Christian Strüber

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 control of nanooptical excitations

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 light–matter interaction in spectroscopic methods as well as in nanophotonic applications.

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.

Angular momentum–induced delays in solid-state photoemission enhanced by intra-atomic interactions

Photoemission with a twist Attosecond time-resolved spectroscopy provides the ability to probe the fastest electronic processes in atoms and solids. Yet the photoemission process from solids is not fully understood. Siek et al. studied photoemission from the layered van der Waals material WSe2 and found that electron emission occurs as a sequence of events that are apparently time-ordered with respect to rising angular momentum of the involved initial states (see the Perspective by Yakovlev and Karpowicz). This result will help provide a more detailed picture of the photoemission process. Science, this issue p. 1274; see also p. 1239 Attosecond time-resolved spectroscopy reveals angular momentum–induced delays in solid-state photoemission. Attosecond time-resolved photoemission spectroscopy reveals that photoemission from solids is not yet fully understood. The relative emission delays between four photoemission channels measured for the van der Waals crystal tungsten diselenide (WSe2) can only be explained by accounting for both propagation and intra-atomic delays. The intra-atomic delay depends on the angular momentum of the initial localized state and is determined by intra-atomic interactions. For the studied case of WSe2, the photoemission events are time ordered with rising initial-state angular momentum. Including intra-atomic electron-electron interaction and angular momentum of the initial localized state yields excellent agreement between theory and experiment. This has required a revision of existing models for solid-state photoemission, and thus, attosecond time-resolved photoemission from solids provides important benchmarks for improved future photoemission models.

Simultaneous Spatial and Temporal Control of Nanooptical Fields

Using time-resolved two-photon photoemission electron microscopy we demonstrate simultaneous spatial and temporal control of nanooptical fields. Cross correlation measurements reveal the ultrafast spatial switching of the local excitation on a subdiffraction length scale.

Delayed Core-Level Photoemission from the van der Waals Crystal WSe2

Attosecond time-resolved XUV streaking experiments are reported for cleaved WSe2 surfaces. The photoemission from Se 3d and W 4f core levels occurs delayed by 50 as with respect to the valence band emission.

Few-Cycle Laser Pulse Induced Plasmon Assisted Thermionic Injection in Metal-Insulator-Metal Junctions

Gold nanoparticles on a metal-insulator-metal junction locally enhance the absorption of few-cycle laser pulses. The locally heated electron gas leads to thermionic emission exceeding multiphoton emission and allows detection of single nanoparticles.

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.

Nanolocalization of ultrashort time-reversed pulses in random nanoparticle assemblies

Localization of time-reversed optical fields in random nano-assemblies is investigated. It is shown that a structural hierarchy of the scatterers (i.e., the presence of a far-field reverberation chamber) improves the nanolocalization of time-reversed waves.