Introduction

Surface plasmons (SPs) are collective excitations of conduction electrons at the metal-dielectric interface, which significantly enhance light-matter interactions, giving rise to novel physical phenomena and fostering the development of innovative applications. This field currently represents a prominent research focus in condensed matter physics and nano-optics. Recently, the team led by Prof. Zheyu Fang at Peking University, was invited to author a review paper entitled "Dual views on plasmonics: from near-field optics to electron nanoscopy", which was published in Issue 2 Volume 4 of Photonics Insights in 2025. (Yuxiang Chen, Han Zhang, Zongkun Zhang, Xing Zhu, Zheyu Fang. Duel views of plasmonics: from near-field optics to electron nanoscopy[J]. Photonics Insights, 2025, 4(2): R04)

This review systematically and comprehensively summarizes the types and properties of surface plasmon polaritons, demonstrates the techniques for characterizing plasmon polaritons, and discusses the manipulation of their spatial distribution, polarization, and directionality via near-field optics and electron beams. It further elaborates on the advances, challenges, and innovative characterization techniques in quantum plasmonics, offering an outlook on future directions in the field.

1. Typology of Surface Plasmons

Plasmons are collective excitations of electrons and are classified as bosonic quasiparticles associated with oscillations of the electron gas plasma. Generally, plasmon polariton modes are primarily divided into two categories: bulk plasmons and surface plasmons. Surface plasmon polaritons are confined to the interface between metals with negative real permittivity and small positive imaginary permittivity and dielectrics with positive real permittivity, and are classified into surface plasmon polaritons (SPPs) and localized surface plasmons (LSPs).

SPPs are surface electromagnetic waves propagating along the metal–dielectric interface, characterized by non-zero magnetic fields and transverse propagation, and exhibiting exponential decay perpendicular to the interface. Because the excitation of conduction electrons in real metals is subject to both free-electron and inter band damping, the propagating SPP undergoes damping characterized by an energy decay length, which typically ranges from 1 to 100 μm in the visible spectrum. Due to the electrostatic induction effect, perfect conductors (such as the behavior of metals at low frequencies) do not support bound electromagnetic modes (including SPPs). In this case, specialized structures are necessary to achieve tightly confined optical fields and new electromagnetic bound modes, such as spoof surface plasmons (spoof SPs). ¹

LSPs are localized oscillations of free electrons on the surface of metal nanoparticles, with resonant behavior dependent on the shape, size, and dielectric environment of nanoparticles. Under excitation of resonant optical waves, LSPs significantly enhance surface electromagnetic field intensity which rapidly decays with increasing distance, confining light to the nanoscale. Nanofabrication techniques have achieved feature sizes below 10 nm, enabling highly localized electromagnetic fields in nanoscale. ²

2. Characterization Technologies

Figure 1. Experimental setups of characterization techniques. Optical microscopes：(a) leakage radiation microscopy (LRM),(b) scanning near-field optical microscopy (SNOM)；electron microscopes：(c) electron energy loss spectroscopy (EELS),(d) cathodoluminescence nano scopy (CL),€ photon-induced near-field electron microscopy (PINEM),(f) photoemission electron microscopy (PEEM)

Advancements in characterization techniques have enabled the realization of multidimensional measurements, encompassing spatial, temporal, spectral, and momentum characterizations, across all methodologies. Despite the differences in fundamental physical principles and the physical quantities detected by different techniques, SNOM and electron microscopes demonstrate a high degree of similarity and complementarity in the quantitative analysis of near-field mode distributions at the nanoscale. Different types of electron nanoscopes can elucidate the interactions between photons and electrons from various perspectives. The distinct advantages of each characterization method become evident under varying experimental conditions, such as wavelength and material properties.

In comparison to optical techniques, electron microscopes exhibit superior spatiotemporal resolution. With regard to spatial resolution, LRM is constrained by the optical diffraction limit. ^3-5 SNOM utilizes a scanning nanoscale tip to realize nanoscale spatial resolution. ^6-9Although SNOM has successfully surpassed the optical diffraction limit, electron microscopes achieve spatial resolution beyond optical methods due to the short de Broglie wavelength of high-energy electrons. As for the temporal resolution, it primarily depends on the duration of the excitation pulse. Currently, an ultrafast electron pulse technology producing pulse durations on the order of hundreds of femtoseconds has been widely applied in EELS, CL, and PINEM. More notably, attosecond electron microscopy has enhanced time resolution to below 1 fs and has been demonstrated in PINEM systems, indicating that this technology can similarly advance the applications of electron microscope techniques such as EELS and CL. ¹⁰ Therefore, the spatiotemporal resolution of advanced electron microscope technologies surpasses that of optical techniques.

3. Near-field manipulation of surface plasmons

The aforementioned advanced optical characterization technologies contribute to a comprehensive investigation of SPs, providing the base of multi-dimensional plasmonic manipulations. Conventional optical manipulations of SPs are constrained by the optical diffraction limit. The advent of nanoscale tips has effectively transcended the conventional limitations, catalyzing a plethora of pioneering advancements in the domain of plasmonic manipulations at the nanoscale.

Figure 2. Examples of plasmonic manipulations. (a) Phase gradient metasurfaces (b) plasmonic devices with the simultaneous near-field and far-field tuning (c) tailorable polarization-dependent plasmonic directional coupler (d) wavelength-multiplexed focusing lens

Phase modulation, which serves as the foundation for near-field modulation of wavefronts, has been extensively researched and widely implemented in the design of various optical devices. Three main types of phase modulation have been investigated deeply and applied in diverse optical devices: resonant phase, geometric phase, and propagation phase. ¹¹These phase modulation methods can be employed to achieve multi-dimensional flexible control of surface plasmons. This includes directionality, real space, polarization, and wavelength multiplexing, contingent upon the selection of an appropriate basic unit structure and reasonable arrangement.

4. Electron Beam-Based Plasmonic Manipulation

In comparison to optical microscopies, electron beams offer supplementary manipulating degrees of freedom, including excitation position and energies. These additional degrees of freedom enable the selective excitation of specific electromagnetic modes, facilitating multidimensional, flexible manipulation of diverse optical parameters. In the context of surface plasmons, selective excitation of the plasmonic resonant mode by electron beams has been demonstrated to induce target far-field radiation. In recent decades, significant advancements have been made in the domain of electromagnetic radiation research, particularly concerning the modulation of wavelength, polarization, radiation direction, and spatial distribution. This control has been achieved for electromagnetic radiation generated by electron beams, whether under normal incidence or grazing incidence.

Figure 3. Regulation of surface plasmons based on electron beam: (a) wavelength (b) polarization (c) angle (d) near-field distribution

For instance, the wavelength and polarization of SP-induced radiation can be directly controlled through the selection of plasmonic resonance modes excited in the near-field. In accordance with the Purcell effect, the selective enhancement of incoherent radiation from various quantum transitions can be achieved at distinct wavelengths through the manipulation of the localized optical density of states. ¹²Specifically, the use of an electron beam to disrupt structural symmetry has been demonstrated to reveal hidden chiral modes in achiral nanostructures, resulting in the controllable generation of chiral surface plasmon resonance. ¹³

By precisely adjusting the relative phase between different modes, the directionality of far-field radiation can be manipulated. The variances in the distribution of near-field modes can also be captured by various electron microscopy techniques, including PINEM and PEEM techniques. ¹⁴ These techniques provide ultrafast time-resolved characterization of the near-field dynamic evolution process of surface plasmons. Moreover, electron beams exhibit a variety of behaviors in their interaction with transition metal dichalcogenide (TMDC) materials. Related research has achieved nanoscale valley polarization manipulation and far-field separation of photons at different valleys. ^{15, 16}A significant application of electron beam manipulation technology is the electron-driven light source. A variety of light sources have been proposed for a range of functionalities, including vortex beam generators and focusing lenses. This provides novel technological routes for on-chip integrated light sources.

5. Summary and Outlook

Prof. Zheyu Fang's team has reviewed the latest advances in plasmonics from perspectives of near-field optics and electron nanoscopies. The review article highlighted the advanced characterization techniques and near-field methods for manipulating surface plasmons. These techniques, including LRM, SNOM, EELS, CL, PINEM, and PEEM, provide insights into the intrinsic nature of surface plasmon resonance and significantly promote its development. They enable precise control over electromagnetic properties, such as field distribution, wavelength, propagation direction, phase, and polarization, and facilitate the design of various functional nanostructures.

Plasmonics based on classical Maxwell's equations has been extensively investigated. These advanced characterization techniques and near-field manipulation methods have laid a solid foundation for plasmonic devices, including integrated circuits, high-speed communication, and biosensing, which are recent, promising applications. Future theoretical research in pure plasmonics will primarily focus on the quantum model. They include but are not limited to quantum interference, quantum recoil, the quantum dynamics of plasmonic systems, and the interaction between electronic matter waves and crystal lattices. As the experimental technology advances, we expect to see more powerful techniques, like the applications of structured electron beams, to further reveal the quantum properties of surface plasmon polaritons. This will overcome the limitations of existing technologies and advance the development of plasmonics.