Standoff explosives trace detection and imaging by selective stimulated Raman scattering

We introduce a sensitive method for laser based standoff detection of chemicals based on stimulated Raman scattering. Selective excitation of a particular Raman transition is detected by measuring the diffusely reflected laser light from a distant surface. The method simultaneously measures stimulated Raman loss and gain within a single laser shot and is insensitive to the optical properties (reflectivity/absorptivity) of the substrate. We demonstrate the specificity and sensitivity by detecting and imaging nanogram analyte micro-crystals on paper, fabric, and plastic substrates at 1 to 10 m standoff distance using only 10 mW of laser power from a single femtosecond laser.