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.

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.”