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Projects mine promise of terahertz radiation


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POSTED: Saturday, June 06, 2009

Modern technology has found some use for most of the electromagnetic (EM) spectrum, but there is a small portion that we are only now beginning to exploit.

Terahertz (THz) radiation, in the range of 100 gigahertz (GHz) to 10,000 GHz, is nestled between the infrared and microwave range in a frequency band hundreds of times greater than can be produced by current electronics.

The THz region of the EM spectrum has been difficult to exploit because conventional semiconductor electronics are sluggish above 100 GHz, and photonic devices still in their infancy falter below 10 THz because thermal noise masks the photon energy.

Terahertz waves are not suitable for long-range communications because the rotational and vibrational resonances of many liquid and gas molecules lie within the THz frequency band, so oxygen and water molecules in the atmosphere attenuate the waves and limit their range.

THz waves, also called T-waves, have properties that make them attractive for other applications. T-waves can pass through fabrics, plastic, wood, ceramics and brick, but they can be blocked by a metal object or a thin layer of water.

Many chemicals exhibit unique spectral signatures in the THz range because the same molecular resonances that absorb THz also allow us to use them to identify molecular structures.

T-wave frequencies have traditionally been considered radio “;no man's land,”; but advances in semiconductor technology are producing transistors that can service the low end of the THz band. Research into various types of “;metamaterials”; is likely to produce higher-frequency oscillators with sufficient power in the not-too-distant future.

The ability to produce and broadcast T-waves would be a breakthrough in short-range data transmission because their high frequencies would increase the bandwidth 500 times over the 2.4 GHz now used for Bluetooth and Wi-Fi.

Passive THz systems already in development can see through concealing barriers such as packaging, corrugated cardboard, clothing, shoes, book bags, and so forth to identify potentially dangerous materials. Many explosives as well as chemical and biological agents have characteristic THz signatures that can be used to identify these materials when concealed in clothing or bags.

NASA used THz technology to inspect the foam on the space shuttle's external tank, part of the return-to-flight requirement following the Columbia tragedy.

The unique combination of attractive traits makes THz rays well suited for many applications, many of which are on the nano-scale and have not even been imagined.

We will see much more of this technology in the early decades of the 21st century, coming soon to an airport near you.

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Richard Brill is a professor of science at Honolulu Community College. E-mail questions and comments to .(JavaScript must be enabled to view this email address).