Tuning Terahertz

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 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 other 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.

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.

But in order to tune to different frequencies, either the capacitance or inductance needs to change. So, as a first attempt, Chen adds silicon to the metal squares to change the capacitance of the structure. Silicon is responsive to light, so when Chen shines a near-infrared laser onto the strips, their capacitance changes, which selects a specific frequency from the incoming terahertz light. By adjusting the power from the infrared laser, Chen was able to tune in to specific frequencies from 850 gigahertz to 1.06 terahertz.

While Chen’s recent work is important for tuning across a range of terahertz frequencies, bringing it closer to a practical application, his approach could also be used for other frequencies. “What’s interesting here is, the way they’ve chosen to achieve [tunability] is versatile,” he says. The same approach of adding silicon strips could be used for devices at different wavelengths, including microwave and optical frequencies. While there are already devices that can tune across these, a tunable metamaterial is of interest to researchers for several reasons. Metamaterials are able to perform bizarre feats: lenses made of metamaterials can focus light tighter than conventional lenses can, and metamaterials have even been used to make a rudimentary cloaking device. A tunable metamaterial for many different wavelengths could expand the potential of these early applications.

Chen says that his next step is to tune the filter using electrical current instead of optical laser pulses. While it was more straightforward to change silicon’s properties by shining light on it, it’s ultimately more practical to use an electric current in an imaging system or wireless device.