Multimode plasmon excitation and in situ analysis in top-down fabricated nanocircuits.

We experimentally demonstrate synthesis and in situ analysis of multimode plasmonic excitations in two-wire transmission lines supporting a symmetric and an antisymmetric eigenmode. To this end we irradiate an incoupling antenna with a diffraction-limited excitation spot exploiting a polarization- and position-dependent excitation efficiency. Modal analysis is performed by recording the far-field emission of two mode-specific spatially separated emission spots at the far end of the transmission line. To illustrate the power of the approach we selectively determine the group velocities of symmetric and antisymmetric contributions of a multimode ultrafast plasmon pulse.

Spectral-interference microscopy for characterization of functional plasmonic elements.

Plasmonic modes supported by noble-metal nanostructures offer strong subwavelength electric-field confinement and promise the realization of nanometer-scale integrated optical circuits with well-defined functionality. In order to measure the spectral and spatial response functions of such plasmonic elements, we combine a confocal microscope setup with spectral interferometry detection. The setup, data acquisition, and data evaluation are discussed in detail by means of exemplary experiments involving propagating plasmons transmitted through silver nanowires. By considering and experimentally calibrating any setup-inherent signal delay with an accuracy of 1 fs, we are able to extract correct timing information of propagating plasmons. The method can be applied, e.g., to determine the dispersion and group velocity of propagating plasmons in nanostructures, and can be extended towards the investigation of nonlinear phenomena.

Ultrafast plasmon propagation in nanowires characterized by far-field spectral interferometry.

Spectral interferometry is employed to fully characterize amplitude and phase of propagating plasmons that are transmitted through silver nanowires in the form of ultrashort pulses. For nanowire diameters below 100 nm, the plasmon group velocity is found to decrease drastically in accordance with the theory of adiabatic focusing. Furthermore, the dependence of the plasmon group velocity on the local nanowire environment is demonstrated. Our findings are of relevance for the design and implementation of nanoplasmonic signal processing and in view of coherent control applications.

Coherent two-dimensional nanoscopy.

We introduce a spectroscopic method that determines nonlinear quantum mechanical response functions beyond the optical diffraction limit and allows direct imaging of nanoscale coherence. In established coherent two-dimensional (2D) spectroscopy, four-wave-mixing responses are measured using three ingoing waves and one outgoing wave; thus, the method is diffraction-limited in spatial resolution. In coherent 2D nanoscopy, we use four ingoing waves and detect the final state via photoemission electron microscopy, which has 50-nanometer spatial resolution. We recorded local nanospectra from a corrugated silver surface and observed subwavelength 2D line shape variations. Plasmonic phase coherence of localized excitations persisted for about 100 femtoseconds and exhibited coherent beats. The observations are best explained by a model in which coupled oscillators lead to Fano-like resonances in the hybridized dark- and bright-mode response.

Analytic coherent control of plasmon propagation in nanostructures.

We present general analytic solutions for optical coherent control of electromagnetic energy propagation in plasmonic nanostructures. Propagating modes are excited with tightly focused ultrashort laser pulses that are shaped in amplitude, phase, and polarization (ellipticity and orientation angle). We decouple the interplay between two main mechanisms which are essential for the control of local near-fields. First, the amplitudes and the phase difference of two laser pulse polarization components are used to guide linear flux to a desired spatial position. Second, temporal compression of the near-field at the target location is achieved using the remaining free laser pulse parameter to flatten the local spectral phase. The resulting enhancement of nonlinear signals from this intuitive analytic two-step process is compared to and confirmed by the results of an iterative adaptive learning loop in which an evolutionary algorithm performs a global optimization. Thus, we gain detailed insight into why a certain complex laser pulse shape leads to a particular control target. This analytic approach may also be useful in a number of other coherent control scenarios.