Broadband optical near-field microscope for nanoscale absorption spectroscopy of organic materials

Conjugated organic materials in the solid state are generally amorphous or polycrystalline, with local order only achieved in mesoscopic domains with size ranging from a few tens to a few hundreds of nanometres. Understanding the interplay between mesoscopic order and macroscopic behaviour of these materials calls for a spatially resolved study of their optical properties. Near-field scanning optical microscopy allows one in principle to beat the diffraction limit in optical imaging. A quantitative measurement of nanoscale absorption spectra is, however, complicated by the difficulty of obtaining broadband near-field illumination with sufficiently high intensity. Here we demonstrate a near-field spectrometer with 100-nm spatial resolution based on an ultrabroadband Ti : sapphire oscillator coupled to an aperture-based near-field scanning optical microscopy, enabling structural phase-selective imaging of organic materials at the nanoscale. In polycrystalline phtalocyanine films we can distinguish between the crystalline and the amorphous phase, thus providing previously unavailable information on their mesoscopic texture.