Towards Single-Shot Detection of Bacterial Endospores via Coherent Raman Spectroscopy*

Recent advances in coherent anti-Stokes Raman scattering (CARS) spectroscopy hold exciting promise to make the most out of now readily available ultrafast laser sources. Techniques have been devised to mitigate the nonresonant four-wave-mixing in favor of informative Raman-resonant signal. In particular, a hybrid technique for CARS (see \textit{Science} \textbf{316}, 265 (2007)) brings together the advantages of coherent broadband pump-Stokes excitation of molecular vibrations and their time-delayed but frequency-resolved probing via a spectrally narrowed and shaped laser pulse. We apply this technique to the problem of real-time detection of warfare bioagents and report single-shot acquisition of a distinct CARS spectrum from a small volume of \textit{B. subtilis} endospores ($\sim $10$^{4}$ spores), a harmless surrogate for \textit{B. anthracis}. We study the dependence of the CARS signal on the energy of the ultrashort preparation pulses and find the limit on the pulse energy fluence ($\sim $0.2 J/cm$^{2})$, imposed by the laser-induced damage of the spores.