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Home » Page 1321

Colour printing hits ultimate resolution

Researchers in Singapore have developed an innovative inkless printing method that uses metal nanostructures to build sharp full-colour images at a resolution of 100,000 dots per inch (dpi) – 10 times the current best resolution. Scaled up, the technique could find applications in anti-counterfeiting, high-density optical data storage or in transmitting hidden messages.

Even with today’s best optical microscopes, there is a hard resolution limit – half the wavelength of the light used for imaging – that dictates how close two juxtaposed colour pixels can be while still being distinguishable from each other. Any closer than this “optical diffraction limit” and the light reflecting from the two elements diffracts, overlaps and the colours blur into one.

For mid-spectrum visible light of about 500 nm, colour pixels are bound by the optical diffraction limit to a minimum size of 250 nm – or a resolution of about 100,000 dpi. But even the best industrial inkjet and laserjet printers struggle to achieve resolutions one-tenth that fine, because of their micron-scale ink spots.

Stained-glass inspiration

Recently, nanotechnologists have looked to the medieval art of glass staining for inspiration. Here, metal-dust additives create characteristically vivid colours when light hits the metal particles and certain wavelengths are absorbed to excite plasmons – coherent oscillations of conduction electrons on the metal’s surface. Based on this knowledge, reflective metal films dotted with selective light-transmitting holes have already been successfully used to make micron-sized colour pixels, but no one has hit upon a method to shrink them any further, until now.

Finely tuned reflectors

Karthik Kumar and colleagues from Singapore’s Agency for Science, Technology and Research combined the idea of nanohole reflectors with another tried-and-tested idea – arrays of isolated metal nanoparticles that absorb or reflect different wavelengths of light according to their diameters. Using electron-beam lithography, they etched silicon-oxide pillars, tens of nanometres wide, on top of a silicon substrate. Next they used a metal evaporation technique to deposit an ultrathin film of plasmonically active silver (15 nm) and gold (5 nm) on to the tips of the pillars and the substrate.

The raised nanodiscs were arranged two-by-two on the back reflector, in pixels measuring 250 nm. Different colours were encoded into each pixel by altering the diameter of the nanodiscs (from 50–140 nm) and the distance between them (30–20 nm). By tweaking the diameters of the discs, the researchers were able to control the frequency of the plasmon resonances across their surfaces – in much the same way that altering the length of a violin string alters its resonant frequencies – and thus control which wavelengths of light were removed from the incident light and which were reflected.

“The distance between the structures also seems to make a difference in what we think is the two structures coupling with one another,” explains Kumar. “Basically they seem to be talking to each other at these small distances, and that is why we also see that we need to have a small group of these structures in order to be able to see the colours effectively.”

Striking demonstration

In demonstrating their novel technique, the team perfectly reproduced the “Lena test image” – a cropped image of a 1972 Playboy centrefold used extensively as an image-processing standard – in all her detailed colour and tone, small enough to fit on a human cell – just 50 µm across.

“[We] built a database of colour that corresponded to a specific nanostructure pattern, size and spacing. These nanostructures were then positioned accordingly,” explains co-author Joel Yang, also from the Agency for Science, Technology and Research, adding that the colours appeared “all at once, almost like magic” when the metal film was applied.

“With the ability to accurately position these extremely small colour dots, we were able to demonstrate the highest theoretical print-colour resolution of 100,000 dpi,” says Kumar. “As far as we could see, we were getting all the colours of the rainbow,” he adds, although mixing in different amounts of other metals such as gold should provide more of the warm red and yellow hues.

Prospects and applications

Possible applications of the technology range from security tags and secret messages, to high-density optical data storage along the lines of Blu-ray discs. According to Kumar, “Reflector colour displays – something like a Kindle in colour should be possible for this technology as well.”

The team has already tried substituting the silicon oxide for quartz, and are now investigating various polymers and alternative lithography techniques that might make the whole architecture easier and more economical to mass-produce.

The research appears in Nature Nanotechnology.

Further proof of extraterrestrial origin of quasicrystals

An international team of researchers has found nine new samples of naturally occurring quasicrystals. The work also provides further proof that quasicrystals were delivered to the Earth by a meteorite. The team’s discovery challenges our understanding of both crystallography and solar-system formation.

Conventional crystal structures are made of atoms, or clusters of atoms, that repeat periodically. These patterns are normally restricted to two, three, four or sixfold rotational symmetry – the numbers corresponding to how many times the crystal appears the same during a rotation through 360°. For a long time these were considered hard and fast rules, and no crystals that broke these conditions were thought to exist.

Ordered, but not periodic

However, Israeli physicist Daniel Shechtman found just such a rule-breaking crystal in 1984 and was awarded the 2011 Nobel Prize for Chemistry for his efforts. Shechtman had discovered a quasicrystal – a crystal that, while ordered, does not contain structures that repeat periodically. Schectman’s crystal also had 10-fold rotational symmetry. Even after his discovery, there was a lot of scepticism about the existence of such a material. But as the years went by, other physicists began to construct quasicrystals of their own and now more than 100 different types have been found. These, however, are synthetic and have been created under precisely controlled laboratory conditions. Just as it was originally assumed that quasicrystals could not exist, after their discovery it was assumed that they could not exist naturally in the wider world.

That assumption was called into question in 2009 when Princeton University’s Paul Steinhardt – the man who originally coined the term “quasicrystal” – appeared to have discovered a naturally occurring variety in a rock sample from Russia. Steinhardt and his colleague Luca Bindi, from the University of Florence, Italy, measured the ratio of oxygen isotopes within the sample and their results suggested that the rock belongs to a class of meteorites known as carbonaceous chondrites. Not only did this rock contain a naturally occurring quasicrystal, it also came from outer space.

Thrilling past

But the scepticism that had followed quasicrystals around since their discovery continued. The rock sample was traced back to Valery Kryachko, a Russian who in 1979 had been panning for platinum in a stream flowing through the Koryak mountains in far-eastern Siberia. The rock had somehow turned up in Bindi’s museum collection in Italy. “People were sceptical of the rock’s back story as the tale of how it got to Florence involves secret diaries, smugglers and KGB agents,” Steinhardt told physicsworld.com.

“The only way to settle the debate was to take a shot at finding more samples,” Steinhardt explains. He put together a team of 10 scientists, two drivers and one cook and set out on a four-day expedition across Siberia back to the stream where Kryachko had found the original sample. Once there, they panned 1.5 tonnes of sediment from the stream bank, eventually isolating a few kilograms for analysis.

Photographs of researchers in the field
In the field

After six weeks of painstaking grain-by-grain analysis, they hit on something special. “We found a grain with a fleck of metal on it. Not only did it contain quasicrystals, but the oxygen-isotope ratio was exactly the same [as the original sample],” says Steinhardt. “It was an incredible moment. Out in the field, no-one bet on a more than 1% chance of successfully finding anything,” he adds. The team isolated a total of nine quasicrystal samples. It is thought these samples all come from the same meteorite, and analysis of the sediment layers suggests it landed within the last 15,000 years.

Extreme formation

As the quasicrystals come from a carbonaceous-chondrite meteorite, they must have formed in the earliest days of the solar system. Carbonaceous chondrites are thought to have collided together to form the cores of the rocky planets, and so Steinhardt’s quasicrystals are older than the Earth itself. However, current models cannot account for the presence of these quasicrystals. “We need a novel kind of geological process to form them and so it challenges our ideas of solar-system formation,” Steinhardt says.

The intense conditions present in the solar system’s youth also challenge the prevailing view of quasicrystals as objects that need a carefully controlled laboratory set-up to produce. “Quasicrystals are not the delicate materials previously thought. The ones we found must have been formed under robust and hardy conditions in the early solar system,” Steinhardt says.

Others agree that the world of quasicrystals could be changed by this 10-fold increase in the number of known naturally occurring examples. “This result emphasizes how normal quasicrystals are and will hopefully make them less of an eccentricity,” Renee Diehl, a surface physics researcher at Pennsylvania State University, US, told physicsworld.com. “It opens our eyes to the fact that they may have been all around us and we just have not noticed,” she explains.

The research is published in Reports on Progress in Physics.

Why would quantum computers be so much faster than classical computers?

In less than 100 seconds, John Rarity explains how quantum mechanics could speed up computing exponentially.

Photon shape could be used to encode quantum information

An international team of researchers has succeeded in measuring the shape of individual photons for the first time. The result could prove extremely useful for secure data transmission using light.

Pulses of light can have almost any shape in space and time, and these shapes depend on the amplitudes and phases of the pulse’s frequency components. Data can be encoded in light pulses by modulating the amplitude or phase of the light. Single photons and other quantum light states can also be generated in a variety of complex shapes and encoding information in these different shapes could be an efficient way to securely transmit data. Indeed, a single photon shape could represent, for example, any letter in the alphabet, or even a quantum combination (or superposition) of several letters.

However, the problem is that once a photon has been sent through apparatus – such as an optical fibre – its shape can become distorted and the information contained within becomes impossible to decipher. A team led by Marco Bellini of the Istituto Nazionale di Ottica in Florence, Italy, and colleagues have now managed to measure the precise shape of the mode of a quantum light state that appears at the receiving end by means of a “mode-selective” detector.

Evolutionary algorithms

The method is based on evolutionary algorithms commonly used in femtochemistry experiments and biology that optimize a particular experimental outcome by adjusting a set of initial parameters. “What is new in our work is that we have applied this approach to the detection of ultrashort quantum light states, combining for the first time very complex and advanced techniques from two distant fields of research: quantum optics and femtosecond coherent control,” explains Bellini.

The researchers begin by “mixing” the photon to be measured with a reference intense laser-light pulse or a “local oscillator” as it is called. The photon and the laser pulse interfere and either reinforce or cancel one another out, depending on their shapes. The closer their shapes, the more likely it is that the photon will be detected.

What Bellini and co-workers do is to continually change the shape of the laser pulse in the detector until it best matches the shape of the photon. “If the shape of the photon is unknown, we start from a set of random shapes for the local oscillator and try them all to find those that perform better in detecting the quantum light state,” explains Bellini. “These best shapes are then slightly modified and mixed among themselves to create a new generation of shapes that we test again against our single photon. The process continues until the best matching shapes are found in a sort of evolutionary adaptation.”

Picture of the researchers
The team

Recovering encoded information

The researchers showed that their scheme could recover information intentionally encoded in the shape of a photon. For example, they created photons that had two separate frequency components with a particular phase difference. The photons could be detected using local oscillator pulses that had a matching phase difference but were not detected when the components of the laser pulse were exactly out of phase with the photons.

Until now, most quantum-optics experiments have relied on generating, manipulating and detecting quantum states of light in one or just a few well-defined modes. For instance, most quantum-communication protocols – such as quantum cryptography – are based on the different polarization directions (horizontal or vertical, for example) of a photon. This means that information is encoded in just two possible states of the photon and all their superpositions – a so-called qubit. “By providing access to the full spatiotemporal-mode structure of a quantum light state, the number of orthogonal modes that a single photon can occupy (the possible letters in the ‘alphabet’) is virtually unlimited, so our approach could greatly enhance the capabilities of a quantum communication or computation system,” Bellini told physicsworld.com.

The researchers says that they are now testing the limits of the technique. “We are also trying to improve the analysis of particular quantum light states and trying to increase the number of independently addressable modes that a single photon can occupy,” adds Bellini.

The work is detailed in Physical Review Letters.

Up, up and away

By Matin Durrani

The Physics World editorial team has been to a fair few places in the last couple of years as we try to make some interesting, entertaining and (hopefully) informative films about the world of physics.

We’ve been inside CERN to investigate the latest in the search for the Higgs boson. We’ve travelled to major international conferences from San Francisco to Boston. And then there was the time we went one mile underground to a dark-matter experiment in the north of England.

Yesterday, however, we shot a set of new films at this year’s Bristol International Balloon Fiesta, where thousands of people gather to watch as a series of hot-air balloons take off over a four-day period.

balloon fiesta

balloon fiesta

So what, you might wonder, is the link with physics? Well, as Alan Watson describes in this new article, this week marks the centenary of the discovery – during a balloon flight – by the Austrian physicist Victor Hess of what we now know as cosmic rays.

Physicists from the University of Bristol, led by David Cussans, decided to use the fiesta as an opportunity to showcase not only the centenary but also a new project that has allowed school pupils to build their own cosmic-ray detector.

The university launched two balloons, one of which you can see being filled with hot air (right). No, don’t ask me the cost in wasted greenhouse gases.

Sadly we didn’t hitch a ride in either of the balloons, but three of the pupils who were involved in the detector-building project were on board, as were three others who won a competition to take part in the flight.

As you can see, the view from the balloon over the festival site was fabulous.

balloon fiesta

Although Physics World editors didn’t manage to thumb a lift, a copy of the August issue of Physics World, which contains Watson’s article, did make the trip.

balloon fiesta

The pupils even took their detector in the balloon, but unfortunately – as is the way with experimental physics – someone had accidentally left the battery running and it had discharged completely so no data could be collected during the flight. Oops.

Apart from that, as we discovered when we returned to the fiesta this morning, the flight was a success and took the pupils and crew to a height of some 3000 m.

We’ll now set about turning our footage into a set of films, so stay tuned.

Meanwhile, for more about the cosmic-ray centenary, don’t forget the Physics World feature.

All pictures courtesy: Beth Cotterell

Why don’t objects on the nanoscale behave as we expect?

In less than 100 seconds, Annela Seddon explains why materials behave in surprising ways at the nanoscale.

Which is the most scientifically interesting planet in the solar system, apart from the Earth?

By Tushna Commissariat

Earlier this week, NASA’s $1bn Curiosity rover landed on Mars and successfully started sending back data. The mission has taken a mammoth team of hundreds of scientists and engineers more than eight years to build and it promises to provide new insights into the Martian landscape while looking for conditions that could host life as we know it.

The rover can travel about 200 m per day and the current mission is expected to last about 687 Earth days or one Martian year. Being much heavier than previous rovers sent to Mars, Curiosity was lowered to the planet’s surface using a retro-rocket-firing “sky crane” that slowly deposited the car-sized rover. The unique and ultimately successful landing had people worldwide excited by the idea of travelling to other worlds once more.

Thumbnail image for Thumbnail image for hands smll.jpg

Around the same time, it was revealed (albeit unofficially) that the Indian government has approved the country’s first ever mission to Mars, with a launch planned for November 2013 from the country’s spaceport at the Satish Dhawan Space Centre on the island of Sriharikota using the Polar Satellite Launch Vehicle. The £70m mission would follow just four years after India’s Chandrayaan-1 lunar mission and the expected 500 kg orbiter would study Martian geology and climate. The mission has already been allocated £26m in the country’s science budget.

In the light of the current interest in sending robots or travelling to other planets in our solar system, this week we are asking you which planet you find the most captivating. Please let us know your opinion by taking part in this week’s Facebook poll.

Which is the most scientifically interesting planet in our solar system, apart from the Earth?

Mercury
Venus
Mars
Jupiter
Saturn
Uranus
Neptune
None of the above – exoplanets are more interesting

Have your say by visiting our Facebook page, and please feel free to explain your response or give us more suggestions by posting a comment below the poll.

In last week’s poll we asked you what would be the most beneficial way of spending $27m on physics, were you feeling very generous, in light of the newly established Fundamental Physics Prize. Of the 216 of you who voted, almost 60% thought that the money would be best spent by investing in a research institute, with another 21% supporting high-school education, 11% voting for funding numerous PhDs, 4% voting for funding goal-oriented competitions and 1% voting for awarding the money to high-achieving scientists – the category that the money is actually used for!

Thank you to everyone who took part and we look forward to hearing from you again in this week’s poll.

India set for Mars mission in 2013

The Indian government has approved a mission to Mars in what would be the country’s first visit to the red planet. The news comes just four years after India launched its maiden mission to the Moon – Chandrayaan-1 – and days after NASA landed the Curiosity car-sized rover on Mars.

The £70m mission will be launched in November 2013 from India’s spaceport at the Satish Dhawan Space Centre on the island of Sriharikota using the Polar Satellite Launch Vehicle. The mission, which will orbit Mars and study the planet’s geology and climate, has already been allocated £26m in the country’s science budget.

Details of the new mission remain scarce as the Indian Space Research Organisation (ISRO) remains tight-lipped about the probe. However, it is expected that more information will be released on 15 August, when India’s prime minister delivers the country’s Independence Day speech.

The Mars orbiter is expected to weigh 500 kg, with a scientific payload of around 25 kg, and be placed in a highly elliptical orbit around the planet. “Not knowing the payload specifics, it is hard to judge [the mission],” says Jeffrey Plescia, a Mars researcher at Johns Hopkins University in the US. “But the Chandrayaan mission had a diverse set of instruments that provided a great deal of unique data. If they do something similar [to the Mars mission], then it could be a substantial contribution to science.”

A source of national pride

There has been a lot of debate in India about whether the country should have a space programme at all. Yet it has been pointed out that the cost of the mission is less than 0.01% of the country’s overall annual budget. Moreover, this money would be spent mostly on hi-tech jobs and advanced technology, as well as supporting and training university students who might study the data that the probe sends back.

The mission would also be a source of pride and excitement to the country, much like the Curiosity mission is doing in the US Jeffrey Plescia, Johns Hopkins University

“I think they look at this in a similar way [to how] the US looked at Apollo – in that they are trying to demonstrate that they are a technologically advanced country and a leader in Asia,” adds Plescia. “Space programmes are not really about science, they are about national prestige and national security. The mission would also be a source of pride and excitement to the country, much like the Curiosity mission is in the US.”

It is not known, however, if other countries will provide payloads for the new mission. Chandrayaan-1 carried instruments from NASA, the European Space Agency and also Bulgaria, so it is hoped that India will open it up for foreign involvement.

What is dark matter?

In less than 100 seconds, Luke Davies explains how we know dark matter exists.

Condensed-matter trio scoop Dirac prize

Three condensed-matter physicists, who have advanced our understanding of a strange type of material known as a “topological insulator”, have won this year’s Dirac medal from the International Centre for Theoretical Physics in Trieste, Italy. Duncan Haldane of Princeton University, Charles Kane of the University of Pennsylvania and Shoucheng Zhang of Stanford University, all in the US, have scooped the $5000 prize, which is named after the British Nobel-prize-winning theorist Paul Dirac. First awarded in 1985, the prize is given each year on 8 August – the day on which Dirac was born in 1902.

At the surface

Topological insulators are currently one of the hottest topics in condensed-matter physics. Insulators on the inside, they manage to conduct electricity on their surface thanks to special surface electronic states that are “topologically protected”, which means that – unlike ordinary surface states – they cannot be destroyed by impurities or imperfections. Moreover, the conducting electrons arrange themselves into spin-up electrons travelling in one direction and spin-down electrons travelling in the other. Such a “spin current” could be useful for anyone wishing to build a practical “spintronic” device that exploits the spin, rather than the charge, of the electrons.

These insulators have an unusual history because – unlike almost every other exotic phase of matter – they were characterized theoretically before being discovered experimentally. Kane was among those who were involved in that early work, which was based on the band theory of solids – the standard quantum-mechanical framework for understanding the electronic properties of materials. The topological insulator states in 2D and 3D materials were predicted theoretically in 2005 and 2007, before being experimentally discovered in 2007.

Novel properties

Given that the 3D topological insulators are fairly standard bulk semiconductors and their topological characteristics can survive to high temperatures, their novel properties could lead to some exciting applications. But as well as constituting a new phase of quantum matter that should keep physicists busy for some time, topological insulators have also aroused interest because they have been shown to harbour quasiparticles resembling “Majorana fermions” – particles that are also their own antiparticle.

But it is the potential applications of topological insulators that drives much of the current interest in these materials. “When developing nanosize electronics and pursuing coveted goals such as quantum computers, the availability of conducting channels that do not spoil and will work no matter what is something that seems quite important,” says ICTP condensed-matter physicist Erio Tosatti. The Dirac medal is awarded annually but anyone who has already won a Nobel prize, Fields medal or Wolf Foundation prize is excluded from the award.

Charles Kane wrote an article on “Topological Insulators” in the February 2011 issue of Physics World, along with Joel Moore. Institute members may access the article here.

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