Metamaterials

Worlds Smallest LED is a Single Molecule

By coaxing light out of a single polymer molecule, researchers have made the world’s tiniest light-emitting diode.

This work is part of an interdisciplinary effort to make molecular scale electronic devices, which hold the potential for creating smaller but more powerful and energy-efficient computers. Guillaume Schull and his colleagues at the University of Strasbourg in France made the device with the conducting polymer polythiophene. They used a scanning tunneling microscope tip to locate and grab a single polythiophene molecule lying on a gold substrate. Then they pulled up the tip to suspend the molecule like a wire between the tip and the substrate.

The researchers report in the journalPhysical Review Letters that when they applied a voltage across the molecule, they were able to measure a nanoampere-scale current passing through it and to record light emitted from it.

(via First Single-Molecule LED - IEEE Spectrum)

A Cloak of Invisibility

Just like in Harry Potter, invisibility in reality is optical-camouflage rather than complete disappearance. Humans are able to see by detecting and processing photons that bounce off objects, and if these are interfered with then objects can seem to “disappear”. The technology is far from perfect, but scientists are making marked steps towards invisibility in many ways. One method uses heated carbon nanotubes to bend light like a desert mirage, while another method uses the principles of green screen, filming and projecting what’s behind an object—but neither is very practical yet. A more feasible method involves the use of metamaterials, which are artificial structures smaller than the wavelength of light, designed to interact with electromagnetic waves—so they can guide light around an object. Scientists have previously only been successful with metamaterials in two dimensions, but now researchers at the University of Texas have successfully “cloaked” a 3-D object (an 18 cm cylindrical tube) standing in free space, making it invisible from any angle. This was achieved by coating the cylinder with plasmonic metamaterials, which cancel out the light bouncing off and render the cylinder invisible—but there’s a catch. The object has only been cloaked from high-frequency wavelengths such as microwaves, and the spectrum of visible light is currently not the researchers’ first priority—they’re more interested in using this technology to improve biomedical imaging. I guess we’ll have to wait a little longer for our invisibility cloaks.

Read the UT team’s original paper in the New Journal of Physics

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Metamaterial Mechanisms

Fabrication research from Hasso Plattner Institute is process to give single 3D printed objects elastic mechanical properties:

Recently, researchers started to engineer not only the outer shape of objects, but also their internal microstructure. Such objects, typically based on 3D cell grids, are also known as metamaterials. Metamaterials have been used, for example, to create materials with soft and hard regions.

So far, metamaterials were understood as materials—we want to think of them as machines. We demonstrate metamaterial objects that perform a mechanical function. Such metamaterial mechanisms consist of a single block of material the cells of which play together in a well-defined way in order to achieve macroscopic movement. Our metamaterial door latch, for example, transforms the rotary movement of its handle into a linear motion of the latch. Our metamaterial Jansen walker consists of a single block of cells—that can walk. The key element behind our metamaterial mechanisms is a specialized type of cell, the only ability of which is to shear.

In order to allow users to create metamaterial mechanisms efficiently we implemented a specialized 3D editor. It allows users to place different types of cells, including the shear cell, thereby allowing users to add mechanical functionality to their objects. To help users verify their designs during editing, our editor allows users to apply forces and simulates how the object deforms in response. 

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New Laser Fabrication Techniques Unlock the Incredible Potential of Metamaterials

Researchers have long believed that it would someday be possible to produce artificial materials, or “metamaterials,” and that they would bring about some stunning, otherworldly technologies—the sort that have figured in science fiction tales for years. These innovations include invisibility cloaks that could mask the presence of objects or their electromagnetic signatures, “unfeelability cloaks” that could mechanically mask the tactile feel of an object, superlenses that could resolve features too small to be seen with ordinary microscope lenses, and power absorbers that could capture essentially all of the sunlight hitting a solar cell.

To achieve these advances we’ll need better metamaterials, and those are on the way. Metamaterials are made up of “meta-atoms”—small two- or three-dimensional structures made of polymer, dielectric material, or metal. When these structures are arranged in regular, repeating crystals, they can be used to manipulate electromagnetic radiation in new ways. Ultimately, the capabilities of a metamaterial are determined by the size, shape, and quality of these structures. And the technology to fabricate meta-atoms has recently turned a corner.

Over the past few years, research groups around the world have succeeded in developing a way to draw meta-atoms using lasers. The resulting structures can now take on nearly any shape and be stacked in three dimensions in dense, crystal-like arrangements. What’s more, they can be made small enough to exhibit unique mechanical and thermal properties and to alter the flow of light in a range of wavelengths—including the long-inaccessible visible chunk of the spectrum. Thanks to this microscopic fabrication technology, we can finally see a path beyond the materials nature has provided us into entirely new realms that are limited only by our imaginations.

(via How to Make a Better Invisibility Cloak—With Lasers - IEEE Spectrum)

THE INVISIBLE BUILDING

In 2009, University of Liverpool researchers theorized that earthquakes’ seismic waves could be deflected from buildings with the use of underground metamaterials. Metamaterials are natural materials that are assembled in a certain pattern. The patterns are based off of calculations for specific wavelengths of certain waves. The patterns help the materials to either “deflect”, “bend”, or “dissipate” its targeted wavelengths. Metamaterials have already proved their usefulness by assisting submarines to hide from sonar, airplanes from radars, and certain locations from laser beams. Each metamaterials are differently patterned.

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newscientist.com
US army calls for ideas on invisible uniforms for soldiers
The US army has said it wants invisibility cloaks for its soldiers within 18 months. How realistic is that?

“The US army wants invisibility cloaks for its soldiers. Not just that – it has announced that it wants to test the best contenders within the next 18 months. Seems a bit unrealistic? Well, we may not be as far away as you think. In 2006, John Pendry, a theoretical physicist at Imperial College London, showed that it should be possible to bend light around an object and hide itusing metamaterials – structures engineered at microscopic levels to channel electromagnetic waves. Since then, many devices trumpeted as invisibility cloaks have been described, but they only work in the lab with specific wavelengths or from certain angles.Now the US army has made a call for proposals from companies for wearable camouflage with a chameleon-like ability to change according to the background. So how will they manage this? Metamaterials are probably the best solution: previous efforts in this field using technology like LEDs were hampered by power and computing requirements. But although they can bend light, metamaterials cannot make things disappear completely. “Complete invisibility of macroscopic objects for all visible colours is fundamentally impossible,” says Martin Wegener of the Karlsruhe Institute of Technology in Germany. His team has created cloaks from photonic crystals that work for certain wavelengths, but bending light over the entire spectrum is forbidden by relativity. “This means that you may see less of something at a particular colour, but see it at all other colours,” says Wegener.The wearer would be effectively transparent at some wavelengths but not all, rendering them as a coloured shadow or ghost image.”

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New Invisibility Cloak Augments Metamaterial Base with Electronics to Improve Performance

Alù’s proposed design consists of a conventional metamaterial base, but with CMOS negative impedance converters (NICs) placed at the corner of each metamaterial square.

A NIC is an interesting electronic component that adds negative resistance to a circuit, injecting energy rather than consuming it. NICs are not widely used as we’re not entirely sure how to use them.

Alù seems to propose that by interspersing NICs (which must be powered) with the metamaterial, multiple frequencies can be cloaked. In the image above, you can see a standard metamaterial cloak (blue), vs Alù’s metamaterial-and-NIC cloak (green).

Alù’s proposed cloak is invisible over a large range of frequencies, while a standard passive cloak is only invisible for a small range, and more visible than non-cloaked devices in other ranges.

(via New invisibility cloak combines metamaterials and fancy electronics to be thinner, lighter, more invisible | ExtremeTech)

Metamaterials Twist Sound

A Chinese-U.S. research team is exploring the use of metamaterials — artificial materials engineered to have exotic properties not found in nature — to create devices that manipulate sound in versatile and unprecedented ways.

In the journal Applied Physics Letters, which is produced by AIP Publishing, the team reports a simple design for a device, called an acoustic field rotator, which can twist wave fronts inside it so that they appear to be propagating from another direction.

Read more: http://www.laboratoryequipment.com/news/2014/03/metamaterials-twist-sound

Magnetic polaron imaged for the first time

Researchers at Aalto University and Lawrence Berkeley National Laboratory have demonstrated that polaron formation also occurs in a system of magnetic charges, and not just in a system of electric charges. Being able to control the transport properties of such charges could enable new devices based on magnetic rather than electric charges, for example computer memories.

Polarons are an example of emergent phenomena known to occur in condensed matter physics. For instance, an electron moving across a crystal lattice displaces the surrounding ions, together creating an effective quasi-particle, a polaron, which has an energy and mass that differs from that of a bare electron. Polarons have a profound effect on electronic transport in materials.

Artificial spin ice systems are metamaterials that consist of lithographically patterned nanomagnets in an ordered two-dimensional geometry. The individual magnetic building blocks of a spin ice lattice interact with each other via dipolar magnetic fields.

Researchers used material design as a tool to create a new artificial spin ice, the dipolar dice lattice.

‘Designing the correct two-dimensional lattice geometry made it possible to create and observe the decay of magnetic polarons in real-time,’ says postdoctoral researcher Alan Farhan from Lawrence Berkeley National Laboratory (USA).

‘We introduced the dipolar dice lattice because it offers a high degree of frustration, meaning that competing magnetic interactions cannot be satisfied simultaneously. Like all systems in nature, the dipolar dice lattice aims to relax and settle into a low-energy state. As a result, whenever magnetic charge excitations emerge over time, they tend to get screened by opposite magnetic charges from the environment,’ explains Dr. Farhan.

The researchers at Berkeley used photoemission electron microscopy, or PEEM, to make the observations. This technique images the direction of magnetization in individual nanomagnets. With the magnetic moments thermally fluctuating, the creation and decay of magnetic polarons could be imaged in real space and time. Postdoctoral researcher Charlotte Peterson and Professor Mikko Alava at Aalto University (Finland) performed simulations, which confirmed the rich thermodynamic behavior of the spin ice system.

‘The experiments also demonstrate that magnetic excitations can be engineered at will by a clever choice of lattice geometry and the size and shape of individual nanomagnets. Thus, artificial spin ice is a prime example of a designer material. Instead of accepting what nature offers, it is now possible to assemble new materials from known building blocks with purposefully designed functionalities,’ says Professor Sebastiaan van Dijken from Aalto University.

‘This concept, which goes well beyond magnetic metamaterials, is only just emerging and will dramatically shape the frontier of materials research in the next decade,’ adds Professor van Dijken.

Research article:
Alan Farhan, Andreas Scholl, Charlotte F. Petersen, Luca Anghinolfi, Clemens Wuth, Scott Dhuey, Rajesh V. Chopdekar, Paula Mellado, Mikko J. Alava & Sebastiaan van Dijken.
Thermodynamics of emergent magnetic charge screening in artificial spin ice.
Nature Communications 7 (2016). http://dx.doi.org/10.1038/ncomms12635

seas.harvard.edu
A chameleon in the physics lab | Harvard School of Engineering and Applied Sciences

LOOKING COOLER WHEN HEATED, A THIN COATING TRICKS INFRARED CAMERAS

Cambridge, Mass. – October 21, 2013 – Active camouflage has taken a step forward at the Harvard School of Engineering and Applied Sciences (SEAS), with a new coating that intrinsically conceals its own temperature to thermal cameras.

In a laboratory test, a team of applied physicists placed the device on a hot plate and watched it through an infrared camera as the temperature rose. Initially, it behaved as expected, giving off more infrared light as the sample was heated: at 60 degrees Celsius it appeared blue-green to the camera; by 70 degrees it was red and yellow. At 74 degrees it turned a deep red—and then something strange happened. The thermal radiation plummeted. At 80 degrees it looked blue, as if it could be 60 degrees, and at 85 it looked even colder. Moreover, the effect was reversible and repeatable, many times over.

Invisibility cloak with photonic crystals

Metamaterials made from metal elements initially proposed for constructing invisibility cloaks, did not solve some important cloaking problems.

There are three challenges remaining. The first is controlling anisotropy – the variable behavior of propagating waves in different directions of the cloak medium. It’s also important to make sure that the cloak materials can operate at microwave and optical wave frequencies. Finally, researchers have to decrease losses that restrict the size of hidden objects.

Elena Semouchkina, an associate professor of electrical and computer engineering, and her graduate students have developed several novel approaches to making invisibility cloaks more practical. Their latest work, published in a special issue of Journal of Optics on transformation optics, looks at a promising new way to manipulate electromagnetic waves to make objects appear invisible. The team developed an approach using photonic crystals.

Read more.

New Chip Design Fuses Electronics and Photonics By Combining Silicon and Graphene

Physicist Swastik Kar and mechanical engineer Yung Joon Jung lay belts of carbon nanotubes on top of a silicon wafer. The junction created by the intersection of the two materials proved to be highly sensitive to light; shining a laser spot on it caused a sharp rise in the light-induced current. That allowed the pair to build logic circuits that could be manipulated both electrically and optically.

“What we’ve done is built a tiny device where one input can be a voltage and the other input can be light,” Kar says. The researchers built an optoelectronic AND gate and a two-bit optoelectronic ADDER/OR gate. They also built a four-bit digital-to-analog converter. Shining spots of light onto an array of these junctions converts the digital signal of the laser into an analog current, with the strength of the current depending on the on/off pattern of the laser.

Jung creates the nanotubes in solution, and they can then be placed on a patterned silicon/silicon oxide substrate, so the technology should be compatible with existing CMOS processes, he says. The process should also be reproducible and scalable to large numbers of junctions.

Using light to both move data around a chip and perform some of the logic operations should save time and make the chip work faster, according to the pair. Just how much faster they can’t say yet, as this is only an early step toward an actual chip.

(via Nanotubes Make Logic Circuits that Use Both Light and Current - IEEE Spectrum)

Year of Light: Now you see it, now you don’t.

Like a Klingon Bird-of-Prey or Harry Potter’s cape, cloaking technology makes the visible become seemingly invisible.

But such technology is not limited to fiction. In recent years, researchers have developed new ways in which light can move around and even through a physical object – making it invisible to parts of the electromagnetic spectrum and undetectable by most sensors.

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Materials are lefties too

Southpaw: Barack Obama, surely the world’s most famous left-hander. 

Leonardo Da Vinci, Isaac Newton, Marie Curie and Alan Turing – there’s a trait that all four trailblazers share apart from their remarkable influence on science, and today we celebrate it. Southpaw throw your hand up with pride – it’s Left Handers Day.

10% of the human population are left-handed – a club that boasts four of the last five US Presidents. The American inventor Christopher Latham Sholes made life a little easier for his fellow lefties when he invented the typewriter – a way to write that eliminated the left-hander’s pen-and-paper fist smudge dilemma. And Fidel Castro is a man of the left in more ways than one.

But it’s not only people that can be left-handed. In fact, you don’t even need hands to be left-handed. Stay with me… I’m talking about left-handed materials, otherwise known as negative index metamaterials (NIMs). This is Materials World, after all.

Refraction: The usual, positive refractive index – show with a ray of light refracted by a plastic block

The Russian physicist Victor Veselago (writing hand unknown) coined the term ‘left-handed material’ in 1967. Most of us will know a little about refraction – the bending of light as it crosses the interface between two materials. It’s a fundamental principle behind optical devices such as camera and microscope lenses – complex optical equipment is designed with materials carefully shaped to refract light in ways that focus and manipulate it as desired.

Every material, including air, has a refractive index – the measurement of how light or any other radiation propagates through it. When any electromagnetic wave (not only visual light) traverses an interface to another material with a differing refractive index, the angle of its trajectory changes.

Negative refractive index: The basic principle of negative refraction.

Glass, air, and all other natural transparent media that we know, have a positive refractive index, which means that light bends, to a different degree, in the same direction across all of them – the same direction as the flow of energy.

Veselago imagined materials with both negative electric permittivity and negative magnetic permeability – the parameters that describe how materials polarise in the presence of electric and magnetic fields. He theorised that a material with such properties would have a negative index, and 33 years later David R. Smith and a team of researchers at the University of California proved his prediction correct, creating the world’s first left-handed material.

What’s the use a left-handed material? For a start, as Eoin Redahan recently wrote, there’s the prospect of invisibility cloaks, which would work by using NIMs to redirect light.

Sir John Pendry: Pioneer of superlens theory. Wikipedia Commons.

Another key strand of research is the superlens  – a lens that can capture detail beyond those possible with materials that have a positive refractive index. This ‘perfect’ lens was first theorised by Sir John Pendry in 1999, and while engineering obstacles still need to be overcome to make it a reality, research is ongoing to develop a lens with substantially higher resolution capabilities than the microscope.

For more on the superlens, watch this video from the Institute of Physics.

So, lefties, enjoy the day in the knowledge that you can claim not only Eminem, Pele, Morgan Freeman, Robert De Niro, Angelina Jolie, Lady Gaga, Bill Gates, Whoopi Goldberg and Ross Kemp as your own, but a whole class of wave-bending-invisibility-enabling-microscopy-revolutionising wonder materials too. I was feeling a bit ‘left’ out, until I read that us right-handers can join in by shifting our allegiance for the day. I don’t think it’s going to stick.