Singapore - Photonics: Graphene’s flexible future

Plots showing that surface plasmons are more confined when propagating along on a monolayer of graphene (G) than they are along a thin film of gold (Au).

Theoretical calculations show graphene’s potential for controlling nanoscale light propagation on a chip

Semiconductors have revolutionized computing because of their efficient control over the flow of electrical currents on a single chip, which has led to devices such as the transistor. Working towards a similar tunable functionality for light, researchers from the A*STAR Institute of High Performance Computing (IHPC), Singapore, have shown how graphene could be used to control light at the nanometer scale, advancing the concept of photonic circuits on chips1.

Graphene, which is made from a single layer of carbon atoms, has excellent electronic properties; some of these are also useful in photonic applications. Usually, only metals are able to confine light to the order of a few nanometers, which is much smaller than the wavelength of the light. At the surface of metals, collective oscillations of electrons, so-called ‘surface plasmons’, act as powerful antennae that confine light to very small spaces. Graphene, with its high electrical conductivity, shows similar behavior to metals so can also be used for plasmon-based applications, explains Choon How Gan of IHPC, who led the research.

Gan and co-workers studied theoretically and computationally how surface plasmons travel along sheets of graphene. Even though graphene is a poorer conductor than a metal, so plasmon propagation losses are higher, it has several key advantages, says team member Hong Son Chu. “The key advantage that makes graphene an excellent platform for plasmonic devices is its large tunability that cannot be seen in the usual noble metals,” he explains. “This tunability can be achieved in different ways, using electric or magnetic fields, optical triggers and temperature.”

The team’s calculations indicated that surface plasmons propagating along a sheet of graphene would be much more confined to a small space than they would traveling along a gold surface (see image). However, the team also showed that surface plasmons would travel far better between two sheets of graphene brought into close contact. Furthermore, by adjusting design parameters such as the separation between the sheets, as well as their electrical conductivity, much better control over surface plasmon properties is possible.

In the future, Gan and his co-workers plan to investigate these properties for applications. “We will explore the potential of graphene plasmonic devices also for the terahertz and mid-infrared regime,” he explains. “In this spectral range, graphene plasmonic structures could be promising for applications such as molecular sensing, as photodetectors, or for optical devices that can switch and modulate light.”

The A*STAR-affiliated researchers contributing to this research are from the Institute of High Performance Computing

References

1. Gan, C. H., Chu, H. S. & Li, E. P. Synthesis of highly confined surface plasmon modes with doped graphene sheets in the midinfrared and terahertz frequencies. Physical Review B 85, 125431 (2012). |  article

Singapore - Electronics: Graphene sheets’ growing attractions

Plasmon energy states in an array of four graphene sheets. Each plane represents different plasmon energy states resulting from different numbers of electrons in each sheet.

A theoretical and numerical study of graphene sheets reveals a property that may lead to novel opto-electric devices and circuits

One-atom-thick sheets of carbon — known as graphene — have a range of electronic properties that scientists are investigating for potential use in novel devices. Graphene’s optical properties are also garnering attention, which may increase further as a result of research from the A*STAR Institute of Materials Research and Engineering (IMRE). Bing Wang of the IMRE and his co-workers have demonstrated that the interactions of single graphene sheets in certain arrays allow efficient control of light at the nanoscale1.

Light squeezed between single graphene sheets can propagate more efficiently than along a single sheet. Wang notes this could have important applications in optical-nanofocusing and in superlens imaging of nanoscale objects. In conventional optical instruments, light can be controlled only by structures that are about the same scale as its wavelength, which for optical light is much greater than the thickness of graphene. By utilizing surface plasmons, which are collective movements of electrons at the surface of electrical conductors such as graphene, scientists can focus light to the size of only a few nanometers.

Wang and his co-workers calculated the theoretical propagation of surface plasmons in structures consisting of single-atomic sheets of graphene, separated by an insulating material. For small separations of around 20 nanometers, they found that the surface plasmons in the graphene sheets interacted such that they became ‘coupled’ (see image). This theoretical coupling was very strong, unlike that found in other materials, and greatly influenced the propagation of light between the graphene sheets.

The researchers found, for instance, that optical losses were reduced, so light could propagate for longer distances. In addition, under a particular incoming angle for the light, the study predicted that the refraction of the incoming beam would go in the direction opposite to what is normally observed. Such an unusual negative refraction can lead to remarkable effects such as superlensing, which allows imaging with almost limitless resolution.

As graphene is a semiconductor and not a metal, it offers many more possibilities than most other plasmonic devices, comments the IMRE’s Jing Hua Teng, who led the research. “These graphene sheet arrays may lead to dynamically controllable devices, thanks to the easier tuning of graphene’s properties through external stimuli such as electrical voltages.” Graphene also allows for an efficient coupling of the plasmons to other objects nearby, such as molecules that are adsorbed on its surface. Teng therefore says that the next step is to further explore the interesting physics in graphene array structures and look into their immediate applications.

The A*STAR-affiliated researchers contributing to this research are from the Institute of Materials Research and Engineering

VIDEO CAPTION:

The propagation of surface plasmons. The plasmons move from the bottom of the screen to the top as a function of the angle of incoming light.

References

1. Wang, B., Zhang, X., García-Vidal, F. J., Yuan, X. & Teng, J. Strong coupling of surface plasmon polaritons in monolayer graphene sheet arrays. Physical Review Letters 109, 073901 (2012). | article

Singapore - Data storage: Electrically enhanced recall

A voltage applied across a magnetic tunnel junction increases the device’s energy efficiency, thus enabling smaller devices — potentially as small as 5 nanometers — for higher density data storage.

Operating tiny magnetic memories under electrical fields reduces power demand and could enable storage and retrieval of data at much higher speeds than conventional devices

Random-access memory (RAM) is a fast electronic device used in computers to temporarily store data. Traditional RAM is based on the flow of electrical current for data processing. To make RAM faster, more energy efficient and capable of storing more information in a smaller volume, hardware developers are investigating RAM based on magnetic fields. Miniaturization of these devices, however, is hampered by thermal instabilities. Hao Meng and his co-workers at the A*STAR Data Storage Institute have now shown how electric fields can help to circumvent this instability in tiny magnetic memories, as well as reduce operating power1. “This means more information can be stored in a single chip at a cheaper price,” says Meng.

Meng and his team investigated a type of memory that incorporates so-called ‘magnetic tunnel junctions’ (MTJs). Other researchers have previously observed electric-field induced improvements in MTJs, but only in fairly large devices — about 7 micrometers across. Large structures limit the writing speed and suffer from poor compatibility with other electronic components. Meng and his team demonstrated that the concept is also applicable to smaller and faster MTJs that can be integrated more easily.

MTJs are an ideal building block for magnetic memories because of their simplicity and large output signal. In general, they consist of just two magnetic layers separated by a thin insulating barrier (see image). A current passing through the device writes the binary information by controlling the direction of the magnetization in one of the magnetic layers. This process stores information as either a ‘one’ or a ‘zero’, depending on whether the induced magnetization is parallel or antiparallel to the magnetization of the second magnetic layer. A measurement of the resistance across the intermediate barrier can then read out the information as it is needed.

The researchers are working to make MTJs smaller so that they can squeeze in more information. However, smaller devices require larger current densities to switch the magnetization: this leads to heating and makes them less efficient. As a workaround, Meng and his co-workers applied just 0.2 volts across electrodes attached to each side of a 150-nanometer MTJ made of CoFeB-MgO. This reduced the magnetic field required to switch the magnetization by as much as 30% which, in turn, decreased the writing current density.

“Such devices could improve the data transfer rate; that is, how fast you can copy your files from one device to another,” says Meng.

The A*STAR-affiliated researchers contributing to this research are from the Data Storage Institute

References

1. Meng, H., Sbiaa, R., Akhtar, M. A. K., Liu, R. S., Naik, V. B. & Wang, C. C. Electric field effects in low resistance CoFeB-MgO magnetic tunnel junctions with perpendicular anisotropy. Applied Physics Letters 100,122405 (2012). | article

USA - The decades-long search for the Higgs

It was a little over two years ago that the Large Hadron Collider kicked off its search for the Higgs boson. But the hunt for the Higgs really began decades ago with the realization of a puzzle to be solved, one that involved more than just the Higgs.

An intriguing asymmetry

The quest started with symmetry, the aesthetically pleasing notion that something can be flipped and still look the same. It’s a matter of everyday experience that the forces of nature work the same way if left is swapped with right; scientists found this also held true, at the subatomic level, for swapping plus-charge for minus-charge, and even for reversing the flow of time. This principle also seemed to be supported by the behavior of at least three of the four major forces that govern the interactions of matter and energy.

In 1956, Tsung-Dao Lee of Columbia University and Chen-Ning Yang of Brookhaven National Laboratory published a paper questioning whether a particular form of symmetry, known as parity or mirror symmetry, held for the fourth force, the one governing the weak interactions that cause nuclear decay. And they suggested a way to find out.

Experimentalist Chien-Shiung Wu, a colleague of Lee's at Columbia, took up the challenge. She used the decay of Cobalt-60 to show that the weak interactions did indeed distinguish between particles spinning to the left and to the right.

This knowledge, combined with one more missing piece, would lead theorists to propose a new particle: the Higgs.

Where does mass comes from?

In 1957, another clue came from a seemingly unrelated field. John Bardeen, Leon Cooper and Robert Schrieffer proposed a theory that explained superconductivity, which allows certain materials to conduct electricity with no resistance. But their BCS theory, named after the three inventors, also contained something valuable to particle physicists, a concept called spontaneous symmetry breaking.

Superconductors contain pairs of electrons that permeate the metal and actually give mass to photons traveling through the material. Theorists suggested that this phenomenon could be used as a model to explain how elementary particles acquire mass.

In 1964, three sets of theorists published three separate papers in Physical Review Letters, a prestigious physics journal. The scientists were Peter Higgs; Robert Brout and Francois Englert; and Carl Hagen, Gerald Guralnik and Tom Kibble. Taken together, the papers showed that spontaneous symmetry breaking could indeed give particles mass without violating special relativity.

In 1967, Steven Weinberg and Abdus Salam put the pieces together. Working from an earlier proposal by Sheldon Glashow, they independently developed a theory of the weak interactions, known as GWS theory, that incorporated the mirror asymmetry and gave masses to all particles through a field that permeated all of space. This was the Higgs field. The theory was complex and not taken seriously for several years. However, in 1971 Gerard `t Hooft and Martinus Veltman solved the mathematical problems of the theory, and suddenly it became the leading explanation for the weak interactions.

Now it was time for the experimentalists to get to work. Their mission: to find a particle, the Higgs boson, that could exist only if this Higgs field does indeed span the universe, bestowing mass upon particles.

The hunt begins

Concrete descriptions of the Higgs and ideas of where to look for it began to appear in 1976. For example, SLAC physicist James Bjorken proposed looking for the Higgs in the decay products of the Z boson, which had been theorized but would not be discovered until 1983.

Einstein's best-known equation, E=mc2, has profound implications for particle physics. It basically means that mass equals energy, but what it really means for particle physicists is that the greater the mass of a particle, the more energy required to create it and the bigger the machine needed to find it.

By the '80s, only the four heaviest particles remained to be found: the top quark and the W, Z and Higgs bosons. The Higgs was not the most massive of the four – that honor goes to the top quark – but it was the most elusive, and would take the most energetic collisions to ferret out. Particle colliders would not be up to the job for a long time. But they began sneaking up on their quarry with experiments that began to rule out various possible masses for the Higgs and narrow the realm where it might exist.

In 1987, the Cornell Electron Storage Ring made the first direct searches for the Higgs boson, excluding the possibility that it had a very low mass. In 1989, experiments at SLAC and CERN carried out precision measurements of the properties of the Z boson. These experiments bolstered the GWS theory of weak interactions and set more limits on the possible range of masses for the Higgs.

Then, in 1995, physicists at Fermilab’s Tevatron found the most massive quark, the top, leaving only the Higgs to complete the picture of the Standard Model.

Closing in

During the 2000s, particle physics was dominated by a search for the Higgs using any means available, but without a collider that could reach the necessary energies, all glimpses of the Higgs remained just that – glimpses. In 2000, physicists at CERN's Large Electron-Positron Collider (LEP) searched unsuccessfully for the Higgs up to a mass of 114 GeV. Then LEP was shut down to make way for the Large Hadron Collider, which steers protons into head-on collisions at much higher energies than ever achieved before.

Throughout the 2000s, scientists at the Tevatron made heroic efforts to overcome their energy disadvantage with more data and better ways to look at it. By the time the LHC officially began its research program in 2010, the Tevatron had succeed in narrowing the search, but not in discovering the Higgs itself. When the Tevatron shut down in 2011 scientists were left with massive amounts of data, and extensive analysis, announced earlier this week, offered a slightly closer glimpse of a still-distant Higgs.

In 2011, scientists at the two big LHC experiments, ATLAS and CMS, had announced they were also closing in on the Higgs. 

Yesterday morning, they had another announcement to make: They have discovered a new boson – one that could, upon more study, prove to be the long-sought signature of the Higgs field.

The discovery of the Higgs would be the start of a new era in physics. The puzzle is much bigger than just one particle; dark matter and dark energy and the possibility of supersymmetry will still beckon searchers even after the Standard Model is complete. Since the Higgs field is connected to all the other puzzles, we will not be able to solve them until we know its true nature. Is it the blue of the sea or the blue of the sky? Is it garden or pathway or building or boat? And how does it truly connect to the rest of the puzzle?

The universe awaits.

Provided by SLAC National Accelerator Laboratory 

http://phys.org/

Switzerland - Scientists Find Evidence Of The Higgs Boson. The God Particle Exists!

Scientists from CERN in Geneva unveiled today preliminary data that provides evidence of the long sought after Higgs particle.

AsianScientist (Jul. 4, 2012) – Scientists from CERN in Geneva unveiled today preliminary data that provides evidence of the long sought after Higgs particle.

In December last year, scientists working independently on two giant detectors at the Large Haldron Collider (LHC) reported that they had found hints of the existence of the Higgs boson.

Although tantalizing hints were seen in the ranges 116-130 GeV by the ATLAS experiment and 115-127 GeV by the CMS experiment, the claims were not strong enough at that point.

At a highly-anticipated press conference today, both the ATLAS and CMS experiments reported strong indications for the presence of a new particle in the mass region around 126 gigaelectronvolts (GeV) after analyzing trillions of proton-proton collisions from the LHC in 2011 and 2012.

The particle is named after British physicist Peter Higgs, who postulated in 1964 that a field somewhat similar to an electromagnetic field might give particles their mass. It is sometimes also referred to as the ‘God particle.’

Together with colleagues Robert Brout and François Englert, Higgs postulated that all particles had no mass just after the Big Bang. As the Universe cooled and the temperature fell below a critical value, an invisible force field called the ‘Higgs field’ was formed together with the associated ‘Higgs boson.’

The field prevails throughout the cosmos: any particles that interact with it are given a mass via the Higgs boson. The more they interact, the heavier they become, whereas particles that never interact are left with no mass at all.

This idea provided a satisfactory solution and fitted well with established theories and phenomena. But until now no one has ever observed the Higgs boson in an experiment to confirm the theory.

“The results are preliminary but the 5 sigma signal at around 125 GeV we’re seeing is dramatic. This is indeed a new particle. We know it must be a boson and it’s the heaviest boson ever found,” said CMS experiment spokesperson Joe Incandela.

The next step will be to determine the precise nature of the particle and its significance for our understanding of the universe.

Positive identification of the new particle’s characteristics will take considerable time and data, and the teams say they will be “extremely diligent” in further studies and cross-checks.

Publication of the analyses shown today is expected around the end of July, and more data is being collected at the LHC.

Both experiments involved thousands of scientists from over a hundred institutes in 30 countries, including Australia, China, India, Japan, and Taiwan.

——

Source: CERN.

Juliana Chan | Top News

http://www.asianscientist.com/

India - ‘God Particle’ Discovery Met With Cheers From India’s TIFR

The discovery of the Higgs particle was met with excitement among scientists at India’s Tata Institute of Fundamental Research, who participated in the project through the CMS experiment.

Wednesday July 4, which incidentally coincides with America’s Independence Day, will always remain an important day in the world of science because of the much-awaited announcement about the discovery of the elusive Higgs Boson at Geneva, the headquarters of CERN.

The discovery is a historic moment in modern physics as it is expected to throw light onto one’s understanding about the universe. It marks a major milestone as it will explain why atoms in the galaxies, stars, and the earth have mass.

Peter Higgs, the 83-year-old British physicist who first proposed the existence of the Higgs Boson in the 60s was at CERN on Wednesday to receive the news.

On hearing the announcement, he became emotional and shed a few tears of joy and said that he and his family will uncork the champagne bottle!

An announcement from CERN said that two experiments designated as Atlas and CMS found hints of the new particle by analyzing trillions of proton-proton collisions from the Large Hadron Collider (LHC) in 2011-2012. The 10 billion dollar LHC is located in a tunnel below the Swiss-French border.

Quoting the Atlas spokesperson, Fabiola Giaotti, the announcement said: “We observe in our data clear signs of a new particle.”

CMS experiment spokesperson, Joe Incandela said: “The results are preliminary, but the five sigma signal at around 125 GeV we’re seeing is dramatic. This is indeed a new particle. We know it must be boson and it is the heaviest boson ever found,” he said.

The finding caused a lot of excitement among scientists at the Tata Institute of Fundamental Research (TIFR) in Mumbai because the institute, the cradle of India’s space and nuclear program, participated in the project through the CMS experiment.

A few hours after the announcement, a former chairman of the Atomic Energy Commission (AEC), Rajgpopal Chidambaram, said: “It is the most exciting thing which has happened today, and in the days ahead more analysis and research will be done.”

He was happy about India’s participation in the project though the CMS detector in which TIFR has partnered.

Another former AEC chief Anil Kakodkar said that many people were looking forward to this discovery. “Yes, today is a big day for science,” he added.

M. R. Srinivasan, member of AEC and former chairman of the commission said that the discovery will help in providing a better understanding of the physical world and “explain some unexplained phenomena” in the universe such as Dark Energy.

He said: “Unquestionably, it is a great achievement and Prof Peter Higgs, the British physicist who proposed the existence of the Higgs Boson in the 60′s, is in line for getting the Nobel Prize.”

Asked if it was a small step since more analysis was required, N. K. Mondal of TIFR’s High Energy Physics department disagreed and called the discovery a “big step.” In a brief interaction with the media he said: “We feel great because TIFR has played a role in this project.”

Amol Dighe, a professor in TIFR’s department of theoretical physics, explained that the Higgs Particle was very different from other particles. “Today it is 99.99 percent sure that it had been discovered. Earlier it was 90 percent. I was all along optimistic that it will be discovered,” he said.

Dighe said that further experiments to find out how the particle interacts will be done. “The strength of the interaction should be proportionate to the mass,” he added.

India and CERN signed an agreement for collaboration in 1991 and the country was elevated to an observer status by CERN in 2002. Indian labs over the years have delivered sub-systems and provided expert help.

Srinivas Laxman | Top News

http://www.asianscientist.com/