Electrical engineers at Rice University in Houston have demonstrated the world's first endoscope for terahertz imaging, a discovery that could extend the reach of terahertz-based sensors for applications as wide-ranging as explosives detection, cancer screening and industrial and post-production quality control.
The research appears in the Nov. 18 issue of the journal Nature. It presents the emerging terahertz sensing industry with a unique new technology for transporting terahertz waves from a source and directing them at a particular target of interest. "Our wave guide opens up a whole new class of capabilities because it offers a way to get terahertz energy into places it could never reach before," said lead researcher Daniel Mittleman, associate professor of electrical and computer engineering. "Wave guide technology frees you to look around corners and get into tight places."
Terahertz waves, also known as T-waves or T-rays, fall between microwaves and infrared light in the least-explored region of the electromagnetic spectrum. Metals and other electrical conductors are opaque to T-rays, but they can penetrate plastic, vinyl, paper, dry timber and glass like X-rays. Unlike X-rays, T-rays are not hazardous radiation, and in some cases T-wave sensors can reveal not only the shape of a hidden object but also its chemical composition. This unique combination of traits make T-waves perfect for applications like explosive detection, and several companies are already working on T-wave security applications, developing systems that can look inside people's shoes, bags and clothing for guns, bombs and contraband.
T-rays lie between microwaves, whose wavelengths measure from centimeters to millimeters, and light, with wavelengths measured in nanometers, or billionths of a meter. The gap between -- the so-called terahertz gap -- contains wavelengths from 30 to 3000 microns, or 100 GHz to 10 THz when measured in frequency. The terahertz gap has been called the "final frontier" of the electromagnetic spectrum because there's never been an easy or cheap way to either generate or measure T-rays, something that's only begun to change with the advent of new technology in the past decade.
The development of "wave guides" is a key element in the technical maturation of T-ray technology. Wave guides -- like fiber optic cables for lasers and coaxial cables for microwaves -- allow design flexibility because they move and direct energy where it's needed. This is particularly useful if the beam generator is bulky or temperamental. Both fiber optic cables and coaxial cables work by confining the energy of the beam in a small space, causing it to propagate down the cable.
Coaxial cables aren't good guides for T-waves because the metal sheath absorbs T-wave energy very quickly, and fiber optics don't transmit T-waves. By blending some aspects of both these technologies, Mittleman's team devised a system to guide T-waves in and out of a confined space.
This could prove useful for sensing applications that range from the mundane -- scanning packages of cookies or cereal to make sure fruit and nuts are distributed evenly in every carton -- to the space age -- checking for defects under the insulation and heat shield of NASA's space shuttle.
In all T-wave applications today, the beam must be aimed directly from the wave generator at the spot to be sensed. Anything that needs to be scanned has to be moved in front of the beam. Moving the beam isn't practical because the beam has to be fine-tuned each time it's set up, and worse, the whole apparatus is very sensitive to bumps and vibrations, which can easily knock the beam out of alignment.
Mittleman and his student, Kanglin Wang, stumbled upon the idea for the wave guide when they noticed T-waves were moving down a wire during an experiment on a new form of terahertz microscopy. In follow-up experiments, they found they could move T-waves along a bare wire, direct those waves onto a surface, catch the reflected waves on another wire, carry those reflected waves back to a receiver and analyze the return waves to reveal information about the surface the original wave was shined upon.
"There are lots of places where T-waves would be handy but where they're difficult to use today," said Mittleman. "Free-space beams are notoriously temperamental -- a shortcoming that's kept them off of some factory floors -- and our endoscope technology has the potential to change that."
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