Metamaterial plumbs terahertz region

SRRs function as artificial atoms in a metamaterial, but are actually constructed from concentric planar copper rings. By shrinking a microwave (millimeter wavelengths) SRR from 5 millimeters to 50 microns, researchers said Friday (March 5)they demonstrated a magnetic response that bridges the terahertz gap (micron wavelengths), thereby opening the door to solid-state sensors that can see through solid objects.

"Many materials appear transparent in the terahertz range, and by using two different frequencies — say half a terahertz and 1 terahertz — you get very good contrast between similar materials, making bridging the terahertz gap important for automated inspection, zero visibility navigation, biomedical imaging and security applications," said Willie Padill, doctoral candidate in the laboratory of University of California at San Diego (UCSD) professor of physics Dimitri Basov.

"What we have demonstrated here is that metamaterials are a natural for terahertz sensors," Padill added.

In the terahertz gap — roughly between the wavelengths of 1 and 300 microns (or from 100 GHz and 30 THz) — neither solid-state optical nor silicon solutions exist today. The region is too fast for silicon, which peaks out above 100 GHz (300 microns wavelength), but too slow for optical, which bottoms out at less than 30 terahertz (1 micron wavelength).

As a result, the search is on for materials that can bridge the terahertz gap between 100 GHz and 30 THz. UCSD's metamaterials may fill the bill, according to Padill.

Metamaterials use repeated composite structures like SRRs with properties specifically engineered to enable permeability and permitivity to take on both positive and negative values. The two properties determine how a material will interact with electromagnetic radiation, from high-frequency light down to terahertz waves, microwaves and radio waves. Natural materials have both positive permeability and permitivity, but metamaterials can have negative permeability and permitivity which is unheard-of in nature.

To downsize from microwave to terahertz, the UCSD researchers imprinted a conductive pattern lithographically in the shape of copper SRRs and wires were arranged into a 2-D structure with a repeated 50-micron lattice. The SRRs were arranged in groups of two resonators each, one slightly smaller and located inside the boundaries of the larger one. The wires were opposite the SRRs.

While copper is not magnetic, and in the 50-micron SRRs had positive permeability and permitivity, the downsized metamaterial reacted magnetically to terahertz signals in the same way as their 5 mm big-brothers do for microwaves.

Next the UCSD researchers will demonstrate that micron-sized SRRs can also detect absorption patterns in the terahertz range, enabling chip-sized SRRs to "see through" clothing and baggage to identify weapons and explosives, or to guide an airplane through fog.

Their work was supported by the Defense Advanced Research Projects Agency through the Office of Naval Research and the U.S. Army Research Office along with the National Science Foundation.