite and the ability of the Herschel infrared telsecope to understand how the earliest galaxies formed.

The interview with Rees took place at the Royal Society’s “presidential flat” – a kind of up-market crash-pad at the society’s headquarters at Carlton House Terrace in central London. The flat has great views out onto the London Eye, Big Ben and the Houses of Parliament, where Rees – as a member of the House of Lords — had spent the morning giving evidence to a scientific committee. That was followed by a radio interview and then us.

Add in his duties as master of Trinity College Cambridge, and it’s not surprising that Rees only has time for research at weekends. But, as he explains, he is “in a style of life that is fascinating”.

Astronomer Royal says Obama is right about space

By Matin Durrani, Editor, Physics World

The Royal Society — perhaps the world’s oldest and most prestigious scientific society — is celebrating its 350th anniversary this year. It was founded in 1650 by a group of 12 natural philosophers, including Robert Boyle, best known for his law describing how the pressure of a gas rises as it is compressed at constant temperature.

Over the years, the society has had plenty of links with physics — past presidents include Isaac Newton, J J Thomson, Lord Kelvin and Ernest Rutherford and the current president is the Cambridge University astrophysicist and cosmologist Martin Rees.

Speaking in an exclusive video interview with physicsworld.com, Rees explains why he thinks the Royal Society still has an essential role to play in the modern world. After all, if scientists can communicate quickly and easily via online discussion groups, Facebook and Twitter, a society with a limited and admittedly elite membership might not be totally in tune with today’s world. For Rees, however, the society’s strengths lie in its ability to promote and disseminate science — and in the increasing amount of scientific advice it offers to politicians on topics like energy and climate change.

In a wide-ranging discussion, Rees also welcomes President Obama’s decision not to return astronauts to the Moon.

“Given the financial constraints, if I were an American taxpayer I would entirely support it,” he says. “I think it is very important we pursue science in space [but] the case for sending people into space is getting weaker all the time with every advance in robotics and miniaturization. I still believe in the long run that there is a role for people in space, but that’s just for an adventure – not for any practical purpose.”

As for what are the mostexciting developments in astronomy, Rees cites the search for Earth-like extrasolar planets, the study of the cosmic microwave background by the Planck satellite and the ability of the Herschel infrared telsecope to understand how the earliest galaxies formed.

The interview with Rees took place at the Royal Society’s “presidential flat” — a kind of up-market crash-pad at the society’s headquarters at Carlton House Terrace in central London. The flat has great views out onto the London Eye, Big Ben and the Houses of Parliament, where Rees — as a member of the House of Lords — had spent the morning giving evidence to a scientific committee. That was followed by a radio interview and then us.

Add in his duties as master of Trinity College Cambridge, and it’s not surprising that Rees only has time for research at weekends. But, as he explains, he is “in a style of life that is fascinating”.

First light for germanium laser

The first infrared-emitting germanium laser has been created by researchers in the US. The development could be an important step towards creating optical components such as lasers from silicon – which like germanium is an indirect-gap semiconductor – rather than direct-gap materials such as indium phosphide. The breakthrough could lead to cheaper and more efficient optical communications systems and even optical computers.

One way of making computers run faster is to use light to transfer data rather than electrical currents. The simplest way to do this would be to make the lasers from silicon, because they could then be integrated directly with other chips. However, today’s commercial diode lasers use comparatively expensive direct-gap semiconductors, which cannot be easily integrated with conventional silicon chips.

Semiconductors emit light when electrons transfer from a higher energy conduction band to recombine with holes in the lower energy valence band. The energy of electrons within these bands also varies according to a property known as crystal momentum. For each band within a material, electrons adopt the lowest energy crystal momentum – and the recombination of electrons and holes with different momentum is forbidden by quantum mechanics.

In direct-gap semiconductors the crystal momentum of electrons in the valence and conduction bands match. By contrast, electrons in the conduction band of indirect-gap semiconductors such as silicon and germanium usually have a different crystal momentum to those in their valence band. This means a quantized lattice vibration – or phonon – must combine simultaneously with the electron and hole to allow light to be emitted. “The probability of that happening is low,” explained Jurgen Michel of the Massachusetts Institute of Technology (MIT), who was involved in the research. “All the other recombination processes that don’t emit light will win out because they’re much faster,” he added.

Direct approach

Now, Michel and colleagues at MIT have shown that lasers can be made that exploit direct-gap transitions in germanium. Michel’s team produced their germanium device on a silicon wafer. The silicon and germanium have different thermal properties, so heating them during fabrication created strain in the germanium that changed its band structure. They also doped the laser with phosphorus impurities, which added enough electrons to ensure that some possessed the correct crystal momentum to make a direct transition. “The energy difference between indirect and direct bandgap in germanium is just small enough that you can overcome it with doping and strain,” Michel said.

The MIT team stimulated light emission in this device at room temperature with a separate pulsed 1064 nm pump laser. When the pumping power was raised above 6 µJ per pulse, the device began lasing, emitting light at different wavelengths around 1600 nm, a wavelength that can be used for optical communications.

Pump it up

Although silicon Raman lasers have been made, these can only emit light when optically pumped with another laser, making them unsuitable for chip-to-chip data transfer. Leading semiconductor firm Intel has also produced hybrid silicon lasers that use direct-gap indium phosphide to produce the light, and silicon to create the laser cavity around it. Such devices currently have the upper hand over the MIT germanium-silicon device as they emit light when electric currents are applied to them. However, Michel says that his team has already developed a design for its own more readily fabricated electrically pumped laser, which is soon to be published.

Douglas Paul of the University of Glasgow, who is developing terahertz frequency silicon-germanium quantum cascade lasers, says that the Michel’s paper “is a significant step forward towards a useful electrically pumped laser on silicon”. He points out that the amount of energy needed to make the device emit laser light is currently high, but that this is expected for a first design and should decrease.

This work will be published in Optics Letters.

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