Retrieving the spatial distribution of cavity modes in dielectric resonators by near-field imaging and electrodynamics simulations.

For good performance of photonic devices whose working principle is based on the enhancement of electromagnetic fields obtained by confining light into dielectric resonators with dimensions in the nanometre length scale, a detailed knowledge of the optical mode structure becomes essential. However, this information is usually lacking and can only be indirectly obtained by conventional spectroscopic techniques. Here we unraveled the influence of wire size, incident wavelength, degree of polarization and the presence of a substrate on the optical near fields generated by cavity modes of individual hexagonal ZnO nanowires by combining scanning near-field optical microscopy (SNOM) with electrodynamics calculations within the discrete dipole approximation (DDA). The near-field patterns obtained with very high spatial resolution, better than 50 nm, exhibit striking size and spatial-dispersion effects, which are well accounted for within DDA, using a wavevector-dependent dipolar interaction and considering the dielectric anisotropy of ZnO. Our results show that both SNOM and DDA simulations are powerful tools for the design of optoelectronic devices able to manipulate light at the nanoscale.

Thickness-dependent structural transitions in fluorinated copper-phthalocyanine (F16CuPc) films.

The detailed structure of F16CuPc films on SiO2 has been determined by means of in situ grazing incidence X-ray diffraction from the first monolayer to thicker films. In contrast to films of the homologous H16CuPc molecule, the F16CuPc films exhibit the same structure independently from the deposition temperature. The films show a thickness-dependent polymorphism manifested in the in-plane crystal structure, which implies large differences in the molecular tilt within the cofacial stacking of the molecules.

In situ study of the growth of nanodots in organic heteroepitaxy.

We report the self-organization of organic nanodots with high crystallinity during the growth of organic heterostructures of Di-indenoperylene (DIP) onto copper-hexadecafluorophthalocyanine (F16CuPc), donor and acceptor molecules, respectively. The process is related to the Stranski-Krastanov growth mode, which is accompanied by a novel type of structural reconstruction of the underlying organic film. This reconstruction affects three monolayers adjacent to the organic interface. In spite of the close resemblance to the formation of semiconductor nanostructures for inorganic heteroepitaxy, the present results conclusively demonstrate a distinctly different growth mechanism for organic heteroepitaxy whose understanding demands further theoretical studies.

Structural rearrangements during the initial growth stages of organic thin films of F16CuPc on SiO2.

We present an experimental study on the first stages of the thin film growth of the organic molecule F(16)CuPc (hexadecafluoro-copper-phthalocyanines) on SiO(2). By means of in situ X-ray reflectivity, in situ grazing incidence X-ray diffraction (GIXD), and ex situ atomic force microscopy (AFM), we provide a detailed picture of the film growth mode and its structural evolution at the nanometer scale. We discovered the formation of a low-density layer of molecular aggregates with heights between 5 and 10 A at the interface with the SiO(2) and show that, on top of this interfacial layer, the nucleation and two-dimensional growth of elongated islands of upright standing molecules take place. Structural changes are observed, pointing to significant relaxations of the lattice parameters within the first layers of standing molecules.