Near Field Optical Microscopy Imaging with a Free Electron Laser in the 1-10 Micron Spectral Range: Overview and Perspectives

The need to image objects on increasingly finer scales and spatially localized specific molecules can be met by the combination of infrared, visible and Raman spectroscopy with scanning near-field microscopy, giving rise to a powerful nanospectroscopic tool used to perform simultaneous topographical measurements and optical/chemical characterization with subwavelength resolution, overcoming the diffraction limit of light. Conventional spectroscopy is often not enough sensitive and spatially resolved to detect specific elements or domains in a sample. Scanning near-field optical microscopy is a sensitive and flexible technique that achieves an enhanced optical resolution through the very close placement of the sensing element to the object. We present several results of near field spectroscopy with infrared radiation emitted by a free electron laser to investigate material science and biological samples. The local reflectivity revealed features, as a function of photon energy, that were not present in the corresponding shear-force (topology) images and were due to localized changes in the bulk properties of the sample. The size of the smallest detected features clearly demonstrated that near-field conditions were reached, with an optical spatial resolution well below the diffraction limit.