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Terahertz rays track down drugs

28 Oct 2003

Drug smuggling may become much harder in the future thanks to a terahertz imaging system being developed in Japan.

Kodo Kawase and his team from RIKEN (The Institute of Physical and Chemical Research) have demonstrated a system that detects the presence of illicit drugs that are concealed within an envelope. (Optics Express 11 2549)

Tests to date have shown that the imaging system can successfully detect and identify a range of substances including ecstasy (MDMA) and methamphetamine. The researchers are now working with companies to develop a mail screening system that could suit use in post offices and airports.

The scheme works by spectral fingerprinting -- illuminating a target envelope with tunable terahertz radiation and analyzing the absorption spectra of the resulting image. The results are cross-referenced with a database of spectra to check for the chemicals of interest.

“The ability to check the contents of a suspect envelope without violating the correspondence rights has been long sought after,” said Kawase. “Since ink is generally transparent to terahertz waves the privacy of the correspondence is not violated while the identification of concealed drugs is possible.”

At the heart of the Japanese system is a compact and tunable optical parametric oscillator (OPO) that emits terahertz waves. The OPO is made from a nonlinear crystal (MgO:LiNbO3) that is pumped by a Q-switched Nd:YAG laser. It emits terahertz radiation that is tunable between 1 and 2.5 THz.

Terahertz waves of several different frequencies are scanned over the envelope and the transmitted radiation is picked up by a pyroelectric bolometer and analyzed by a computer.

In tests, the system correctly identified the contents of three small (10x10mm) polyethylene bags that were placed inside an airmail envelope and contained 20 mg of ecstasy, methamphetamine and aspirin. The detection limit of the system is estimated at around 3 mg/cm2.

Perhaps the biggest drawback of the current system is its measurement time of 10 minutes. Kawase now plans to speed this up.

“With the present technology, one minute per image is achievable, however we will continue our efforts to obtain higher speeds and we can see several ways to try this,” he told Optics.org. “We are now working on the development of a parallel (CCD) imaging system that can be used much like a regular camera and can output images at a much higher rate than the present scanning system.”

RIKEN is not the only research institute to have investigated the use of terahertz spectroscopy for analysing suspect powders in envelopes. Last year, a group lead by Xi Cheng Zhang at Rensselaer Polytechnic, US, demonstrated a system that could detect bacterial spores.

Oliver Graydon is editor of Optics.org and Opto & Laser Europe magazine.

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