Tuning Terahertz

A new device gives researchers unprecedented control over an unwieldy part of the spectrum.

For years, engineers have had their eyes on terahertz radiation--a band of radio waves that could be used to capture images as clear as x-rays without the harmful radiation, and that could be used in short-range, ultrafast wireless communication. But terahertz applications remain few because scientists haven't yet developed all the tools needed to modify and otherwise control the waves. Now, researchers at Los Alamos National Laboratory, Boston College, and Boston University have built a device--similar to a tuning dial on a radio--that could finally help make terahertz radiation practical.

Terahertz tuner: In order to pick a particular terahertz frequency from a broadband source, researchers use metamaterials that are made of periodic arrays of metal structures (bottom). Hou-Tong Chen and his colleagues added silicon strips, whose properties can be changed by shining laser light onto them, to the individual squares (top). Changing the properties of silicon changes the properties of the entire metamaterial, allowing researchers to tune in to a range of specific terahertz frequencies. Credit: Hou-Tong Chen, LANL

"Terahertz is the last spectrum band to be explored," says Hou-Tong Chen, a physicist at Los Alamos National Laboratory and lead researcher on the project. It has great promise as a security-imaging tool because its frequencies, which range from 300 gigahertz to 3 terahertz, easily pass through clothes but reflect off biological tissue. And since the waves don't have the energy that x-rays do, they don't pose the health risks. In addition, terahertz waves oscillate much faster than microwaves used in Wi-Fi do, which means that they can carry thousands of times more information than today's wireless signals can, albeit over shorter distances.

In 2006, Chen and his colleagues developed a terahertz amplifier made of metamaterials, specially designed materials whose periodic structures affect incoming radiation. And others researchers have made filters out of metamaterials that pick out specific frequencies. Such filters are crucial to, say, encoding information on a certain terahertz frequency. But the problem with these structures is that once they are designed and built, the frequency is set. And that means that in order to access a range of frequencies, separate filters need to be built and assembled, which could be costly and lead to bulky terahertz devices.

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Chen's recent work, published in Nature Photonics, expands the usable frequency range of the structure, essentially adding a tuning knob to the material. The trick, he says, is to integrate silicon strips into the structure. The electronic properties of these strips can be modified on the fly, so that they change the overall properties of the metamaterial, and thereby change the frequency of terahertz radiation that passes through.

Chen began with the same basic premise he used to make a terahertz filter: build an array of metal structures, such as squares or rectangles, on a flat surface. These structures must be much smaller than the wavelength of the incoming radiation in order to affect it; in this case, he built squares with sides 37 micrometers long. When terahertz radiation hits a flat surface with these metal squares, its electromagnetic field interacts with the electrons in the metal, "feeling the inductance and capacitance of the little metal structures," says Daniel Mittleman, a professor of electrical and computer engineering at Rice University, who's not associated with the work. Depending on the inductance (the electric force around the structure) and the capacitance (the amount of electric charge the structure can hold), the incoming broadband terahertz radiation is filtered at a narrow frequency.