Science News

Terror-Hertz Scanners

Tue, 4th Dec 2007

Scientists have developed a compact way to produce a safe form of electromagnetic waves - known as "T-rays" - that could revolutionise airport security scanners and also give rise to powerful new diagnostic techniques.

The terahertz rays are radio waves sandwiched between the infrared and microwave frequencies.  They are attractive as a medical and security imaging system because the waves are harmless - they are not sufficiently powerful to ionise or knock electrons off other chemicals, which means that they cannot damage DNA or trigger cancers.  But they are highly penetrating, which means that they can be used to "see" through varying thicknesses of skin, leather, cardboard and clothing, although they are stopped by water and metals.  They can also be used to "chemically fingerprint" materials because certain substances absorb specific characteristic frequencies of the waves.

Unfortunately these waves are very difficult to produce because there were no materials available to generate the required frequencies in a portable fashion.  But now a team of researchers in the US, Turkey and Japan have taken advantage of a crystal trick called a Josephson Junction to produce a highly compact terahertz radiation source.  These microscopic junctions, which are 10,000 times thinner than a human hair, consist of a sandwich of superconducting copper oxide insulated by layers of bismuth oxide and strontium oxide.  When a voltage is applied across the junction it spits out photons of light (electromagnetic waves) at terahertz frequencies, although the intensity is very low.  So, to produce a more powerful signal, the researchers stacked up 1000 of these junctions on top of each other.  Then, to make all of the junctions emit photons at the same time (so that the signal would add together), they arranged the junctions into resonant cavities - the quantum effect of an echo chamber.  In this way the signal builds up and sustains itself, whilst also emitting some electromagnetic radiation that can be used.  And by varying the length and thickness of the cavities the team were able to generate frequencies from 0.4-0.8 terahertz at a power of 0.5 microwatts.  To push up the output power yet futher - to an ideal 1 milliwatt, which would have useful applications, the team are now experimenting with arranging the cavities into arrays so that their outputs can be added together.

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