Wire guides terahertz waves

By Eric Smalley, Technology Research News

Terahertz radiation, which falls between microwaves and infrared light on the electromagnetic spectrum, is gaining attention from researchers because it shows great promise for medical imaging, chemical sensing and communications.

Terahertz radiation puts much less energy into biological tissue than x-rays, and terahertz medical imaging systems can be tuned to highlight specific types of tissue such as skin cancers. Because terahertz waves can penetrate plastic and cloth, they can be used to detect concealed objects. Terahertz radiation is also capable of detecting chemicals like toxic gases and explosives.

Among the challenges to making terahertz sensing and imaging applications more practical is finding ways to direct the waves to specific targets. Researchers are working to develop terahertz wave guiding devices that are similar to the waveguides used to channel microwaves and lightwaves.

Researchers at Rice University have come up with a particularly simple solution. They have shown that a piece of stainless steel wire nine tenths of a millimeter in diameter causes terahertz waves to propagate in the space around wire. "A very simple structure -- just a bare metal rod -- permits us to guide terahertz pulses around corners and into tight spots," said Daniel Mittleman, an associate professor of electrical and computer engineering at Rice University.

Previous efforts at developing terahertz waveguides focused on producing terahertz equivalents of the optical fibers that guide light or the metal tubes that guide microwaves, said Mittleman. Neither approach has panned out, he said. Fibers tend to absorb terahertz radiation and terahertz waves create electric currents that heat metal, a process that takes energy away from the terahertz waves.

The researchers' metal wire does not absorb terahertz waves, and its small surface area limits the current the waves produce, said Mittleman. Previous attempts at terahertz waveguides also tended to cause short terahertz pulses to spread out. The researchers' waveguide did not show any pulse dispersion for distances up to 24 centimeters, he said.

The waveguide could be used to image objects inside the body. To that end, the researchers made a terahertz endoscope from a pair of wire waveguides configured into a Y. One of the wires channels terahertz waves onto a sample and the other channels waves reflected by a metal plate positioned behind the sample to a detector.

The waveguide could also be used to detect substances inside containers. "If we take this straight piece of wire and dip it into a cargo container... then this becomes a dipstick sensor for trace gas analysis," said Mittleman. "Many different materials have specific chemical fingerprints in the terahertz range," he said.

Substances absorb or scatter different wavelengths of electromagnetic radiation in different ways, creating unique patterns. This makes it possible to identify substances and study their properties using light.

The researchers' prototype consists of a terahertz transmitter, an input coupler and a waveguide. The input coupler is a second piece of wire positioned perpendicularly to the waveguide with half a millimeter of space between the two wires. The terahertz transmitter focuses a beam at the intersection of the two wires, which causes terahertz waves to travel through the space around the waveguide.

The current method of getting the terahertz radiation onto the waveguide transfers less than one percent of the energy, according to Mittleman. "This is a big limitation because it means that the total energy we're starting with is very small," he said. The researchers are working on new method that should boost that to more than 50 percent, he said.

The researchers' waveguide could be incorporated very quickly into either of the two commercial terahertz medical imaging systems, said Mittleman. This would allow medical imaging of small areas inside the body.

Mittleman's research colleague was Kanglin Wang. The work appeared in the November 18, 2004 issue of Nature. The research was funded by the National Science Foundation (NSF), the Robert A. Welch foundation, and Advanced Micro Devices, Inc. (AMD).

Timeline:   < 1 year
Funding:   Government, Private, Corporate
TRN Categories:  Optical Computing, Optoelectronics and Photonics
Story Type:   News
Related Elements:  Technical paper, "Metal Wires for Terahertz Waveguiding," Nature, November 18, 2004


March 9/16, 2005

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