Waveguides bridge ‘terahertz gap’

PORTLAND, Ore. — Wires that carry terahertz electromagnetic radiation could carry 1,000-GHz signals. That's 20 times faster than the speed of the world’s fastest microprocessor, which IBM Corp. announced last week.

Until now, the terahertz gap has prevented circuitry from rising much above 60 GHz—the speed of next-generation wide personal-area networks, or WPANs. Now, researchers at the University of Utah have demonstrated a method of building wires that act as terahertz-caliber waveguides atop stainless-steel foil by using lines of micron-scale perforations. The technique was shown to transmit, bend, split and combine terahertz radiation at 0.3 THz (300 GHz) but could be extended up to 10 THz, according to electrical-engineering professor Ajay Nahata at the university.

“It's a speed issue,” said Nahata. At terahertz speeds, it would be possible “to download a movie in a few seconds.”

Terahertz frequencies exist in the gap between microwaves and the infrared wavelengths used by optical fibers. Terahertz electromagnetic signals have properties of both electrical signals and optical signals, requiring waveguide-like structures, but in a form factor similar to the wiring used by an electrical signal.

Nahata has now demonstrated a wiring method that focuses radiation into a perforated metallic “wire” that acts like the splitter used to feed a cable-TV signal to two televisions, an essential element of future terahertz circuitry. Nahata estimates that within 10 years, his demonstration will lead the way to terahertz-speed circuitry for computers and communications.

“We've demonstrated the first step toward making circuits that use terahertz radiation,” said Nahata, who performed the work with Wenqi Zhu and Amit Agrawal, two doctoral candidates in the university's electrical and computer engineering department.

The University of Utah demonstration builds on the theoretical results of Stefan Maier, a researcher at Imperial College (London), who collaborated with scientists at the University of Bath, the University of Madrid and the University of Zaragoza, Spain. Maier and colleagues demonstrated earlier this year that surface plasmon polaritons—electromagnetic surface waves supported at the interface between a metal and a dielectric (copper and a polymer)—could guide terahertz signals.

Nahata and his colleagues at the University of Utah fabricated their waveguide and signal splitter using steel as the conductor and air as the dielectric, by virtue of perforating a stainless-steel foil with rectangular holes measuring 50 x 500 microns. The waveguide was about an inch long and the foil about 625 microns thick. The researchers say they have shown that the grooves enabled 0.3-THz radiation to be focused into the waveguide, where it traveled to within 1.7 mm of the foil's surface, spreading horizontally only about 2 mm as it followed the pattern of perforations.

The shape of the pattern began with two terahertz wires separated by about a centimeter, then angled together like the leftmost strokes of an “X.” But instead of crossing, the scheme ran the two wires alongside each other, then angled them again like the rightmost stokes of the elongated “X.” Because the two paths ran alongside each other, the 2-mm spread of the signal on each side of the waveguides coupled the two wires, so that the rightmost exiting wires both carried 50 percent of the combined signal.

To test the waveguide, a semicircular score in the foil focused broadband 0.3-THz radiation into just one of the leftmost wires. After coupling in the middle section, where the second wire ran alongside the first to within about 50 microns, the signal exited the rightmost branch of the splitter with 50 percent power in each wire.

“All we've done is made the wires,” said Nahata. “Now the issue is, how do we make devices at terahertz frequencies?”

Their report was published Friday, April 18, in the free online journal Optics Express.

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