Near-field Raman dichroism of azo-polymers exposed to nanoscale dc electrical and optical poling

Azobenzene-functionalized polymer films are functional materials, where the (planar vs. homeotropic) orientation of azo-dyes can be used for storing data. In order to characterize the nanoscale 3D orientation of the pigments in sub-10 nm thick polymer films we use two complementary techniques: polarization-controlled tip-enhanced Raman scattering (TERS) microscopy and contact scanning capacity microscopy. We demonstrate that the homeotropic and planar orientations of the azo-dyes are produced by applying a local dc electrical field and a resonant longitudinal optical near-field, respectively. For a non-destructive probe of the azo-dye orientation we apply a non-resonant optical near-field and compare the intensities of the Raman-active vibrational modes. We show that near-field Raman dichroism, a characteristic similar to the absorption dichroism used in far-field optics, can be a quantitative indicator of the 3D molecular orientation of the azo-dye at the nanoscale. This study directly benefits the further development of photochromic near-field optical memory that can lead to ultrahigh density information storage

Atomic force and shear force based tip-enhanced Raman spectroscopy and imaging

Underlying near-field optibal effects on the nanoscale have stimulated the development of apertureless vibrational spectroscopy and imaging with ultrahigh spatial resolution. We demonstrate tip-enhanced Raman spectra of single-walled carbon nanotubes (SWCNTs), recorded with a scanning near-field optical spectrometer using both atomic force (AF) and shear force (SF) feedback lock-in regulation, and critically discuss the advantages and drawbacks of both operation modes. For accurate calculation of the enhancement factor obtained, we have analysed the tip shape and diameter by means of scanning electron and transmission electron microscopy (SEM and TEM). In our experiments we reproducibly attain diameter-corrected and area-corrected enhancement factors of up to ~10 4 and 10 5, respectively, estimated as the linear ratio of near- and far-field intensities, and we are able to demonstrate near-field Raman imaging if SWCNTs with spatial resolution better than 50 nm

Tip-enhanced Raman spectroscopy and imaging : Nanoscopic imaging of single-walled carbon nanotubes

A possibility to not only visualize but to locally probe a chemical structure, composition, conformational state and stresses on the nanoscale has stimulated the development of apertureless near-field vibrational spectroscopy and imaging with ultrahigh spatial resolution laying beyond the diffraction limit [1–3]. It has become possible due to the delocalization of evanescent waves (near-field) existing in the proximity of nano-sized objects with a sharp metal probe