Magnetic circular dichroism (MCD) in near-threshold photoemission is measured for a perpendicularly magnetized Cs/Co/Pt(111) film with work function adjusted by Cs adsorption. For one-photon photoemission (1PPE) the MCD asymmetry is recorded at a fixed photon energy of hν = 3.06 eV and varying work function Φ. The asymmetry shows a nonmonotonous behavior in dependence of the excess energy hν-Φ with a maximum value of A1PPE = 6.2 % at Φ = 2.45 eV. The measurement explores the first excitation step of a former two-photon photoemission (2PPE) measurement with A2PPE = 8.4% demonstrating that in 2PPE from Co(111) the first excitation step is the dominant asymmetry-generating process. An energy-dependent measurement in 2PPE at reduced work function (Φ≈ 3 eV) yields a constant asymmetry of about 17% in the photon energy range between hν = 1.53–1.66 eV. It reveals that for Co(111) the involvement of a real intermediate state is crucial for enlarged MCD asymmetries. Both results are discussed in the framework of direct interband transitions in directions deviating from the direction of normal electron emission Γ-L. The 1PPE measurement is in reasonable agreement with calculations on the basis of this model. This reveals that an ab initio calculation considering all directions of excitation with an additional restriction in energy due to the existence of the sample work function in the photoemission process adequately describes MCD asymmetries in near-threshold photoemission.

Strongly coupled plasmons in a system of individual gold nanoparticles placed at subnanometer distance to a gold film (nanoparticle-onplane,
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.

We apply colloidal lithography to construct stacked nanocrescent dimer structures with an exact vertical alignment and a separation distance of approximately 10 nm. Highly ordered, large arrays of these nanostructures are accessible using nonclose-packed colloidal monolayers as masks. Spatially separated nanocrescent dimers are obtained by application of spatially distributed colloids. The polarization dependent optical properties of the nanostructures are investigated in detail and compared to single crescents. The close proximity of the nanocrescents leads to a coupling process that gives rise to new optical resonances which can be described as linear superpositions of the individual crescents’ plasmonic modes. We apply a plasmon hybridization model to explain the spectral differences of all polarization dependent resonances and use geometric arguments to explain the respective shifts of the resonances. Theoretical calculations are performed to support the hybridization model and extend it to higher order resonances not resolved experimentally.

We introduce a new optical nanoantenna structure
with three-fold rotational symmetry. The proposed gold
nanoantenna can produce a hot spot with circular polarization
in the gap. The effect of the shape of the arms and geometrical
imperfection on the optical response is examined.
We introduce a figure of merit for practical applications that
utilize the circular polarization. The figure of merit is more
sensitive to changes of the geometry than the maximum
near field amplitude or the maximum circular polarization.
The relatively equivalent optical response of the proposed
nanoantenna compared to its counterparts and its reasonable
stability against small changes accompanied with its simpler
structure design makes it more appropriate for experimentalists.

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.

We report the observation of magnetic circular dichroism (MCD) in two-photon photoemission (2PPE). The Heusler alloys Ni2MnGa and Co2FeSi were investigated by excitation with femtosecond laser light, showing MCD asymmetries of A=(3.5±0.5)×10-3 for Ni2MnGa and of A=(2.1±1.0)×10-3 for Co2FeSi, respectively. A theoretical explanation is provided based on local spin-density calculations for the magnetic dichroic response; the computed 2PPE MCD agrees well with the experiment. The observed 2PPE magnetic contrast represents an interesting alternative for future time-resolved photoemission studies on surface magnetism practicable in the laboratory.