IEEE Spectrum July, 2007 - 38

Pulses drive
a photoconductor
antenna.

T-rAY SCANNEr
There are many methods under
development to scan people for
weapons, drugs, and explosives
using terahertz radiation. one
particularly promising method
works by detecting the spatial
frequencies of an image and
reconstructing the picture
from them in a computer.

lasers mix to make
1-thz pulses.
transmitter

laser frequency
=x

optical
fiber

receiver
the arrangement of and distances between
detectors on a receiving dish represent the
spatial frequency components of the image.

laser frequency
= x + 1 thz

Spatial frequencies are added to produce the image.

News, Va. Forcing a fast-moving electron to make a sharp turn drive an electronic circuit with a picosecond pulse of laser light.
produces a broad spectrum of T-rays instead of the single fre- Such a T-ray generator is basically a photosensitive semiconductor
quency of a laser beam. At many tens of watts, the Jefferson with a pair of antennas etched onto its surface. A voltage on the
machine is still orders of magnitude more powerful than most antennas sets up a strong electric field across the semiconductor
other sources. In fact, it may be powerful enough to penetrate between them. When the laser pulse strikes the semiconductor it
some distance into the ground and discover land mines and IEDs creates pairs of charge carriers: electrons and holes. These accelat a distance; because of this the U.S. military has contracted erate across the semiconductor and through the antennas. For a
Advanced Energy Systems to design an electron accelerator and femtosecond-long pulse, the rush of current lasts about a picoT-ray generator compact enough to fit in a Humvee and capable second, about the period of one cycle of 1-THz radiation.
of producing 1 W of radiation. The portable version would have
The resulting T-ray pulse is weak, with an average power only
a design more akin to a free-electron laser, but it would produce somewhere around a microwatt, but it's still bright enough to
a broader spectrum of T-rays than a laser can.
produce still images. And the pulses have a couple of interestThere are many types of T-ray sources that have smaller foot- ing side benefits. First, as with radar, timing the pulse's echo
prints than the enormous electron racetracks in Newport News as it bounces off an object gives the range to that object. Range
and Novosibirsk. These depend on combining electronics with is useful in processing multilevel T-ray images, such as a scan
lasers, befitting the radiation's straddling of the two worlds.
of a suitcase that might be difficult to interpret unless it had
Gigahertz-frequency oscillation is no big deal-the inexpen- been scrutinized layer by layer. Second, pulses let you perform
sive circuits in your cellphone are a testament to that fact. But spectroscopy, the identification of a substance by the waveit's quite another thing to build a circuit that oscillates trillions lengths of light it reflects. This capability comes from the fact
of times per second at terahertz rates. Even the 2006 record holder that a single pulse actually comprises a broad swath of T-ray
for the fastest-switching transistor in the world, the 845-GHz frequencies. You need only analyze the shape of the pulse's echo
device made by Milton Feng's group at the University of Illinois to calculate which frequencies were absorbed and then look up
at Urbana-Champaign, is barely in the terahertz range. However, what substances produce that absorption pattern.
for over a decade, scientists have been able to generate laser pulses
The problem with pulses is that they are quickly absorbed and
so short that 10 trillion or more could fit in a single second. So "smeared" in air, particularly humid air. After only a few meters in
the most common commercial method of making T-rays is to moist air, a 1-ps pulse lasts 30 ps, and the resolution of an image

IEEE Spectrum | July 2007 | NA

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BryAN chrIStIE dESIGN

2 spatial frequencies

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Table of Contents for the Digital Edition of IEEE Spectrum July, 2007