We study the conditions to perform micro-spectroscopy with a sub-wavelength lateral
resolution in the wavelength spectral range from 3 to 20 μm, taking advantage of the infrared
spectral signature of different chemical species. We utilised CLIO, a free electron laser, as the
photon source. The transmitted photons were collected by either fluoride or chalcogenide
glass fibres. Fibre tips were obtained through chemical etching by organic solvents.
Metallisation of the tips permits to achieve lateral resolution of the order of the tip size.
However, parasitic propagation of the light in the film reduces the contrast between irradiated
and non-irradiated zones. We exemplify our set up with near-field infrared spectra obtained for
polymer thin films deposited onto silicon wafers.