A new class of ‘invisible’ materials is within sight

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The theoretical discovery of transparent particles that break the previously accepted limit of visibility opens a new door in the search for perfect transparency

Transparent particles with extraordinarily high refractive indices can become almost invisible at wavelengths longer than the particle size, an A*STAR-led theoretical study has shown1. The discovery challenges the accepted wisdom around the limits of light scattering and visibility, and could lead to a new class of ‘invisible’ materials.

The scattering of sunlight from gas molecules in the atmosphere is what makes the sky look blue, allowing us to effectively see what would other be a transparent medium. This process, known as Rayleigh scattering, occurs when molecules or particles are smaller than the wavelength of light that hits them. It has long been accepted that all particles undergo Rayleigh scattering, and that the minimum amount of scattering occurs when the refractive index — a measure of the ‘slowness’ of light passing through a medium compared with a vacuum — is less than two. Water, air and glass all meet this condition, suggesting that the Rayleigh scattering that makes the sky blue is the least visible state physically achievable.

Boris Luk’yanchuk and colleagues from the A*STAR Data Storage Institute, in collaboration with researchers from the Australian National University, have now upset this status quo with the discovery that Rayleigh scattering can be suppressed in transparent particles at wavelengths longer than the particle scale if their refractive index is extraordinarily high.

“There have been many attempts to reduce scattering,” says Luk’yanchuk. “For example, suppression of the back reflection of radar signals has been widely studied as part of the development of stealth technology. Yet even very small transparent particles have some degree of scattering. We have been able to reveal a new phenomenon that could be used to design ultra-transparent optical materials.”

Rayleigh scattering occurs when light is absorbed by a molecule — producing a separation of positive and negative charges known as an electric dipole — and re-emitted by the dipole at the same energy. This can occur at all wavelengths, but is more efficient at short wavelengths, which is why the sky is more blue (short wavelength) than red (long wavelength).

“In our theoretical study we found that for very high refractive index materials, the contribution of the electric dipole becomes vanishingly small,” explains Luk’yanchuk. “Specifically, we found that the electric dipole mode in small particles of such materials is suppressed by the emergence of another dipole mode, resulting in ultra-weak scattering below the Rayleigh limit. The challenge now is to find or develop materials with a high enough refractive index at the wavelength of interest to suppress Rayleigh scattering.”

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New cloaking technology uses The Beam of Invisibility

A material with random irregularities scatters an incident light wave into all directions.

A new cloaking technology has been developed at TU Wien: a special kind of material is irradiated from above in such a way that another beam of light can pass completely uninhibited.

How do we make an object invisible? Researchers from TU Wien (Vienna), together with colleagues from Greece and the USA, have now developed a new idea for a cloaking technology. A completely opaque material is irradiated from above with a specific wave pattern – with the effect that light waves from the left can now pass through the material without any obstruction. This surprising result opens up completely new possibilities for active camouflage. The idea can be applied to different kinds of waves, it should work with sound waves just as well as with light waves. Experiments are already in the planning.

Outwitting the Scattering of Light

“Complex materials such as a sugar cube are opaque, because light waves inside them are scattered multiple times”, says Professor Stefan Rotter (TU Wien). “A light wave can enter and exit the object, but will never pass through the medium on a straight line. Instead, it is scattered into all possible directions.”

For years many different attempts have been made to outwit this kind of scattering, creating a “cloak of invisibility”. Special materials have been worked out, for example, which are able to guide light waves around an object. Alternatively, also experiments have been performed with objects that can emit light by themselves. When an electronic display sends out exactly the same light as it absorbs in the back, it can appear invisible, at least when looked at in the right angle.

At TU Wien a more fundamental approach has now been chosen. “We did not want to reroute the light waves, nor did we want to restore them with additional displays. Our goal was to guide the original light wave through the object, as if the object was not there at all”, says Andre Brandstötter, one of the authors of the study. “This sounds strange, but with certain materials and using our special wave technology, it is indeed possible.”

The Laser Material

The team at TU Wien has spent years working on optically active materials, which are used for building lasers. To make the laser shine, energy has to be supplied by means of a pump beam. Otherwise, the laser material behaves just like any other material – it absorbs part of the incident light.

“The crucial point is to pump energy into the material in a spatially tailored way such that light is amplified in exactly the right places, while allowing for absorption at other parts of the material”, says Professor Konstantinos Makris from the University of Crete (previously TU Wien). “To achieve this, a beam with exactly the right pattern has to be projected onto the material from above – like from a standard video projector, except with much higher resolution.”

If this pattern perfectly corresponds to the inner irregularities of the material which usually scatter the light, then the projection from above can effectively switch off the scattering, and another beam of light travelling through the material from one side can pass without any obstruction, scattering or loss.

“Mathematically, it is not immediately obvious that it is at all possible to find such a pattern”, says Rotter. “Every object we want to make transparent has to be irradiated with its own specific pattern – depending on the microscopic details of the scattering process inside. The method we developed now allows us to calculate the right pattern for any arbitrary scattering medium.”

Light or Sound

Computer simulations have shown that the method works. Now the idea should be confirmed in experiments. Stefan Rotter is confident that this will be successful: “We are already discussing with experimentalists how this could be done. As a first step, we may test this technology with sound instead of light waves. Experimentally, they are easier to handle, and from a mathematical point of view, the difference does not matter significantly.”

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Discovery increases LED efficiency by 50% and could even lead to invisibility cloaking devices

The inside of the main concourse of the molecular beam epitaxy apparatus, which University of Michigan engineering researchers used to make the advanced nanoparticle-infused gallium nitride semiconductors. The semiconductors could boost LED efficiency by up to 50 percent, and even lead to invisibility cloaking devices.
Image credit: Joseph Xu, Michigan Engineering

In an advance that could boost the efficiency of LED lighting by 50 percent and even pave the way for invisibility cloaking devices, a team of University of Michigan researchers has developed a new technique that peppers metallic nanoparticles into semiconductors.

It’s the first technique that can inexpensively grow metal nanoparticles both on and below the surface of semiconductors. The process adds virtually no cost during manufacturing and its improved efficiency could allow manufacturers to use fewer semiconductors in finished products, making them less expensive.

The metal nanoparticles can increase the efficiency of LEDs in several ways. They can act as tiny antennas that alter and redirect the electricity running through the semiconductor, turning more of it into light. They can also help reflect light out of the device, preventing it from being trapped inside and wasted.

 

The main growth chamber of the molecular epitaxy beam apparatus used to make the nanoparticle-infused gallium nitride semiconductors. The semiconductors could boost LED efficiency by up to 50 percent, and even lead to invisibility cloaking devices.

Image credit: Joseph Xu, Michigan Engineering

The process can be used with the gallium nitride that’s used in LED lighting and can also boost efficiency in other semiconductor products, including solar cells. It’s detailed in a study published in the Journal of Applied Physics.

“This is a seamless addition to the manufacturing process, and that’s what makes it so exciting,” said Rachel Goldman, U-M professor of materials science and engineering, and physics. “The ability to make 3-D structures with these nanoparticles throughout is going to open a lot of possibilities.”

The key innovation

The idea of adding nanoparticles to increase LED efficiency is not new. But previous efforts to incorporate them have been impractical for large-scale manufacturing. They focused on pricey metals like silver, gold and platinum. In addition, the size and spacing of the particles must be very precise; this required additional and expensive manufacturing steps. Furthermore, there was no cost-effective way to incorporate particles below the surface.

 

Former materials science PhD student Sunyeol Jun prepares the molecular beam epitaxy apparatus that’s used to make the nanoparticle-infused gallium nitride semiconductors. The semiconductors could boost LED efficiency by up to 50 percent, and even lead to invisibility cloaking devices.

Image credit: Joseph Xu, Michigan Engineering

Goldman’s team discovered a simpler way that integrates easily with the molecular beam epitaxy process used to make semiconductors. Molecular beam epitaxy sprays multiple layers of metallic elements onto a wafer. This creates exactly the right conductive properties for a given purpose.

The U-M researchers applied an ion beam between these layers—a step that pushes metal out of the semiconductor wafer and onto the surface. The metal forms nanoscale particles that serve the same purpose as the pricey gold and platinum flecks in earlier research. Their size and placement can be precisely controlled by varying the angle and intensity of the ion beam. And applying the ion beam over and over between each layer creates a semiconductor with the nanoparticles interspersed throughout.

“If you carefully tailor the size and spacing of nanoparticles and how deeply they’re embedded, you can find a sweet spot that enhances light emissions,” said Myungkoo Kang, a former graduate student in Goldman’s lab and first author on the study. “This process gives us a much simpler and less expensive way to do that.”

 

A microscopy photo showing an array of precisely placed metallic nanoparticles on the surface of a gallium arsenide semiconductor.

Image courtesy: Rachel S. Goldman, Michigan Engineering

Researchers have known for years that metallic particles can collect on the surface of semiconductors during manufacturing. But they were always considered a nuisance, something that happened when the mix of elements was incorrect or the timing was off.

“From the very early days of semiconductor manufacturing, the goal was always to spray a smooth layer of elements onto the surface. If the elements formed particles instead, it was considered a mistake,” Goldman said. “But we realized that those ‘mistakes’ are very similar to the particles that manufacturers have been trying so hard to incorporate into LEDs. So we figured out a way to make lemonade out of lemons.”

Toward invisibility cloaks
Because the technique allows precise control over the nanoparticle distribution, the researchers say it may one day be useful for cloaks that render objects partially invisible by inducing a phenomenon known as “reverse refraction.”

Gallium arsenide nanoparticles forming on the surface of a semiconductor. Image courtesy: Rachel S. Goldman

Reverse refraction bends light waves backwards in a way that doesn’t occur in nature, potentially directing them around an object or away from the eye. The researchers believe that by carefully sizing and spacing an array of nanoparticles, they may be able to induce and control reverse refraction in specific wavelengths of light.

“For invisibility cloaking, we need to both transmit and manipulate light in very precise ways, and that’s very difficult today,” Goldman said. “We believe that this process could give us the level of control we need to make it work.”

The team is now working to adapt the ion beam process to the specific materials used in LEDs—they estimate that the higher-efficiency lighting devices could be ready for market within the next five years, with invisibility cloaking and other applications coming further in the future.

Learn more: Nanoparticles could spur better LEDs, invisibility cloaks

 

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Invisible sensor can hide from both thermal and electric detection at the same time

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Made of pure copper, the ultra-thin ‘shell’ conceals sensors from remote inspection while still allowing them to probe the exterior environment

A team of researchers from the National University of Singapore (NUS) has invented a novel camouflage technique that effectively hides thermal and electronic sensors without compromising performance. Led by Assistant Professor Qiu Cheng-Wei from the Department of Electrical & Computer Engineering at NUS Faculty of Engineering, the team created the world’s first multifunctional camouflage shell that renders sensors invisible in both thermal and electric environments.

Current technologies which make sensors ‘invisible’ usually also make them ineffective, while others only work in specific physical fields (i.e. either thermal or electrical). Over the past ten months, the NUS team has experimentally demonstrated that they could hide sensors in both thermal and electric fields without them being detected. The invisible sensors are also able to continue to probe on the environment while ‘under cover’.

Asst Prof Qiu explained, “We have designed a camouflage ‘shell’ that not only mimics surrounding thermal fields but also electric fields, both at the same time. The object under camouflage becomes truly invisible as its shape and position cannot be detected in terms of both thermal and electric images.”

In their experiment, they created an ideal invisible sensor by covering it with a thin shell which is made of pure copper. The shell is designed to drastically reduce the perturbation of heat flux and electric current simultaneously. The thickness of the shell is fabricated based on detailed calculations to allow precise manipulation of external multi-physical fields to insulate the sensor and hence render it invisible and yet allows it to receive incoming signals from outside.

“Our camouflaging shell will open up a new avenue for advanced sensing and security systems. Sensors which are used to monitor current and heat flow in strong voltage or high temperature environments are easily damaged. Our camouflaging shell hence protect such sensors from the harsh environment and at the same time enhance the accuracy of the hidden sensor, as the shell will eliminate any distortion around the sensor. This attribute is significant in our study of other applications such as using the camouflaging shell on special mission fieldtrips. The team is also working on developing multifunctional invisible sensors that have instantaneous stealth ability,” added Dr Qiu.

Drawing a comparison with the chameleon, from which the team had drawn inspiration to develop the novel camouflaging shell, Dr Qiu said, “The skin of a chameleon is made up of several layers of specialised cells containing various pigment while the outermost layer is transparent. The cells beneath the skin change colour based on light intensity and temperature as well as the chameleon’s mood. Our team’s invention can be seen as an improved “skin” for the chameleon such that it will become invisible when it appears in front of thermal and electric signal detectors!”

Learn more: NUS Engineering team designs novel multi-field invisible sensor

 

 

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Engineers give invisibility cloaks a slimmer design

An extremely thin cloaking device is designed using dielectric materials. The cloak is a thin Teflon sheet (light blue) embedded with many small, cylindrical ceramic particles (dark blue). Credit: Li-Yi Hsu/UC San Diego.

An extremely thin cloaking device is designed using dielectric materials. The cloak is a thin Teflon sheet (light blue) embedded with many small, cylindrical ceramic particles (dark blue). Credit: Li-Yi Hsu/UC San Diego.

Researchers have developed a new design for a cloaking device that overcomes some of the limitations of existing “invisibility cloaks.”

In a new study, electrical engineers at the University of California, San Diego have designed a cloaking device that is both thin and does not alter the brightness of light around a hidden object. The technology behind this cloak will have more applications than invisibility, such as concentrating solar energy and increasing signal speed in optical communications.

“Invisibility may seem like magic at first, but its underlying concepts are familiar to everyone. All it requires is a clever manipulation of our perception,” said Boubacar Kanté, a professor in the Department of Electrical and Computer Engineering at the UC San Diego Jacobs School of Engineering and the senior author of the study. “Full invisibility still seems beyond reach today, but it might become a reality in the near future thanks to recent progress in cloaking devices.”

As their name implies, cloaks are devices that cover objects to make them appear invisible. The idea behind cloaking is to change the scattering of electromagnetic waves — such as light and radar — off an object to make it less detectable to these wave frequencies.

One of the drawbacks of cloaking devices is that they are typically bulky.

“Previous cloaking studies needed many layers of materials to hide an object, the cloak ended up being much thicker than the size of the object being covered,” said Li-Yi Hsu, electrical engineering Ph.D. student at UC San Diego and the first author of the study, which was recently published in the journal Progress In Electromagnetics Research. “In this study, we show that we can use a thin single-layer sheet for cloaking.”

The researchers say that their cloak also overcomes another fundamental drawback of existing cloaking devices: being “lossy.” Cloaks that are lossy reflect light at a lower intensity than what hits their surface.

“Imagine if you saw a sharp drop in brightness around the hidden object, it would be an obvious telltale. This is what happens when you use a lossy cloaking device,” said Kanté. “What we have achieved in this study is a ‘lossless’ cloak. It won’t lose any intensity of the light that it reflects.”

Many cloaks are lossy because they are made with metal particles, which absorb light. The researchers report that one of the keys to their cloak’s design is the use of non-conductive materials called dielectrics, which unlike metals do not absorb light. This cloak includes two dielectrics, a proprietary ceramic and Teflon, which are structurally tailored on a very fine scale to change the way light waves reflect off of the cloak.

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The reflection pattern from an uncloaked object on a flat surface (top) compared to the reflection pattern of the same object covered with the cloaking device (bottom), which effectively mimics the reflection from a completely flat surface. Credit: Li-Yi Hsu/UC San Diego.

 

 

In their experiments, the researchers specifically designed a “carpet” cloak, which works by cloaking an object sitting on top of a flat surface. The cloak makes the whole system — object and surface — appear flat by mimicking the reflection of light off the flat surface. Any object reflects light differently from a flat surface, but when the object is covered by the cloak, light from different points is reflected out of sync, effectively cancelling the overall distortion of light caused by the object’s shape.

“This cloaking device basically fools the observer into thinking that there’s a flat surface,” said Kanté.

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