Tag Archives: metamaterial

Scientists count microscopic particles without a microscope

Counting microscopic particles is hard, but researchers from Russia and Australia believe they’ve found a way to make it easier.

The process - image via ITMO University.

The process – image via ITMO University.

The principle is fairly simple. A research team from ITMO University, Ioffe Institute, and Australian National University passed a laser beam through an optic material riddled with microscopic particles and then projected it onto a screen. The screen showed a characteristic pattern consisting of numerous bright spots and the number of these bright spots corresponded to exactly to the number of scattering microscopic particles in the optical material.

“The light senses heterogeneity,” says Mikhail Rybin, first author of the paper, senior researcher at the Department of Nanophotonics and Metamaterials at ITMO University.

“We found out that by looking at the pattern it is possible to determine the precise number of scatterers in the material. This helps understand not only the type of the sample lattice (square, triangular), but also establish its structure (20 to 20 particles, or 30 to 15) just by counting the light spots on the screen”.

It’s very simple but also very effective. You shine a laser beam through the material and then onto a projector. The different structure of the material will cause light to diffract, and the result will be visible on the projector.

“Depending on the shape and relative position of the scatterers, the light wave continues to propagate differently behind the sample. In other words, the structure of the sample affects the diffraction pattern, which will be projected on the screen.

Mikhail Rybin at work. Photo via ITMO University.

Mikhail Rybin at work. Photo via ITMO University.

The new method is a much more affordable option to expensive electron or atomic force microscopy and doesn’t damage the sample at all. It’s also extremely easy to use.

“Even a schoolboy can buy a laser pointer, adapt a small lens to focus the light better, fix the sample and shine a laser beam on it,” notes Mikhail Rybin. “In addition, our method makes it possible to study optical materials without changing their structure, in contrast to electron microscopy, where the sample surface has to be covered with conductive metal layer, which impairs optical properties of the sample”.

The method can be extremely useful for constructing and assessing metamaterials. They tested it for two classes of such materials: photonic crystals and metasurfaces. Photonic crystals are periodic optical nanostructures which affect the motion of photons in much the same way that ionic lattices affect electrons in solids. When light passes through one, it generates a fancy pattern on the screen behind the sample.

In the case of a metasurface – an artificial sheet material – things are a bit more complicated because the material has to be constructed in such a way that the distance between particles is significantly smaller than the wavelength of light. In fact, the difference between a photonic crystal and a metasurface depends on the wavelength of the light.

“For one wavelength, the material will act as a photonic crystal and as a metasurface for another. That is why designing such structures, we can evaluate maximum lattice period with laser,” concludes Mikhail Rybin.

Islamic art inspires metamaterial that grows when stretched

A new type of metamaterial that can grow when stretched, with possible applications for medical equipment and satellites, was inspired by an unlikely source — ancient Islamic art.

Most materials, such as cotton, plastic or rubber, stretch in one direction and become thinner in another when you pull on them. Some metamaterials however, a class of materials engineered specifically to have properties that don’t occur naturally, can be designed to grow as you pull on them.

It all comes down to the way they’re structured at a microscopic level. If you zoom in enough, you’ll see that they’re typically made up of a series of interconnected squares. When pulled apart, these squares turn relative to one another, increasing the total volume of the material — in essence, becoming larger. But this comes at the price of losing the original shape of the material as it expands.

Ahmad Rafsanjani and Damiano Pasini of McGill University in Montreal, Canada, set out to create a material that would grow when stretched but keeps its form. And they turned to the beautifully intricate geometry of ancient Islamic arhitecture.

Some of the designs that the team used as inspiration.
Image credits A Rafsanjani/McGill University.

 

“There is a huge library of geometries when you look at Islamic architectures,” says Rafsanjani.

The team picked their designs out of the over 70 patterns adorning the walls of Iran’s Kharragan towers, two mausoleums built in 1067 and 1093 in the northern part of the country. The mausoleums are decorated with intricate patterns of both repeating and alternating shapes, separated by either parallel or circular cuts.

Based on the same design features, the team fashioned the new metamaterial from natural rubber. When pulled on, it can expand into a larger volume that the original by leaving open spaces in between the shapes.

“I introduced some cuts and some hinges, but the pattern is exactly the same,” says Rafsanjani.

The new material, based on designs nearly a thousand years old.
Image credits A Rafsanjani/McGill University

Rafsanjani presented the materials at a meeting of the American Physical Society in Baltimore, Maryland, on 15 March.

The ability of the material could make it useful for an array of applications, such as inserting medical devices inside veins and arteries, or deploying new satellites that unfold in space.

Magnetic Mirror reflects Light like No Other. Opens new suit of Optical Applications

In Lewis Caroll’s Through the Looking-Glass, and What Alice Found There (1871), the sequel to the classic Alice’s Adventures in Wonderland, Alice again enters a fantastical world, this time by climbing through a mirror into the world that she can see beyond it. Though far from Alice’s spectacular feat, scientists at the Sandia National Laboratories in Albuquerque, New Mexico demonstrated a new type of mirror that behaves like no other.

A mirror without metals

magnetic_mirror

Artist’s impression of a comparison between a magnetic mirror with cube shaped resonators (left) and a standard metallic mirror (right). The incoming and outgoing electric field of light (shown as alternating red and white bands) illustrates that the magnetic mirror retains light’s original signature while a standard metallic mirror reverses it upon reflection. Credit: S. Liu et al.

A conventional, metal coated mirror not only reverses the image, but also the light’s electric fields as well. This is because the mirror interacts with the electrical component of electromagnetic radiation. Of course, for those of us who use mirrors casually this physical alteration makes no difference, but it can become a real nuisance for physicists working on various optical and light absorbing/reflecting materials like solar cells, lasers and such. This becomes most intruding at the mirror’s surface, at the point of reflection where the opposite incoming and outgoing electrical fields produce a canceling effect. Yet, scientists in the US have made a breakthrough after they placed nanoscale antennas at or very near the surface of so-called “magnetic mirrors.”

“We have achieved a new milestone in magnetic mirror technology by experimentally demonstrating this remarkable behavior of light at infrared wavelengths. Our breakthrough comes from using a specially engineered, non-metallic surface studded with nanoscale resonators,” said Michael Sinclair, co-author on the Optica paper and a scientist at Sandia National Laboratories in Albuquerque, New Mexico, USA who co-led a research team with fellow author and Sandia scientist Igal Brener.

Unlike silver and other metals, however, there is no natural material that reflects light magnetically. Magnetic fields can reflect and even bottle-up charged particles like electrons and protons. But photons, which have no charge, pass through freely. To overcome this predicament, the researchers devised a matematerial – a material that can’t be found in nature, artificially created to suit certain needs – made up of nanoscale cube-shaped resonators, based on the element tellurium, each considerably smaller than the width of a human hair and even tinier than the wavelengths of infrared light, which is essential to achieve magnetic-mirror behavior at these incredibly short wavelengths.

 “The size and shape of the resonators are critical,” explained Sinclair “as are their magnetic and electrical properties, all of which allow them to interact uniquely with light, scattering it across a specific range of wavelengths to produce a magnetic mirror effect.”

Typically, this sort of solution is practical only for long microwave frequencies, which limits the scope of applications to microwave antennas, mainly. The  two-dimensional array of non-metallic dielectric resonators, however, overcomes these limitations – all without loss of signal. To prove their design actually works like a magnetic mirror, the Sandia scientists used a technique called time-domain spectroscopy.

“Our results clearly indicated that there was no phase reversal of the light,” remarked Sheng Liu, Sandia postdoctoral associate and lead author on the Optica paper. “This was the ultimate demonstration that this patterned surface behaves like an optical magnetic mirror.”

Next, the researchers plan on demonstrating magnetic mirrors at even shorter wavelengths.  where extremely broad applications can be found.

“If efficient magnetic mirrors could be scaled to even shorter wavelengths, then they could enable smaller photodetectors, solar cells, and possibly lasers,” Liu concluded.

Touch invisibility cloak prevents objects from being felt

With the finger or a force measurement instrument, no information is obtained about the bottom side of the material. Credit: T Bückmann / KIT

In the past years, several types of invisibility cloaks have been developed, hiding objects not only from light, but also from sound and even heat. But this is the first time an invisibility cloak for touch has been developed.

Recently, we’ve written quite a lot about invisibility cloaks – how they work, how they can be improved, and what real life applications they have (aside from being really cool) – like for example protecting cities from earthquakes (yeah, really). But what about a  “touch invisibility cloak” ? Naturally, it is constructed very differently – based on a metamaterial that consists of a polymer with a special structure.

“We build the structure around the object to be hidden. In this structure, strength depends on the location in a defined way,” explains Tiemo Bückmann, KIT, the first author of the article. “The precision of the components combined with the size of the complete arrangement was one of the big obstacles to the development of the mechanical invisibility cloak.”

The metamaterial in case is actually in a crystalline material, built with extreme sub-micrometer accuracy; it basically consists of needle-shaped cones, whose tips meet. The size of the contact points is calculated precisely to reach the mechanical properties desired. The resulting structure is built in such a way that a finger or a measuring instrument cannot feel its way through it.

“It is like in Hans-Christian Andersen’s fairy tale about the princess and the pea. The princess feels the pea in spite of the mattresses. When using our new material, however, one mattress would be sufficient for the princess to sleep well,” Bückmann explains.

So far, this is pure research – no end goal in sight. However, since it paves the way for producing materials with freely selectable mechanical properties, it could have a myriad of applications. The first that pop to mind are comfier mattresses or rugs under which you can sneak a cable or two and not feel them; however, investing in such a high-end technology for this type of results is not something doable at the moment.

Journal Reference:
T. Bückmann, M. Thiel, M. Kadic, R. Schittny, M. Wegener. An elasto-mechanical unfeelability cloak made of pentamode metamaterials. Nature Communications, 2014; 5 DOI: 10.1038/ncomms5130

This five-cell metamaterial converts stray microwave energy, as from a WiFi hub, into more than 7 volts of electricity with an efficiency of 36.8 percent—comparable to a solar cell. (c) Duke University

Microwave harvester converts wireless energy into direct current with solar cell-like efficiency

This five-cell metamaterial converts stray microwave energy, as from a WiFi hub, into more than 7 volts of electricity with an efficiency of 36.8 percent—comparable to a solar cell. (c) Duke University

This five-cell metamaterial converts stray microwave energy, as from a WiFi hub, into more than 7 volts of electricity with an efficiency of 36.8 percent—comparable to a solar cell. (c) Duke University

Using a range of cheap materials arranged in a specific manner, researchers at Duke University have demonstrated a device that captures microwave signals, such as those relayed by WiFi or even satellites high up above in Earth’s orbit, an converts this free, lost energy into electrical current. The harvesting and conversion efficiency of the device is on par with currently photo-voltaic solar panels. In the future, such circuits could be used to always wirelessly power your phone when it’s not in use or charge a battery in your home.

It’s enough to open  your notebook just about anywhere downtown in a developed city to spot tens and tens of public WiFi hotspots. There are maybe tens of thousands of WiFi networks in a developed city, and all of these waste energy by constantly relaying microwave signals. The same applies to satellites, sound signals and other sources. What if you could harvest and cut the waste a bit?

This is what the Duke team of engineers, comprised of undergraduate engineering student Allen Hawkes, working with graduate student Alexander Katko and lead investigator Steven Cummer, set out to do. Using simple materials like five fiberglass and copper energy conductors wired together on a circuit board  to form a five-cell metamaterial array. The resulting device harnesses microwave energy and converts it into direct current 7.3V electricity. For reference, your  Universal Serial Bus (USB)  charges small electronic devices with 5V, so a system coupled with the Duke metamaterial circuit could provide free charging for your mobile devices.

“We were aiming for the highest energy efficiency we could achieve,” said Hawkes. “We had been getting energy efficiency around 6 to 10 percent, but with this design we were able to dramatically improve energy conversion to 37 percent, which is comparable to what is achieved in solar cells.”

“It’s possible to use this design for a lot of different frequencies and types of energy, including vibration and sound energy harvesting,” Katko said. “Until now, a lot of work with metamaterials has been theoretical. We are showing that with a little work, these materials can be useful for consumer applications.”

Microwave harvester

Another application could be to improve the energy efficiency of appliances by wirelessly recovering power that is now lost during use.

“The properties of metamaterials allow for design flexibility not possible with ordinary devices like antennas,” said Katko. “When traditional antennas are close to each other in space they talk to each other and interfere with each other’s operation. The design process used to create our metamaterial array takes these effects into account, allowing the cells to work together.”

A metamaterial coating could be applied to the ceiling of your living room, for instance, to harvest free energy from microwaves that litter space all around us. A more interesting, and maybe more practical, application might be coating smartphones with a thin layer of mematerial directly, so your phone charges constantly with significant benefits for your battery life. It’s unclear though how much voltage a thin and small surface metamaterial circuit might provide.

“Our work demonstrates a simple and inexpensive approach to electromagnetic power harvesting,” said Cummer.  “The beauty of the design is that the basic building blocks are self-contained and additive. One can simply assemble more blocks to increase the scavenged power.”

In remote locations, like in the desert or the middle of the wilderness, a series of power-harvesting blocks could be assembled to capture the signal from a known set of satellites passing overhead. The generated power would be far from impressive, of course, but it might still be enough to power a small array of sensors, the researchers believe.

Their device and findings were reported in the journal Applied Physics Letters

 

Invisibility cloak

Human-sized invisibility cloak makes use of magic trick to hide large objects

Invisibility cloaks have become the object of study for many research institutes out of a number of considerations. There’s the simple scientific thrill of breaching the SciFi barrier, something every researcher dreams of. Then of course, there’s always the corporate and military tail that always likes to meddle with concepts that shine with the possibility of profit or strategic edge, and is always quick to offer incentives – money. Advancements in this direction have been nothing short of mind boggling, as we’ve reported in the past about a few working models, developed at great pace in the past few years alone. The latest invisibility cloak comes from scientists at University of Rochester in New York, and differentiates itself by extremely simple design, reminiscent of tricks magicians use on stage to hide objects, and its high potential for scaling the cloak for large objects like the human body or even satellites.

Most invisibility cloaks today work by exploiting two core ideas: transformation optics (bending light around a region of space to make it look as if it weren’t there) and metamaterials ( synthetic substances with optical properties that can’t be found anywhere else in nature designed specifically for cloaking). The ultimate goal is to produce an invisibility cloak that can hide a human from optical observations, Harry Potter-cloak style. This hasn’t been possible as of now, though.

The problem lies in compromise. So far, cloaks have been made that can hide objects in 3-D only at a single microwave frequency. Then, there are cloaks that can hide objects over a wide range of optical frequencies, but only work from a certain viewing angle. Apparently, you can’t have both – not just yet at least.

Invisibility cloakThe  University of Rochester cloak unveiled just recently had to make a compromise as well, and the researchers chose settled for a limited viewing angle for their device. It makes up for it, however, by functioning over the entire optical spectrum and, most importantly, through its potential of being scaled for extremely large objects.

Besides, their approach is extremely simple and doesn’t require expensive and difficult to implement metamaterials. To cloak, the device makes use of an array of lenses or mirrors that steer light around the region of space they want to hide. Mirrors can be fashioned into any size, easily and cheaply.

To demonstrate the cloaking ability of their device, the researchers cloaked a chair with a trash can behind it. “This volume is sufficient to cloak a human, albeit with not as much convenience as Harry Potter’s cloak,” say John Howell at the University of Rochester in New York.

Can their work have any potential application? The researchers seem to think so. Despite being unidirectional, the cloak can be scaled to very large sizes and could have value, for example, in cloaking satellites in mid- to high-earth orbit, making them optically invisible from Earth-based observations. A system might be employed to constantly shift the angles of the mirrors as the satellite swirls through orbit.

Findings were reported in a published paper.

New invisibility cloak takes things to the next level

It may be a little off to talk about invisibility when we’re not even in the visible spectrum, but ‘invisibility cloak‘ sounds just to awesome not to use it. Using a new kind of cloak that uses a very thin multilayer dielectric coating made of natural material (as opposed to metamaterials which are often used), researchers from Michigan University demonstrated better cloaking efficiency than a similarly sized metamaterial cloak designed with a different technology.

cloaked

The materials they used are relatively simple – dielectric materials such as ceramics having differing dielectric permittivity (the measure of resistance created when forming an electric field; usually with invisibility studies, researchers use metamaterials – which are artificially developed and have different characteristic to anything found in nature. Yet using this simple technique, they were able to cloak larger cylindrical objects more effectively than ever before, with any kind of material.

The so-called metascreen they created has a fishnet like structuer, covering an 18 cm cylindrical rod from microwaves. The reason why we see things is because light bounces on and off of them; previous such technologies used metamaterials to divert, or bend, the incoming waves (including light) around an object, this uses an ultrathin metallic metascreen to cancel out the waves as they are scattered off the cloaked object.

“When the scattered fields from the cloak and the object interfere, they cancel each other out and the overall effect is transparency and invisibility at all angles of observation,” said co-author of the study Professor Andrea Alu.

According to their report in the Applied Physics Letters article, Semouchkina and Wang used a very thin multilayer dielectric coating and compared it with cloaks of a similar size which use a different principle, called transformation optics.

“The new cloak demonstrated better cloaking efficiency than did a similarly sized metamaterial cloak designed by using transformation optics relations,” said Semouchkina.

Not only does the new cloak outperform previous cloaks, which caused more reflection and more shadows, as well as distortion of the electromagnetic waves, but it is also more scalable and cheaper to produce; about 8-9 times cheaper, they noted.

“The multi-layer dielectric cloak could easily be scaled to work in a variety of frequency ranges,” Semouchkina said. “The design procedure developed in this work could be used to further advance the cloak parameters and for adjusting it to practical needs.”

Airport microwave scanner.

Microwave metamaterial camera images in real time. It’s only a fraction of the size current devices are

Scientists at Duke University have devised a metamaterial that uses microwaves to image objects or scenes in real time, all through a set-up no larger than a book. Currently, the same imaging is being made with robust, huge machinery – the kind you see in airports used to scan people before they board flights – that are very expensive and contain moving parts. Applications for the new metamaterial are boundless, according to the researchers, ranging from security to self-driving cars.

A digital camera, much like the human eye as well, images by focusing light through a lens which then hits millions of individual silicon detectors, one for each pixel in the final photograph. The system works great for visible light since optical wavelengths are short and can fit in tiny spaces, like the back of the eye (retina).

Airport microwave scanner.

Airport microwave scanner.

When delving in longer wavelengths, such as microwaves or millimeter waves, things aren’t that easy since imaging currently requires detectors that are slow, expensive to construct, and require continuous reorientation to capture targets. You see, building a microwave camera that works similarly to an optical camera would require millions of detectors as well, which for microwaves however are orders of magnitude more expensive and larger. So engineers developed a single detector or a line of them and moved them from point to point across the plane you wish to capture. This works fairly well, but the resulting imaging solution is a behemoth in size, pricing and maintenance efforts.

Researchers at Duke University, however, have devised a metamaterial – a material whose properties can’t be found in nature – that seeks to replace walls and walls of materials or extremely cumbersome machinery. The device is only 40 centimeters long and consists of a single aperture that doesn’t move and which can capture a scene at various microwave frequencies. This is then fed to a computer which processes the information and retrieves an image.

Slimming down millimeter wave cameras

In broader detail, the detector consists of three plates – two copper plates with a plastic one sandwiched between – each etched with a pattern of box-like structures, 2 millimeters long that permit different lengths of microwaves to pass through. This design allows for focusing on a narrow range of frequencies in the microwave range and, ultimately, capturing an image.

“It has a large aperture, which is good for resolution, but it’s thin, has no moving parts, and it made of relatively cheap materials,” John Hunt, a graduate student at Duke and corresponding author.

The Duke researchers have a fast experience in working with metamaterials. Not long ago, scientists from the prestigious university announced they had built the first working invisibility cloak.

Light doesn’t pass through walls, clothing or weather like fog – microwaves do. So you can imagine what kind of interest is invested in microwave imaging technology, especially in the field of security. The technology is employed in many other applications as well, like self-driving cars which use a smaller, yet still huge  and expensive microwave aperture that can sense obstacles in front of the vehicle.

“To my knowledge, this is probably the first example of metamaterials and compressive imaging in the same paper,” said Kevin Kelly, an electrical engineering professor at Rice University who helped develop the first single-pixel camera.

Hunt and his team demonstrated their work after their reconstructed the placement of various objects scattered in a room that was bombarded with microwaves. Since the metamaterial consists of a single slab, the images were cross sections of a horizontal plane through the scene, showing range only. Still, the objects were easily identified. More research will allow for developing a 2-D aperture capable of capturing 3-D images – then we’re pretty much open for business. Imagine a device in the future no bigger than a smartphone today that you can use to image through walls or clothing. It will be interesting to see how such technology will be controlled and enforced.

“This is an elegant solution to a lot of problems in microwave, millimeter-wave, and terahertz imaging,” says Eddie Jacobs, an electrical and computer engineering professor at the University of Memphis, in Tennessee

The system was reported in the journal Science.

via IEE

The orientation of 4,000 S-shaped units forms a metamaterial lens that focuses radio waves with extreme precision, and very little energy lost. (c) Dylan Erb

New metamaterial focuses radio waves with extreme precision similar to Star Wars’ Death Star

Researchers at MIT have created a new metamaterial that they used to fashion a concave lens capable of focusing radio waves with extreme precision. The result lens is extremely lightweight compared to its counterparts developed from conventional materials, and could see promising applications in satellite telecommunications and space exploration of distant stars.

In many ways metamaterials are supernatural, that’s because by definition it is a material artificially engineered by man to have properties that can never be encountered in nature. It’s  an extremely exciting field, since you’re basically building new, unique compounds and structures. The most interesting applications of metamaterials we’ve been granted to see comes in the form of invisibility cloaks and what’s commonly referred to as “super lenses” – extremely potent lenses that focus light beyond the range of optical microscopes to image objects at nanoscale detail.

Building the metamaterial lens

The latter is what MIT scientists have been going for with their negative refraction concave lens, which bends electromagnetic waves — in this case, radio waves — in exactly the opposite sense from which a normal concave lens would work. These properties are given off by the structure of the metamaterial, and how individual cells are arranged. In this case, the researchers built a blocky, S-shaped “unit cell” only a few millimeters wide whose shape refracts radio waves in particular directions – 4000 of these were arranged to form the concave negative refraction lens. Each of these cells only bends radio waves slightly, but together they focus the wave.

The orientation of 4,000 S-shaped units forms a metamaterial lens that focuses radio waves with extreme precision, and very little energy lost. (c) Dylan Erb

The orientation of 4,000 S-shaped units forms a metamaterial lens that focuses radio waves with extreme precision, and very little energy lost. (c) Dylan Erb

Isaac Ehrenberg, an MIT graduate student in mechanical engineering, shaped the lens via 3-D printing layer by intricate layer from a polymer solution. He then washed away any residue with a high-pressure water jet and coated each layer with a fine mist of copper to give the lens a conductive surface.

“There’s no solid block of any material in the periodic table which will generate this effect,” Ehrenberg says. “This device refracts radio waves like no other material found in nature.”

In an experiment, two radio antennas were positioned between the metamaterial lens. The resulting energy transmitted through it was found to travel through the lens almost in its entirety, with very little being lost with the metamaterial- significant improvement in energy efficiency when compared with past negative-refraction design. The team also found that radio waves converged in front of the lens at a very specific point, creating a tight, focused beam.

Star Wars’ Death Star laser beam?

As an analogy, Ehrenberg sees the design and functionality of the lens much in the same way as the Death Star’s concave dish that focuses a powerful laser beam to destroy nearby planets in the movie Star Wars. Again George Lucas’ awesome saga offers invaluable inspiration to scientists.

Since it weighs less than a pound, the lens could be used to focus radio waves precisely on molecules to create the same high-resolution images currently produced by very heavy and bulky lenses. Mass is one of the main factors taken into account for space applications, and future space satellites would definitely benefit from this. In addition, Ehrenberg says its fabrication is simple and easily replicated, allowing other scientists to investigate 3-D metamaterial designs.

“You can really fully explore the space of metamaterials,” Ehrenberg says. “There’s a whole other dimension that now people will be able to look into.”

His findings were documented in the Journal of Applied Physics.

source

 

Nanomaterial converts light and heat into electricity

Yi-Hsuan Tseng et al./Nanotechnolog

(c) Yi-Hsuan Tseng et al./Nanotechnolog

Previously, scientists have managed to devise material that can convert light into electricity, and other materials that can convert heat into electricity. Now, a group of researchers  at University of Texas at Arlington have managed to create a hybrid material that can convert both forms of energy at the same time into electricity. This double spanned function gives it an edge over existing solutions, and since it’s very cheap, if the technology is improved it might even act as an alternative to conventional solar panels.

Single-walled carbon nanotubes (SWNTs) have been used in the construction of transparent solar cells and all-carbon solar cells, however when compared to conventional photovoltaic cells these are still highly inefficient. However, when also converting heat, not just light, into electricity, the resulting material becomes of novel interest.

“If we can convert both light and heat to electricity, the potential is huge for energy production,” said UT Arlington associate physics professor Wei Chen. “By increasing the number of the micro-devices on a chip, this technology might offer a new and efficient platform to complement or even replace current solar cell technology.”

The thermoelectric generator was made after the researchers combined copper sulfide nanoparticles and single-walled carbon nanotube. In lab tests, the new thin-film structure showed increases by as much at 80 percent in light absorption when compared to single-walled nanotube thin-film devices alone, making it a more efficient generator. The researchers soon plan an assembling a prototype thermoelectric generator that they hope can eventually produce milliwatts of power. The technology could be used in devices such as self-powering sensors, low-power electronic devices and implantable biomedical micro-devices.

Findings were documented in the Journal of Biomedical Nanotechnology

Invisibility cloak now a reality – perfect demonstration

It may still be a while until we get our Harry Potter-like invisibility cloak, but researchers have managed, for the first time, to render a cylinder invisible to microwaves.

Science, not magic

Invisibility is a notion that fascinated humans for centuries or even millennia; it’s just one of those things which never gets old, regardless of the period. We now know that, since we see objects because light bounces of their surface, if we want to make something invisible, you have to prevent it from reflecting light – simple in theory, extremely hard to practice. However, in 2006, invisibility became more of a practical matter, with the suggestion that artificially structured metamaterials could enable a new electromagnetic design paradigm – something which is now called transformation optics.

Since this term was coined, transformation optics has developed significantly – so greatly that it forced researchers to make simplifying approximations to achieve even a subset of the desired functionality. The problems with most devices spawned here – despite many ‘invisibility cloaks’ being tested successfully, they all reflected some of the incident light, which made the illusion incomplete.

This new research, published in Nature Materials showed how to pull that trick off flawlessly. Nathan Landy & David R. Smith teamed up in an effort to take things to the first level. Despite the perfection of their method, this only works from one direction, and with microwaves. But this is no spherical cow approach – it holds great promise for the future.

Where’s my invisible cloak?

The material structures which can create such remarkable features are all but impossible to create, and again, some approximations were made. So when you would use your invisibility cloak, while you would see the image behind your object, you would see it distorted, mostly darkened – not very useful to make something invisible, but noticeable.

Previous example of invisibility cloak. Source

Now, Prof Smith and his Duke colleague have used a different approach. They have reworked the edges of the cloak, ensuring the light passes with around the cloak with completely no reflections. The trick was to used a diamond shaped cloak with properties carefully matched at the diamond’s corners.

“This to our knowledge is the first cloak that really addresses getting the transformation exactly right to get you that perfect invisibility,” Prof Smith told BBC News.

However, it was a “win some lose some” approach. While the illusion is perfect, it only works from one direction.

“It’s like the card people in Alice in Wonderland,” Prof Smith explained. “If they turn on their sides you can’t see them but they’re obviously visible if you look from the other direction.”

Lengthy problems

This wouldn’t even be the biggest problem. It would be extremely hard to make the device function at optical wavelengths, which are much larger than those of the microwaves. However, this could be extremely useful in many applications, principally telecommunications and radar, for which it could greatly improve the performance.

“The cloak we demonstrated in 2006 as a kind of microwave device would be very poor, but this one gets us to something that could be potentially useful,” Prof Smith said. “I think it’s something that a lot of people can build on. Everything in this field is going to come down to what you can make, what you can design. And I think this steps up the design.”

Microwaves can be seen being blocked and scattered without (l), and "reconstructed" (r) with the cloak

Scientists cloak 3D object in microwave spectrum

The much dreamed off invisibility cloak is just a few tiny steps away, after remarkable research in the field, many backed by military interests, have sparked some amazing advances. In the last few years alone, scientists have managed to successfully cloak various objects either using meta-materials that bent light around an object to conceal it or electrically stimulated nanotubes which cause the human eye to perceive a mirage-like effect and thus conceal the object. Just a few weeks ago, scientists manage to hide an event in time after they developed a time cloak.

Microwaves can be seen being blocked and scattered without (l), and "reconstructed" (r) with the cloak

Microwaves can be seen being blocked and scattered without (l), and "reconstructed" (r) with the cloak

However, we’re still in a highly incipient state as far as a full-on invisibility cloak in its all rightful manner is concerned. You see, these devices are only capable of rending a particular object only in 2D, from a particular angle, which although doesn’t seem particularly useful, it’s still been a remarkable progress. Now, in a recently published paper, University of Texas scientists describe how they’ve been able to use plasmonic meta-materials to make an 18-inch cylindrical tube invisible – a full 3-D cloak.

What we actually perceive with our eyes is actually information transmitted by light which bounces off objects in our surroundings, as its constituent atoms absorb, transmit or reflect electric and magnetic fields. One might say that the world around us, as we visually see it, is not the real one, but its reflection. Bearing in mind this, if one can manipulate or stop light from bouncing off an object altogether, than that object would become invisible.

” That means the object is invisible, from any angle of observation.

“This object’s invisibility is independent of where the observer is,” Professor Andrea Alu, the study’s co-author, tells Danger Room. “So you’d walk right around it, and never see it.”

Plasmonic materials can be designed to have effects on the fields that are precisely opposed to those of the object, and thus cancel out the light scattering from an object. When the plasmonic shell was coated on a cylinder, the two cancelled each other out, and became invisible in the high-frequency wavelengths, like the microwave spectrum – it remained perceivable as always in the visual wavelength spectrum, however.

The plasmonic material shell is, in essence, a photo-negative of the object being cloaked, so for this to work the shell needs to be tailored specifically for the object to be cloaked. Cloaking in visible light, hiding more complex shapes and materials, is still extremely distant, however these recent advances, with this latest one to bolster as well, proves that it’s far from being impossible.

“We have some ideas to make it work,” Alu says. “But the human eye is not our priority. Right now, we’re focused on improving biomedical imaging.”

 

The study was presented in a recent edition of the New Journal of Physics.

Scientists have been able to manipulate light beams so they are unable to detect an event. This is an depiction of a sculpture by Sun Shaoqun entitled "Beyond Space Time". (c) China Photos/stringer/AFP/Getty Images

‘Time Cloaking’ remarkable experiment hides event in time

Cloaking used to be one of my favorite SciFi themes. James Bond supercars that would show up or disappear instantly at the flick of an alarm key, the hallow man, objects rendered completely invisible to the human eye and lost in the surroundings. I say used to be because spatial cloaking has transcended for some time now in the realm of reality, after researchers successfully created an invisibility cloak from metamaterials. Cornell University physicists have taken this concept, however, to a whole new level – temporal cloaking.

Remarkably, the Cornell scientists managed to basically create a wrinkle in time in which, albeit for a trillionth of a second, any objects that passed through it became invisible and events went unrecorded. Now this is far from being magic, of course, and like spatial cloaking, the time cloak experiment is based on the same principles of physics which discuss the manipulation of light.

 

Its hard to find a working analogy, but you could view this as a video tape with a missing frame, only that missing millisecond in video isn’t lost in the time cloak experiment, but hidden – physical information can never be lost, maybe except for black holes.

The team of researchers used a system of two half time-lenses connected with one another at their tips, thus forming a split time lens. A continuous beam of green light was fired from a laser which became slowed down when it passed through the first half lens, and sped up when it went through the second – a 50 picosecond gap in the beam was thus opened. During this exact window the researchers shot a stream or red light though through fiber optical cables thinner than a human hair which went on to be undetected.

“You kind of create a hole in time where an event takes place,” said study co-author Alexander Gaeta, director of Cornell’s School of Applied and Engineering Physics. “You just don’t know that anything ever happened.”

Sure using this tech won’t give you enough time to pull the perfect museum heist, however it’s truly an amazing first step. According to Cornell scientists, it’d take a machine 18,600 miles long to produce a time mask that lasts a single second. When data transmissions are concerned, this kind of temporal cloaking might prove to be incredibly useful for information security, as high speed packets of information might be able to travel undetected without disrupting the actual flow.

“We think of time in the way that other people think of space. What other people are doing in space, we can do it in time,” said Moti Fridman, a researcher at the School of Applied and Engineering Physics at Cornell University .

“I think it’s a big step forward,” said Vladimir M. Shalaev, a professor of electrical and computer engineering at Purdue University, who has worked on spatial cloaking. “It’s another example of the beauty of ‘transformational optics,’ which is behind all these ideas.”