April 21, 2008

Non-metallic metamaterials able to tune and guide terahertz radiation


Scanning electron microscopy (SEM) images of the frequency-tunable planar metamaterial. An individual unit cell (a. above), and periodically patterned square array (b. below). All dimensions are shown in microns and materials are indicated in the images. The polarization of the incident linearly-polarized THz radiation is also indicated in b. (Credit: Image courtesy of Nature Photonics)

There were two separate significant advances in capabilities to control terahertz radiation. Los Alamos developed frequency tuning and Utah developed waveguides.

Researchers formed a single layer of metamaterial and semiconductor that allowed the team to tune terahertz resonance across a range of frequencies in the far-infrared spectrum. Most previous metamaterials were metallic structures. Applications include imaging and screening, terahertz switches, modulators, lenses, detectors, high bit-rate communications, secure communications, the detection of chemical and biological agents and characterization of explosives, according to Los Alamos National Laboratory.

University of Utah engineers took an early step toward building superfast computers that run on far-infrared light instead of electricity. Ajay Nahata and colleagues designed stainless steel foil sheets with patterns of perforations that successfully served as wire-like waveguides to transmit, bend, split or combine terahertz radiation. "A waveguide is something that allows you to transport electromagnetic radiation from one point to another point, or distribute it across a circuit," Nahata says. If terahertz radiation is to be used in computing and communication, it not only must be transmitted from one device to another, "but you have to process it," he adds. "This is where terahertz circuits are important. The long-term goal is to develop capabilities to create circuits that run faster than modern-day electronic circuits so we can have faster computers and faster data transfer via the Internet."

They made these waveguides on a flat surface so that you can make circuits just like electronic circuits on silicon chips." The researchers used pieces of stainless steel foil about 4 inches long, 1 inch wide and 625 microns thick, or 6.25 times the thickness of a human hair. They perforated the metal with rectangular holes, each measuring 500 microns (five human hair widths) by 50 microns (a half a hair width). The rectangular holes were arranged side by side in three different patterns to form "wires" for terahertz radiation. "All we've done is made the wires" for terahertz circuits, Nahata says. "Now the issue is how do we make devices [such as switches, transistors and modulators] at terahertz frequencies?"

The Los ALamos team's first-generation device achieved 20 percent tuning of the terahertz resonance to lower frequencies -- those in the far-infrared region --addressing the critical issue of narrow band response typical of all metamaterial designs to date.

Constructed on the micron-scale, metamaterials are composites that use unique metallic contours in order to produce responses to light waves, giving each metamaterial its own unique properties beyond the elements of the actual materials in use.

Within the past decade, researchers have sought ways to significantly expand the range of material responses to waves of electromagnetic radiation -- classified by increasing frequency as radio waves, microwaves, terahertz radiation, infrared radiation, visible light, ultraviolet radiation, X-rays and gamma rays. Numerous novel effects have been demonstrated that defy accepted principles.

"Metamaterials demonstrated negative refractive index and up until that point the commonly held belief was that only a positive index was possible," said Padilla. "Metamaterials gave us access to new regimes of electromagnetic response that you could not get from normal materials."

Prior research has shown that because they rely on light-driven resonance, metamaterials experience frequency dispersion and narrow bandwidth operation where the centre frequency is fixed based on the geometry and dimensions of the elements comprising the metamaterial composite. The team believes that the creation of a material that addresses the narrow bandwidth limitations can advance the use of metamaterials.

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