Philip Kahl

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.

Revealing the subfemtosecond dynamics of orbital angular momentum in nanoplasmonic vortices

Putting plasmons in a spin The ability of light to carry angular momentum provides an additional degree of freedom for applications such as optical tweezing and optical communication. Spektor et al. show that the optical angular momentum modes of light can be shrunk down to the nanometer scale through plasmonic transfer. They patterned spiral-like structures into an atomically smooth layer of gold, which allowed them to launch plasmons with controlled amounts of angular momentum. Science, this issue p. 1187 Rotating plasmonic excitations can be launched with controlled amounts of optical angular momentum. The ability of light to carry and deliver orbital angular momentum (OAM) in the form of optical vortices has attracted much interest. The physical properties of light with a helical wavefront can be confined onto two-dimensional surfaces with subwavelength dimensions in the form of plasmonic vortices, opening avenues for thus far unknown light-matter interactions. Because of their extreme rotational velocity, the ultrafast dynamics of such vortices remained unexplored. Here we show the detailed spatiotemporal evolution of nanovortices using time-resolved two-photon photoemission electron microscopy. We observe both long- and short-range plasmonic vortices confined to deep subwavelength dimensions on the scale of 100 nanometers with nanometer spatial resolution and subfemtosecond time-step resolution. Finally, by measuring the angular velocity of the vortex, we directly extract the OAM magnitude of light.

Direct Observation of Surface Plasmon Polariton Propagation and Interference by Time-Resolved Imaging in Normal-Incidence Two Photon Photoemission Microscopy

Time-resolved imaging of the propagation and interference of isolated ultrashort surface plasmon polariton wave packets is demonstrated using two photon photoemission microscopy. The group- and phase velocity of individual wave packets are determined experimentally. Using two counter-propagating surface plasmon polariton pulses, the transient formation of a standing surface plasmon polariton wave is imaged in time and space. We demonstrate that using a normal incidence geometry in time-resolved photoemission microscopy provides great advantages for in-situ imaging of surface plasmon polaritons in arbitrary plasmonic structures. A simple 1D wave-simulation is used to confirm the experimental results.

Short-range surface plasmonics: Localized electron emission dynamics from a 60-nm spot on an atomically flat single-crystalline gold surface

We demonstrate nanofocusing down to 60 nm with 800-nm light in atomically flat single-crystalline 22-nm-thick gold flakes. We experimentally and theoretically visualize the propagation of short-range surface plasmon polaritons using atomically flat single-crystalline gold platelets on silicon substrates. We study their excitation and subfemtosecond dynamics via normal-incidence two-photon photoemission electron microscopy. By milling a plasmonic disk and grating structure into a single-crystalline gold platelet, we observe nanofocusing of the short-range surface plasmon polariton. Localized two-photon ultrafast electron emission from a spot with a smallest dimension of 60 nm is observed. Our novel approach opens the door toward reproducible plasmonic nanofocusing devices, which do not degrade upon high light intensity or heating due to the atomically flat surface without any tips, protrusions, or holes. Our nanofoci could also be used as local emitters for ultrafast electron bunches in time-resolved electron microscopes.

Spatio-Temporal Imaging of Surface Plasmon Polaritons in Two Photon Photoemission Microscopy

A two-photon photoemission microscopy experiment with femtosecond time-resolution for imaging of propagating surface plasmon polaritons is discussed. The experimental setup of an actively Pancharatnam’s phase stabilized interferometer is described, and a temporal stability in time-resolved two-photon photoemission microscopy of less than 20 attoseconds is demonstrated. The time-resolved setup is applied to investigate the interaction of a surface plasmon polariton wave packet with a plasmonic beam-splitter. Pump-probe data recorded at times before and after the interaction of the surface plasmon polariton wave packet with the beam-splitter indicate transmission and reflection coefficients of T≈0.3 and R≈0.4, respectively.