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11/28/2007
Seeing in the Dark
If you watched Macy’s Thanksgiving Day parade in New York,
you know it was a sunny, balmy day here in the New York/New
Jersey area. The red and yellow maple trees were still in full
color as Brian Trumbore drove my wife and me to his club for a
much-appreciated Thanksgiving dinner. The temperature hit 65
degrees Fahrenheit on our thermometer. The next day the
temperature fell some 25 or so degrees and by Monday the
yellow leaves had fallen to form a brilliant yellow carpet on our
neighbors’ lawn. Unfortunately, the oak leaves on the trees that
influence the state of our gutters hang on stubbornly overhead.
The challenge to clean out gutters and to clean up the lawn
before the first major snowfall remains.
Last week we talked about another vastly more significant
challenge, addressing the problem of arsenic contaminated water
in Bangladesh. Specifically, we discussed Abul Hussam and his
invention that won the Grainger Sustainability Challenge and one
million dollars. In passing, I also mentioned in that column an
experiment at Bell Labs with arsenic and germanium. This
week, an article by Charles Choi in the December Scientific
American alerted me to another challenge involving germanium,
two former Bell Labs guys and a substantial monetary prize, the
Global Security Challenge 2007 prize of 500,000 dollars.
The award, announced on November 8, was given to the
NoblePeak Vision Corporation, a start-up company co-founded
by Conor Rafferty and Clifford King. At Bell Labs, Rafferty and
King were involved in developing devices for long-distance
telecommunications applications. In modern optical fiber
telecommunications, the glass fibers carry voice and data via
packets of light that lie in the infrared region of the spectrum.
We see our visible world through eyes sensitive to light with
wavelengths of some 400-700 nanometers. Of interest here are
two regions of light lying in the near-infrared (NIR) with
wavelengths of 750-1050 nanometers and the short-wave
infrared (SWIR) with wavelengths of 1100-2000 nanometers.
In optical communications, you have to detect the light that’s
been transmitted. Silicon is good for visible light – witness its
use in cameras and camcorders and in telescopes. It’s also
marginally OK for part of the NIR region. Germanium, on the
other hand, is good for detecting light in the visible, NIR and a
large portion of the SWIR range. (I found curves showing the
sensitivities of silicon and germanium to light in all three regions
on the NoblePeak Web site and found other details on the Web
sites of the London Business School and of the Global Security
Challenge itself.) NoblePeak won the award for its night vision
work, which involves the use of germanium in night vision
devices such as the devices worn by our troops in Iraq.
Using germanium instead of silicon, Rafferty and King and the
NoblePeak team have come up with a night vision system that
works in visible and in both NIR and SWIR infrared light. This
means that a camera or other device using their germanium
sensor will work day and night. Which brings up the subject of
nightglow. Even at night the sky has a faint glow due to
molecules excited by sunlight or by cosmic rays. The amount of
light is roughly equal to the light from a full moon, but spread
over the whole night sky. However, most of this nightglow lies
in the invisible SWIR region. Just as we can see objects by
reflected moonlight, the germanium night vision device can
“see” by the reflected nightglow in the SWIR region of the
spectrum.
The use of nightglow can have a distinct advantage in military
applications. With a very sensitive detector, the nightglow in
essence illuminates the target much as a spotlight will reveal that
target. With more conventional night vision systems, one
approach is to use the infrared equivalent of a spotlight, namely
illuminating the target with invisible infrared light. This is fine if
your enemy is not technically advanced. If the enemy is savvy,
however, he or she will detect your illumination with their night
vision devices and will likely end up killing you! With
nightglow, you don’t reveal your presence.
In order to produce these germanium devices at a reasonable cost
and in quantity, NoblePeak apparently has relied on some very
nice work at Bell Labs. There, Rafferty, King and colleagues
worked on growing thin layers of germanium on silicon wafers.
This approach let them use the vast silicon technology and
procedures developed over decades for manufacture of the
ubiquitous silicon chips. But there’s a problem. The structures
of germanium and silicon don’t match; that is, although both
have the crystal structure of diamond, the distances between the
atoms differ significantly. When you grow layers of germanium
on silicon, typically from a vapor of some sort, this mismatch
causes defects to form. You need germanium free of these
defects to maintain the high sensitivity for detecting the visible
and infrared light.
Well, the Bell Labs team observed that these defects grew
diagonally upward from the silicon base. Very cleverly, they
found that, if they grew the initial germanium in microscopic pits
in the silicon, the defects hit the walls of the pits and the rest of
the germanium grew upwards, free of the defects. Unfortunately,
at the time of their discovery came the big telecommunications
crash, a crash I remember vividly, my Lucent stock falling from
about 80 to a buck or less! For Rafferty and King, the crash
spurred them to switch their attention from communications to
imaging and NoblePeak was born.
The company plans to target first the security camera and the
military markets but hopes eventually to “open a new window on
the electromagnetic spectrum”, as Rafferty is quoted in the Choi
article. One possible application would be to use night vision
devices to improve a driver’s vision at night. I certainly would
join millions of other “senior” drivers in welcoming that
application!
Allen F. Bortrum
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