Graph shows the NIST detector’s linear increase in frequency as a function of time, sweeping from 550-561 Gigahertz in frequency over 100 nanoseconds. Click on the image to see an animation of the process, slowed to 5 seconds and using an audio chirp as an analogy to the terahertz chirp. Credit: Douglass, NIST |
Trace
gas detection, the ability to detect a scant quantity of a particular
molecule—a whiff of formaldehyde or a hint of acetone—in a vast sea of
others, underlies many important applications, from medical tests to air
pollution detectors to bomb sniffers. Now, a sensor recently developed
at the National Institute of Standards and Technology (NIST) that is
hundreds of times faster and more sensitive than other similar
technologies may make such detectors portable, economical and fast
enough to be used everywhere.
According
to the NIST investigators, the new sensor overcomes many of the
difficulties associated with trace gas detection, a technique also used
widely in industry to measure contaminants and ensure quality in
manufacturing. A trace level of a particular gas can indicate a problem
exists nearby, but many sensors are only able to spot a specific type of
gas, and some only after a long time spent analyzing a sample. The NIST
sensor, however, works quickly and efficiently.
“This
new sensor can simultaneously detect many different trace gases at very
fast rates and with high sensitivity,” says NIST chemist Kevin
Douglass. “It’s also built from off-the-shelf technology that you can
carry in your hands. We feel it has great commercial potential.”
The
key to the new sensor is the use of radiation at “terahertz”
frequencies—between infrared and microwaves. Terahertz waves can make
gas molecules rotate at rates unique to each type of gas, which implies
the waves hold great promise for identifying gases and measuring how
much gas is present. The NIST team has developed the technology to
rotate the molecules “in phase”—imagine synchronized swimmers—and detect
the spinning molecules easily as they gradually fall out of phase with
each other.
A
major hurdle the new technology overcomes is that it is now possible to
look at nearly all possible gas molecules instantly using terahertz
frequencies. Previously, it was necessary to expose molecules to a vast
range of terahertz frequencies—slowly, one after another. Because no
technology existed that could run through the entire frequency band
quickly and easily, the NIST team had to teach their off-the-shelf
equipment to “chirp.”
“The
sensor sends a quick series of waves that run the range from low
frequency to high, sort of like the ‘chirp’ of a bird call,” says
Douglass. “No other terahertz sensor can do this, and it’s why ours
works so fast. Teaching it to chirp in a repeatable way has been one of
our team’s main innovations, along with the mathematical analysis tools
that help it figure out what gas you’re looking at.”
The
NIST team has applied for a patent on its creation, which can plug into
a power outlet and should be robust enough to survive in a real-world
working environment.
E.
Gerecht, K.O. Douglass and D.F. Plusquellic. Chirped-pulse terahertz
spectroscopy for broadband trace gas sensing. Optics Express, April 22,
2011, Vol. 19, Issue 9, pp. 8973-8984 (2011), doi:10.1364/OE.19.008973.
