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, taki ng advantage of the infrared spectral signature of different chemical speci es. We utilised CLIO, a free electron laser, as the photon source. The tran smitted photons were collected by either fluoride or chalcogenide glass fib res. Fibre tips were obtained through chemical etching by organic solvents. Metallisation of the tips permits to achieve lateral resolution of the ord er of the tip size. However, parasitic propagation of the light in the lm r educes the contrast between irradiated and non-irradiated zones. We exempli fy our set up with near-field infrared spectra obtained for polymer thin lm s deposited onto silicon wafers.