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.

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.

We show that pulse shaping techniques can be applied to tailor the ultrafast temporal response of the strongly confined and enhanced optical near fields in the feed gap of resonant optical antennas (ROAs). Using finite-difference time-domain (FDTD) simulations followed by Fourier transformation, we obtain the impulse response of a nano structure in the frequency domain, which allows obtaining its temporal response to any arbitrary pulse shape. We apply the method to achieve deterministic optimal temporal field compression in ROAs with reduced symmetry and in a twowire transmission line connected to a symmetric dipole antenna. The method described here will be of importance for experiments involving coherent control of field propagation in nanophotonic structures and of light-induced processes in nanometer scale volumes.PACS numbers: 73.21, 84.40.Ba, 84.40.Az Resonant optical antennas (ROA) are metal nano structures that upon illumination with light of welldefined wavelength and polarization exhibit hot spots in which the local optical field is spatially confined and strongly enhanced due to geometry-dependent plasmon resonances [1,2]. When excited by a pulsed laser such hot spots provide the possibility to e.g. investigate nonlinear behavior of materials at moderate pulse energies [2,3,4]. Vice versa, well designed ROAs are able to enhance and direct the radiation of point-like sources that drive the antenna from within a hot spot [5,6] thus providing the basis for enhanced single photon sources [5] and antenna-enhanced local spectroscopy [7]. An important future application of ROAs is coherent spatiotemporal control of optical fields for antenna-enhanced local spectroscopies. Coherent control of near-fields by amplitude and polarization pulse shaping has recently been theoretically proposed [8,9,10,11] and experimentally realized [12]. In extension of such experiments it is envisaged to perform spectroscopy of complex matter in contact with a ROA assembly, which will allow studying ultrafast temporal dynamics with nanoscale spatial resolution [13]. The well-established techniques of femtosecond quantum control [14,15] can be realized on the nanoscale. To reach this goal it is important to understand how the resonant character of nano antennas affects the pulse shape in the feed gap and which degrees of freedom are available to be exploited for coherent control of their near-fields. So far efficient ROAs have been realized using noble metal nano rods which are produced by microfabrication techniques. The plasmon resonances of individual rods are determined by their dimensions and dielectric function [16] or equivalently by length-dependent FabryPerot resonances of a plasmon wave [17] propagating with an effective wavelength [18] along the rod and being reflected at its ends. While a single rod may be considered a monopole antenna [6], dipole antennas are formed by aligning two nano rods end to end thus creating a very small feed gap between the two rods where optical fields are concentrated [2]. The plasmon ...

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.

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.