Surface plasmons are formally described by Maxwell's equations and boundary conditions, and thus often viewed as a “solved” problem in condensed matter physics ; However, recent progress in them has caused renewed interest in them. This renewed attention stems from their role in plasmonics, where nanoscale structural design and quantum effects introduce new physics and applications. Currently, surface plasmons are key to advances in nanophotonics and sensing. This issue highlights current efforts to harness their functionality and explores how even well-understood excitations can drive new scientific frontiers through changing contexts of design and application.

Plasmons excited in noble metal nanoparticles confine light and amplify the optical field in the vicinity of the nanoparticles. To design plasmonic fields for advanced applications, understanding of plasmons is indispensable. In this report, we describe near-field visualization of plasmons and chiral optical fields around noble metal nanoplates. Near-field transmission imaging revealed that the nanoplate exhibits multiple resonances and transmission images exhibit distinctive spatial features assignable to plasmonic wavefunctions. By using near-field transmission and reflection spectroscopy, we further unraveled the near-field absorption and scattering properties of the nanoplate. We performed three-dimensional near-field polarimetry and found that the observed spatial distribution is ascribable to chiral optical fields. We also revealed that the chiral optical field extends beyond the optical near-field, owing to distinct spatial distributions of electric and magnetic fields around the nanoplate. The findings in this report provide a fundamental basis for future plasmonic applications.

Optical responses of metallic nanostructures supported on graphene substrates are described. Alkali metal thin films exhibit strong absorption in the visible region due to multipole plasmon, which show anomalously long coherence decay time. Deposition of silver and indium atoms leads to their cluster formation, which exhibits localized surface plasmon resonance in the UV and far-UV regions, respectively. Plexciton formations between molecular transitions and the localized surface plasmons of these clusters are studied.

We investigated two types of surface plasmons―localized surface plasmon resonance (LSPR) and surface plasmon polaritons (SPPs)―and their interplay using size-selected Ag, Au, and Cu nanoclusters (NCs) and ultrafast spectroscopy. Atomically precise NCs were prepared by magnetron sputtering with mass selection and deposited on C₆₀-modified substrates. Two-photon photoemission spectroscopy (2PPE) revealed threshold sizes for plasmon emergence (≥9 atoms for Ag, ≥21 for Au) and clarified the influence of d-band structures. Dual-color 2P-PEEM visualized SPP propagation at metal–organic interfaces, showing that LSPR enhancement and molecular modifications can tune SPP characteristics, even at buried interfaces using NC sensitizers. These results provide key insights into size- and structure-dependent plasmonic behavior and inform the design of advanced photoelectronic and nanophotonic devices.

Localized surface plasmons have attracted significant attention for nearly two decades. However, the understanding of their interactions with matter, especially in plasmon-induced chemical reactions, remains limited as it is difficult to experimentally analyze reaction processes due to the ultrafast relaxation and nanoscale localization of plasmons. This article presents our studies on plasmon-induced reactions at a single-molecule level by using a scanning tunneling microscope combined with light illumination. By comparing two molecular systems, we demonstrated that the electronic state of adsorbed molecules formed through hybridization with the metal surface plays a crucial role in determining the reaction mechanism. These findings, enabled by single-molecule analysis, provide experimental validation of previously hypothetical mechanisms and valuable insights for designing plasmonic catalysts.

Tip-enhanced Raman spectroscopy (TERS) is a powerful method for chemically identifying single molecules on surfaces. Although the mechanisms of the Raman signal enhancement have long been proposed, their precise evaluation and understanding remain challenging. This article introduces two key techniques to obtain single-molecule Raman signals with high reproducibility using TERS : (1) electromagnetic enhancement by a strong plasmonic field confined at atomic-scale structures of the tip apex, namely picocavities, and (2) chemical enhancement by forming an atomic point contact between the tip and the target molecule. These techniques have enabled TERS measurements on advanced and challenging samples, including molecular hydrogen physisorbed on metal surfaces and organic molecules chemisorbed on a non-plasmonic semiconductor substrate. Thus, characterizing and controlling plasmonic junctions at the atomic scale would further expand the TERS targetability.

Tip-enhanced vibrational spectroscopy has matured to the point of reliably achieving nanoscale spatial resolution, with tip-enhanced Raman spectroscopy reaching even atomic-level and submolecular sensitivity. Likewise, tip-enhanced infrared methods, such as nano-FTIR and AFM-IR spectroscopy, have enabled chemical analysis at the nanoscale for diverse systems. However, the fundamental spatial resolution and sensitivity limits of infrared nano-spectroscopy remain elusive, highlighting the need for a quantitative understanding of near-field interactions within infrared nanocavities. In this study, we apply nano-FTIR spectroscopy to detect the amide-I vibrational mode of a single protein composed of approximately 500 amino acids. By employing higher-order demodulation harmonics up to the seventh order, we observe a significant increase in vibrational resonance amplitude, attributed to geometrical effects confined below the tip radius and beyond conventional dipole models. This work provides a quantitative description of near-field interactions at the single-nanometer scale, paving the way for sub-nanometer and single-molecule infrared vibrational spectroscopy.

We investigated the fundamental characteristics of welding 0.2 wt% Be-Cu (BeCu) using a Blue＋IR hybrid laser. Butt welding of BeCu plates, welding of a one-sided rotating nipple (ICF70)-which cannot be fabricated by conventional machining-and dissimilar material welding between steel use stainless (SUS) and BeCu were performed. These trials successfully demonstrated the feasibility of fabricating vacuum components that were previously considered unmanufacturable. Furthermore, tensile tests of the SUS/BeCu dissimilar welded joints revealed strengths exceeding that of SUS itself, confirming both the viability and effectiveness of this welding technique.