Ultra-Nonlinear Subcycle Photoemission of Few-Electron States from Sharp Gold Nanotapers.

The generation of ultrashort electron wavepackets is crucial for the development of ultrafast electron microscopes. Recent studies on Coulomb-correlated few-electron number states, photoemitted from sharp metallic tapers, have shown emission nonlinearities in the multiphoton photoemission regime which scale with the electron number. Here, we study few-electron photoemission from gold nanotapers triggered by few-cycle near-infrared pulses, demonstrating extreme 20th-order nonlinearities for electron triplets. We report interferometric autocorrelation traces of the electron yield that are quenched to a single emission peak with subfemtosecond duration due to these high nonlinearities. The modulation of the emission yield by the carrier-envelope phase suggests that electron emission predominantly occurs during a single half cycle of the driving laser field. When applying a bias voltage to the tip, recollisions in the electron trajectories are suppressed and coherent subcycle electron beams are generated with promising prospects for ultrafast electron microscopy with subcycle time resolution.

Ultrafast Coupling of Optical Near Fields to Low-Energy Electrons Probed in a Point-Projection Microscope.

We report the first observation of the coupling of strong optical near fields to wavepackets of free, 100 eV electrons with <50 fs temporal resolution in an ultrafast point-projection microscope. Optical near fields are created by excitation of a thin, nanometer-sized Yagi-Uda antenna, with 20 fs near-infrared laser pulses. Phase matching between electrons and near fields is achieved due to strong spatial confinement of the antenna near field. Energy-resolved projection images of the antenna are recorded in an optical pump-electron probe scheme. We show that the phase modulation of the electron by transverse-field components results in a transient electron deflection while longitudinal near-field components broaden the kinetic energy distribution. This low-energy electron near-field coupling is used here to characterize the chirp of the ultrafast electron wavepackets, acquired upon propagation from the electron emitter to the sample. Our results bring direct mapping of different vectorial components of highly localized optical near fields into reach.

Super-Resolution Mapping of a Chemical Reaction Driven by Plasmonic Near-Fields.

Plasmonic nanoparticles have recently emerged as promising photocatalysts for light-driven chemical conversions. Their illumination results in the generation of highly energetic charge carriers, elevated surface temperatures, and enhanced electromagnetic fields. Distinguishing between these often-overlapping processes is of paramount importance for the rational design of future plasmonic photocatalysts. However, the study of plasmon-driven chemical reactions is typically performed at the ensemble level and, therefore, is limited by the intrinsic heterogeneity of the catalysts. Here, we report an in situ single-particle study of a fluorogenic chemical reaction driven solely by plasmonic near-fields. Using super-resolution fluorescence microscopy, we map the position of individual product molecules with an ∼30 nm spatial resolution and demonstrate a clear correlation between the electric field distribution around individual nanoparticles and their super-resolved catalytic activity maps. Our results can be extended to systems with more complex electric field distributions, thereby guiding the design of future advanced photocatalysts.

A Full Quantum Mechanical Approach Assessing the Chemical and Electromagnetic Effect in TERS.

Tip-enhanced Raman spectroscopy (TERS) is a valuable method for surface analysis with nanometer to angstrom-scale resolution; however, the accurate simulation of particular TERS signals remains a computational challenge. We approach this challenge by combining the two main contributors to plasmon-enhanced Raman spectroscopy and to the high resolution in TERS, in particular, the electromagnetic and the chemical effect, into one quantum mechanical simulation. The electromagnetic effect describes the sample's interaction with the strong, highly localized, and inhomogeneous electric fields associated with the plasmonic tip and is typically the thematic focus for most mechanistic studies. On the other hand, the chemical effect covers the different responses to the extremely close-range and highly position-sensitive chemical interaction between the apex tip atom(s) and the sample, and, as we could show in previous works, plays an often underestimated role. Starting from a (time-dependent) density functional theory description of the chemical model system, comprised of a tin(II) phthalocyanine sample molecule and a single silver atom as the tip, we introduce the electromagnetic effect through a series of static point charges that recreate the electric field in the vicinity of the plasmonic Ag nanoparticle. By scanning the tip over the molecule along a 3D grid, we can investigate the system's Raman response on each position for nonresonant and resonant illumination. Simulating both effects on their own already hints at the achievable signal enhancement and resolution, but the combination of both creates even stronger evidence that TERS is capable of resolving submolecular features.

Enhancement of Magnetic Resonance Imaging with Metasurfaces.

It is revealed that the unique properties of ultrathin metasurface resonators can improve magnetic resonance imaging dramatically. A metasurface formed when an array of metallic wires is placed inside a scanner under the studied object and a substantial enhancement of the radio-frequency magnetic field is achieved by means of subwavelength manipulation with the metasurface, also allowing improved image resolution.

Hot Carrier Extraction with Plasmonic Broadband Absorbers.

Hot charge carrier extraction from metallic nanostructures is a very promising approach for applications in photocatalysis, photovoltaics, and photodetection. One limitation is that many metallic nanostructures support a single plasmon resonance thus restricting the light-to-charge-carrier activity to a spectral band. Here we demonstrate that a monolayer of plasmonic nanoparticles can be assembled on a multistack layered configuration to achieve broadband, near-unit light absorption, which is spatially localized on the nanoparticle layer. We show that this enhanced light absorbance leads to ∼40-fold increases in the photon-to-electron conversion efficiency by the plasmonic nanostructures. We developed a model that successfully captures the essential physics of the plasmonic hot electron charge generation and separation in these structures. This model also allowed us to establish that efficient hot carrier extraction is limited to spectral regions where (i) the photons have energies higher than the Schottky junctions and (ii) the absorption of light is localized on the metal nanoparticles.

General Analytic Correction for Probe-Position Errors in Spherical Near-Field Measurements.

A general theoretical procedure is presented to remove known probe-position errors in spherical near-field data to obtain highly accurate far fields. We represent the measured data as a Taylor series in terms of the displacement errors and the ideal spectrum of the antenna. This representation is then assumed to be an actual near field on a regularly spaced error-free spherical grid. The ideal spectrum is given by an infinite series of an error operator acting on data containing errors of measurement. This error operator is the Taylor series without the zeroth-order term. The n th-order approximation to the ideal near field of the antenna can be explicitly constructed by inspection of the error operator. Computer simulations using periodic error functions show that we are dealing with a convergent series, and the error-correction technique is highly successful. This is demonstrated for a triply periodic function for errors in each of the spherical coordinates. Appropriate graphical representations of the error-contaminated, error-corrected and error-free near fields are presented to enhance understanding of the results. Corresponding error-contaminated and error-free far fields are also obtained.

Probe-Position Error Correction in Planar Near Field Measurements at 60 GHz: Experimental Verification.

This study was conducted to verify that the probe-position error correction can be successfully applied to real data obtained on a planar near-field range where probe position errors are known. Since probe position-error correction is most important at high frequencies, measurements were made at 60 GHz. Six planar scans at z positions separated by 0.03 A were obtained. The correction technique was applied to an error-contaminated near field constructed out of the six scans according to a discretized periodic error function. The results indicate that probe position errors can be removed from real near-field data as successfully as from simulated data; some residual errors, which are thought to be due to multiple reflections, residual drift in the measurement system, and residual probe position errors in all three coordinates, are observed.

A Near Field Microscope in the Tera-hertz Region.

This paper reports the study of aexperimental project for two near fieldmicroscopes to operate in the Tera-Hertz(THz) region of the electromagneticspectrum. The first design is suited moreproperly for the millimetre wave region,while the second is addressed to shorterwavelengths, typically below onemillimetre.