Taming the Terahertz

Devices that can control terahertz waves could pave the way for faster wireless communication.

Terahertz waves have been touted as the next big thing for security and communication devices. Researchers can already generate and detect terahertz radiation, but controlling it has proved difficult. More control could mean faster wireless communication and clearer images for security scans.

Terahertz waves help reveal nonmetallic hidden weapons in this shoe. (The image on the left shows the shoe as scanned by terahertz waves; the shoe in the middle shows how it looks to the naked eye; and the shoe on the right has had the heel removed to show the hidden items.) A new device could improve such images further and spur the development of faster wireless-communication devices operating in the terahertz range. Credit: Teraview

Now U.S. researchers have found a way to control those waves on the fly, using a new class of materials known as metamaterials. "This is the starting point of efficient manipulation of terahertz waves," says Hou-Tong Chen, a physicist who carried out the work with colleagues at the Los Alamos National Laboratory, NM, and the University of California in Santa Barbara.

Also known as T-rays, terahertz waves sit on the electromagnetic spectrum between infrared and microwaves, and they exhibit a range of properties that make them particularly attractive. For example, their ability to pass through clothes and yet be reflected by biological tissue offers some of the benefits of X rays without the inherent risks of using ionizing radiation. Similarly, many chemicals have been shown to exhibit unique spectral signatures in the terahertz range.

Companies such as the Toshiba spin-off Teraview, based in the United Kingdom, have started developing terahertz devices for security, medical, and pharmaceutical applications. For example, terahertz security scanners are being designed to sniff out a range of explosives by detecting specific spectral signatures. Personnel scanners capable of detecting nonmetallic concealed weapons are also in development.

But existing terahertz devices tend to either emit or detect these waves. Finding ways to affect or modify them has remained a challenge. "They are difficult to influence, mainly because most naturally occurring materials lack the useful electronic response at this frequency range," says Chen.

Indeed, with a frequency range of between 300 and 3,000 gigahertz (0.3 to 3.0 terahertz), T-rays sit on the cusp between traditional light waves and radio waves. So for a device to have an effect on them, it would have to operate in a way that straddles both photonics and electronics.

But now Chen and his colleagues believe they have the answer: metamaterials. These are materials that exhibit electromagnetic properties dictated not by their substance so much as by their structure or electronic function.

By applying a voltage in a particular way to a standard electronic component known as a Schottky diode, the group was able to make a section of this component resonate, creating an alternating electromagnetic field. Varying the voltage altered the field. The researchers found that these field changes could increase and decrease the amplitude of the terahertz signal. (The results are published in the latest issue of the journal Nature.)