Two-photon near-field characterization of hexaphenyl nanofibers

Using a two-photon scanning near-field optical microscope (SNOM) working in transmission we observe two-photon induced (TPI) luminescence from hexaphenyl nanofibers deposited on mica sheets. The nanofibers consist of organic hexaphenyl molecules that, due to a surface-dipole-assisted epitaxial growth process on a cleaved mica substrate, form almost perfectly aligned very thin (100 - 400 nm) and very long (up to mm) fibers (nanoneedles) with heights of 20 - 100 nm. We apply the two-photon SNOM for high-resolution mapping of local molecular orientations in nanofibers. Taking the width of nanofibers into account we evaluate the resolution being better than 0.4 mu m in the TPI-images and slightly worse in the topographic and fundamental harmonic images. From the ratio of TPI luminescence signals for two perpendicular polarizations of illumination we determine the local (with the resolution of 0.4 mu m) molecular orientation angles along the nanofibers. The molecular orientation angles obtained are comparable to our recent results obtained using two-photon scanning far-field microscopy.