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Waveguides Control T-rays

Photonics.com
Apr 2008
SALT LAKE CITY, Utah, April 15, 2008 -- Waveguides that can transmit, bend, split and combine terahertz radiation (also known as T-rays) represent a step toward building circuits for light-powered computers that operate 1000 times faster than today's gigahertz-based machines.

The electromagnetic spectrum, which ranges from high to low frequencies (or short to long wavelengths), includes gamma rays and x-rays, as well as ultraviolet, visible, and infrared light. It also includes microwaves, FM radio waves, television, short wave and AM radio. Fiber optic phone and data lines now use near-infrared light and some visible light. The only part of the spectrum not now used for communications or other practical purposes is terahertz-frequency or far-infrared radiation, located on the spectrum between mid-infrared and microwaves.

Ajay Nahata, a professor of electrical and computer engineering at the University of Utah, said he and doctoral students Wenqi Zhu and Amit Agrawal have designed stainless steel foil sheets with patterns of perforations that successfully served as wire-like waveguides to transmit, bend, split or combine terahertz (THz) radiation. Their study will be published April 18 in the online journal Optics Express.
AjayNahata.jpg
Ajay Nahata, a University of Utah professor of electrical and computer engineering, with equipment he uses to test devices aimed at harnessing terahertz radiation -- also known as far-infrared light or T-rays -- to run superfast computers of the future. In a new study, Nahata and his students created waveguides that successfully transmitted, bent, split and combined terahertz radiation. (Image courtesy Lee Siegel)
“A waveguide is something that allows you to transport electromagnetic radiation from one point to another point, or distribute it across a circuit,” Nahata said. “We have taken a first step to making circuits that can harness or guide terahertz radiation. Eventually -- in a minimum of 10 years -- this will allow the development of superfast circuits, computers and communications.”

If terahertz radiation is to be used in computing and communication, it not only must be transmitted from one device to another, but it has to be processed.

“This is where terahertz circuits are important. The long-term goal is to develop capabilities to create circuits that run faster than modern-day electronic circuits so we can have faster computers and faster data transfer via the Internet,” he said. “Electronic circuits today work at gigahertz frequencies -- billions of cycles per second. Electronic devices like a computer chip can operate at gigahertz. What people would like to do is develop capabilities to transport and manipulate data at terahertz frequencies (trillions of hertz.) It’s a speed issue. People want to be able to transfer data at higher speeds. People would like to download a movie in a few seconds.”

With so much of the spectrum clogged by existing communications, engineers would like to harness terahertz frequencies for not only faster computing but communication and antiterrorism scanners and sensors able to detect biological, chemical or other weapons. Nahata says the new study is relevant mainly to computers that would use THz radiation.

In March 2007, Nahata, Agrawal and others published a study in the journal Nature showing it was possible to control a signal of T-rays using thin stainless steel foils perforated with round holes arranged in semiregular patterns.

In February 2008, British researchers reported they used computer simulations and some experiments to show that indentations punched across an entire sheet of copper-clad polymer could hold T-rays close to the sheet’s surface. That led them to conclude the far-infrared light could be guided along such a material’s surface.

But the London researchers did not actually manipulate the direction the T-rays moved, such as by bending or splitting the radiation.

“We have demonstrated the ability to do this, which is a necessary requirement for making terahertz guided-wave circuits,” Nahata said. “In this study, we’ve demonstrated the first step toward making circuits that use terahertz radiation and ultimately might work at terahertz speeds,” or a thousand times faster than today’s gigahertz-speed computers.

“People have been working on terahertz waveguides for a decade. We’ve shown how to make these waveguides on a flat surface so that you can make circuits just like electronic circuits on silicon chips,” he said.
THzWaveguide.jpg
Close-up of a waveguide device (shown with a penny for scale) that "couples" terahertz (THz) radiation, moving it from one wire-like waveguide to another. The device is fabricated on a piece of stainless steel foil. THz radiation is beamed onto the foil within the semicircular etching, which focuses the radiation and sends it down the lower waveguide (the lower part of the "x" shape). Where the two waveguides come near each other (the elongated middle of the "x"), half the radiation jumps from one waveguide to another, so half the radiation comes out of the right-side end of each waveguide. Each waveguide is made of numerous small rectangular punctures in the foil.(Image courtesy Wenqi Zhu)
The researchers used pieces of stainless steel foil about 4 inches long, 1 inch wide and 625-µm (microns) thick, or 6.25 times the thickness of a human hair. They perforated the metal with rectangular holes, each measuring 500 µm (five human hair widths) by 50 µm (half a hair width). The rectangular holes were arranged side by side in three different patterns to form “wires” for terahertz radiation. The straight pattern successfully carried T-rays in a straight line. The other two patterns “changed the direction the terahertz radiation was moving” by splitting it or coupling it, Nahata said.

The design of the waveguide means that it carries THz radiation in the form of surface plasma waves -- also known as plasmons or plasmon polaritons -- which are analogous to electrons in electrical devices or photons of light in optical devices. The surface plasma waves are waves of electromagnetic radiation at a terahertz frequency that are bound to the surface of the steel foil because they are interacting with moving electrons in the metal, Nahata said.

“All we’ve done is made the wires” for terahertz circuits, Nahata said. “Now the issue is how do we make devices (such as switches, transistors and modulators) at terahertz frequencies.”

For more information, visit: www.unews.utah.edu


GLOSSARY
light
Electromagnetic radiation detectable by the eye, ranging in wavelength from about 400 to 750 nm. In photonic applications light can be considered to cover the nonvisible portion of the spectrum which includes the ultraviolet and the infrared.
optical
Pertaining to optics and the phenomena of light.
photon
A quantum of electromagnetic energy of a single mode; i.e., a single wavelength, direction and polarization. As a unit of energy, each photon equals hn, h being Planck's constant and n, the frequency of the propagating electromagnetic wave. The momentum of the photon in the direction of propagation is hn/c, c being the speed of light.
photonics
The technology of generating and harnessing light and other forms of radiant energy whose quantum unit is the photon. The science includes light emission, transmission, deflection, amplification and detection by optical components and instruments, lasers and other light sources, fiber optics, electro-optical instrumentation, related hardware and electronics, and sophisticated systems. The range of applications of photonics extends from energy generation to detection to communications and...
plasmon
Calculated quantity of the entire longitudinal wave of a solid substance's electron gas.
radiation
The emission and/or propagation of energy through space or through a medium in the form of either waves or corpuscular emission.
spectrum
See optical spectrum; visible spectrum.
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