Taming the Terahertz

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.

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.)

It’s a breakthrough because the researchers are able to modify and control the signal continuously, says Xi-Cheng Zhang, director of the Terahertz Center for Research at Rensselaer Polytechnic Institute, in Troy, NY.

Ultimately, the researchers hope to manipulate terahertz waves much the same way they focus, filter, and switch laser beams to encode information onto the signal. Chen and his colleagues have laid the foundation, says Zhang, but he adds that there is still a long way to go.

In practical terms, this should translate into big changes in terahertz devices. For example, they might help enhance the blurry images currently produced by passive terahertz detectors.

Creating filters that can tune to particular frequency ranges may also improve devices designed to detect explosives, says Don Arnone, chief executive of Teraview. Allowing such devices to isolate specific narrow bands of terahertz may prove to be an easier way to detect the chemical signatures for explosives.

According to Chen, the metamaterials could be useful for improving terahertz lenses, but he believes the real benefits will be in wireless communications. Modulation is the very principle that underpins communications. Chen says that by demonstrating that they can modulate a terahertz signal, the researchers have shown that it is possible to encode information on a terahertz carrier wave.

Zhang agrees. There is a natural progression to move beyond microwaves and into the terahertz band. While terahertz are useless for long-range communications because they are so easily absorbed by atmospheric water, the improvements in speed and bandwidth they offer make it almost inevitable that terahertz will be used for short-range communications. “Sooner or later, we’re going to have terahertz communications,” Zhang says.