Parcels are see-through in the right light

The ubiquity of postal mail makes it one of the easiest ways to convey harmful and illicit substances.

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by John Wallace

The ubiquity of postal mail makes it one of the easiest ways to convey harmful and illicit substances. Biological agents such as anthrax have joined explosives and illegal drugs as materials targeted for identification within packages and envelopes. Two questions present themselves to developers of scanning methods. How can parcels be made see-through for nondestructive scanning? And how can suspicious-looking material—especially if it is formless, such as a powder—be identified?

Terahertz radiation may be one answer, say researchers at Rensselaer Polytechnic Institute (RPI; Troy, NY), the New York State Department of Health (Albany, NY), and Adelaide University (Adelaide, Australia). Not only are paper and cardboard nearly transparent at that wavelength, but terahertz pulses can excite a substance within a parcel, producing a measurement that contains frequency and phase information that help identify the substance.

In results presented at the Conference on Lasers and Electro-Optics 2002 (May 19-24; Long Beach, CA), the researchers outlined how they have constructed an imaging system based on chirped terahertz pulses. The frequency of a chirped pulse varies with time; if a certain frequency is affected by passage of the pulse through a substance, the effect is encoded in a slice in time of the pulse. Dispersing the pulse in the frequency domain allows the information to be reconstructed.

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Terahertz imaging classifies powdered substances. Four different powders—salt, baking soda, flour, and Chinese seasoning—were placed in strips between two pieces of paper (left) and placed in an envelope, which was then scanned with chirped terahertz pulses. An image of the Fourier transform of the terahertz response at each pixel displays the amplitude at a 0.3-THz frequency as the grayscale intensity (center). The scanned data gives information on the frequency-dependent scattering, absorption, and refractive indices of the substances. Operating on the 0.3-Thz data, a simple classification algorithm classifies each pixel, resulting in the identification of all four powders as well as the paper (right, shown as five shades of gray).

The radiation for the group's terahertz imaging system is generated by a Ti:sapphire laser emitting 130-fs pulses at a rate of 1 kHz. The pulses are split into a pump beam, used to generate broadband terahertz pulses with a photoconductive antenna, and a probe beam, used to detect the pulses after transmission through the target. "In normal terahertz time-domain spectroscopy, the short 130-fs probe pulse is used to measure one instant of the terahertz pulse," says Brad Ferguson, one of the researchers. "The delay between pump and probe pulses is then adjusted to allow the full terahertz pulse to be measured with sequential measurements. Our system accelerates this process by allowing the full terahertz pulse to be measured simultaneously."

A simple grating pair applies a linear chirp to the probe pulse, expanding it from 130 fs to 30 ps. The terahertz pulse is detected using electro-optic sampling in a zinc telluride crystal. The temporal profile of the terahertz pulse is encoded on the probe-pulse frequency components; in a way, says Ferguson, this can be seen as wavelength-division multiplexing. A spectrometer and a charge-coupled-device camera are then used to separate out the different wavelength components to recover the terahertz pulse.

Detecting, but slowly
The system scans in two dimensions. As a test, the researchers adhered four different powdered substances to double-sided tape, then sandwiched the tape between two pieces of paper. The specimen was placed inside an envelope, which was scanned. All five substances, including the paper, were identified (see figure). In preliminary results, the researchers have also tested samples of a benign bacteria within envelopes, visualizing the spore flakes.

The two-dimensional (2-D) scanner takes 8 min to scan a single envelope—far too slow to be practical. A faster one-dimensional (1-D) version is possible, says Ferguson. "Our 2-D scanner requires the target to be scanned in x and y," he explains. "The terahertz beam is focused to a single point on the target. In the proposed 1-D scanning imager, the terahertz beam will be focused to a vertical line on the target. The target then only has to be scanned horizontally to acquire the full image. This method is an order of magnitude faster but has a degraded signal-to-noise ratio, since the available terahertz power is spread over more pixels. We are limited by the available terahertz power of our sources and do not plan to build a 1-D imager in the immediate future." Ferguson notes that higher-power terahertz sources are under development by a number of groups, making possible the construction of 1-D imagers with comparable sensitivity to the current 2-D system.

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