Infrared near-field study of a localised absorption in a thin film

We study the conditions to perform micro-spectroscopy with a sub-wavelength lateral resolution in the wavelength spectral range from 3 to 20 mum, 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 lm reduces the contrast between irradiated and non-irradiated zones. We exemplify our set up with near-field infrared spectra obtained for polymer thin lms deposited onto silicon wafers.