1970 1990 2010
ADVANCES IN IMAGING
Terahertz imaging: A technology
in search of applications
VIACHESLAV M. MURAVEV, GOMBO E. TSYDYNZHAPOV, IGOR V.
KUKUSHKIN, IAN MCNEE, and VLADIMIR G. KOZLOV
The first advances in terahertz sources, operating at frequencies of 0.1 to
3.0 THz and corresponding to a wavelength range of 3.0 to 0.1 mm, date
back to the 1970s. These were vacuum electronic devices called backward
wave oscillators (BWOs), producing up
to 100 m W of power at 0.1–0.3 THz
and up to 1 m W at higher frequencies.
Although more compact and reliable
solid-state terahertz electronic devices developed in the 1990s deliver similar performance now, reaching higher power levels remains a challenge.
Development of time-domain terahertz technology based on generation
of terahertz waves by ultrafast lasers
created the second round of excitement
about the potential of terahertz imaging
in the 1990s. Images of skin cancer pro-
duced by this method set up high expec-
tations for the use of terahertz technolo-
gy in medical applications, but progress
has remained slow, as strong absorption of terahertz waves by water limits their penetration depth in tissue to
1 mm at most. And time-domain terahertz imaging requires raster scanning
of samples since data is acquired one
pixel at the time—another challenge
for real-time diagnostics.
Video-rate terahertz imaging enabled by microbolometer detector
arrays developed by
INO (Québec City,
QC, Canada) and
NEC (Tokyo, Japan) around 2005
caused the next wave of excitement
in terahertz imaging technologies.
Terahertz microbolometer cameras
leveraged technology developed for the
mid-infrared (mid-IR) spectral range,
but improved the sensitivity of these
devices at lower frequencies, reaching
down to 1 THz.
Development of terahertz sources
using quantum cascade lasers (QCLs)
by extending the performance of mid-
IR QCLs to the terahertz range also
occurred around 2005, and seemed
to be perfect for microbolometer ter-
ahertz cameras in terms of spectral
range. But even though a combina-
tion of these devices delivered a viable
solution for video-rate terahertz im-
aging with sufficiently high contrast
at frequencies above 2. 8 THz with
cooled QCLs, the problem is
that the transmission of a majority of
materials deteriorates at such high fre-
quencies and, again, water absorption
becomes very strong.
The high cost of QCLs and cryogenic
cooling has hampered applications de-
velopment, even as terahertz technology
advances. And like any other photonics technology, development activity in
terahertz imaging has fluctuated, with
current levels of product development
being modest (see Fig. 1).
In 2015, one of the leading suppliers of time-domain terahertz systems—
Zomega Terahertz (Troy, N Y)—
discontinued operations, and other vendors
reduced their staffing levels. However,
this situation is improving.
Terahertz to the rescue
Opportunities for terahertz product
developers have often coincided with
disasters. The first one was the tragic
events of September 11, 2001. Potential
applications of terahertz imagers for
airport security scanners attracted a
lot of government funding in 2002-
2010, but high-frequency microwave
Despite losing traction for cancer and
security screening, terahertz imaging
continues to target several agricultural
and industrial applications that may finally
benefit terahertz equipment vendors.