Dynamics of electron-emission currents in plasmonic gaps induced by strong fields

Quantum surface effects in the electromagnetic coupling between a quantum emitter and a plasmonic nanoantenna: time-dependent density functional theory vs. semiclassical Feibelman approach

We use time-dependent density functional theory (TDDFT) within the jellium model to study the impact of quantum-mechanical effects on the self-interaction Green's function that governs the electromagnetic interaction between quantum emitters and plasmonic metallic nanoantennas. A semiclassical model based on the Feibelman parameters, which incorporates quantum surface-response corrections into an otherwise classical description, confirms surface-enabled Landau damping and the spill out of the induced charges as the dominant quantum mechanisms strongly affecting the nanoantenna-emitter interaction. These quantum effects produce a redshift and broadening of plasmonic resonances not present in classical theories that consider a local dielectric response of the metals. We show that the Feibelman approach correctly reproduces the nonlocal surface response obtained by full quantum TDDFT calculations for most nanoantenna-emitter configurations. However, when the emitter is located in very close proximity to the nanoantenna surface, we show that the standard Feibelman approach fails, requiring an implementation that explicitly accounts for the nonlocality of the surface response in the direction parallel to the surface. Our study thus provides a fundamental description of the electromagnetic coupling between plasmonic nanoantennas and quantum emitters at the nanoscale.

Energy and Momentum Distribution of Surface Plasmon-Induced Hot Carriers Isolated via Spatiotemporal Separation

Understanding the differences between photon-induced and plasmon-induced hot electrons is essential for the construction of devices for plasmonic energy conversion. The mechanism of the plasmonic enhancement in photochemistry, photocatalysis, and light-harvesting and especially the role of hot carriers is still heavily discussed. The question remains, if plasmon-induced and photon-induced hot carriers are fundamentally different or if plasmonic enhancement is only an effect of field concentration producing these carriers in greater numbers. For the bulk plasmon resonance, a fundamental difference is known, yet for the technologically important surface plasmons, this is far from being settled. The direct imaging of surface plasmon-induced hot carriers could provide essential insight, but the separation of the influence of driving laser, field-enhancement, and fundamental plasmon decay has proven to be difficult. Here, we present an approach using a two-color femtosecond pump-probe scheme in time-resolved 2-photon-photoemission (tr-2PPE), supported by a theoretical analysis of the light and plasmon energy flow. We separate the energy and momentum distribution of the plasmon-induced hot electrons from that of photoexcited electrons by following the spatial evolution of photoemitted electrons with energy-resolved photoemission electron microscopy (PEEM) and momentum microscopy during the propagation of a surface plasmon polariton (SPP) pulse along a gold surface. With this scheme, we realize a direct experimental access to plasmon-induced hot electrons. We find a plasmonic enhancement toward high excitation energies and small in-plane momenta, which suggests a fundamentally different mechanism of hot electron generation, as previously unknown for surface plasmons.

From SERS to TERS and Beyond: Molecules as Probes of Nanoscopic Optical Fields

A detailed understanding of the interaction between molecules and plasmonic nanostructures is important for several exciting developments in (bio)molecular sensing and imaging, catalysis, as well as energy conversion. While much of the focus has been on the nanostructures that generate enhanced and nano-confined optical fields, we herein highlight recent work from our groups that uses the molecular response in surface and tip enhanced Raman scattering (SERS and TERS, respectively) to investigate different aspects of the local fields. TERS provides access to ultra-confined volumes, and as a result can further explore and explain ensemble-averaged SERS measurements. Exciting and distinct molecular behaviors are observed in the quantum limit of plasmons, including molecular charging, chemical conversion, and optical rectification. Evidence of multipolar Raman scattering from molecules additionally provides insights into the inhomogeneous electric fields that drive SERS and TERS and their spatial and temporal gradients. The time scales of these processes show evidence of cooperative nanoscale phenomena that altogether contribute to SERS and TERS.

Suppressing Molecular Charging, Nanochemistry, and Optical Rectification in the Tip-Enhanced Raman Geometry

Classical versus quantum plasmons are responsible for the recorded signals in non-contact-mode versus contact-mode tip-enhanced Raman spectroscopy (TERS) and lead to distinct observables. Under otherwise identical experimental conditions, we illustrate the concept through tapping- and contact-mode TERS mapping of chemically functionalized silver nanocubes. Whereas molecular charging, chemical transformations, and optical rectification are prominent observables in contact-mode TERS, the same effects are suppressed using tapping-mode feedback. In effect, this work demonstrates that nanoscale physical and chemical processes can be accessed and/or suppressed on demand in the TERS geometry. The advantages of tapping-mode TERS are otherwise highlighted with the latter in mind.

An asymmetric aluminum active quantum plasmonic device

Plasmonic metal nanostructures support intense nanoscale electromagnetic hotspots that can be modulated in an active plasmonic device.