We demonstrate the ultrafast generation of electrons from tailored metallic nanoparticles and unravel the role of plasmonic field enhancement in this process by comparing resonant and off-resonant particles, as well as different particle geometries. We find that electrons become strongly accelerated within the evanescent fields of the plasmonic nanoparticles and escape along straight trajectories with orientations governed by the particle geometry. These results establish plasmonic nanoparticles as versatile ultrafast, nanoscopic sources of electrons.

We theoretically investigate electron energy loss spectroscopy (EELS) of metallic nanoparticles in the optical frequency domain. Using a quasistatic approximation scheme together with a plasmon eigenmode expansion, we show that EELS can be rephrased in terms of a tomography problem. For selected single and coupled nanoparticles we extract the three-dimensional plasmon fields from a collection of rotated EELS maps. Our results pave the way for a fully three-dimensional plasmon-field tomography and establish EELS as a quantitative measurement device for plasmonics.

We theoretically investigate gap plasmons for two silver nanocubes coupled through a molecular tunnel junction. In absence of tunneling, the red-shift of the bonding mode saturates with decreasing gap distance. Tunneling at small gap distances leads to a damping and slight blue-shift of the bonding mode, but no low-energy charge transfer plasmon mode appears in the spectra. This finding is in stark contrast to recent work of Tan et al. [Science 343, 1496[Science 343, (2014 [1,[3][4][5][6]. Molecular tunnel junctions enable tunneling over larger gap distances in the nanometer regime [7,8], and thus establish a novel platform for hybrid structures reconciling molecular electronics with plasmonics.Recent years have seen significant research efforts to understand the properties of gap plasmons, and have highlighted the importance of the tunneling strength as a trigger for the CTP appearance [9] and of the gap morphology which strongly influences the CTP spectral position [10]: for rounded gap terminations the bonding mode red-shifts with decreasing gap separation, until tunneling sets in, as evidenced by the appearance of a low-frequency CTP together with a broadening and blueshift of the bonding mode [1,3,6]. In contrast, for flat terminations the red-shift of the bonding mode saturates with decreasing gap distance, while at the same time the transversal cavity plasmon (TCP) modes shift to the red; here, the onset of tunneling has no significant impact on the bonding mode and no low-frequency CTP appears in the spectra.In this paper, we theoretically investigate the plasmonic properties of two coupled silver nanocubes, similarly to the electron energy loss spectroscopy (EELS) experiments of Tan et al. [7] for two nanocubes coupled through a molecular tunnel junction. We compute EEL and extinction spectra using the MNPBEM toolbox [11][12][13], supplemented with the quantum corrected model (QCM) [14] to account for quantum tunneling. We find that the red-shift of the bonding mode saturates with decreasing distance and an additional tunnel conductivity in the gap region leaves the spectral position unaffected. The TCP modes shift with decreasing gap distance to the red, and the tunnel conductivity damps these modes. All these findings are in perfect agreement with the observations of Esteban et al. [6] for flat gap terminations and would qualify our work as a systematic research paper, if it was not for this single point: despite serious efforts we were unable to confirm the emergence of the low-energy CTP observed by Tan et al. [7] and could not reproduce their simulation results. We will argue why we believe that our results are valid within the electrodynamic and QCM model under consideration, and why a re-interpretation of the experiments might be needed.In our simulations we model the cubes with rounded edges and corners as superellipsoides, whose boundaries are parameterized through u ∈ [0, π) and v ∈ [−π, π) according towhere a determines the cube size (we use side lengths of 35 nm throughout), r is a roundi...

The photonic local density of states (LDOS) governs the enhancement of light–matter interaction at the nanoscale, but despite its importance for nanophotonics and plasmonics experimental local density of states imaging remains extremely challenging. Here we introduce a tomography scheme based on electron microscopy that allows retrieval of the three-dimensional local density of states of plasmonic nanoparticles with nanometre spatial and sub-eV energy resolution. From conventional electron tomography experiments we obtain the three-dimensional morphology of the nanostructure, and use this information to compute an expansion basis for the photonic environment. The expansion coefficients are obtained through solution of an inverse problem using as input electron-energy loss spectroscopy images. We demonstrate the applicability of our scheme for silver nanocuboids and coupled nanodisks, and resolve local density of states enhancements with extreme sub-wavelength dimensions in hot spots located at roughness features or in gaps of coupled nanoparticles.

Electron tomography in combination with electron energy-loss spectroscopy (EELS) experiments and simulations was used to unravel the interplay between structure and plasmonic properties of a silver nanocuboid dimer. The precise 3D geometry of the particles fabricated by means of electron beam lithography was reconstructed through electron tomography, and the full three-dimensional information was used as an input for simulations of energy-loss spectra and plasmon resonance maps. Excellent agreement between experiment and theory was found throughout, bringing the comparison between EELS imaging and simulations to a quantitative and correlative level. In addition, interface mode patterns, normally masked by the projection nature of a transmission microscopy investigation, could be unambiguously identified through tomographic reconstruction. This work overcomes the need for geometrical assumptions or symmetry restrictions of the sample in simulations and paves the way for detailed investigations of realistic and complex plasmonic nanostructures.