Nanoscale polymer recognition by spectral signature in scattering infrared near-field microscopy

We demonstrate—for a typical polymer vibrational infrared line—that scattering-type “apertureless” optical near-field microscopy features a spectral signature that differs characteristically from far-field absorption. Theory predicts a dispersion-like amplitude spectrum (besides an absorption-like, bell-shaped phase spectrum). This signature is experimentally verified for a vibrational resonance of PMMA, by probing with a CO laser tuned from 5.5 to 6 μm. We apply this signature to identify PMMA in the near-field imaging of a nanostructured PMMA/PS polymer blend, at <70nm resolution. Our results suggest a potentially quantitative chemometry based on near-field infrared vibrational fingerprints with spatial resolution that could reach 10 nm.