Physicists have joined forces to perfect a powerful new weapon in the war on terrorism – a laser technique to identify deadly anthrax spores. //Their results are published in the prestigious journal Science, due to hit newsstands tomorrow.
“Our report shows how to use lasers to detect anthrax in real time as opposed to the cumbersome and wieldy way it is done now,” says Marlan Scully, Distinguished Professor of Physics at Texas A&M and holder of a joint faculty appointment at Princeton’s Applied Physics and Materials Science Group. “We do our experiments ‘on the fly’, so we can get a signature within a tiny fraction of a second. Our procedure can work for monitoring anthrax in mail, but it can also scan the whole atmosphere. And there are a lot of other potential applications -- monitoring glucose in the blood, for example.”
The technique was developed by Scully and a group of 11 other researchers, including Texas A&M physicists Dmitry Pestov, Gombojav O. Ariunbold, Xi Wang, Miaochan Zhi, Alexei V. Sokolov, and Vladimir A. Sautenkov, Princeton physicists Arthur Dogariu and Yu Huang, and joint Texas A&M and Princeton scientists Robert K. Murawski and Yuri V. Rostovtsev. The group co-authored the Science article.
The new technique is based on coherent anti-Stokes Raman scattering (CARS), a phenomenon that measures the light scattering that occurs when a molecule is bombarded by light energy (photons). Molecules are composed of two or more atoms, and the subatomic particles which make up these atoms are in constant motion, producing vibration patterns unique to each substance. When a molecule is hit by an appropriate sequence of laser pulses, it gives off light in a specific ‘fingerprint’ pattern. If three laser pulses are used, the resultant emitted light yields a coherent signature at a particular frequency.
“Unfortunately, however, when anthrax molecules are subjected to such study, their CARS signature can be obscured
by background signals from other molecules present in the medium containing the anthrax spores,” Scully explains.
Scientists have tried a variety of methods to counteract or neutralize these background vibrations, with only limited success, he notes.
But the Texas A&M-Princeton group has developed new techniques for minimizing the background “noise” from extraneous molecules and maximizing the coherent molecular oscillations crucial to detecting endospores of Bacillus anthracis (anthrax). They have accomplished this by using a succession of femtosecond pulses (one femtosecond is one billionth of one millionth of a second) so that the first two laser pulses in the CARS process prepare a coherent molecular vibration, then time-delaying the third laser pulse, which is scattered off the molecular oscillation, resulting in the anti-Stokes fingerprint. This technique in effect cancels the noise from vibrations of non-anthrax molecules.
“The combination of shared preparation pulses and an ultrashort time-delayed probe pulse maximized the signal and lessens the background contribution,” Scully says. They call this approach femtosecond adaptive spectroscopic techniques via CARS (FAST-CARS).
The research reported in Science represents further refinement of the FAST-CARS method, using ultrashort broadband pulses for the first two laser bursts and a longer, tailored narrowband pulse for the third laser probe.
“This combination of broadband preparation and time-delayed narrowband probing yields a very sensitive and robust technique that allows us to identify bacterial endospores, such as anthrax, in real time,” Scully explains. “We refer to this technique as hybrid CARS for short.”
Using the new method, Scully and his colleagues used the dipicolinic acid (DPA) marker molecule to identify anthrax spores.
“DPA, in the form of its salt, calcium DPA, accounts for 10 to 17 percent of the bacterial spo
re dry weight,” he notes. “In our research, we worked with sodium DPA, which is similar to the calcium salt, but easier to work with.”
The significance of this project has been recognized by other researchers.
“This paper is a remarkable achievement in that it makes very clever use of modern techniques for coherent control to detect molecular signals with high sensitivity. Many important practical applications can be foreseen for this technology,” says Harvard professor of physics Mikhail Lukin.
Szymon Suckewer, Princeton professor of mechanical and aerospace engineering, also was impressed by the efforts of Scully and his colleagues.
“Scully has convinced a number of very skeptical scientists (including some of our own at Princeton) that his theory and experimental approach are correct and has convinced DARPA to provide several millions in funding to do experiments,” Suckewer notes. “He found several excellent scientists to conduct experiments and was a key person in arranging two very sophisticated experimental set-ups, one at Texas A&M University and the second at Princeton University’s School of Engineering and Applied Sciences in conjunction with the Princeton Institute of the Science and Technology of Materials (PRISM).
“By running two experiments, it was possible to test several parts of the idea in parallel in order to obtain a final result faster. And the excellent results, just published today in Science, were achieved in less than three years! Many experimentalists cannot believe that a ‘pure theoretician’ could lead such difficult experiments.”
Scully, a modest man who’s always ready to give credit to other physicists, nonetheless is excited about the group’s current research.
“This has been a very challenging problem,” he notes. “So many people have said this could not work. People have left the program because they were afraid their careers would be scuttled. But we hav
e succeeded beyond our hopes and are now ready to take it to the next leel.”
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