Stalking the Stealthy Atom

Have you ever stood in a long line at airport security, thinking to yourself, "there has got to be a better way?" We all understand the need for caution. A pressurised aluminum shell at 12,000 meters cannot take very much stress before it disintegrates, spilling its precious cargo across the countryside--that means you. Finding the explosives before the explosives find you, becomes very important to a frequent flyer. That is one application of cavity ring-down spectroscopy--a type of laser absoption spectroscopy. Cavity ring-down spectroscopy is an excellent way of detecting materials, but it is limited to measuring a narrow spectrum of 1 nm at a time.

A recent breakthrough described in this newsrelease allows the simultaneous scanning of wavelengths spanning 100 nm (750-850nm)--from the visible into the near infra-red.

Described 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.

...."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 and molecules.

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 simultaneously.

* 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.

By 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 trace gases.

In a previous posting I discussed the use of mass spectrometry for similar purposes. Both techniques have their advantages, and each will find niches of opportunity.

It is important that we be able to sense the world around us in very fine detail, given the potential dangers we face in our lives and travels. As we delve more deeply in the worlds of bio-engineering and nanotechnology, the need for more sensitive detection instruments becomes even more acute.