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Center for Nonlinear Studies |
Standoff detection of explosives in order to protect public spaces is extremely challenging. Explosives generally have very low vapor pressures, limiting the effectiveness of air sampling and promoting research into optical methods to detect thin residues or micro-particles on surfaces as indicators of concealed danger. With excellent chemical specificity, Raman spectroscopy is capable of detecting particular compounds within the chemically complex background manifest in everyday surfaces, but the weak signal of the spontaneous process prohibits quickly detecting such small quantities.
I will discuss our use of stimulated Raman scattering to quickly detect and image trace quantities of explosives in a standoff configuration.1 Stimulated Raman microscopy techniques generally employ two synchronized laser pulses and wavelength scanning to tune to the appropriate vibrational frequency. Our approach uses a single femtosecond laser and pulse shaper to selectively excite a particular transition with the broad bandwidth. The transition is detected by simultaneously measuring stimulated Raman gain and loss using the diffusely reflected laser light from a single pulse. I will present images showing detection of single microcrystals of NH4NO3 on a variety of real world surfaces using a few laser shots and collecting the strong signal at ten meters.
1. Bremer, M. T. & Dantus, M. Standoff explosives trace detection and imaging by selective stimulated Raman scattering. Appl. Phys. Lett. 103, 061119–061119–5 (2013).
Host: Andrei Piryatinski