Researchers Create Terahertz Magnetism From Non-magnetic Materials

"Creating a magnetic activity at the edge of optical frequencies is the first milestone toward realizing optical magnetism, which is not found in natural materials due to the lack of a magnetic monopole," said project leader Xiang Zhang, a professor in the UCLA Henry Samueli School of Engineering and Applied Science. "It will allow us to begin developing materials and devices that operate in the gap between optical frequencies and microwave frequencies. It opens the door to new applications in areas such as medicine, bio-sensing and security imaging."

The field of metamaterials is essentially based on designer's physics — researchers design and create new materials with a set of desired physical properties that do not exist in nature. By manipulating the structures, scientists can create materials with properties not found in the parent material. Recent advances in this field made it possible for Zhang's team to construct a system that exhibits magnetic properties at higher frequencies.

"The range of materials to be engineered is unlimited, despite the relatively small number of elements found in nature," Zhang said.

There has been growing interest in the possibility of applications operating at higher frequencies in biological and security imaging, biomolecular fingerprinting, and remote sensing and guidance in zero-visibility weather. Materials that exhibit a magnetic response at terahertz (THz) and optical frequencies are rarely found in nature, but Zhang's metamaterial bridges this gap. It exhibits magnetic activity that is wide bandwidth and tunable throughout THz frequencies.

"At higher frequencies, it would be possible to develop new tools for security or medical imaging," Zhang said. "The tools would become smaller, and could also detect organic threats such as anthrax or plastic knives that current security methods, such as X-ray machines, can't identify. We're not there yet, but we're getting closer."

The breakthrough is the culmination of four years of collaborative research at UCLA, UCSD and Imperial College. Funded by the Office of Naval Research and the U.S. Defense Advanced Research Projects Agency MURI program, the UCLA researchers initiated the project, which is based on theories proposed by their colleague at Imperial College.

The magnetic activity of natural materials tends to fade away at higher frequencies, making it difficult to sustain magnetism at optical frequencies. To address this, the research team developed a structure that extends the frequency range of metamaterials by more than two orders of magnitude.

The new properties were created by opening a gap that allows the structure to resonate at higher frequencies. By mimicking the magnetic effect at a much smaller scale, the researchers were able to create magnetic activity at nearly optical frequencies using common non-magnetic materials such as copper.

The split ring resonators that make up the periodic array were fabricated using a unique self-aligned microfabrication technique called photo-proliferate-process. UCLA researchers are among the first to develop and demonstrate successfully the use of this technique, which produces a well-defined shape with sharp edges and a very high filling density.

The team also discovered that by adjusting the parameters of the split ring resonators, they could tune the bandwidth of the magnetic response to a specific frequency.

"Designing THz or optical devices and components has many challenges," Zhang said. "Our work provides a new foundation for materials selection and device design, and we think it has the potential to enable an entirely new array of applications."

Before researchers can realize the full potential of applications operating at these higher frequencies, they must address such challenges as the limits of current nano-fabrication techniques and electron scattering on the surface of the materials.

The recently established National Science Foundation Nano-scale Science and Engineering Center headed by Zhang at UCLA is bringing new approaches to solving these problems. The Center for Scalable and Integrated Nano Manufacturing is developing novel nano‑manufacturing technologies and tools that will enable cost-effective nano-devices and systems.