imaging technology emerged as the winner in that race.
Many airports are now equipped with
millimeter-wave scanners that use 20–
30 GHz ( 15–10 mm wavelength) sources and detectors to produce three-dimensional (3D) images of passengers
revealing hidden objects. The next generation of these scanners is likely to use
80–90 GHz technology, reaching clos-
er to the terahertz range. Use of millimeter-wave technology in wireless
communications, a much larger mar-
ket, has helped to bring more invest-
ment for technology development and
reduce the cost of products.
A second event of note was the Space
Shuttle Columbia disaster of 2003 that
was linked to defects in the adhesion
of thermal insulating foam to the body
of the spacecraft. Terahertz imaging
was brought in as one of the diagnostic
tools by NASA to ensure safety on the
missions that followed. Cost was not
an issue in this project and Picometrix
(Roanoke, VA; now a division of Luna)
delivered several time-domain scanners
to NASA. Foam is very transparent in
the terahertz range and in this case,
terahertz imaging was a viable complementary technology to x-ray and ultrasound imagers.
While companies developing time-do-
main terahertz imaging systems have
had more success in applications for
industrial process and product quality controls, competition from x-ray
and ultrasound imaging is fierce. More-established technologies tend to win on
price and reliability. And even though
terahertz systems often deliver better
performance, customers tend to settle
for more proven methods and are unwilling to take a risk by dealing with
a new technology.
Disasters aside, demand for terahertz
Plasmonic detectors and
imaging for agricultural and industri-
al applications seems to be picking up.
Progress in technology development also
remains steady. Two new approaches for
video-rate imaging include two-dimen-
sional (2D) arrays of plasmonic detectors
and upconversion of terahertz images to
the near-infrared (near-IR) region, re-
vealing an unseen world that eludes oth-
er spectral bands.
Recent progress in the purity of semi-
conductor aluminum gallium arse-
nide/gallium arsenide (AlGaAs/GaAs)
nanostructures has made it possible
to adapt the plasmonic concepts from
the optical region of the spectrum to
the microwave and terahertz bands.
Unfortunately, standard 2D plasmons
are observable only when their frequency is ω > 1/τ, where the momentum re-
laxation time τ essentially decreases
with increasing temperature. Hence, the