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
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