UMD CMNS Physics S1 Color

New Material Becomes Invisible to Microwave Radiation with the Flip of a Switch

University of Maryland physicists have developed a new cloaking material that can become transparent to microwave radiation with the flip of a switch. Because many wireless communication devices rely on microwaves, the new material could be used to design more efficient communications networks. Additionally, the material has unique properties that could help bridge the gap between modern digital computers and next-generation “quantum” computers.

The new material can be selectively tuned to respond to a wide range of microwave wavelengths, making it more versatile than many previous attempts at cloaking technology. The achievement is described in a paper published on December 18, 2015 in the journal Physical Review X. The UMD researchers teamed with HYPRES, an advanced electronics company based in Elmsford, NY, on the project.

“Prior to this work, other cloaking materials were only effective at a single wavelength, which is not realistically useful,” said Daimeng Zhang, lead author of the study and a graduate student in electrical and computer engineering at UMD. “Our material is transparent across a broad range of microwave wavelengths. Also, we can turn the microwave transparency on and off. This hasn’t been explored before.”

This schematic illustrates a new self-cloaking metamaterial developed by University of Maryland physicists in collaboration with HYPRES, Inc. An array of miniature devices (circular structures) called rf-SQUIDs can become transparent to microwave radiation with the flip of a switch. At left, the material is in transparent mode and allows microwaves to travel freely. At right, the material is in opaque mode and prevents microwaves from traversing the barrier. Image credit: Sean Kelley/JQI (Click image to download hi-res version.)

This schematic illustrates a new self-cloaking metamaterial developed by University of Maryland physicists in collaboration with HYPRES, Inc. An array of miniature devices (circular structures) called rf-SQUIDs can become transparent to microwave radiation with the flip of a switch. At left, the material is in transparent mode and allows microwaves to travel freely. At right, the material is in opaque mode and prevents microwaves from traversing the barrier. Image credit: Sean Kelley/JQI (Click image to download hi-res version.)

The cloaking material developed at UMD cannot make other objects (or people) disappear. Instead, by selectively becoming transparent to microwave radiation, it can either shield or expose a target to incoming microwaves. For this reason, the researchers use the terms “auto-cloaking” or “self-cloaking” to describe the material.

“In that sense, our material could be said to work in reverse. When the transparency is turned on, any object behind it is visible to microwave detection,” said Steven Anlage, senior author of the study and a professor of physics at UMD. “But when the transparency is turned off, the material becomes a barrier and conceals anything behind it. It’s a good hider.”

The cloaking material is considered a metamaterial, or a “smart” material engineered to have properties not found in nature. Metamaterials are made of an array of “meta-atoms,” which are not atoms in the true chemical sense, but rather the smallest component parts of a metamaterial. The meta-atoms used in the UMD cloaking material are tiny devices—not much wider than a human hair—called Radio Frequency Superconducting QUantum Interference Devices (rf-SQUIDs). Each rf-SQUID exhibits the same properties as the metamaterial, meaning that the technology theoretically can be scaled up to any size.

“Previous attempts at cloaking technology could only respond to one wavelength,” said Melissa Trepanier, a co-author of the study and a graduate student in physics at UMD. “Perhaps more importantly, the wavelength could not be changed after the material was created. This meant that engineers needed to decide on—and commit to—a target wavelength prior to the design and construction phase.”

The UMD researchers solved this problem by designing the rf-SQUIDs with properties that can be tuned by varying the magnetic field applied to the material and/or the temperature of the material.

Zhang, Trepanier and Anlage co-authored the study with Oleg Mukhanov, chief technology officer of HYPRES. The research was supported by the National Science Foundation’s Grant Opportunities for Academic Liaison with Industry (GOALI) program. The GOALI program is designed to fund high-risk/high-reward research projects and enhance collaborations between academic scientists and industry.

Beyond its use for cloaking, the rf-SQUID-based metamaterial might help solve other technological challenges, including the implementation of quantum computers.

“HYPRES is very interested in the interface between quantum computing and classical digital computing, so we are looking for new technology capable of connecting the two,” Mukhanov said. “This new metamaterial has properties that are sensitive to both quantum processes and superconducting digital logic, so it would most likely be cross-compatible.”

“We’re working on the edges of what anyone has done before,” Anlage added. “It’s wild stuff, but there is a lot of potential to help develop cool new technology.”

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This work was supported by the National Science Foundation through the Grant Opportunities for Academic Liaison with Industry (GOALI) program (Award No. ECCS-1158644). The content of this article does not necessarily reflect the views of this organization.

The research paper, “Tunable Broadband Transparency of Macroscopic Quantum Superconducting Metamaterials,” Daimeng Zhang, Melissa Trepanier, Oleg Mukhanov and Steven Anlage, was published on December 18, 2015 in the journal Physical Review X.

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UMD Discovery Could Enable Portable Particle Accelerators

Conventional particle accelerators are typically big machines that occupy a lot of space. Even at more modest energies, such as that used for cancer therapy and medical imaging, accelerators need large rooms to accommodate the required hardware, power supplies and radiation shielding.

A new discovery by physicists at the University of Maryland could hold the key to the construction of inexpensive, broadly useful, and portable particle accelerators in the very near future. The team has accelerated electron beams to nearly the speed of light using record-low laser energies, thus relieving a major engineering bottleneck in the development of compact particle accelerators. The work appears in the November 6, 2015 issue of the journal Physical Review Letters.

“We have accelerated high-charge electron beams to more than 10 million electron volts using only millijoules of laser pulse energy. This is the energy consumed by a typical household lightbulb in one-thousandth of a second.” said Howard Milchberg, professor of Physics and Electrical and Computer Engineering at UMD and senior author of the study. “Because the laser energy requirement is so low, our result opens the way for laser-driven particle accelerators that can be moved around on a cart.”

This schematic illustrates the laser-driven electron accelerator experiment at the University of Maryland. The three images at the top directly depict three key phases of the process. At left, a laser pulse is directed into a dense jet of hydrogen gas, where it ionizes the gas to form a plasma and initiates an effect called relativistic self-focusing. (See left inset.) Electrons within the plasma are rapidly accelerated to nearly the speed of light, which produces a brief, intense flash of visible light. (See middle inset.) The accelerated ultra-short bunch of electrons continues to gain energy and then exits the plasma, where it produces intense radiation that can be used for ultra-fast, high-energy imaging applications. (See right inset.) Image credit: Howard Milchberg/George Hine (Click image to download hi-res version.)

This schematic illustrates the laser-driven electron accelerator experiment at the University of Maryland. The three images at the top directly depict three key phases of the process. At left, a laser pulse is directed into a dense jet of hydrogen gas, where it ionizes the gas to form a plasma and initiates an effect called relativistic self-focusing. (See left inset.) Electrons within the plasma are rapidly accelerated to nearly the speed of light, which produces a brief, intense flash of visible light. (See middle inset.) The accelerated ultra-short bunch of electrons continues to gain energy and then exits the plasma, where it produces intense radiation that can be used for ultra-fast, high-energy imaging applications. (See right inset.) Image credit: Howard Milchberg/George Hine (Click image to download hi-res version.)

“As an unexpected bonus, the accelerator generates an intense flash of optical light so short that we believe it represents only one-half of a wavelength cycle,” Milchberg added. These ultrashort light flashes could lead to the development of optical strobe lights that can capture the motion of electrons as they swarm across their atomic orbits—a potentially important development for materials science and nanotechnology.

The UMD team began with a technique known as laser-driven plasma wakefield acceleration and pushed it to the extreme. Generally speaking, the approach works by shooting a laser pulse into plasma, which is a gas (in this case, hydrogen) that has been fully ionized to remove all the electrons from the gas atoms. An intense laser pulse can create a plasma wake that follows the laser, much like the water wake that trails a speedboat. A bunch of electrons following the initial laser pulse can “surf” the waves of this wake, accelerating to nearly the speed of light in millionths of a meter.

“Unless your laser pulse can induce the plasma wake in the first place—and it takes a very intense pulse to do that—you’re out of luck,” Milchberg explained. Prior efforts needed much bigger laser energies to accomplish this effect. So Milchberg and his team tried a different approach, instead forcing the plasma itself to transform a weak laser pulse into a very intense one.

When a laser pulse passes through plasma, the laser causes the electrons to wiggle back and forth in the laser field. The electrons in the center experience the most intense part of the beam, so they wiggle the fastest. As they do they become more massive, as dictated by Einstein’s law of relativity, which says that faster objects must increase in mass. The result is that the center of the beam—where the electrons become heaviest—slows down compared to the outer parts of the beam. This causes the beam to self-focus, gaining intensity as it collapses, finally generating a strong plasma wake. This effect is known as relativistic self-focusing, and becomes more pronounced as the plasma density increases.

The UMD team took advantage of this self-focusing effect, drastically increasing the density of the plasma to as much as 20 times that used in typical experiments. In the process, they dramatically reduced the laser pulse energy needed to initiate relativistic self-focusing and thereby generate a strong plasma wake.

“If you increase the plasma density enough, even a pipsqueak of a laser pulse can generate strong relativistic effects,” Milchberg added.

“From a practical standpoint, the key difference here is the footprint of the accelerator. What once required a room full of equipment and a very powerful laser could eventually be done with a small machine on a movable cart, with a standard wall-socket plug,” said Andrew Goers, a graduate student in Physics at UMD and the study’s lead author. “We started with a very powerful laser and found that we were able to keep dialing the energy back. Eventually we got down to about 1 percent of the laser’s peak energy, but we were still seeing an effect. We were blown away by this.”

The UMD laser-driven accelerator produces a beam of electrons and radiation, including gamma rays, which can be used for safe medical imaging and other applications without the need for significant levels of radiation shielding outside the beam path. The secondary effect of bright, extremely brief flashes of light is the result of the initial accelerations of electrons within the plasma wake, as they are accelerated from rest to almost the speed of light in less than 1 millionth of a meter.

“Such a violent acceleration means they radiate like crazy,” Goers said. “As much as 3 percent of the initial laser radiation is emitted in the flash in a millionth of a billionth of a second.”

In terms of sheer acceleration, laser-driven particle accelerators have a long way to go before they are ready for applications in high-energy physics, where facilities such as Fermilab and CERN reign supreme. But for more immediate applications, such as ultra-fast medical and scientific imaging, the main barriers to laser-driven acceleration are cost, complexity, and portability.

“We may have found a solution to overcome all three of these barriers,” Milchberg said.