Infrared near-field spectroscopy of trace explosives using an external cavity quantum cascade laser
By: Ian M. Craig, Matthew S. Taubman, A. Scott Lea, Mark C. Phillips, Erik E. Josberger, and Markus B. Raschke
Optics Express 21, 30401 (2013)
Utilizing a broadly-tunable external cavity quantum cascade laser for scattering-type scanning near-field optical microscopy (s-SNOM), we measure infrared spectra of particles of explosives by probing characteristic nitro-group resonances in the 7.1-7.9 μm wavelength range. Measurements are presented with spectral resolution of 0.25 cm−1, spatial resolution of 25 nm, sensitivity better than 100 attomoles, and at a rapid acquisition time of 90 s per spectrum. We demonstrate high reproducibility of the acquired s-SNOM spectra with very high signal-to-noise ratios and relative noise of <0.02 in self-homodyne detection.