Elusive Terahertz Wavelengths Captured by New Detector

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Xin Zhang, Tiger Tao and Rick Averitt
Xin Zhang, Tiger Tao and Rick Averitt

Materials science researchers have combined their engineering and physics expertise to create a novel device that detects a range of electromagnetic wavelengths that has historically eluded detection. The device — a terahertz detector — could have future applications ranging from airport security to medical imaging.

Graduate student Hu “Tiger” Tao, Associate Professor Xin Zhang, Assistant Professor of Physics Richard Averitt and colleagues at Boston College developed the device by combining the expertise of Tao and Zhang in fabrication of microelectromechanical systems (MEMS) with Averitt’s work in terahertz research.

The terahertz, or far-infrared, falls between the microwave and mid-infrared to visible region of the electromagnetic spectrum. Electron based technologies abound at microwave frequencies, while photon-based technologies are well developed at higher frequencies. However, closing in on the terahertz region using either of these approaches is challenging, leading to the “terahertz gap.”

“There is a lack of terahertz detectors, so people cannot find what’s happening in this region. We want to make a terahertz detector to see what’s inside this terahertz region –people are curious, because it’s a gap,” said Tao.

Their original research paper, “A metamaterial absorber for the terahertz regime: Design, fabrication and characterization,” was published in the May 12 issue of Optics Express and highlighted in Science and Nature magazines.

Almost everything around us gives off radiation, including terahertz radiation, but very weakly. To elicit meaningful data for medical, security or other future applications, researchers hit an object with a radiation source, letting the detector then collect higher levels of reflected radiation.

“With terahertz radiation one can see through many materials — plastics, clothes, ceramics — which are opaque to infrared and optical radiation. That’s very unique of the terahertz range,” said Tao. “It’s a perfect candidate for safety scanning. If there is a dangerous material, for example plastic explosives, it has a unique spectrum along this terahertz frequency, so we can not only see through a bag to what’s inside, but can know exactly what the material is we’re looking at.”

To make their detector, the team had to find a material that absorbed terahertz radiation extremely well. No natural material did this, so they turned to working with metamaterials – materials composed in the laboratory with properties not found in the naturally occurring materials. Their detector absorbs 70 percent of terahertz radiation, a vast improvement on natural materials’ one to two percent absorption. The device is made of a metamaterial with two metal features, an electrical ring resonator and a cut wire, separated by a polyimide spacer.

These metal threads give the detector one of its unique features, allowing the researchers to tune the electrical and magnetic resonance of the device to a specific frequency. This means the detector is potentially useful in a number of different scenarios to seek unique terahertz signatures from a variety of materials  — a type of explosives in one region of the terahertz range or a cancerous tumor at another frequency. Other detectors without this tunability only function over a broad spectral range which does not easily permit spectral-based identification, limiting their applicability. 

The engineering and physics group members say this is just the beginning of their collaboration. They will continue to improve fabrication of the detector as well as focusing on the fundamental research in this area, such as making unique structures and devices for use at terahertz frequencies, such as flexible or 3D terahertz metamaterials.

Zhang and Averitt recently won a new grant from the National Science Foundation to continue their collaboration, working on MEMS-based sensors and detectors.

“This is the very best kind of collaboration,” said Zhang, “because Rick knows something I don’t know and vice versa, so we have complementary expertise. Every day there is something fresh to learn.”