Graduate Student Bogdan Popa Puts Exotic Metamaterials to Action

When communism fell in Romania 20 years ago, it was as if the people had moved from jail to a jungle, according to Bogdan Popa, a Romanian citizen and recent graduate of Duke University's Pratt School of Engineering. Afterwards, he said the biggest change was that people "could leave the country and visit other countries. They had freedom to move, and you could say anything you wanted." Popa took advantage of his newfound mobility some years later, setting out for the United States in pursuit of a doctorate degree in electrical and computer engineering. Now, with his dissertation recently completed, he is well on his way to expanding the possibilities afforded by metamaterials—exotic composites perhaps best known for making an invisibility cloak for microwaves possible, although imperfectly. That cloaking discovery was made by Pratt's David R. Smith in collaboration with Steven Cummer, Popa's advisor at Duke, and others, making the Pratt School about the best place to be for metamaterials research in Popa's estimation. As a result, he has decided to stay on at Pratt and continue his work for another year or two as a postdoctoral researcher.

The Road to Graduate School

As a high school student in Romania, Popa focused his studies on math and science. He said he didn't think about engineering, but liked computers. Not wanting to spend his days sitting in front of one, however, he opted for a different path. He majored as an undergraduate in Bucharest in a subject that combined elements of electrical and computer engineering and computer science. Immediately after completing his bachelor's degree, he made the move overseas to pursue an interest in research. In Romania, a funding shortage makes research opportunities scarce, he noted. After stepping off the plane into a hot and humid North Carolina at midnight, Popa said he "didn't have time to wonder" what he had gotten himself into. The following day, he had an exam to test his proficiency in English. As a result of the jetlag, he failed the test he said he otherwise would have passed. But the setback proved to be a blessing in disguise. He was required to take an English writing course that he credits with improving his papers.

Unlike most graduate students, Popa began his studies at Duke before he had selected a lab and adviser. He spent the first year taking classes and then his qualifying exams. After passing his qualifiers, he said that a few professors, including Cummer, were interested in welcoming him to their labs.

He said he liked Cummer's style right away and was intrigued by a new project in the emerging field of metamaterials. "In 2003, it was a really new area," Popa said. "People didn't know much about them. Some still didn't even believe in them."

On a Metamaterials Mission

Popa's first project was to see if indeed metamaterials could really be made "left-handed," meaning that they reverse the usual properties of electromagnetic waves. That quality, also known as negative refractive index, had first been demonstrated by Smith in 2000, before he joined the faculty at Duke. But at the time, some scientists maintained that such a phenomenon would break the laws of physics. Popa built his own version of a negative index metamaterial "to figure out if they worked," and they did. He examined how electromagnetic waves passed through the material and found that they began to flow in the opposite direction. It's similar to seeing the waves in the ocean start moving away from the shore, he explained.

He was particularly amazed that his metamaterial performed almost exactly as expected based on computer simulations he had run earlier. He also found that the new materials had other unique properties. For example, rather than decaying exponentially, waves inside the metamaterial actually grew. That ability could lead to a "perfect lens" able to focus on objects no matter how small they are, Popa said. But the passive metamaterials, made up of split rings and wires in precise patterns, do have some drawbacks, he said. They have significant losses, meaning that they absorb some of the electromagnetic energy from the waves with which they interact. They also only work within a narrow range of frequencies. Such limitations could be a stumbling block for the development of an invisibility cloak that would work well enough for a real application.

In a step toward solving the problem, Popa and Cummer recently reported in Microwave and Optical Technology Letters the development and validation of a design for an active metamaterial that absorbs little to no energy. The individual "cells" in the active metamaterial include an electromagnetic field sensing element, amplifier and a driven element that produces the metamaterial's response. "We presented an idea for how one can embed active elements in a metamaterial particle, and we showed in a proof-of-concept experiment that this architecture can in principle eliminate losses and give one much more control over the bandwidth of the resulting metamaterial," Cummer explained.

With that discovery in hand, Popa said, "I'm optimistic about the future for metamaterials. With passive structures, I don't know that commercial applications would be possible. But, if you could make an invisibility cloak really work using active metamaterials, you can imagine the impact."