Infrared scattering scanning near-field optical microscopy using an external cavity quantum cascade laser for nanoscale chemical imaging and spectroscopy of explosive residues

Abstract

Infrared scattering scanning near-field optical microscopy (s-SNOM) is an apertureless superfocusing technique that uses the antenna properties of a conducting atomic force microscope (AFM) tip to achieve infrared spatial resolution below the diffraction limit. The instrument can be used either in imaging mode, where a fixed wavelength light source is tuned to a molecular resonance and the AFM raster scans an image, or in spectroscopy mode where the AFM is held stationary over a feature of interest and the light frequency is varied to obtain a spectrum. In either case, a strong, stable, coherent infrared source is required. Here we demonstrate the integration of a broadly tunable external cavity quantum cascade laser (ECQCL) into an <i>s</i>-SNOM and use it to obtain infrared spectra of microcrystals of chemicals adsorbed onto gold substrates. Residues of the explosive compound tetryl was deposited onto gold substrates. <i>s</i>-SNOM experiments were performed in the 1260-1400cm^-1 tuning range of the ECQCL, corresponding to the N0₂ symmetric stretch vibrational fingerprint region. Vibrational infrared spectra were collected on individual chemical domains with a collection area of ~500 nm² and compared to ensemble averaged far-field reflection-absorption infrared spectroscopy (RAIRS) results.