Probing the Polarization of Low-Energy Excitations in 2D Materials from Atomic Crystals to Nanophotonic Arrays using Momentum-Resolved Electron Energy Loss Spectroscopy
Andrew W. Rossi, Marc R. Bourgeois, Caleb Walton, David J. Masiello
Spectroscopies utilizing free electron beams as probes offer detailed information on the reciprocal-space excitations of 2D materials such as graphene and transition metal dichalcogenide monolayers. Yet, despite the attention paid to such quantum materials, less consideration has been given to the electron-beam characterization of 2D periodic nanostructures such as photonic crystals, metasurfaces, and plasmon arrays, which can exhibit the same lattice and excitation symmetries as their atomic analogs albeit at drastically different length, momentum, and energy scales. Due to their lack of covalent bonding and influence of retarded electromagnetic interactions, important physical distinctions arise that complicate interpretation of scattering signals. Here we present a fully-retarded theoretical framework for describing the inelastic scattering of wide field electron beams from 2D materials and apply it to investigate the complementarity in sample excitation information gained in the measurement of a honeycomb plasmon array versus angle-resolved optical spectroscopy in comparison to single monolayer graphene.
Momentum resolved electron energy-loss spectroscopy was used to measure the dispersions of excitons in a free-standing monolayer of WSe2. Besides the parabolically dispersed valley excitons, a sub-gap dispersive exciton was observed at specific q values for the first time. The simultaneous STEM-ADF imaging of the local probed areas suggests that this sub-gap exciton is attributed to the prolific Se vacancies.
Time-resolved electrical spin transport is used to generate and probe spin currents in GaAs electrically. We use high bandwidth current pulses to inject phase-coherent spin packets from Fe into n-GaAs. By means of time-resolved Faraday rotation we demonstrate that spins are injected with a clearly defined phase by the observation of multiple Larmor precession cycles. We furthermore show that spin precession of optically created spin packets in n-GaAs can be probed electrically by spin-polarized photo-current pulses. The injection and detection experiments are not direct reciprocals of each oth…
We present two-dimensional plasmonic Doppler grating (PDG) for broadband azimuthal angle-resolved nanophotonic applications. The PDG consists of a set of non-concentric circular rings mimicking the wavefronts of a moving point source that exhibits Doppler Effect and thereby offers continuous azimuthal angle-dependent lattice momentum for photon-plasmon coupling. The center and span of the working frequency window are fully designable for optimal performance in specific applications. We detail the design, fabrication and optical characterization of the PDG. The design of Doppler grating provide…
The dynamics of photo-generated electron-hole pairs in solids are dictated by many-body interactions such as electron-electron and electron-phonon scattering. Hence, understanding and controlling these scattering channels is crucial for many optoelectronic applications, ranging from light harvesting to optical amplification. Here we measure the formation and relaxation of the photo-generated non-thermal carrier distribution in monolayer graphene with time- and angle-resolved photoemission spectroscopy. Using sub 10fs pulses we identify impact ionization as the primary scattering channel, which…
Electron energy-loss spectroscopy (EELS) performed in transmission electron microscopes is shown to directly render the photonic local density of states (LDOS) with unprecedented spatial resolution, currently below the nanometer. Two special cases are discussed in detail: (i) 2D photonic structures with the electrons moving along the translational axis of symmetry and (ii) quasi-planar plasmonic structures under normal incidence. Nanophotonics in general and plasmonics in particular should benefit from these results connecting the unmatched spatial resolution of EELS with its ability to probe …
The controlled introduction of covalent sp$^{3}$ defects into semiconducting single-walled carbon nanotubes (SWCNTs) gives rise to exciton localization and red-shifted near-infrared luminescence. The single-photon emission characteristics of these functionalized SWCNTs make them interesting candidates for electrically driven quantum light sources. However, the impact of sp$^{3}$ defects on the carrier dynamics and charge transport in carbon nanotubes remains an open question. Here, we use ultrafast, time-resolved optical-pump terahertz-probe spectroscopy as a direct and quantitative technique …
Using polarization-resolved transient reflection spectroscopy, we investigate the ultrafast modulation of light interacting with a metasurface consisting of coherently vibrating nanophotonic meta-atoms in the form of U-shaped split-ring resonators, that exhibit co-localized optical and mechanical resonances. With a two-dimensional square-lattice array of these resonators formed of gold on a glass substrate, we monitor the visible-pump-pulse induced gigahertz oscillations in intensity of reflected linearly-polarized infrared probe light pulses, modulated by the resonators effectively acting as …
Current advances in ultrafast electron microscopy make it possible to combine optical pumping of a nanostructure and electron beam probing with subångstrom and femtosecond spatiotemporal resolution. We present a theory predicting that this technique can reveal a rich out-of-equilibrium dynamics of plasmon excitations in graphene and graphite samples. In a disruptive departure from the traditional probing of nanoscale excitations based on the identification of spectral features in the transmitted electrons, we show that measurement of angle-resolved, energy-integrated inelastic electron scatter…
We demonstrate that the dispersion of surface plasmon polaritons in a periodically perforated gold film can be efficiently manipulated by femtosecond laser pulses with the wavelengths far from the intrinsic resonances of gold. Using a time- and frequency- resolved pump-probe technique we observe shifting of the plasmon polariton resonances with response times from 200 to 800 fs depending on the probe photon energy, through which we obtain comprehensive insight into the electron dynamics in gold. We show that Wood anomalies in the optical spectra provide pronounced resonances in differential tr…
Time-Resolved Photoemission Spectroscopy of Quantum Materials Using High Harmonic Generation: Probing Electron-Phonon Interactions and Non-Equilibrium Dynamics
A recent article by Sassa et al. [Phys. Rev. B 91, 045114 (2015)] reports on a soft x-ray angle-resolved photoemission study of MgB2. The analysis and/or presentation of the collected data and the corresponding calculations appear to be partially inconsistent. The aim of this comment is to provide a guide to these inconsistencies and to discuss their influence on the presented conclusions.
We report angle-resolved electron transmission measurements through freestanding graphene sheets in the energy range of 18 to 30 eV above the Fermi level. The measurements are carried out in a low-energy electron point source microscope, which allows simultaneously probing the transmission for a large angular range. The characteristics of low-energy electron transmission through graphene depend on its electronic structure above the vacuum level. The experimental technique described here allows mapping the unoccupied band structure of freestanding two-dimensional materials as a function of ener…
Hyperbolic polariton modes are highly appealing for a broad range of applications in nanophotonics, including surfaced enhanced sensing, sub-diffractional imaging and reconfigurable metasurfaces. Here we show that attenuated total reflectance micro-spectroscopy (ATR) using standard spectroscopic tools can launch hyperbolic polaritons in a Kretschmann-Raether configuration. We measure multiple hyperbolic and dielectric modes within the naturally hyperbolic material hexagonal boron nitride as a function of different isotopic enrichments and flake thickness. This overcomes the technical challenge…
Control of structural couplings at the complex-oxide interfaces is a powerful platform for creating new ultrathin layers with electronic and magnetic properties unattainable in the bulk. However, with the capability to design and control the electronic structure of such buried layers and interfaces at a unit-cell level, a new challenge emerges to be able to probe these engineered emergent phenomena with depth-dependent atomic resolution as well as element- and orbital selectivity. Here, we utilize a combination of core-level and valence-band soft x-ray standing-wave photoemission spectroscopy,…
We present a flexible and efficient ultrafast time-resolved spontaneous Raman spectroscopy setup to study collective excitation and quasi-particle dynamics in quantum matter. The setup has a broad energy tuning range extending from the visible to near infrared spectral regions for both the pump excitation and Raman probe pulses. Additionally, the balance between energy and time-resolution can be controlled. A high light collecting efficiency is realized by high numerical aperture collection optics and a high-throughput flexible spectrometer. We demonstrate the functionality of the setup with a…
Observing signatures of light-induced Floquet topological states in materials has been shown to be very challenging. Angle-resolved photoemission spectroscopy (ARPES) is well suited for the investigation of Floquet physics, as it allows to directly probe the dressed electronic states of driven solids. Depending on the system, scattering and decoherence can play an important role, hampering the emergence of Floquet states. Another challenge is to disentangle Floquet side bands from laser-assisted photoemission (LAPE), since both lead to similar signatures in ARPES spectra. Here, we investigate …
Metal-corroles are macrocycle organic molecules with numerous practical applications. In particular, copper corroles exhibit an interesting saddled geometry, which has attracted significant attention from theoreticians and experimentalists over the years. The present work is dedicated to understand the effect of structural saddling in a copper corrole on potential probe signals via imaging ultrafast coherent electron dynamics. A linearly polarized pulse is used to trigger the electron dynamics and time-resolved x-ray diffraction is employed to image the triggered dynamics. It is found that the…
The kinetics and thermodynamics of first order transitions is universally controlled by defects that act as nucleation sites and pinning centers. Here we demonstrate that defect-domain interactions during polarization reversal processes in ferroelectric materials result in a pronounced fine structure in electromechanical hysteresis loops. Spatially-resolved imaging of a single defect center in multiferroic BiFeO3 thin film is achieved, and the defect size and built-in field are determined self-consistently from the single-point spectroscopic measurements and spatially-resolved images. This met…
Near-field mapping has been widely used to study hyperbolic phonon-polaritons in van der Waals crystals. However, an accurate measurement of the polaritonic loss remains challenging because of the inherent complexity of the near-field signal and the substrate-mediated loss. Here we demonstrate that large-area monocrystalline gold flakes, an atomically-flat low-loss substrate for image polaritons, provide a platform for precise near-field measurement of the complex propagation constant of polaritons in van der Waals crystals. As a topical example, we measure propagation loss of the image phonon…
The electronic excitation occurring on adsorbates at ultrafast time scales from optical lasers that initiate surface chemical reactions is still an open question. Here, we report the ultrafast temporal evolution of X-ray absorption spectroscopy (XAS) and X-ray emission spectroscopy (XES) of a simple well known adsorbate prototype system, namely carbon (C) atoms adsorbed on a nickel (Ni(100)) surface, following intense laser optical pumping at 400 nm. We observe ultrafast (~100 fs) changes in both XAS and XES showing clear signatures of the formation of a hot electron-hole pair distribution on …
Numerous efforts have been devoted to improve the electronic and optical properties of III-V compound materials via reduction of their nonradiative states, aiming at highly-efficient III-V sub-micrometer devices. Despite many advances, there is still a controversial debate on which combination of chemical treatment and capping dielectric layer can best reproducibly protect the crystal surface of III-Vs, while being compatible with readily available plasma deposition methods. This work reports on a systematic experimental study on the role of sulfide ammonium chemical treatment followed by diel…
The investigation of ultrafast electronic and structural dynamics in low-dimensional systems like nanowires and two-dimensional materials requires femtosecond probes providing high spatial resolution and strong interaction with small volume samples. Low-energy electrons exhibit large scattering cross sections and high sensitivity to electric fields, but their pronounced dispersion during propagation in vacuum so far prevented their use as femtosecond probe pulses in time-resolved experiments. Employing a laser-triggered point-like source of either divergent or collimated electron wave packets,…
Time-resolved soft X-ray transmission microscopy is applied to image the current-induced resonant dynamics of the magnetic vortex core realized in a micronsized Permalloy disk. The high spatial resolution better than 25 nm enables us to observe the resonant motion of the vortex core. The result also provides the spin polarization of the current to be 0.67 +/-0.16 for Permalloy by fitting the experimental results with an analytical model in the framework of the spin-transfer torque.
We demonstrate that the magnetic nanoskyrmion lattice on the Fe monolayer on Ir(111) and the positions of the Fe atoms can be resolved simultaneously using magnetic exchange force microscopy. Thus, the relation between magnetic and atomic structure can be determined straightforwardly by evaluating the Fourier transformation of the real space image data. We further show that the magnetic contrast can be mapped on a Heisenberg-like magnetic interaction between tip and sample spins. Since our imaging technique is based on measuring forces, our observation paves the way to study skyrmions or other…
Angle-resolved photoemission spectroscopy (ARPES), an experimental technique based on the photoelectric effect, is arguably the most powerful method for probing the electronic structure of solids. The past decade has witnessed notable progress in ARPES, including the rapid development of soft-X-ray ARPES, time-resolved ARPES, spin-resolved ARPES and spatially resolved ARPES, as well as considerable improvements in energy and momentum resolution. Consequently , ARPES has emerged as an indispensable experimental probe in the study of topological materials, which have characteristic non-trivial b…
High-harmonic spectroscopy has become an essential ingredient in probing various ultrafast electronic processes in solids with sub-cycle temporal resolution. Despite its immense importance, sensitivity of high-harmonic spectroscopy to phonon dynamics in solids is not well known. This work addresses this critical question and demonstrates the potential of high-harmonic spectroscopy in probing intertwined phonon-electron dynamics in solids. A pump pulse excites in-plane optical phonon modes in monolayer graphene and a circularly polarised pulse is employed to probe the excited phonon dynamics th…
Ultrafast optical pulses are an increasingly important tool for controlling quantum materials and triggering novel photo-induced phase transitions. Understanding these dynamic phenomena requires a probe sensitive to spin, charge, and orbital degrees of freedom. Time-resolved resonant inelastic X-ray scattering (trRIXS) is an emerging spectroscopic method, which responds to this need by providing unprecedented access to the finite-momentum fluctuation spectrum of photoexcited solids. In this Perspective, we briefly review state-of-the-art trRIXS experiments on condensed matter systems, as well …
