Ultrafast Photoemission Electron Microscopy: Imaging Plasmons in Space and Time

Angular momentum transfer from swift electrons to non-spherical nanoparticles within the dipolar approximation

In this work, we study the angular momentum transfer from a single swift electron to non-spherical metallic nanoparticles, specifically investigating spheroidal and polyhedral (Platonic Solids) shapes. While previous research has predominantly focused on spherical nanoparticles, our work expands the knowledge by exploring various geometries. Employing classical electrodynamics and the small particle limit, we calculate the angular momentum transfer by integrating the spectral density, ensuring causality through Fourier-transform analysis. Our findings demonstrate that prolate spheroidal nanoparticles exhibit a single blueshifted plasmonic resonance, compared to spherical nanoparticles of equivalent volume, resulting in lower angular momentum transfer. Conversely, oblate nanoparticles display two resonances - one blueshifted and one redshifted - resulting in a higher angular momentum transfer than their spherical counterparts. Additionally, Platonic Solids with fewer faces exhibit significant redshifts in plasmonic resonances, leading to higher angular momentum transfer due to edge effects. We also observe resonances and angular momentum transfers with similar characteristics in specific pairs of Platonic Solids, known as duals. These results highlight promising applications, particularly in electron tweezers technology.

Near-field induced local excitation dynamics of Na10 and Na10-N2 from real-time TDDFT

Electron dynamics of the Na10 chain and the Na10-N2 complex locally excited by an atomistic optical near-field are investigated using real-time time-dependent density functional theory calculations on real-space grids. Ultrafast laser pulses were used to simulate the near-field excitation under on- and off-resonance conditions. Off-resonance excitation did not lead to the propagation of the excitation through the Na10 chain. In contrast, under the resonance conditions, the excited state is delocalized over the entire Na chain. Analysis of the local dipole moment of each atom in Na10 indicates that this behavior is consistent with the transition density. Adding an N2 molecule to the opposite end of the local excitation region results in energy transfer via the Na10 chain. The energy transfer efficiency of the N2 molecule is well correlated with the absorption spectrum of Na10. The present study paves the way for realizing remote excitation and photonic devices at the atomic scale.

Imaging of ultrafast photoexcited electron dynamics in pentacene nanocrystals on a graphite substrate

Understanding molecular film growth on substrates and the ultrafast electron dynamics at their interface is crucial for advancing next-generation organic electronics. We have focused on studying the ultrafast photoexcited electron dynamics in nanoscale organic crystals of an aromatic molecule, pentacene, on a two-dimensional material of graphite substrate. Through the use of time-resolved two-photon photoelectron emission microscopy (2P-PEEM), we have visualized the ultrafast lateral evolution of photoexcited electrons. By resonantly tuning the incident photon to excite pentacene molecules, polarization-dependent 2P-PEEM has revealed that pentacene nanocrystals (sub- to several μm) on the substrate exhibit a preferential orientation, in which a molecular π-orbital contacts the substrate in a "lying flat" orientation, facilitating electron transfer to the substrate. The time-resolved 2P-PEEM captures the motion of excited electrons in a femto- to pico-second timescale, clearly imaging the ultrafast charge transfer and lateral expansion two-dimensionally on the graphite substrate. Moreover, we found that the lying-flat molecular orientation of pentacene nanocrystals is transformable into a "standing-up" one through gentle heating up to 50 °C. These experimental insights using time-resolved 2P-PEEM will be highly valuable in enhancing the photofunctionalities of organic electronic devices by controlled molecular deposition.

Laser-induced electron diffraction: Imaging of a single gas-phase molecular structure with one of its own electrons

Among the many methods to image molecular structure, laser-induced electron diffraction (LIED) can image a single gas-phase molecule by locating all of a molecule's atoms in space and time. The method is based on attosecond electron recollision driven by a laser field and can reach attosecond temporal resolution. Implementation with a mid-IR laser and cold-target recoil ion-momentum spectroscopy, single molecules are measured with picometer resolution due to the keV electron impact energy without ensemble averaging or the need for molecular orientation. Nowadays, the method has evolved to detect single complex and chiral molecular structures in 3D. The review will touch on the various methods to discuss the implementations of LIED toward single-molecule imaging and complement the discussions with noteworthy experimental findings in the field.

Few-Cycle Surface Plasmon Polaritons

Surface plasmon polaritons (SPPs) can confine and guide light in nanometer volumes and are ideal tools for achieving electric field enhancement and the construction of nanophotonic circuitry. The realization of the highest field strengths and fastest switching requires confinement also in the temporal domain. Here, we demonstrate a tapered plasmonic waveguide with an optimized grating structure that supports few-cycle surface plasmon polaritons with >70 THz bandwidth while achieving >50% light-field-to-plasmon coupling efficiency. This enables us to observe the─to our knowledge─shortest reported SPP wavepackets. Using time-resolved photoelectron microscopy with suboptical-wavelength spatial and sub-10 fs temporal resolution, we provide full spatiotemporal imaging of co- and counter-propagating few-cycle SPP wavepackets along tapered plasmonic waveguides. By comparing their propagation, we track the evolution of the laser-plasmon phase, which can be controlled via the coupling conditions.

'When is a hotspot a good nanospot' - review of analytical and hotspot-dominated surface enhanced Raman spectroscopy nanoplatforms

Substrate development in surface-enhanced Raman spectroscopy (SERS) continues to attract research interest. In order to determine performance metrics, researchers in foundational SERS studies use a variety of experimental means to characterize the nature of substrates. However, often this process would appear to be performed indiscriminately without consideration for the physical scale of the enhancement phenomena. Herein, we differentiate between SERS substrates whose primary enhancing structures are on the hundreds of nanometer scale (analytical SERS nanosubstrates) and those whose main mechanism derives from nanometric-sized gaps (hot-spot dominated SERS substrates), assessing the utility of various characterization methods for each substrate class. In this context, characterization approaches in white-light spectroscopy, electron beam methods, and scanning probe spectroscopies are reviewed. Tip-enhanced Raman spectroscopy, wavelength-scanned SERS studies, and the impact of surface hydrophobicity are also discussed. Conclusions are thus drawn on the applicability of each characterization technique regarding amenability for SERS experiments that have features at different length scales. For instance, while white light spectroscopy can provide an indication of the plasmon resonances associated with 10 s-100 s nm-scale structures, it may not reveal information about finer surface texturing on the true nm-scale, critical for SERS' sensitivity, and in need of investigation via scanning probe techniques.

Real-time tracking of coherent oscillations of electrons in a nanodevice by photo-assisted tunnelling

Coherent collective oscillations of electrons excited in metallic nanostructures (localized surface plasmons) can confine incident light to atomic scales and enable strong light-matter interactions, which depend nonlinearly on the local field. Direct sampling of such collective electron oscillations in real-time is crucial to performing petahertz scale optical modulation, control, and readout in a quantum nanodevice. Here, we demonstrate real-time tracking of collective electron oscillations in an Au bowtie nanoantenna, by recording photo-assisted tunnelling currents generated by such oscillations in this quantum nanodevice. The collective electron oscillations show a noninstantaneous response to the driving laser fields with a T2 decay time of nearly 8 femtoseconds. The contributions of linear and nonlinear electron oscillations in the generated tunnelling currents were precisely determined. A phase control of electron oscillations in the nanodevice is illustrated. Functioning in ambient conditions, the excitation, phase control, and read-out of coherent electron oscillations pave the way toward on-chip light-wave electronics in quantum nanodevices.

Photodiode-based time zero determination for ultrafast electron microscopy

Pump-probe experiments in ultrafast electron microscopy require temporal overlap between the pump and probe pulses. Accurate measurements of the time delay between them allows for the determination of the time zero, the moment in time where both pulses perfectly overlap. In this work, we present the use of a photodiode-based alignment method for these time zero measurements. The cheap and easy-to-use device consists of a photodiode in a sample holder and enables us to temporally align individual, single-electron pulses with femtosecond laser pulses. In a first device, a temporal resolution of 24 ps is obtained, limited by the photodiode design. Future work will utilize a smaller photodiode with a lower capacitance, which will increase the temporal resolution and add spatial resolution as well. This upgrade will bring the method toward the micrometer and picosecond spatiotemporal resolution.

Angle-resolved photoelectron spectroscopy in a low-energy electron microscope

Spectroscopic photoemission microscopy is a well-established method to investigate the electronic structure of surfaces. In modern photoemission microscopes, the electron optics allow imaging of the image plane, momentum plane, or dispersive plane, depending on the lens setting. Furthermore, apertures allow filtering of energy-, real-, and momentum space. Here, we describe how a standard spectroscopic and low-energy electron microscope can be equipped with an additional slit at the entrance of the already present hemispherical analyzer to enable an angle- and energy-resolved photoemission mode with micrometer spatial selectivity. We apply a photogrammetric calibration to correct for image distortions of the projective system behind the analyzer and present spectra recorded on Au(111) as a benchmark. Our approach makes data acquisition in energy-momentum space more efficient, which is a necessity for laser-based pump-probe photoemission microscopy with femtosecond time resolution.

Ultrafast Electronic Relaxation Dynamics of Atomically Thin MoS2 Is Accelerated by Wrinkling

Strain engineering is an attractive approach for tuning the local optoelectronic properties of transition metal dichalcogenides (TMDs). While strain has been shown to affect the nanosecond carrier recombination dynamics of TMDs, its influence on the sub-picosecond electronic relaxation dynamics is still unexplored. Here, we employ a combination of time-resolved photoemission electron microscopy (TR-PEEM) and nonadiabatic ab initio molecular dynamics (NAMD) to investigate the ultrafast dynamics of wrinkled multilayer (ML) MoS2 comprising 17 layers. Following 2.41 eV photoexcitation, electronic relaxation at the Γ valley occurs with a time constant of 97 ± 2 fs for wrinkled ML-MoS2 and 120 ± 2 fs for flat ML-MoS2. NAMD shows that wrinkling permits larger amplitude motions of MoS2 layers, relaxes electron-phonon coupling selection rules, perturbs chemical bonding, and increases the electronic density of states. As a result, the nonadiabatic coupling grows and electronic relaxation becomes faster compared to flat ML-MoS2. Our study suggests that the sub-picosecond electronic relaxation dynamics of TMDs is amenable to strain engineering and that applications which require long-lived hot carriers, such as hot-electron-driven light harvesting and photocatalysis, should employ wrinkle-free TMDs.

Imaging and Controlling Ultrafast Electron Pulses Emitted from Plasmonic Nanostructures

We experimentally study photoemission from gold nanodisk arrays using space-, time-, and energy-resolved photoemission electron microscopy. When excited by a plasmonic resonant infrared (IR) laser pulse, plasmonic hotspots are generated owing to local surface plasmon resonance. Photoelectrons emitted from each plasmonic hotspot form a nanoscale and ultrashort electron pulse. When the system is excited by an extreme ultraviolet (EUV) laser pulse, a uniformly distributed photoelectron cloud is formed across the sample surface. When excited by the IR and EUV laser pulses together, both the photoemission image and kinetic energy vary significantly for the IR laser-generated electrons depending on the time delay between the two laser pulses. These observations are well explained by the Coulomb interaction with the EUV laser-generated electron cloud. Our study offers a feasible approach to manipulate the energy of electron pulse emitted from a plasmonic nanostructure on an ultrafast time scale.

Revealing low-loss dielectric near-field modes of hexagonal boron nitride by photoemission electron microscopy

Low-loss dielectric modes are important features and functional bases of fundamental optical components in on-chip optical devices. However, dielectric near-field modes are challenging to reveal with high spatiotemporal resolution and fast direct imaging. Herein, we present a method to address this issue by applying time-resolved photoemission electron microscopy to a low-dimensional wide-bandgap semiconductor, hexagonal boron nitride (hBN). Taking a low-loss dielectric planar waveguide as a fundamental structure, static vector near-field vortices with different topological charges and the spatiotemporal evolution of waveguide modes are directly revealed. With the lowest-order vortex structure, strong nanofocusing in real space is realized, while near-vertical photoemission in momentum space and narrow spread in energy space are simultaneously observed due to the atomically flat surface of hBN and the small photoemission horizon set by the limited photon energies. Our approach provides a strategy for the realization of flat photoemission emitters.

Two-Photon Photoemission Spectroscopy and Microscopy for Electronic and Plasmonic Characterizations of Molecularly Designed Organic Surfaces

Functional surfaces decorated with organic molecules and/or nanoclusters (NCs) composed of several tens of atoms are promising for use in future photoelectronic substrates, whose functionalities are governed by molecular local electronic/plasmonic excitations at the interfaces. Here, we combine two-photon photoemission spectroscopy (2P-PES) and microscopy (2P-PEEM) to investigate the local excited-state dynamics at organic surfaces functionalized with NCs. The 2P-PES and 2P-PEEM for organic fullerene (C₆₀) layers on graphite and Au substrates demonstrated photophysical characterization of electronic and plasmonic properties, including propagating surface plasmon polaritons (SPPs). The SPP propagation at the Au interface buried by overlayered C₆₀ can be visualized by Ag_n NC deposition, which enhances plasmon-induced hot electrons, where the threshold number of Ag atoms (n ≥ 9) for the plasmonic response is revealed by the size dependence of 2P-PES for Ag_n NCs on C₆₀ layers.

Plate-Like Colloidal Metal Nanoparticles

The pseudo-two-dimensional (2D) morphology of plate-like metal nanoparticles makes them one of the most anisotropic, mechanistically understood, and tunable structures available. Although well-known for their superior plasmonic properties, recent progress in the 2D growth of various other materials has led to an increasingly diverse family of plate-like metal nanoparticles, giving rise to numerous appealing properties and applications. In this review, we summarize recent progress on the solution-phase growth of colloidal plate-like metal nanoparticles, including plasmonic and other metals, with an emphasis on mechanistic insights for different synthetic strategies, the crystallographic habits of different metals, and the use of nanoplates as scaffolds for the synthesis of other derivative structures. We additionally highlight representative self-assembly techniques and provide a brief overview on the attractive properties and unique versatility benefiting from the 2D morphology. Finally, we share our opinions on the existing challenges and future perspectives for plate-like metal nanomaterials.

Light-Induced Subnanometric Modulation of a Single-Molecule Electron Source

Single-molecule electron sources of fullerenes driven via constant electric fields, approximately 1 nm in size, produce peculiar emission patterns, such as a cross or a two-leaf pattern. By illuminating the electron sources with femtosecond light pulses, we discovered that largely modulated emission patterns appeared from single molecules. Our simulations revealed that emission patterns, which have been an intractable question for over seven decades, represent single-molecule molecular orbitals. Furthermore, the observed modulations originated from variations of single-molecule molecular orbitals, practically achieving the subnanometric optical modulation of an electron source.

Optical manipulation of matter waves

Light is used to steer the motion of atoms in free space, enabling cooling and trapping of matter waves through ponderomotive forces and Doppler-mediated photon scattering. Likewise, light interaction with free electrons has recently emerged as a versatile approach to modulate the electron wave function for applications in ultrafast electron microscopy. Here, we combine these two worlds, theoretically demonstrating that matter waves can be optically manipulated via inelastic interactions with optical fields. This allows us to modulate the translational part of the wave function and produce temporally and spatially compressed atomic beam pulses. We realize such modulation through stimulated photon absorption and emission by atoms traversing phase-matching evanescent optical fields generated upon light scattering by a nanostructure and via stimulated Compton scattering in free space without any assistance from material media. Our results support optical manipulation of matter waves as a powerful tool for microscopy, spectroscopy, and exploration of fundamental phenomena associated with light-atom interactions.

Polarization-Resolved Electron Energy Gain Nanospectroscopy With Phase-Structured Electron Beams

Free-electron-based measurements in scanning transmission electron microscopes (STEMs) reveal valuable information on the broadband spectral responses of nanoscale systems with deeply subdiffraction limited spatial resolution. Leveraging recent advances in manipulating the spatial phase profile of the transverse electron wavefront, we theoretically describe interactions between the electron probe and optically stimulated nanophotonic targets in which the probe gains energy while simultaneously transitioning between transverse states with distinct phase profiles. Exploiting the selection rules governing such transitions, we propose phase-shaped electron energy gain nanospectroscopy for probing the 3D polarization-resolved response field of an optically excited target with nanoscale spatial resolution. Considering ongoing instrumental developments, polarized generalizations of STEM electron energy loss and gain measurements hold the potential to become powerful tools for fundamental studies of quantum materials and their interaction with nearby nanostructures supporting localized surface plasmon or phonon polaritons as well as for noninvasive imaging and nanoscale 3D field tomography.

Nanoimaging of the Edge-Dependent Optical Polarization Anisotropy of Black Phosphorus

The electronic structure and functionality of 2D materials is highly sensitive to structural morphology, not only opening the possibility for manipulating material properties but also making predictable and reproducible functionality challenging. Black phosphorus (BP), a corrugated orthorhombic 2D material, has in-plane optical absorption anisotropy critical for applications, such as directional photonics, plasmonics, and waveguides. Here, we use polarization-dependent photoemission electron microscopy to visualize the anisotropic optical absorption of BP with 54 nm spatial resolution. We find the edges of BP flakes have a shift in their optical polarization anisotropy from the flake interior due to the 1D confinement and symmetry reduction at flake edges that alter the electronic charge distributions and transition dipole moments of edge electronic states, confirmed with first-principles calculations. These results uncover previously hidden modification of the polarization-dependent absorbance at the edges of BP, highlighting the opportunity for selective excitation of edge states of 2D materials with polarized light.

Ultrafast infrared nano-imaging of far-from-equilibrium carrier and vibrational dynamics

Ultrafast infrared nano-imaging has demonstrated access to ultrafast carrier dynamics on the nanoscale in semiconductor, correlated-electron, or polaritonic materials. However, mostly limited to short-lived transient states, the contrast obtained has remained insufficient to probe important long-lived excitations, which arise from many-body interactions induced by strong perturbation among carriers, lattice phonons, or molecular vibrations. Here, we demonstrate ultrafast infrared nano-imaging based on excitation modulation and sideband detection to characterize electron and vibration dynamics with nano- to micro-second lifetimes. As an exemplary application to quantum materials, in phase-resolved ultrafast nano-imaging of the photoinduced insulator-to-metal transition in vanadium dioxide, a distinct transient nano-domain behavior is quantified. In another application to lead halide perovskites, transient vibrational nano-FTIR spatially resolves the excited-state polaron-cation coupling underlying the photovoltaic response. These examples show how heterodyne pump-probe nano-spectroscopy with low-repetition excitation extends ultrafast infrared nano-imaging to probe elementary processes in quantum and molecular materials in space and time.

Coherent Excitation and Control of Plasmons on Gold Using Two-Dimensional Transition Metal Dichalcogenides

The hybrid combination of two-dimensional (2D) transition metal dichalcogenides (TMDs) and plasmonic materials open up novel means of (ultrafast) optoelectronic applications and manipulation of nanoscale light-matter interaction. However, control of the plasmonic excitations by TMDs themselves has not been investigated. Here, we show that the ultrathin 2D WSe₂ crystallites permit nanoscale spatially controlled coherent excitation of surface plasmon polaritons (SPPs) on smooth Au films. The resulting complex plasmonic interference patterns are recorded with nanoscale resolution in a photoemission electron microscope. Modeling shows good agreement with experiments and further indicates how SPPs can be tailored with high spatiotemporal precision using the shape of the 2D TMDs with thicknesses down to single molecular layers. We demonstrate the use of WSe₂ nanocrystals as 2D optical elements for exploring the ultrafast dynamics of SPPs. Using few-femtosecond laser pulse pairs we excite an SPP at the boundary of a WSe₂ crystal and then have a WSe₂ monolayer wedge act as a delay line inducing a spatially varying phase difference down to the attosecond time range. The observed effects are a natural yet unexplored consequence of high dielectric functional values of TMDs in the visible range that should be considered when designing metal-TMD hybrid devices. As the 2D TMD crystals are stable in air, can be defect free, can be synthesized in many shapes, and are reliably positioned on metal surfaces, using them to excite and steer SPPs adds an interesting alternative in designing hybrid structures for plasmonic control.

Revealing the Chiroptical Response of Plasmonic Nanostructures at the Nanofemto Scale

The spatiotemporal origin of plasmonic chiroptical responses in nanostructures remains unexplored and unclear. Here, two orthogonally oriented Au nanorods as a prototype were investigated, with a giant chiroptical response caused by antisymmetric and symmetric mode excitations for obliquely incident left-handed circular polarization (LCP) and right-handed circular polarization (RCP) light. Time-resolved photoemission electron microscopy (PEEM) was employed to measure the near-field spatial distributions, spectra, and spatiotemporal dynamics of plasmonic modes associated with the chiroptical responses at the nanofemto scale, verifying the characteristic near-field distributions at the resonant wavelengths of the two modes and a very large spectral dichroism for LCP and RCP. More importantly, eigenmode excitations and their contributions to the ultrafast plasmonic chiroptical response in the space-time domain were directly revealed, promoting a full understanding of the ultrafast chiral origin in complex nanostructures. These findings open a way to design chiroptical nanophotonic devices for spatiotemporal control of chiral light-matter interactions.

Optical Excitations with Electron Beams: Challenges and Opportunities

Free electron beams such as those employed in electron microscopes have evolved into powerful tools to investigate photonic nanostructures with an unrivaled combination of spatial and spectral precision through the analysis of electron energy losses and cathodoluminescence light emission. In combination with ultrafast optics, the emerging field of ultrafast electron microscopy utilizes synchronized femtosecond electron and light pulses that are aimed at the sampled structures, holding the promise to bring simultaneous sub-Å-sub-fs-sub-meV space-time-energy resolution to the study of material and optical-field dynamics. In addition, these advances enable the manipulation of the wave function of individual free electrons in unprecedented ways, opening sound prospects to probe and control quantum excitations at the nanoscale. Here, we provide an overview of photonics research based on free electrons, supplemented by original theoretical insights and discussion of several stimulating challenges and opportunities. In particular, we show that the excitation probability by a single electron is independent of its wave function, apart from a classical average over the transverse beam density profile, whereas the probability for two or more modulated electrons depends on their relative spatial arrangement, thus reflecting the quantum nature of their interactions. We derive first-principles analytical expressions that embody these results and have general validity for arbitrarily shaped electrons and any type of electron-sample interaction. We conclude with some perspectives on various exciting directions that include disruptive approaches to noninvasive spectroscopy and microscopy, the possibility of sampling the nonlinear optical response at the nanoscale, the manipulation of the density matrices associated with free electrons and optical sample modes, and appealing applications in optical modulation of electron beams, all of which could potentially revolutionize the use of free electrons in photonics.

Controlling the Spatial and Momentum Distributions of Plasmonic Carriers: Volume vs Surface Effects

Spatial and momentum distributions of excited charge carriers in nanoplasmonic systems depend sensitively on optical excitation parameters and nanoscale geometry, which therefore control the efficiency and functionality of plasmon-enhanced catalysts, photovoltaics, and nanocathodes. Growing appreciation over the past decade for the different roles of volume- vs surface-mediated excitation in such systems has underscored the need for explicit separation and quantification of these pathways. Toward these ends, we utilize angle-resolved photoelectron velocity map imaging to distinguish these processes in gold nanorods of different aspect ratios down to the spherical limit. Despite coupling to the longitudinal surface plasmon, we find that resonantly excited nanorods always exhibit transverse (sideways) multiphoton photoemission distributions due to photoexcitation within volume field enhancement regions rather than at the tip hot spots. This behavior is accurately reproduced via ballistic Monte Carlo modeling, establishing that volume-excited electrons primarily escape through the nanorod sides. Furthermore, we demonstrate optical control over the photoelectron angular distributions via a screening-induced transition from volume (transverse/side) to surface (longitudinal/tip) photoemission with red detuning of the excitation laser. Frequency-dependent cross sections are separately quantified for these mechanisms by comparison with theoretical calculations, combining volume and surface velocity-resolved photoemission modeling. Based on these results, we identify nanomaterial-specific contributions to the photoemission cross sections and offer general nanoplasmonic design principles for controlling photoexcitation/emission distributions via geometry- and frequency-dependent tuning of the volume vs surface fields.

Plasmonic topological quasiparticle on the nanometre and femtosecond scales

At the interface of classical and quantum physics, the Maxwell and Schrödinger equations describe how optical fields drive and control electronic phenomena to enable lightwave electronics at terahertz or petahertz frequencies and on ultrasmall scales^1-5. The electric field of light striking a metal interacts with electrons and generates light-matter quasiparticles, such as excitons⁶ or plasmons⁷, on an attosecond timescale. Here we create and image a quasiparticle of topological plasmonic spin texture in a structured silver film. The spin angular momentum components of linearly polarized light interacting with an Archimedean coupling structure with a designed geometric phase generate plasmonic waves with different orbital angular momenta. These plasmonic fields undergo spin-orbit interaction and their superposition generates an array of plasmonic vortices. Three of these vortices can form spin textures that carry non-trivial topological charge⁸ resembling magnetic meron quasiparticles⁹. These spin textures are localized within a half-wavelength of light, and exist on the timescale of the plasmonic field. We use ultrafast nonlinear coherent photoelectron microscopy to generate attosecond videos of the spatial evolution of the vortex fields; electromagnetic simulations and analytic theory confirm the presence of plasmonic meron quasiparticles. The quasiparticles form a chiral field, which breaks the time-reversal symmetry on a nanometre spatial scale and a 20-femtosecond timescale (the 'nano-femto scale'). This transient creation of non-trivial spin angular momentum topology pertains to cosmological structure creation and topological phase transitions in quantum matter^10-12, and may transduce quantum information on the nano-femto scale^13,14.

Linewidth Narrowing with Ultimate Confinement of an Alkali Multipole Plasmon by Modifying Surface Electronic Wave Functions with Two-Dimensional Materials

This work demonstrates significant line narrowing of a surface multipole plasmon (MP) by modifying the surface electronic wave function with two-dimensional materials (2DMs): graphene and hexagonal boron nitride. This is found in an optical reflectivity of alkali atoms (Cs or K) on an Ir(111) surface covered with the 2DMs. The reduction in reflectivity induced by deposition of the alkali atoms becomes as large as 20% at ∼2  eV, which is ascribed to a MP of a composite of alkali/2DM/alkali/Ir multilayer structure. The linewidth of the MP band becomes as narrow as 0.2 eV by the presence of the 2DM between the two alkali layers. A numerical simulation by time-dependent density functional theory with a jellium model reveals that the density of states of the surface localized state is sharpened remarkably by the 2DMs that decouple the outermost alkali layer from the Ir bulk. Consequently, a local field enhancement of an order of 10^{5} is achieved by ultimate confinement of the MP within the outermost alkali layer. This work leads to a novel strategy for reducing plasmon dissipation in an atomically thin layer via atomic scale modification of surface structure.

Continuous angular control over anisotropic photoemission from isotropic gold nanoshells

A variety of applications rely on the efficient generation of hot carriers within metal nanoparticles and charge transfer to surrounding molecules or materials. The optimization of such processes requires a detailed understanding of excited carrier spatial, temporal, and momentum distributions, which also leads to opportunities for active optical control over hot carrier dynamics on nanometer and femtosecond scales. Such capabilities are emerging in nanoplasmonic systems and typically rely on tuning optical polarization and/or frequency to selectively excite one or more discrete hot spots defined by the particle geometry. Here, we introduce a unique case in which hot electron excitation and emission distributions can instead be continuously controlled via linear laser polarization in the azimuthal plane of a gold nanoshell supported on a substrate. In this configuration, it is the laser field that breaks the azimuthal symmetry of the supported nanoshell and determines the plasmonic field distribution. Using angle-resolved photoelectron velocity map imaging, we find that the hot electrons are predominantly emitted orthogonal to the nanoshell dipolar surface plasmon resonance axis defined by the laser polarization. Furthermore, such anisotropic emission is only observed for nanoshells, while solid gold nanospheres are found to be isotropic emitters. We show that all of these effects are recapitulated via simulation of the plasmonic electric field distributions within the nanoparticle volume and ballistic Monte Carlo modeling of the hot electron dynamics. These results demonstrate a highly predictive level of understanding of the underlying physics and possibilities for ultrafast spatiotemporal control over hot carrier dynamics.

Femtosecond photoemission electron microscopy of surface plasmon polariton beam steering via nanohole arrays

Directional control over surface plasmon polariton (SPP) waves is a prerequisite for the development of miniaturized optical circuitry. Here, the efficacy of single and dual component SPP steering elements is explored through photoemission electron microscopy. Our imaging scheme relies on two-color photoemission and counter-propagating SPP generation, which collectively allow SPPs to be visualized in real space. Wave-vector difference mixing between the two-dimensional nanohole array and photon momenta enables SPP steering with directionality governed by the array lattice constant and input photon direction. In our dual component configuration, separate SPP generation and Bragg diffraction based steering optics are employed. We find that array Bragg planes principally influence the SPP angles through the array band structure, which allows us to visualize both positive and negative refractory waves.