PORTLAND, Ore. Metamaterial experts bridged the terahertz gap recently by demonstrating a magnetic sensor based on split-ring resonators.
SRRs function as artificial atoms in a metamaterial, but are actually constructed from concentric planar copper rings. By downsizing a microwave SRR from 5 millimeters to 50 microns, the researchers demonstrated a magnetic response that bridges the terahertz gap, thereby opening the door to solid-state sensors that can see through solid objects.
In the terahertz gap-roughly between the wavelengths of 1 micron and 300 microns (or in the 100-GHz and 30-THz frequency range)-neither solid-state optical nor silicon solutions exist today. The region is too fast for silicon, which peaks out above 100 gigahertz (300-micron 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.
"Many materials appear transparent in the terahertz range," said doctoral candidate Willie Padill, who works in the laboratory of physics professor Dimitri Baso at the University of California at San Diego (UCSD), "and by using two different frequencies-say half a terahertz and one 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. What we have demonstrated here is that metamaterials are a natural for terahertz sensors." Also contributing were professors David Smith and Xiang Zhang, as well as project scientist David Vier and graduate students Ta-Jen Yen and Nicholas Fang.
In 1968, Russian theorist Victor Veselago predicted that metamaterials could someday be engineered that would interact with their environment in a manner precisely the opposite of the way natural materials react. Metamaterials use repeated composite structures like SRRs with properties specifically engineered to enable permeability and permitivity to take on both positive and negative values. Those two properties determine how a material will interact with electromagnetic radiation, from high-frequency light down to terahertz waves, microwaves and radio waves. All 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 frequencies, the UCSD researchers had a conductive pattern lithographically imprinted in the shape of copper split-ring resonators arranged into a 2-D structure with a repeated 50-micron lattice. The SRRs were arranged in groups of two resonators each, one smaller and located inside the boundaries of the larger one.
Even though copper is not magnetic and the 50-micron SRRs had positive permeability and permitivity, nevertheless the downsized metamaterial reacted magnetically to terahertz signals. This was the same reaction already demonstrated by the 50-micron SSRs' 5-millimeter big brothers.
Next, the UCSD researchers want to 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 on a foggy night.
The work was supported by the Multidisciplinary University Re-search Initiative sponsored by the Defense Advanced Research Projects Agency through the Office of Naval Research and the U.S. Army Research Office, as well as the National Science Foundation.