Pascal Melchior

Coherent Two-Dimensional Nanoscopy

Coherent electronic states excited by ultrafast laser pulses were imaged at subwavelength resolution with photoelectrons. 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.

Spatiotemporal Characterization of SPP Pulse Propagation in Two-Dimensional Plasmonic Focusing Devices

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.

Normal-Incidence Photoemission Electron Microscopy (NI-PEEM) for Imaging Surface Plasmon Polaritons

We introduce a novel time-resolved photoemission-based near-field illumination method, referred to as femtosecond normal-incidence photoemission microscopy (NI-PEEM). The change from the commonly used grazing-incidence to normal-incidence illumination geometry has a major impact on the achievable contrast and, hence, on the imaging potential of transient local near fields. By imaging surface plasmon polaritons in normal light incidence geometry, the observed fringe spacing directly resembles the wavelength of the plasmon wave. Our novel approach provides a direct descriptive visualization of SPP wave packets propagating across a metal surface.

Optimal open-loop near-field control of plasmonic nanostructures

Optimal open-loop control, i.e. the application of an analytically derived control rule, is demonstrated for nanooptical excitations using polarization-shaped laser pulses. Optimal spatial near-field localization in gold nanoprisms and excitation switching is realized by applying a shift to the relative phase of the two polarization components. The achieved near-field switching confirms theoretical predictions, proves the applicability of predefined control rules in nanooptical light-matter interaction and reveals local mode interference to be an important control mechanism.

Photoelectron imaging of modal interference in plasmonic whispering gallery cavities

We use three-photon photoemission electron microscopy (PEEM) to investigate the interference of coherently excited dipolar and quadrupolar resonant modes of plasmonic whispering gallery resonators formed by circular grooves patterned into a flat Au surface. Optical scattering and cathodoluminescence spectroscopy are used to characterize the cavity resonance spectra for a wide range of cavity radii and groove depths. Using PEEM, we directly resolve the interference between the modal field distribution of dipolar and quadrupolar modes that are coherently excited at λ = 795 nm under oblique incidence. Characteristic asymmetries in the photoelectron images for both TM and TE excitation are a direct consequence of the coherent excitation of the resonant modes.

Nano-Optical Control of Hot-Spot Field Superenhancement on a Corrugated Silver Surface

Coherent control of ultrafast nano-optical excitations of a corrugated silver surface is demonstrated by means of predetermined few-parameter scans and adaptive polarization laser pulse shaping. “Hot spots” in the multiphoton photoemission signals are enhanced and manipulated with a high contrast. Switching between separated and closely spaced hot spots is shown. The latter allows controlling the shape of hot spots and yields improved nanofocusing and “purification” of the photoemission signals. Complex pulse shapes were obtained in adaptive optimizations whose features were reproducible in repeated runs. Predetermined few-parameter control scans provide insight into the interpretation of optimal pulse shapes. The results indicate the existence of long coherence lifetimes on a corrugated silver surface. This combination of collective strong nanoplasmonic near-field enhancement and long-lived coherence may be used to achieve an even stronger field enhancement (“superenhancement”) making these hot spots ideal candidates for future nanophotonic, spectroscopic, sensor and quantum information applications. In addition the observation of such long coherence lifetimes is relevant to the understanding of surface-enhanced spectroscopies such as single-molecule Raman spectroscopy.

Near-Field Imaging and Spectroscopy of Gold Nanoantenna

Near-field imaging and spectroscopy of ring resonators is performed with photoemission electron microscopy and a tuneable femtosecond laser source. Phase- and time-resolved near-field imaging is achieved with an actively stabilized interferometer.

Probing And Imaging Of Optical Antennas With PEEM

Multi‐photon photoemission electron microscopy (PEEM) is an excellent tool for mapping the local near‐field distribution around nanostructures. The combination of PEEM with a pump‐probe laser setup enables the investigation of the dynamics of plasmonic excitations. The phase‐sensitive superposition of different plasmonic modes leads to a spatially controllable enhancement of the near‐field inside and in the vicinity of a metallic nanostructure. By controlling the relative phase Θ between two orthogonally polarized light pulses the spatial distribution of the near‐field is manipulated on the basis of the interference of the near‐field modes.

Deterministic control of subwavelength field localization in plasmonic nanoantennas

Subwavelength photoemission localization and switching in plasmonic bowtie nanoantennas is achieved experimentally. Analytic and adaptive control schemes are investigated, and agreement between both approaches is demonstrated.