Routes to enabling Raman detection of in-theatre biological contaminants over extended range: Spatial heterodyne spectroscopy, time-resolved Raman measurements, and the march towards the deep-UV
We report on an investigation addressing the challenge of the rapid detection of in-theatre surface chemical, biological and explosive (CBE) contaminants at a stand-off distance (<1m). The techniques we will describe are fundamentally underpinned by highly characteristic, molecule-specific Raman scattering. The implementation of Raman-at-range is problematic due to the extremely weak scattering cross-sections associated with this process, particularly when undertaken at the near-infrared wavelengths usually mandated by the need to suppress fluorescence. Excitation at shorter (near-UV) wavelengths can result in a two-order increase in scatter and this, combined with the extremely high throughput associated with Spatial Heterodyne Spectrometer (SHS) instrumentation, proves a viable route to Raman-at-range. We then implement time resolved spectral measurements on the ~100ps time scale to exploit the difference in generation timescale associated with Raman scatter and fluorescence generation; once so divorced the characteristics (both temporal and spectral) of the previously-troublesome fluorescent light can be embraced as an additional detection tool. We will show how SHS instrumentation, coupled with low-noise detector technology, can offer over four orders of magnitude improvement in spectral signal-to-noise level compared to conventional Czerny-Turner 'slitted' spectrometers using lower-cost linear CCD detectors. Finally, we show how a move to the deep-UV ""Resonance-Raman"" excitation region of sub- 250nm excitation leads both to enormous improvements in generated Raman signal, and spectral separation of the precious Raman from the troublesome fluorescence signal. We show the viability of this approach with biological spore simulant samples provided by DSTL.