‘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,’ said 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.
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 photons. 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 explained.
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 anthrax (Bacillus anthracis). They have accomplished this by using a succession of femtosecond pulses 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 said. They call this approach femtosecond adaptive spectroscopic techniques via CARS (FAST-CARS).