Backscattered coherent anti-Stokes Raman scattering for at range detection of dipicolinic acid and four-wave mixing multiple scattering

We investigate femtosecond coherent anti-Stokes Raman scattering (CARS) signals collected at range from dipicolinic acid (DPA), one of the marker molecules within anthrax spores. The backscattered CARS signal from the DPA solution recorded versus probe pulse delay exhibits molecular coherence beating. We show a signal-to-noise ratio better than two orders of magnitude for a sample placed 12 m away, and infer the possibility of detection at range over distances of hundreds of metres. We also study the backscattered CARS signals from DPA powder. The beat signal exhibits two vibrational modes with a level spacing of 7010 cm- 1, which matches very well with the literature value of 69 cm- 1. We quantitatively study the effects of multiple scattering in four-wave mixing from powder samples and estimate an 8 m mean distance between the scattering events.