at JILA have designed and demonstrated a highly sensitive
new tool for real-time analysis of the quantity,
structure and dynamics of a variety of atoms and
molecules simultaneously, even in minuscule gas samples.
The technology could provide unprecedented capabilities
in many settings, such as chemistry laboratories,
environmental monitoring stations, security sites
screening for explosives or biochemical weapons,
and medical offices where patients' breath is analyzed
to monitor disease.
in the March 17 issue of Science,* the new technology
is an adaptation of a conventional technique, cavity
ring-down spectroscopy, for identifying chemicals
based on their interactions with light. The JILA
system uses an ultrafast laser-based "optical
frequency comb" as both the light source and as a
ruler for precisely measuring the many different
colors of light after the interactions. The technology
offers a novel combination of a broad range of frequencies
(or bandwidth), high sensitivity, precision and speed.
A provisional patent application has been filed.
JILA is a joint institute of the National Institute
of Standards and Technology (NIST), a non-regulatory
agency of the U.S. Department of Commerce, and the
University of Colorado at Boulder.
"What a frequency comb can do beautifully is offer
a powerful combination of broad spectral range and
fine resolution," says NIST Fellow Jun Ye, who led
the work described in the paper. "The amount of information
gathered with this approach was previously unimaginable.
It's like being able to see every single tree of
an entire forest. This is something that could have
tremendous industrial and commercial value."
Frequency combs are an emerging technology designed
and used at JILA, NIST and other laboratories for
frequency metrology and optical atomic clocks, and
are being demonstrated in additional applications.
NIST/JILA physicist John (Jan) Hall shared the 2005
Nobel Prize in physics in part for his contributions
to the development of frequency combs [www.nist.gov/public_affairs/newsfromnist_frequency_combs.htm].
In the application described in Science, the frequency
comb is used to precisely measure and identify the
light absorption signatures of many different atoms
The JILA system described in Science offers exceptional
performance for all four of the primary characteristics
desired in a cutting-edge spectroscopic system:
- The system currently spans 125,000 frequency
components of light, or 100 nanometers (750-850
nm) in the visible and near-infrared wavelength
range, enabling scientists to observe all the energy
levels of a variety of different atoms and molecules
- High resolution or precision allows scientists
to separate and identify signals that are very
brief or close together, such as individual rotations
out of hundreds of thousands in a water molecule.
The resolution can be tweaked to reach below the
limit set by the thermal motion of gaseous atoms
or molecules at room temperature.
- High sensitivity--currently 1 molecule out of
100 million--enables the detection of trace amounts
of chemicals or weak signals. With additional work,
the JILA team foresees building a portable tool
providing detection capability at the 1 part per
billion level. Such a device might be used, for
example, to analyze a patient's breath to monitor
diseases such as renal failure and cystic fibrosis.
- A fast data-acquisition time of about 1 millisecond
per 15 nm of bandwidth enables scientists to observe
what happens under changing environmental conditions,
and to study molecular vibrations, chemical reactions
and other dynamics.
comparison, conventional cavity ring-down spectroscopy
offers comparable sensitivity but a narrow bandwidth
of about 1 nanometer. A more sensitive "optical nose" technique
developed at NIST can identify one molecule among
1 trillion others, but can analyze only one frequency
of light at a time. Other methods, such as Fourier
transform infrared spectroscopy, provide large bandwidths
and high speed but are not sensitive enough to detect
The research at JILA is supported by the Air Force
Office of Scientific Research, NIST, Office of Naval
Research, National Aeronautics and Space Administration,
and National Science Foundation.
As a non-regulatory agency of the Commerce Department's
Technology Administration, NIST promotes U.S. innovation
and industrial competitiveness by advancing measurement
science, standards and technology in ways that enhance
economic security and improve our quality of life.
* M.J. Thorpe, K.D. Moll, R.J. Jones, B. Safdi,
and J. Ye. 2006. Broadband cavity ringdown spectroscopy
for sensitive and rapid molecular detection. Science.
Background: Using a Frequency Comb to Enhance
Cavity ring-down spectroscopy identifies atoms or
molecules by the way they absorb laser light as it
is repeatedly reflected and dissipates inside a mirrored
JILA system uses a laser that emits a broad range
of colors. The laser generates about 380 million
pulses per second, each lasting about 20 femtoseconds
(quadrillionths of a second). The laser light is
tuned to the "resonant frequency" of the cavity,
such that all of the many different wavelengths of
light--all "harmonics" of a single basic wave size--fit
perfectly between two special mirrors. The distance
between the mirrors is adjusted using tiny motors
to select the resonant frequency of the cavity. The
mirrors inside the laser are then rotated to match
the laser frequencies to those of the cavity.
The light is repeatedly reflected inside the cavity
until the laser is turned off, after which all of
the energy is gradually lost in a few microseconds.
If atoms or molecules are placed inside the cavity,
they absorb some of the light energy at frequencies
where they switch energy levels, vibrate, or rotate,
and the light dissipates faster at those frequencies.
beam of "white light" is emitted from the cavity
during the dissipation process and separated into
a rainbow of colors, which are detected in sets of
color bands. Computer software can analyze the change
in the decay time of selected channels of different
frequencies simultaneously. The results are rapidly
matched against a catalog of absorption signatures
of known atoms and molecules.
The JILA method was demonstrated by conducting a
variety of experiments with argon atoms and acetylene,
water, oxygen, and ammonia molecules. The scientists
demonstrated real-time, quantitative measurements
of traces of gas, the frequencies and strength of
signals signifying changes in energy levels, and
other changes due to collisions and temperature changes
inside the cavity.
For instance, the system identified a change in
the acetylene signal, detected as a faster dissipation
time, as the pressure of the background argon gas
was increased and collisions between the gases increased.
The signal resolution was sufficient to reveal spectral
information that is difficult to access because it
is below the physical limits set by the thermal motion
of the gas molecules. In addition, analyses of water,
ammonia, and oxygen demonstrated that nearly the
entire 100 nm spectral range can be probed simultaneously.
This combination of high resolution and broad bandwidth