First demonstration of a metal-free metamaterial that can absorb electromagnetic energy
Electrical engineers at Duke University have created the world’s first electromagnetic metamaterial made without any metal. The device’s ability to absorb electromagnetic energy without heating up has direct applications in imaging, sensing and lighting.
Metamaterials are synthetic materials composed of many individual, engineered features that together produce properties not found in nature. Imagine an electromagnetic wave moving through a flat surface made of thousands of tiny electrical cells. If researchers can tune each cell to manipulate the wave in a specific way, they can dictate exactly how the wave behaves as a whole.
For researchers to manipulate electromagnetic waves, however, they’ve typically had to use electrically conducting metals. That approach, however, brings with it a fundamental problem of metals—the higher the electrical conductivity, the better the material also conducts heat. This limits their usefulness in temperature-dependent applications.
In a new paper, electrical engineers at Duke University demonstrate the first completely dielectric (non-metal) electromagnetic metamaterial—a surface dimpled with cylinders like the face of a Lego brick that is designed to absorb terahertz waves. While this specific frequency range sits between infrared waves and microwaves, the approach should be applicable for almost any frequency of the electromagnetic spectrum.
The results appeared online on Jan. 9 in the journal Optics Express.
“People have created these types of devices before, but previous attempts with dielectrics have always been paired with at least some metal,” said Willie Padilla, professor of electrical and computer engineering at Duke University. “We still need to optimize the technology, but the path forward to several applications is much easier than with metal-based approaches.”
Padilla and his colleagues created their metamaterial with boron-doped silicon—a non-metal. Using computer simulations, they calculated how terahertz waves would interact with cylinders of varying heights and widths.
A closer look at one of the cylinders comprising a new non-metal metamaterial. The arrows depict how different aspects of an electromagnetic field interact with the cylinder.
The researchers then manufactured a prototype consisting of hundreds of these optimized cylinders aligned in rows on a flat surface. Physical tests showed that the new “metasurface” absorbed 97.5 percent of the energy produced by waves at 1.011 terahertz.
Efficiently absorbing energy from electromagnetic waves is an important property for many applications. For example, thermal imaging devices can operate in the terahertz range, but because they have previously included at least some metal, getting sharp images has been challenging.
“Heat propagates fast in metals, which is problematic for thermal imagers,” said Xinyu Liu, a doctoral student in Padilla’s laboratory and first author of the paper. “There are tricks to isolate the metal during fabrication, but that becomes cumbersome and costly.”
Another potential application for the new technology is efficient lighting. Incandescent light bulbs make light but also create a significant amount of wasted heat. They must operate at high temperatures to produce light—much higher than the melting point of most metals.
“We can produce a dielectric metasurface designed to emit light, without producing waste heat,” Padilla said. “Although we’ve already been able to do this with metal-based metamaterials, you need to operate at high temperature for the whole thing to work. Dielectric materials have melting points much higher than metals, and we’re now quickly trying to move this technology into the infrared to demonstrate a lighting system.”
When a material is made, you typically cannot change whether that material is hard or soft. But a group of University of Michigan researchers have developed a new way to design a “metamaterial” that allows the material to switch between being hard and soft without damaging or altering the material itself.
Metamaterials are man-made materials that get their properties—in this case, whether a material is hard or soft—from the way the material is constructed rather than the material that constructs it. This allows researchers to manipulate a metamaterial’s structure in order to make the material exhibit a certain property.
In the group’s study, published in the journal Nature Communications, the U-M researchers discovered a way to compose a metamaterial that can be easily manipulated to increase the stiffness of its surface by orders of magnitude—the difference between rubber and steel.
Since these properties are “topologically protected,” meaning that the material’s properties come from its total structure, they’re easily maintained even as the material shifts repeatedly between its hard and soft states.
“The novel aspect of this metamaterial is that its surface can change between hard and soft,” said Xiaoming Mao, assistant professor of physics. “Usually, it’s hard to change the stiffness of a traditional material. It’s either hard or soft after the material is made.”
For example, a dental filling cannot be changed after the dentist has set the filling without causing stress, either by drilling or grinding, to the original filling. A guitar string cannot be tightened without putting stress on the string itself, according to Mao.
Mao says the way an object comes in contact with the edge of the metamaterial changes the geometry of the material’s structure, and therefore how the material responds to stress at the edge. But metamaterial’s topological protection allows the inside of the metamaterial remains damage free.
The material could one day be used to build cars or rocket launch systems. In cars, the material could help absorb impacts from a crash.
“When you’re driving a car, you want the car to be stiff and to support a load,” Mao said. “During a collision, you want components to become softer to absorb the energy from the collision and protect the passenger in the car.”
The researchers also suggest the material could be used to make bicycle tires that could self-adjust to ride more easily on soft surfaces such as sand, or to make damage-resistant, reusable rockets.
An electric current will not only heat a hybrid metamaterial, but will also trigger it to change state and fade into the background like a chameleon in what may be the proof-of-concept of the first controllable metamaterial device, or metadevice, according to a team of engineers.
“Previous metamaterials work focused mainly on cloaking objects so they were invisible in the radio frequency or other specific frequencies,” said Douglas H. Werner, John L. and Genevieve H. McCain Chair Professor of electrical engineering, Penn State. “Here we are not trying to make something disappear, but to make it blend in with the background like a chameleon and we are working in optical wavelengths, specifically in the infrared.”
Metamaterials are synthetic, composite materials that possess qualities not seen in natural materials. These composites derive their functionality by their internal structure rather than by their chemical composition. Existing metamaterials have unusual electromagnetic or acoustic properties. Metadevices take metamaterials and do something of interest or value as any device does.
“The key to this metamaterial and metadevice is vanadium dioxide, a phase change crystal with a phase transition that is triggered by temperatures created by an electric current,” said Lei Kang, research associate in electrical engineering, Penn State.
The metamaterial is composed of a base layer of gold thick enough so that light cannot pass through it. A thin layer of aluminum dioxide separates the gold from the active vanadium dioxide layer. Another layer of aluminum dioxide separates the vanadium from a gold-patterned layer that is attached to an external electric source. The geometry of the patterned mesh screen controls the functional wavelength range. The amount of current flowing through the device controls the Joule heating effect, the heating due to resistance.
“The proposed metadevice integrated with novel transition materials represents a major step forward by providing a universal approach to creating self-sufficient and highly versatile nanophotonic systems,” the researchers said in today’s (Oct. 27) issue of Nature Communications.
As a proof of concept, the researchers created a .035 inch by .02 inch device and cut the letters PSU into the gold mesh layer so the vanadium dioxide showed through. The researchers photographed the device using an infrared camera at 2.67 microns. Without any current flowing through the device, the PSU stands out as highly reflective. With a current of 2.03 amps, the PSU fades into the background and becomes invisible, while at 2.20 amps, the PSU is clearly visible but the background has become highly reflective.
The response of the vanadium dioxide is tunable by altering the current flowing through the device. According to the researchers, vanadium dioxide can change state very rapidly and it is the device configuration that limits the tuning.
Researchers have designed a device that uses light to manipulate its mechanical properties. The device, which was fabricated using a plasmomechanical metamaterial, operates through a unique mechanism that couples its optical and mechanical resonances, enabling it to oscillate indefinitely using energy absorbed from light.
This work demonstrates a metamaterial-based approach to develop an optically-driven mechanical oscillator. The device can potentially be used as a new frequency reference to accurately keep time in GPS, computers, wristwatches and other devices, researchers said. Other potential applications that could be derived from this metamaterial-based platform include high precision sensors and quantum transducers. The research was published Oct. 10 in the journal Nature Photonics.
Researchers engineered the metamaterial-based device by integrating tiny light absorbing nanoantennas onto nanomechanical oscillators. The study was led by Ertugrul Cubukcu, a professor of nanoengineering and electrical engineering at the University of California San Diego. The work, which Cubukcu started as a faculty member at the University of Pennsylvania and is continuing at the Jacobs School of Engineering at UC San Diego, demonstrates how efficient light-matter interactions can be utilized for applications in novel nanoscale devices.
Metamaterials are artificial materials that are engineered to exhibit exotic properties not found in nature. For example, metamaterials can be designed to manipulate light, sound and heat waves in ways that can’t typically be done with conventional materials.
Metamaterials are generally considered “lossy” because their metal components absorb light very efficiently. “The lossy trait of metamaterials is considered a nuisance in photonics applications and telecommunications systems, where you have to transmit a lot of power. We’re presenting a unique metamaterials approach by taking advantage of this lossy feature,” Cubukcu said.
The device in this study resembles a tiny capacitor—roughly the size of a quarter—consisting of two square plates measuring 500 microns by 500 microns. The top plate is a bilayer gold/silicon nitride membrane containing an array of cross-shaped slits—the nanoantennas—etched into the gold layer. The bottom plate is a metal reflector that is separated from the gold/silicon nitride bilayer by a three-micron-wide air gap.
When light is shined upon the device, the nanoantennas absorb all of the incoming radiation from light and convert that optical energy into heat. In response, the gold/silicon nitride bilayer bends because gold expands more than silicon nitride when heated. The bending of the bilayer alters the width of the air gap separating it from the metal reflector. This change in spacing causes the bilayer to absorb less light and as a result, the bilayer bends back to its original position. The bilayer can once again absorb all of the incoming light and the cycle repeats over and over again.
The device relies on a unique hybrid optical resonance known as the Fano resonance, which emerges as a result of the coupling between two distinct optical resonances of the metamaterial. The optical resonance can be tuned “at will” by applying a voltage.
The researchers also point out that because the plasmomechanical metamaterial can efficiently absorb light, it can function under a broad optical resonance. That means this metamaterial can potentially respond to a light source like an LED and won’t need a strong laser to provide the energy.
“Using plasmonic metamaterials, we were able to design and fabricate a device that can utilize light to amplify or dampen microscopic mechanical motion more powerfully than other devices that demonstrate these effects. Even a non-laser light source could still work on this device,” said Hai Zhu, a former graduate student in Cubukcu’s lab and first author of the study.
“Optical metamaterials enable the chip-level integration of functionalities such as light-focusing, spectral selectivity and polarization control that are usually performed by conventional optical components such as lenses, optical filters and polarizers. Our particular metamaterial-based approach can extend these effects across the electromagnetic spectrum,” said Fei Yi, a postdoctoral researcher who worked in Cubukcu’s lab.