Nanoplasmonic excitations as generated by few-cycle laser pulses on metal nanostructures undergo ultrafast dynamics with timescales as short as a few hundred attoseconds (1 as = 10(-18) s). So far, the spatiotemporal dynamics of optical fields localized on the nanoscale (nanoplasmonic field) have been hidden from direct access in the real space and time domain. An approach which combines photoelectron emission microscopy and attosecond streaking spectroscopy and which provides direct and non-invasive access to the nanoplasmonic field with nanometer-scale spatial resolution and temporal resolution of the order of 100 as has been proposed (Stockman et al 2007 Nat. Photon. 1 539). To implement this approach, a time of flight-photoemission electron microscope (TOF-PEEM) with ∼25 nm spatial and ∼50 meV energy resolution, which has the potential to detect a nanoplasmonic field with nanometer spatial and attosecond temporal resolution, has been developed and characterized using a 400 nm/60 ps pulsed diode laser. The first experimental results obtained using this newly developed TOF-PEEM in a two-photon photoemission mode with a polycrystalline Cu sample and an Ag microstructure film show that the yield and the kinetic energy of the emitted photoelectrons are strongly affected by the nanolocalized plasmonic field.

High harmonic radiation is meanwhile nearly extensively used for the spectroscopic investigation of electron dynamics with ultimate time resolution. The majority of high harmonic beamlines provide linearly polarized radiation created in a gas target. However, circular polarization greatly extends the spectroscopic possibilities for high harmonics, especially in the analysis of samples with chirality or prominent spin polarization. We produced a free-standing multilayer foil as a transmission EUV quarter waveplate and applied it for the first time to high harmonic radiation. We measured a broadband (4.6 eV FWHM) ellipticity of 75% at 66 eV photon energy with a transmission efficiency of 5%. The helicity is switchable and the ellipticity can be adjusted to lower values by angle tuning. As a single element it can be easily integrated in any existing harmonic beamline without major changes.

The direct detection of the spatiotemporal dynamics of nanolocalized optical near-fields on nanostructured metal surfaces, for example, imaging of localized surface plasmons (cf. Chapter 1) on rough or nanostructured metal films or the imaging of propagating surface plasmon polaritons at a vacuum-metal or metal-dielectric interface is a prerequisite to further control and optimize surface-plasmon based ultrafast nanooptics for future device development and applications [1][2][3][4].While free electrons in metals collectively respond to excitation from a light pulse, which is resonant to the surface plasmon frequency of the system, and squeeze and amplify the field intensity of the incoming plane light field into a subwavelength spatial volume, the typically broad frequency bandwidth of surface plasmon resonances supports an ultrafast response of these fields with rapid field changes on sub-femtosecond time scales [5].The sub-wavelength nanoscaled localization of optical fields in the vicinity of metal nanostructures and the ultrafast temporal evolution of such fields on a 0.1-100 fs time scale require the invention and development of new experimental methodologies, which combine nanometer (sub-optical) spatial resolution, sub-femtosecond temporal resolution, and optionally further nanospectroscopic information.Resolving the spatial distribution of such fields requires a microscopic technique with sub-optical spatial resolution, for example, in the 10-100 nm range. Scanning near-field optical microscopy (SNOM) techniques have been successfully applied with spatial resolutions of about ∼100 nm; however, the combination with ultrashort light pulses is still very difficult. Photoemission electron microscopy (PEEM) is a technique capable of resolving the spatial emission distribution of photoelectrons with an ultimate resolution of ∼10 nm.

We report a time-resolved normal-incidence photoemission electron microscope with an imaging time-of-flight detector using ∼7-fs near-infrared laser pulses and a phase-stabilized interferometer for studying ultrafast nanoplasmonic dynamics via nonlinear photoemission from metallic nanostructures. The interferometer’s stability (35 ± 6 as root-mean-square from 0.2 Hz to 40 kHz) as well as on-line characterization of the driving laser field, which is a requirement for nanoplasmonic near-field reconstruction, is discussed in detail. We observed strong field enhancement and few-femtosecond localized surface plasmon lifetimes at a monolayer of self-assembled gold nanospheres with ∼40 nm diameter and ∼2 nm interparticle distance. A wide range of plasmon resonance frequencies could be simultaneously detected in the time domain at different nanospheres, which are distinguishable already within the first optical cycle or as close as about ±1 fs around time-zero. Energy-resolved imaging (microspectroscopy) additionally revealed spectral broadening due to strong-field or space charge effects. These results provide a clear path toward visualizing optically excited nanoplasmonic near-fields at ultimate spatiotemporal resolution.