26 Nov 2004
Scientists in the US discover that a simple waveguide made from a metal wire can efficiently transport terahertz pulses.
Making terahertz waveguides could be much simpler than scientists first thought thanks to a discovery at Rice University in the US. Researchers there have demonstrated that a bare metal wire can guide terahertz pulses with virtually no dispersion and low attenuation. (Nature 432 376)
Bridging the gap between microwave and optical regions in the electromagnetic spectrum, terahertz (THz) radiation can penetrate plastics, clothing, cardboard and semiconductors. Less hazardous than X-rays, THz radiation offers an attractive way to screen for concealed items at airports or mail depots.
However, finding a waveguide that operates in this spectral region has proved a challenge. Scientists have turned to dielectric fibers, metal tubes, plastic ribbons and even photonic crystal fibers in an attempt to overcome the high attenuation and dispersion that plagues transmission of THz waves.
"We actually stumbled on the idea quite by accident," Daniel Mittleman of Rice University's electrical and computer engineering department told Optics.org. "We were using apertureless near-field microscopy to improve the spatial resolution in THz imaging." The technique places a small metal tip in close proximity to a sample surface to induce enhanced scattering.
The breakthrough came when Mittleman's student, Kanglin Wang, moved the illumination spot along the shaft of the near-field tip. "[We observed] the same scattered THz signal, only shifted in time according to the propagation delay along the metal tip." described Mittleman. "This was firm evidence that there was a propagating guided mode along the tip."
According to Mittleman, most of the previous work on THz waveguides has evolved from earlier designs for guiding at microwave frequencies or in the infrared. "The idea of a bare wire is not one that has been tried at other frequencies," he commented. "So it is not something that one would naturally have thought to try."
The team has demonstrated the success of their bare metal waveguide by constructing a THz endoscope and analyzing the interior of a glass flask and metal tube.
Motivated by their initial results, the researchers are now busy optimizing their scheme for coupling THz waves into their waveguide. "[Currently] we are using a scattering process, which is very inefficient," said Mittleman. "This means that the amplitude of the radiation propagating along the wire is very small to begin with, and that limits what we can do with it."
Once perfected, Mittleman believes that their THz wires would be compatible with commercial THz imaging systems, available from firms such as Picometrix and TeraView, and could lead to licensing opportunities.
James Tyrrell is reporter on Optics.org and Opto & Laser Europe magazine.
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