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.

The most general investigation and exploitation of light-induced processes require simultaneous control over spatial and temporal properties of the electromagnetic field on a femtosecond time and nanometer length scale. Based on the combination of polarization pulse shaping and time-resolved two-photon photoemission electron microscopy, we demonstrate such control over nanoscale spatial and ultrafast temporal degrees of freedom of an electromagnetic excitation in the vicinity of a nanostructure. The time-resolved cross-correlation measurement of the local photoemission yield reveals the switching of the nanolocalized optical near-field distribution with a lateral resolution well below the diffraction limit and a temporal resolution on the femtosecond time scale. In addition, successful adaptive spatiotemporal control demonstrates the flexibility of the method. This flexible simultaneous control of temporal and spatial properties of nanophotonic excitations opens new possibilities to tailor and optimize the lightmatter interaction in spectroscopic methods as well as in nanophotonic applications.coherent control | nanophotonics | plasmonics | ultrafast spectroscopy T he interaction of light with matter is of fundamental importance in many areas of nature, science, and engineering, and the dynamics and efficiency of light-induced processes are determined by the properties of the optical field as a function of space and time at the location of interaction. Hence their most general investigation and exploitation would require the generation of light fields that can be specified at will in both their spatial and temporal degrees of freedom at all length and time scales. In the past, significant progress has been made toward realizing either of these two manipulation objectives separately. For temporal field properties, femtosecond laser pulse shaping (1) offers flexible control over the field amplitude, phase, and polarization (2, 3) on an ultrashort time scale. This has been exploited for coherent control over numerous quantum-mechanical systems (4, 5). For the case of spatial light-field properties, on the other hand, emerging nanooptical techniques (6) have made available spectroscopy beyond the Abbe diffraction limit, as, for example, nanoantenna-assisted addressing of individual molecules (7). Combination with femtosecond excitation offers high resolution in space and time (8-15) and opens routes toward novel applications (16,17). In particular, deliberate spatial manipulation of optical near-field distributions was realized with adaptive and coherent control methods (9,(11)(12)(13)(18)(19)(20)(21)(22). In our recent demonstration of adaptive control of nanooptical fields (13), only spatial properties of optical near-field distributions were accessed. In the present work, in contrast, we directly measure and control also the temporal evolution of the nanoscale excitation. This information is obtained and exploited here using time-resolved cross-correlation measurements with one polarization-shaped "pump" light pu...

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The spatiotemporal evolution of a SPP wave packet with femtosecond duration is experimentally investigated in two different plasmonic focusing structures. A two-dimensional reconstruction of the plasmonic field in space and time is possible by the numerical analysis of interferometric time-resolved photoemission electron microscopy data. We show that the time-integrated and time-resolved view onto the wave packet dynamics allow one to characterize and compare the capabilities of two-dimensional components for use in plasmonic devices operating with ultrafast pulses.

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Excitation, orientation and alignment of a Na atomic beam is studied in a laser optical pumping experiment for the 32S1/2ff=2+-~32P3/2F=3 transition. For practical application in collision experiments with laser excited atom beams, we relate the observable parameters to several alternative descriptions by multipole moments, and discuss the experimental geometries most suitable for their determination. The dependences on laser power, atom beam density, small magnetic fields and spatial profiles are measured. They can be understood by simple rate equation models.

We employ a recently developed purpose-made technique based on spin-resolved two-photon photoemission spectroscopy to study the influence of alkali-metal doping (Cs and Na) on the spin functionality of the interface between a cobalt thin film and the organic semiconductor copper phthalocyanine. We find two alkali-metal-induced effects. First, alkali-metal atoms act as impurities and increase the spin-flip probability for the electrons crossing the interface (detrimental effect). Second, they allow one to modify the interface energy level alignment and, consequently, to enhance the efficiency of spin injection at an arbitrary energy above the Fermi level of the cobalt (intrinsic effect). We show that the intrinsic effect dominates over the detrimental one, opening the possibility to actively tailor the spin functionality of the considered hybrid metal-organic interface by changing the doping concentration.