Hydrodynamic boundary condition of water on hydrophobic surfaces.

By combining total internal reflection fluorescence cross-correlation spectroscopy with Brownian dynamics simulations, we were able to measure the hydrodynamic boundary condition of water flowing over a smooth solid surface with exceptional accuracy. We analyzed the flow of aqueous electrolytes over glass coated with a layer of poly(dimethylsiloxane) (advancing contact angle Θ = 108°) or perfluorosilane (Θ = 113°). Within an error of better than 10 nm the slip length was indistinguishable from zero on all surfaces.

Field emission of electrons generated by the near field of strongly coupled plasmons.

Field emission of electrons is generated solely by the ultrastrong near-field of strongly coupled plasmons without the help of a noticeable dc field. Strongly coupled plasmons are excited at Au nanoparticles in subnanometer distance to a Au film by femtosecond laser pulses. Field-emitted electrons from individual nanoparticles are detected by means of photoelectron emission microscopy and spectroscopy. The dependence of total electron yield and kinetic energy on the laser power proves that field emission is the underlying emission process. We derive a dynamic version of the Fowler-Nordheim equation that yields perfect agreement with the experiment.

Near field of strongly coupled plasmons: uncovering dark modes.

Strongly coupled plasmons in a system of individual gold nanoparticles placed at subnanometer distance to a gold film (nanoparticle-on-plane, NPOP) are investigated using two complementary single particle spectroscopy techniques. Optical scattering spectroscopy exclusively detects plasmon modes that couple to the far field via their dipole moment (bright modes). By using photoemission electron microscopy (PEEM), we detect in the identical NPOPs near-field modes that do not couple to the scattered far field (dark modes) and are characterized by a strongly enhanced nonlinear electron emission process. To our knowledge, this is the first time that both far- and near-field spectroscopy are carried out for identical individual nanostructures interacting via a subnanometer gap. Strongly resonant electron emission occurs at excitation wavelengths far off-resonant in the scattering spectra.

Visible mie scattering in nonabsorbing hollow sphere powders.

Hollow silica nanoparticles (HSNP) with diameters comparable to visible wavelengths and with thin shells (<15 nm) feature an unexpected color effect. Single particle and powder spectroscopy, as well as calculations based on Mie theory were used to investigate this phenomenon. The use of HSNPs increases the transport mean free path of light significantly, which reduces multiple scattering, and thus the Mie resonances become visible to the bare eye.

Fluorescence enhancement from individual plasmonic gap resonances.

We studied the fluorescence enhancement of a dye-loaded polyphenylene dendrimer in a gap of 2-3 nm between a silver film and single silver particles with an average diameter of 80 nm. This sphere-on-plane geometry provides a controllable plasmonic resonator with a defined dye position. A strong fluorescence signal was seen from all particles, which was at least 1000 times stronger than the signal from the plane dye-coated metal surface. The fluorescence emission profile varied between the particles and showed light emission at higher energies than the free dye, which we assigned to hot luminescence. The maximum fluorescence emission peak shifted along with the scattering maximum of the plasmonic resonance. Two classes of scattering resonators could be distinguished. Up to a significant line-broadening, the response of the "sphere-on-plane"-like cases resembled the theoretical prediction for a perfect sphere-on-plane geometry. Resonators which deviate strongly from this ideal scenario were also found. Electron microscopy did not show significant differences between these two classes, suggesting that the variations in the optical response are due to nanoscale variations of shape and roughness in the gap region. The strong modifications of the dye emission spectrum suggested the presence of physical mechanisms at very small metal/dye separations, which are beyond a simple wavelength-dependent enhancement factor.

Plasmon mediated confocal dark-field microscopy.

An efficient mode for scanning confocal dark-field microscopy through a thin gold film is established that takes advantage of the intermediate excitation of surface plasmons both in the excitation and in the emission process. This concept is verified by experimental investigation of the effective point-spread function, the intensity distribution of the scattered radiation and by comparison with a classical dark-field geometry. The wavelength-dependence of both the signal strength and the point-spread function are discussed.