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07 Jun 2007
Detecting bacterial spores by using coherent Raman scattering could help to improve national security.
Coherent anti-stokes Raman Scattering (CARS) is helping scientists at Princeton University, US, to identify bacterial spores such as anthrax in milliseconds. They hope that the real-time nature of this method could significantly improve national security.
"We have successfully detected about 300 spores in 50 milliseconds," Arthur Dogariu, Research Scholar at Princeton University, told optics.org. "Being a Raman spectroscopy method, CARS allows for identification of molecules, but the coherent enhancement allows it to happen much faster."
CARS detects bacterial spores by collecting vibrational spectra from molecules in their chemical fingerprint spectral region. Previous attempts at detecting a signal from spores yielded low efficiency.
"This is partly due to the molecules of interest being located in the core of the spore and much of the signal being lost to scattering at the interface," commented Dogariu. "CARS enhances the signal by several orders of magnitude."
Dogariu adds that the choice of excitation wavelength is also important. "Near-infrared beams allow higher energy without destroying the molecules and without generating background light such as multiphoton induced luminescence," he explained.
The team used an amplified Ti:Sapphire laser emitting 100fs pulses at 800nm, a total power of 700 mW and a repetition rate of 1 kHz. "500 mW is used to pump an optical parametric amplifier to produce a pump beam at 1.4 microns and a Stokes beam at 1.8 microns. Both are 10 µJ/pulse," explained Dogariu. "200 mW is used for the probe beam, which is spectrally narrowed by a pulse shaper to less than 1 nm. The probe pulse is about 1 µJ/pulse." According to Dogariu this method can detect spores on materials such as glass, mica, copper and even no substrate at all.
Another factor which can make detecting spores difficult is a high background signal from effects such as non-resonant four-wave mixing. This means that the background signals are often stronger than the signal the scientists are looking for. To reduce this, the team employed a time delay between the pump and Stokes pulses.
"The length of the delay depends on the lifetime of the vibrational levels, which needs to be less than the decoherence time," explained Dogariu. "For spores this seems to be 0.5-1 ps, which means that the delay is at most 1 ps. Collecting the light in a backscattered configuration, which is required for remote detection, also reduces the signal-to-noise ratio significantly."
The team now plans to further enhance the signal by using different pumping schemes to open up the possibility of detecting at range. "We are also looking into molecular detection in general, not only spores," concluded Dogariu. "We have so far obtained good results with various applications ranging from national security to biology and medicine."
The team presented these latest real-time results at the postdeadline session at CLEO in May. The work is the result of a two-year collaboration between two groups run by Prof. Marlan Scully at Texas A&M and Princeton University.
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