Tera-X: IMAGING BENEATH THE VISIBLE.

While there are many applications for terahertz technology, perhaps the most beneficial to society are for security, weapon/bomb detection, and biological agent detection.

We have made several reflection measurements of metallic and non-metallic targets in our laboratory, and have observed predictably high contrast against a skin background. In particular, we have taken small quantities of energetic materials such as plastic explosives and TNT, and measured them against both reflecting and absorbing backgrounds at the focus of a broadband electronic THz reflectometer. The patterns of reflection versus frequency give rise to signatures that are remarkably specific to the chemical composition of the target, even though the target’s morphology and position is varied. While more fundamental work is ongoing to understand the response of organic crystals such as these in the low-THz regime, the results provide intriguing motivation to push further for a new type of screening system that would operate in tandem with existing X-ray and metal-detecting screeners.

The heightened sense of vulnerability to concealed threats—whether non-metallic weapons, explosives or even biological agents such as anthrax—has motivated a wide-ranging response from the scientific community. THz technology, with its ability to “see through” many types of enclosures (and to distinguish metal from plastics) is a promising candidate to address the problem of detecting such concealed threats.

Laboratories around the world including ours have measured a variety of targets both in transmission and in reflection to map out potential routes to field-deployable sensing systems for concealed weapons, whether on people or in packages and envelopes. The ability to penetrate envelopes and see useful dielectric contrast with THz radiation may enable rapid screening and identification of suspicious contents

To examine the contents of envelopes with an eye toward distinguishing common powders from potentially dangerous ones, we prepared samples of sugar, starch, flour, and talcum powder, and used our electronic THz system on them in both reflection and transmission. Additional transmission measurements were done through envelopes with the same four powders and with B. cereus, a close relative of B. anthracis, or anthrax.

While the magnitude of transmission from 8-300 GHz showed interesting patterns for all powders, the phase signal was most conclusive, demonstrating the value of a broadband system to accurately discriminate the spores from other powders. The negative dispersion exhibited in this measurement is readily distinguished, and may be due to the particular water content and size of the spores.

An intriguing aspect of broadband THz sensors would their multivalent capability for sensing multiple gas species with one system. While mid- and near-infrared sensors are usually employed for gas sensing and spectroscopy, rotational lines of molecules with a permanent dipole moment are often accessible to THz spectrometers, given sufficient signal-to-noise ratios. We have measured absorption lines of species such as nitric oxide and carbonyl sulfide, but only in low-pressure tubes where atmospheric pressure broadening of these lines is non-existent. The idea of monitoring automotive exhaust gas streams for pollution is an intriguing high-volume application for field-deployable THz technology, but will only work at frequencies far from the ubiquitous water-vapor absorption lines that dominate the spectra. This challenge is further compounded by the ~ 400 K temperature of exhaust gas, which further enhances thermal background.

The outlook for THz technology is bright, especially for field-deployable systems that can be compact and even battery-operated. While optoelectronic THz technology is more refined, in part because of its larger number of practitioners, electronic THz technology will catch up as more high-volume applications are pursued.