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Electron tomography peers into the nanoworld

Electron tomography is a powerful way to image materials with extremely high resolution but the method cannot provide 3D images on the atomic scale for several reasons. Now, researchers in the US are saying that they may have overcome some of the technique’s limitations by taking a new approach to the problem. The advance could be a boon for those imaging nanomaterials, including biological samples.

Being able to visualize how atoms are arranged in materials has played a crucial role in the evolution of modern science and technology. Crystallography techniques have long been employed to reveal 3D atomic structures and scanning probe microscopes can determine surface structures on the atomic scale. Electron microscopes, for their part, can routinely resolve atoms in 2D projections of 3D crystalline samples.

However, currently there is no direct way to determine the 3D structure of samples on the atomic scale without first knowing the material’s lattice structure and assuming that the atoms fit rigidly on that lattice. These conditions do not apply to a nanoparticle, which can have a different lattice structure than a bulk sample of the same material.

Now, Jianwei Miao and colleagues at the University of California, Los Angeles (UCLA) and the Lawrence Berkeley National Laboratory say that they have performed the first experiments in which they can directly image 3D local structures without relying on a priori structural information. “We have succeeded in observing individual atoms inside some regions of a gold nanoparticle 10 nm in size and identifying several grains therein at a resolution of just 2.4 Å in three dimensions,” Miao says. “This has never been done before.”

Overcoming old challenges

One major drawback of conventional tomography is that samples have to be tilted a number of times and images taken at each tilt to build up the overall structure. But the problem is that prolonged exposure to the electron beams used in the microscope damages the sample and therefore limits the number of projections possible from a single object. Another disadvantage of the technique is that it is technically challenging to align the sample along a common axis and then tilting it with respect to this axis with atomic-level precision. Finally, samples cannot normally be tilted beyond ±79°, which means that data cannot be acquired from the “missing wedge”.

Miao and colleagues have now overcome all of these problems by combining a new alignment method with an iterative tomographic reconstruction technique that uses an annular dark-field scanning transmission electron microscope (ADF-STEM).

The team says that it has been working towards this goal since 2005, when it decided to improve on conventional tomography by developing an “equally sloped tomography” (EST) technique. This method allows the researchers to tilt their samples by small, and as the technique’s name suggests, equally sloped, increments and perform 3D reconstructions on them using a Fourier-based iterative algorithm. The EST helped to significantly reduce the required number of projections from a given sample, which automatically meant less beam damage, while maintaining the same image resolution, image contrast and image quality.

“In our new work, we have combined this EST method with another new projection alignment approach, which relies on the centre of mass of a particle, to image a 10 nm gold nanoparticle in 3D at the atomic scale using only 69 projections,” explains Miao.

According to the UCLA researchers, their general method could, in principle, be used to determine the 3D local structure of crystalline, polycrystalline and even disordered nanomaterials at the atomic scale. It might also help to improve the resolution and image quality when performing electron tomography of biological samples.

The work is presented in Nature.

How to hide from a magnetic field

Researchers in Europe have built a magnetic cloak that, in theory, is reasonably practical to manufacture. An object concealed by the new cloak, the researchers claim, is magnetically undetectable, while the cloak itself is made from materials available in many physics labs the world over. This means that it is, in principle, the first cloak that should be reasonably practical to manufacture.

Cloaks and shields

In 2011 Alvaro Sanchez and colleagues at Universitat Autònoma de Barcelona, Spain, developed a theory for a type of magnetic cloak they called an “antimagnet” that would have two crucial properties. One is that any magnetic field created within the cloak would not leak outside the cloaked region and the other is that the cloak and the cloaked region would be undetectable by an external magnetic field; that is, the field would not be distorted by the cloak. Now, Sanchez along with Fedor Gömöry and colleagues from the Slovak Academy of Sciences, has designed and demonstrated a modified version of the cloak proposed last year.

The new cloak is a simple bi-layer cloak made up of two common materials – an inner superconducting layer made up of a high-temperature superconducting tape and an outer ferromagnetic layer composed of a few turns of a thick FeNiCr commercial alloy sheet. “The cloak we proposed last year was more of an ideal cloak,” explains Sanchez. “But it was complicated with 10 layers and included superconducting plates. This new cloak, while not perfect, is a much simpler design for achieving similar results using a static uniform magnetic field.” He adds that it is fair to say that this is the first cloak that is an exact cloak that can be feasibly implemented in practice.

The superconducting layer on its own repels the magnetic field, while a ferromagnetic layer on its own attracts the magnetic field lines; so both independent layers distort the field. The cloak is the accurate combination of the two layers, determined by a specific radius, which adjusts for the permittivity (μ) such that there is no external field distortion at all. This radius is calculated using Maxwell equations. “It is quite amazing that almost 160 years after Maxwell equations were first developed, we are still finding new solutions based solely on them!” says Sanchez.

Perfection problems

Sanchez tells physicsworld.com that the entire team is highly inspired by the initial work on building invisibility cloaks using transformation optics carried out by John Pendry and colleagues at Imperial College London since 2006. “There are generally two ways of achieving a cloak – either using transformation optics or using plasmonics. The problem with the first is that, while it is theoretically the perfect cloak, it is nearly impossible to physically create. With plasmonics, while the materials are available, you get a slight shadowing or scattering effect, not a complete cloak at all. This is the first time that you get both using commercially viable materials,” Sanchez explains.

Sanchez points out that an advantage in developing a cloak for a static magnetic field is that, for such a field, the magnetic and electric effects decouple and the researchers only have to consider the magnetic permeability. The team tested its cloak using a static field of 40 mT – which is greater than the Earth’s magnetic field. Currently, the cloak has been built on a small but reasonable scale – 12.5 × 12 mm. Sanchez explains that another advantage is that, for a static magnetic field, the cloak can work on any length scale – from microns to metres – as there is no intrinsic cut-off, unlike other cloaks that work at fixed wavelengths.

Because the cloak is capable of running under relatively strong magnetic fields and relatively warm liquid-nitrogen temperatures, and as it is made from commercially available materials, it could be readily put to practical use, the researchers say. The team is also looking at other methods to manipulate and control magnetic fields into different “shapes”, for purposes other than cloaking, in the coming months.

The research is published in Science 335 1466.

Proteins wander through whorls and vortices

A multidisciplinary team of physicists and biologists has discovered a new system of proteins that organizes itself spontaneously into a spectacular pattern of whorls and vortices. But unlike similar systems studied in the past, the individual protein molecules move freely throughout the pattern, while the pattern itself remains fixed. The scientists believe that further study of the system could shed light on the collective behaviour of living organisms.

The spontaneous, self-organized collective motion of entities as diverse as flocks of birds and colonies of bacteria have long fascinated humans. Scientists, however, have struggled to understand the underlying mechanisms that allow hundreds, thousands or even millions of individuals to act as one. It is often the case that the dynamics of a group cannot be predicted in a straightforward manner from observing the behaviour of individual members.

Difficult to disentangle

While this collective behaviour in living organisms is fascinating, it is also extremely difficult to model mathematically because of the complicated ways in which living things can interact. These range from one bird hearing another’s distress call to a person being influenced by a friend’s political ideas. It is often difficult to identify all these interactions, let alone to disentangle their influences on a system’s behaviour. Furthermore, conducting interventional experiments can be problematic, as many of the parameters may not be under the experimenter’s control.

Fortunately for scientists, self-organization is not limited to living organisms. By studying often simpler inanimate systems, important insights into collective behaviour can be gleaned. In 2010, two research groups turned their attention to the formation of patterns by protein filaments made from actin, which are driven by motor proteins. These filaments are used by cells to convert chemical energy into kinetic energy by propelling the tiny protein filaments like the outboard motor of a speedboat. Above a certain critical concentration, the researchers discovered that protein filaments on a glass slide organized themselves into various ordered structures such as waves and spirals.

Whorls and vortices

Now, scientists in Japan have looked at a different combination of filament and motor, and found that the proteins then organize themselves into an elaborate pattern of whorls and vortices when placed on a glass plate. The research provides an important new demonstration of complex behaviour arising in a simple system.

The new research is a collaboration involving Yutaka Sumino at the Aichi University of Education, Ken Nagai at the University of Tokyo, Kazuhiro Oiwa at the Advanced ICT Research Centre in Kobe, Hugues Chaté at CEA-Saclay in France and their respective teams. The researchers used a different kind of protein filament called a microtubule and a different type of motor protein called dynein. As with actin, they found that when the density of motor proteins was high enough, the filaments organized themselves into patterns. However, unlike the previous examples of relatively simple patterns, the pattern here was a far more elaborate, repeating “lattice” of adjacent vortices.

Crucially, however, while the established pattern remained stable, a single microtubule was not stuck in one vortex. Individual molecules wandered across the surface of the substrate from vortex to vortex, sometimes rotating clockwise and sometimes anticlockwise, all without disturbing the stability of the overall pattern. Although the researchers were able to construct a computer model of the behaviour, it is not yet clear whether the behaviour is the result of a specific property of the microtubule molecule or whether there may be a more general explanation.

Not frozen-in

Biophysicist Andreas Bausch of the Technical University of Munich in Germany. leader of one of the research groups that originally discovered pattern formation in protein filaments, is fascinated. “These are not frozen-in structures as, for example, we observed last year,” he explains. “You’ve got the constant exchange of material between the vortices and yet they’re stable over very long timescales.”

Tamas Vicsek of Eotvos University in Hungary, an expert on complexity theory and author of an accompanying commentary on the paper describing the work, believes the research marks a significant step forward in the study of complex behaviour in protein filaments. “They just observed a new phase in the study of collective motion that has not been seen before,” he says. “It seemed that searching for new patterns was over, but this shows that the whole system of self-propelled particles is a much richer world than we had assumed.”

Sumino hopes that the work may one day provide a basis for understanding complexity theory more widely. “In our model all you have is a short-range collisional interaction,” he explains. “Depending on the system, sometimes there is more detailed interaction. So if you think about the collective motion of fish, fish have eyesight in one direction only. Such detailed information can be taken into the model later.”

The research is published in Nature.

Should we engineer the climate to counter the effect of global warming?

By James Dacey

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One of the more radical proposals is to intervene with the Earth’s solar-energy balance by deploying technologies to reflect sunlight. Suggestions include painting buildings white to make them more reflective, injecting reflective aerosols into the atmosphere, or even deploying a fleet of shields into the Earth’s orbit to directly intercept incoming sunlight.

The other main approach to geoengineering is to try to directly remove carbon dioxide from the atmosphere. One area already being developed is carbon capture and storage (CCS), a three-stage process that involves harvesting, transporting and then storing the carbon dioxide in suitable underground locations such as vast saline aquifers. A more radical approach is to fertilize the ocean with a limiting nutrient such as iron to promote more marine flora, which will draw more carbon out of the atmosphere during photosynthesis.

Earlier this week we published an interview with the high-profile geophysicist Ken Caldeira of the Carnegie Institution for Science in the US. Caldeira has some severe reservations about geoengineering, specifically concerning: its environmental impact; how the presence of a “plan B” that may prove unreliable could affect efforts to cut carbon emissions; and who on the global stage should regulate use of the technology, particularly when it may reduce rainfall in some areas.

We want to know your opinion on this issue, via this week’s Physics World Facebook poll.

Should we engineer the climate to counter the effect of global warming?

Let’s do it!
We should prepare to do it as a “plan B” if carbon emissions continue to rise
No way! The environmental risks are too high
No, because it won’t work anyway

Have your say by casting your vote on our Facebook page. As always, please feel free to explain your response by posting a comment.

In last week’s poll we looked at the issue of university ranking exercises. The issue was on our minds because the Times Higher Education (THE) had just released its annual list of the top 100 universities, which was dominated by institutions in English-speaking countries. We asked whether you think these university ranking exercises are inherently biased. The outcome was highly conclusive, with 96% of respondents opting for “yes”.

Thank you for your participation and we look forward to hearing from you in this week’s poll.

Physics for students, not poets

My immediate response to the title of Quantum Physics for Poets is “I am not worthy.” Although I have written a couple of limericks and a particularly dire sonnet, I am hardly a poet. Luckily, the book’s target audience is not actually so limited. Instead, the authors’ stated aim is to introduce quantum physics in a way that enables arts students – and presumably poets are regarded as the ultimate of that ilk – to get their heads around this truly mind-bending subject.

With this principle in mind, I was a little disappointed with the verbose introduction, in which drawn-out parallels are made with revolutions in the arts and politics – as if to prove that quantum physics is particularly suited to the intellectual rebel and make it more palatable to arty types. This seems rather condescending. The introduction also reflects a problem that reoccurs throughout the book. Although their intent is to present information in a non-technical way, authors Leon Lederman and Christopher Hill struggle to detach themselves from their jargon. Theoretical physicist Hill and Nobel-prize-winning particle physicist Lederman have had a long involvement in the public understanding of science, but demonstrate here how difficult it is for science professionals to understand the worldview of the non-scientist.

As an example, I find it difficult to believe that anyone with a non-scientific background would be comfortable with this sentence from the introduction: “Since the location of June can be deduced without measuring the electron Molly, whose properties are correlated by the initial quantum state of the radioactive parent particle, the properties of the particle arriving at Alpha Centauri must seemingly have an objective reality.” I can imagine an awful lot of poets (and other people) going “Huh?”.

After the introduction, we are eased into the quantum world with a brief historical exploration of classical physics. Galileo and Newton feature heavily here, providing a good mix of historical context and basic science. Occasionally, though, the history is something of a caricature; for example, we are told that Galileo dropped balls off the leaning tower of Pisa, an event that most historians of science consider unlikely. The exploration then moves on to cover light, which introduces the reader to the “ultraviolet crisis” – the prediction from 19th-century electromagnetic theory that all atoms should emit vast quantities of high-energy light – and the origins of quantum physics.

As the book’s scientific side comes to the fore, the historical context is downplayed, though we do get occasional snippets. I found it particularly delightful to discover that Max Born was Olivia Newton-John’s grandfather. But again, there is something of a tendency to tiptoe around historical accuracy. So, for example, we hear that in 1685 the Danish astronomer Ole Rømer’s calculations “yielded the first precise measurement of the speed of light, a whopping 300,000,000 m s^–1“. In reality, Rømer’s value was closer to 220,000,000 m s^–1. Suggesting otherwise condenses history a little too much.

Once we enter the 20th century, the science is given considerably more opportunity to develop, so the reader is taken with some care through Planck’s idea that radiation should be split up into “bunches, or quanta”. An interesting revelation in this section is that Planck did not really see this as an observation about light itself, but rather a description of the action of the atoms in a black body that is radiating light. Soon, Einstein enters the picture, and from this point on, a key part of the book’s message is the “shock of the new”. Looking back, it is hard to imagine just how much of a departure from classical thinking was required to begin to grasp quantum theory, and Lederman and Hill make sure that we really understand that the culture shock among physicists was immense. Indeed, some – Einstein and Schrödinger being two obvious examples – were never comfortable with its implications.

To get this far has taken only around one-third of the book. Now we plunge into the structure of the atom, matrix mechanics, the uncertainty principle and the Schrödinger equation. A whole chapter is dedicated to quantum entanglement and its implications, with an unusually detailed exploration of Bell’s theorem – a topic that is often considered too confusing for the general reader, as the authors demonstrate here. After exploring Dirac’s relativistic expansion of the Schrödinger equation and a quick tour of Feynman’s sum-over-paths approach, the book concludes with a rapid crescendo of supersymmetry, holographic universes, quantum gravity and string theory, climaxing with a brief introduction to some of the new quantum technologies of quantum cryptography and quantum computing.

Throughout the book, I get the impression that it is essentially a collection of physics lectures for arts students, generated by simplifying standard introductory physics lectures. This is acceptable for an actual course, for students who are prepared to sit through it to get their credits, but it does not work as well as a science book for the general reader. Such “science-for-the-arts” courses are quite common at US universities, but even if this were the target audience of this book, the authors could take lessons in how to go about it from Richard Muller’s superb Physics for Future Presidents, while a more general audience would benefit much more from the approach of a title such as Marcus Chown’s Quantum Theory Cannot Hurt You. Unfortunately, poets have not been well served here.

This is, nonetheless, a good book. Lederman and Hill provide the reader with plenty of introductory meat on the development of quantum physics and they really bring out the startling surprises at the heart of it. But the approach they take is not for poets. It would be much better targeted at high-school physics students to help prepare them for university physics. Rather than quantum physics for poets, this is quantum physics 101 lite. That is a useful book, and in that role I would heartily recommend it. But it doesn’t do what it says on the tin.

Robot jellyfish fuelled by hydrogen

A robotic swimmer that mimics the motion of a jellyfish has been built by researchers in the US. Dubbed “Robojelly”, the swimmer propels itself using an “artificial muscle” built in part from carbon nanotubes and powered by hydrogen. Such robots could be used in a number of scientific, military and commercial marine applications, the researchers say.

Robojelly has been built by a team led by Yonas Tadesse from Virginia Tech and the University of Texas at Dallas that is claiming “the first successful powering of an underwater robot using external hydrogen as a fuel source”. As well as only producing water as a waste product, hydrogen is attractive for remote marine vehicles because, in principle, the fuel could be obtained from seawater using energy from the Sun.

Deform and reform

At Robojelly’s heart is a commercially available nickel-titanium shape-memory alloy (SMA) – a deformable material that returns to its original shape when heated. The SMA is wrapped in a sheet of carbon nanotubes that itself is coated in titanium particles, which catalyse the reaction between hydrogen and oxygen. The heat produced as a result of this oxidation then allows the SMA to revert to its original shape.

Robojelly uses carbon nanotubes because they are highly porous, which allows the hydrogen and oxygen to reach the catalyst. And apart from being lightweight and very robust, nanotubes also conduct heat well, which is good because heat must be transferred quickly to and from the SMA during operation.

The robot has an umbrella-like structure that mimics the propulsion strategy of the common jellyfish Aurelia aurita. The “bell” of the robot is made from silicone and is supported by eight springy steel ribs, with a string running alongside each rib from the edge of the bell to a pulley at the centre. Each string then goes down into a tube that holds the SMA actuator.

In one design, all the strings are attached to one central SMA actuator, while in another design there are eight different actuators. One benefit of the latter design is that, in principle, different segments of the bell can be controlled individually. “This should allow the robot to be controlled and moved in different directions,” explains Tadesse.

Flapping bell

To demonstrate Robojelly in action, the team placed it in a tank of water. A fixed amount of hydrogen and oxygen is then introduced to the tube, which warms up the SMA and makes it change shape. As it deforms, the alloy pulls on the strings, causing the bell to flap in one direction. As the SMA cools, the restoring force of the steel ribs makes the bell flap in the opposite direction. According to the team, an entire cycle can occur in less than 10 s.

The team measured the deformation of the bell – defined as the distance moved by the edge of the bell divided by the length of its curve – and got a value of about 14%. This is smaller than the 29% achieved when the robot was electrically powered and the 42% that is typical of a living jellyfish. Although the researchers have so far only operated the robot jellyfish clamped at the bottom of a water tank, they are looking at ways of boosting the performance and efficiency of the system.

The research is described in Smart Materials and Structures 21 045013 and a video of Robojelly can be viewed below.

Geoengineering: the pitfalls and politics

Geoengineering is the idea of controlling the weather and climate by the large-scale engineering of the environment. The idea has come to prominence in recent years as concerns about man-made global warming have increased and governments have faltered on negotiations to restrict carbon-dioxide emissions. But many people are concerned about geoengineering: its environmental impact; how the presence of a “plan B” that may prove unreliable could affect efforts to cut carbon emissions; and who on the global stage should regulate use of the technology, particularly when it may reduce rainfall in some areas.

One high-profile geophysicist who has reservations about geoengineering is Ken Caldeira of the Carnegie Institution for Science in the US. Caldeira believes that geoengineering efforts should focus on existing areas of science and technology research. This includes the removal of carbon dioxide from smokestacks in power plants, no-till agriculture and other soil amendments, and stratospheric particle and chemistry research based on volcanoes and the ozone layer. Activities such as whitening clouds over the ocean, meanwhile, could be carried out by those who are already studying marine clouds.

Caldeira is in favour of environmental-science studies into the option of sunlight reflection by distributing reflective particles into the atmosphere. But he is against work that strays into the engineering development of implementation techniques.

Take the engineering out of geoengineering

While geoengineering is not yet on most people’s agendas, future events could cause a radical shift in public opinion in the US, says Caldeira. In turn, this would put pressure on politicians to implement the technology. But Caldeira is aware that what is best from a political point of view is not necessarily best for the environment, given the variations in timescales between political and environmental cycles.

Politics versus environment

As well as altering the environment, geoengineering will affect human systems such as agriculture. This gives poor people in tropical countries perhaps the biggest incentive of all to implement geoengineering, says Caldeira, as crop yields in the tropics are more likely to be badly affected by heat stress than those in northern climes, where yields may even improve.

Developing gains

But are countries likely to go it alone when it comes to geoengineering, without complying with any international agreements that may be set up? On this point, at least, Caldeira is sure that when citizens and environmental resources are under threat, politicians will not refrain from acting in their national interests.

Taking sides

New Einstein online archive

By James Dacey

He is still regularly quoted as a supreme authority on a range of topics from fundamental physics to Eastern religion, along with his more whimsical asides on people, places and experiences. Now, the thoughts of Albert Einstein have just become more accessible, thanks to a new website launched yesterday that collates 2000 digitized items, including his scientific writings and his personal correspondence. The website also contains various curios, newspaper clippings and even some of Einstein's poetry.

The new collection has been assembled by the Albert Einstein Archives, a subdivision of The Hebrew University of Jerusalem in Israel, which has teamed up with the Einstein Papers Project (EPP) based in California. Funding has come from the UK-based Polonsky Foundation, an organization that previously digitized the writings of Sir Isaac Newton for the University of Cambridge. The new Einstein collection is designed to replace an earlier database launched in 2003, which contained just 900 digitized papers.

Personally, I think the new site could still be a little easier to navigate. But the best thing to do is to take the tour of the gallery, which showcases some of the most interesting items. It includes an image of the 46-page manuscript presented to the Hebrew University on its opening in 1925, which is said to be Einstein's first systematic exposé of his general theory of relativity. It also includes a letter that Einstein wrote in to his mother in 1919 after hearing the news that Eddington's preliminary findings appeared to confirm general relativity. There is an image of the original letter in German, and there is also an English translation (see below).

Dear Mother,

Good news today. H.A. Lorentz has telegraphed me that the British expeditions have definitely confirmed the deflection of light by the Sun. Unfortunately, Maja has written me that you're not only in a lot of pain but that you've also had gloomy thoughts. How I would like to keep you company again so that you're not left to ugly brooding. But I will have to stay here a while and work. I will also be travelling to Holland for a few days to show my gratitude to Ehrenfest, even though the delay is rather painful.

I wish you good days.

Affectionately yours,

Albert.

The archive also includes some of Einstein's writings on the plight of the Jewish people. One powerful example is this extract taken from a speech Einstein delivered in 1921 to a Zionist audience in Berlin.

Palestine is for us Jews not a matter of charity or colonization: it is a problem of paramount importance for the Jewish people. Palestine is first and foremost not a refuge for east European Jews, but the incarnation of a reawakening sense of national solidarity for all Jews. Is it opportune and necessary to waken and strengthen this feeling of community? I believe I can answer this question with an absolute "yes", based on not only on [sic] spontaneous emotion but on sound reason.

Einstein goes on to discuss the history of the Jews in Germany during the previous 100 years, and the respect he has for the strong sense of community that his forefathers sustained.

German speakers may also enjoy the quirky feature of being able to leaf through a digital version of Einstein's travel journal.

Physicists look round corners in 3D

The ability to see round corners would be nice – especially for police offers chasing criminals in a city or for motorists careering down winding country lanes. Although that prospect is still a long way off, researchers in the US have come up with a way of looking round corners using ultrafast laser pulses. Their technique creates a 3D image of what is round a corner by looking at how light reflects from objects in the local environment.

The technique has been developed by Andreas Velten and colleagues at the Massachusetts Institute of Technology, who hid an artist's mannequin roughly 20 cm tall from the view of a camera by placing it behind an opaque wall that they called the occlude (see image above). A second wall – called a diffuser – was placed at a right angle to the occluder and a small distance away from the end of it. Light travels from the mannequin, scatters from the diffuser and is then captured by the camera – but because the light is scattered diffusely, the image of the mannequin does not appear on the diffuser wall.

This 2D image of the mannequin was captured by the MIT system

What the camera sees

In their experiment, Velten's team positioned an ultrafast laser next to the camera before firing pulses lasting 50 femtoseconds at a beamsplitter. Half of the pulse goes on to a photodetector and the other half continues to the diffuser, from which it scatters and illuminates the mannequin. Some of the light bounces off the mannequin, scatters from the diffuser and finds its way back to the camera.

Arrival times

The distance travelled by the light can be calculated from the time differences between the arrival of the pulse at the photodetector and the light arriving at the camera. The device is, in fact, a specialized instrument called a streak camera, which records the arrival time of light to within 2 ps, which is the time that it takes light to travel about 0.6 mm. The camera also records the intensity of the light as a function of position along one direction.

To build up a 3D image of the mannequin, the position at which the laser pulse first strikes the diffuser is changed in a systematic way and 60 images are acquired, each at a different position. Finally, a computer algorithm is used to reconstruct the image, which can be rendered in 2D or with depth information to provide a 3D image. According to Velten, the algorithms can easily be implemented on a personal computer.

Because the camera only captures light over 2 ps intervals, any background noise from ambient light is small and Velten believes that it should be possible to use the technique even in broad daylight.

Peering into lungs

One possible application of the technology, according to the team, is in endoscopy, in which a tiny camera is inserted into the body. Seeing round corners could be useful in probing organs such as the lungs or heart, which contain corner-like structures. Other applications include locating survivors in an emergency situation and avoiding vehicle collisions at blind corners.

While the current set-up is not portable, Velten tells physicsworld.com that ongoing improvements in semiconductor laser and camera technologies should soon make it possible to create compact systems that could be deployed with firefighters or military personnel.

The system is described in Nature Communications.

Laser writer makes graphene supercapacitors

A routine laser-writing technique has been used to create sheets of graphene on the surface of a DVD. The work was done by researchers in the US, who then joined the sheets together to make electrochemical capacitors (or supercapacitors). The devices can store as much energy as a conventional battery but can be charged 100–1000 times faster. According to the researchers, the capacitors are completely flexible and robust, which makes them ideal energy-storage systems for flexible and portable electronics.

Electrochemical capacitors – also known as supercapacitors or electric double-layer capacitors – can store much more electrical charge than standard capacitors. This is thanks to the double layer formed at an electrolyte–electrode interface when voltage is applied to the device. Although promising energy-storage materials, supercapacitors still lag behind traditional batteries in terms of energy densities: 4–5 Wh/kg and 10–150 Wh/kg, respectively. They do, however, have a much longer shelf – and cycle life than batteries and can deliver large amounts of energy much more quickly.

Now, a research team led by Richard Kaner and Maher El-Kady at the University of California, Los Angeles, says that they have developed a graphene-based device that combines both the power performance of capacitors with the high energy density of batteries. The researchers have come up with a new process that involves coating an ordinary DVD with a film of graphite oxide supported on a sheet of plastic.

Brown fades to black

Kaner and colleagues begin by reducing the graphite oxide to graphene using a standard "LightScribe" DVD drive head – usually used optically to etch labels and images on DVD discs. The process can easily be monitored as the golden-brown-coloured graphite oxide turns into black-coloured graphene. The graphene-coated plastic is then peeled off and cut with scissors to make different devices (see video below).

The graphene sheets can be used as electrodes without the need for any additional binders or additives. Electrochemical capacitors are made by simply "gluing" together two identical pieces of graphene sheet with a little polymer gel electrolyte that is placed between them. "We also tested a variety of other electrolytes confirming that the material can be used in a number of device systems for different applications," says Kaner.

Large surface area

The LightScribe graphene sheets have a large area per unit mass – greater than 1500 m²/g – which helps to increase their energy-storage capacity, and a high electrical conductivity of more than 1700 S/m. They are also flexible mechanically and can be bent thousands of times without suffering any damage to their electrical properties. All of these characteristics make them ideal not only for making supercapacitors but also for a host of other electronic devices, says El-Kady, who also lectures at Cairo University.

The LightScribe process overcomes another important problem, too, in that it produces non-stacked graphene sheets. Graphene sheets have a natural tendency to stack during production, something that reduces the overall surface area of the material.

"We believe that our work will help pave the way to making flexible supercapacitors for use in bendable electronic equipment for the upcoming boom in flexible portable electronics," El-Kady tells physicsworld.com. "Applications include roll-up computer displays, wearable electronics that harvest and store energy produced by body movement, electronic wallpaper and energy-storage systems that can be combined with flexible photovoltaic cells."

The team now plans to look at scaling up their production technique in a cost-effective manner. "Our initial calculations show that the price of the precursor, graphite oxide and the whole process is completely viable for commercial applications," adds El-Kady.

Details of the current work are published in Science.

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