Fig. 1. (a) Schematics of SPPs propagating along the given metal-dielectric interface. (b) Calculated dispersion relationship of SPPs in the interface (red dashed line) and free-space light (blue solid line).

Fig. 2. (a) The experimental setup of LRM. (b) Experimental schematics of leakage radiation interferometer. The fs pulsed laser beam is split into two beams. One beam is sent through a leakage radiation microscope, and the other is optically delayed to regulate the phase difference and ensure proper beam overlap. The image at the back focal plane (BFP) of the objective shows the delay induced by the SP mode propagating on the metallic film. Reproduced with permission from Ref. [145], © 2016 Optical Society of America (OSA). (c) Unidirectional surface-plasmon excitation in a spatially symmetric structure. The results of angle-resolved LRM demonstrate that LCP and RCP excitations generate SPPs propagating in different directions. Reproduced with permission from Ref. [146], © 2013 American Association for the Advancement of Science (AAAS). (d) Unidirectional propagation of SPPs characterized by LRM in real space (top) and the momentum space (bottom). Reproduced with permission from Ref. [147], © 2016 American Chemical Society (ACS).

Fig. 3. (a) The experimental setup of a-SNOM. (b) The experimental setup of s-SNOM. (c) Schematic of a high-performance plasmon nanofocusing tip under internal illumination. The Au spiral-grating conical tip transfers the optical energy to the outer SPP modes, resulting in a superfocusing spot at the apex. The plasmonic enhancement accomplishes an SNOM tip with high resolution, throughput, and SNR. The Au spiral grating offers flexible momentum-matching conditions for the excitation of SPPs. Reproduced with permission from Ref. [60], © 2023 the authors. (d) SNOM images of an Ag nanoprism for excitations with different linear polarizations. Reproduced with permission from Ref. [159], © 2008 ACS. (e) The spatial distributions of amplitude $S_3$ and phase ${\varphi }_3$ measured by s-SNOM. Reproduced with permission from Ref. [160], © 2015 ACS. (f) Time-resolved pump-probe near-field images of the waveguide exciton polaritons in ${\mathrm{WSe}}_2$. Reproduced with permission from Ref. [161], © 2019 AAAS.

Fig. 4. (a) A comparison of both EELS (left) and CL (right) spectra distinguishes the lossy and the trapped optical modes, characterizing true photonic BICs with high spatial precision. By systematically breaking the antenna symmetry, the quasi-BIC resonances become visible. Reproduced with permission from Ref. [256], © 2022 Nature Publishing Group (NPG). (b) Schematics of the EELS experimental setup. Left: The use of a round aperture obtains spectral information with a larger beam convergence angle. With the beam scanning in two spatial dimensions, a 3D EELS ($x-y-\omega$) dataset can be recorded. Right: With a slot aperture, the dispersion diagram can be recorded in parallel, yielding a 4D EELS ($x-y-\omega -q$) dataset. (c) The EELS measurement of phonon dispersion. Left: schematics of the experimental geometry, which illustrates the beam position and the diffraction plane. Right: phonon dispersion line profiles at different positions. Reproduced with permission from Ref. [257], © 2021 NPG. (d) 3D Time-resolved EELS plot. The dynamics of the chemical bonding of graphite can be pictured. Reproduced with permission from Ref. [258], © 2009 AAAS.

Fig. 5. (a) The optical path of the CL detection platform for (b) the total CL mapping or (c) the angular distribution imaging. A quarter-wave plate combined with a linear polarizer was utilized to extract the LCP and RCP components of CL emissions. Circularly polarized resolved CL mapping and angular distribution can be obtained by employing bandpass filters with varying central wavelengths. BP: bandpass. (b) Reproduced with permission from Ref. [277], © 2018 ACS. (c) Reproduced with permission from Ref. [278], © 2019 American Physical Society (APS). (d) The CL signal can also be detected by a spectrometer after a slit for (e) CL spectrum or (f) energy-momentum mapping. (e) Reproduced with permission from Ref. [279], © 2021 ACS. (f) Reproduced with permission from Ref. [280], © 2023 NPG. (g) Schematics of two different configurations of time-resolved CL microscopy. Left: The ultrafast electron pulse is driven by an ultrafast laser pulse. Right: A set of electrostatically deflected plates is mounted inside the electron column and driven by square voltages from an electron shape generator. Reproduced with permission from Ref. [281], © 2023 ACS. (h) The 2D histogram of time delay versus electron energy loss can be reconstructed using time-resolved CL and EELS microscopy to identify electrons that are within $\pm 25\mathrm{ns}$ of a detected photon. Reproduced with permission from Ref. [282], © 2022 AAAS.

Fig. 6. (a) Schematic of PINEM setup. Ultrashort electron pulses generated by nanotip photoemission interact with the electromagnetic field of the nanostructure, exchanging energy in integer multiples of the photon energy. (b) Typical spectral shape in PINEM: the energy comb. This result reveals multiphoton absorption and emission events. Reproduced with permission from Ref. [305], © 2010 ACS. (c) Schematic of electron-near-field interaction in PINEM. 200 keV STEM-EELS, 20 keV SEM-CL, and 200 keV STEM-PINEM spectra of the Au nanostar for different excitation positions, which are indicated by the color-matched dots in the insets. Reproduced with permission from Ref. [306], © 2021 NPG. (d) Top: continuous-wave modulation of electron wave functions in the PINEM experiment. Highly efficient electron-light interaction facilitated by an inverse-designed silicon-photonic nanostructure, consisting of a Bragg mirror and a periodic channel that achieves quasi-phase-matching of electron and quantum light. Two types of light states are revealed by the electron energy spectrum. Bottom: Free-electron-light interactions imprint the quantum photon statistics on the electron energy spectra, demonstrating the quantum walk of a free electron. Reproduced with permission from Ref. [307], © 2021 AAAS.

Fig. 7. (a) Schematic of the PEEM setup with pump-probe measurements. (b) PEEM micrographs of the identical region on the Ag grating obtained with 254 nm line of a Hg lamp (left), and p-polarized (middle) and s-polarized (right) 400 nm femtosecond laser excitation microscopic interferometric two-pulse correlation scans from individual hot spots. Intensities are enhanced for p-polarized light by more than two orders of magnitude compared with the emission excited with s-polarized light. Reproduced with permission from Ref. [326], © 2005 ACS. (c) The momentum distributions of emitted electrons from individual Au nanorods is studied with a momentum-resolved PEEM, which reveals two distinct emission mechanisms: a coherent photoemission process from the optically heated electron gas and an additional emission process resulting from the optical field enhancement at both ends of the nanorod. Reproduced with permission from Ref. [327], © 2017 ACS. (d), (e) Schematic (d) and experimental results (e) of the dispersive and dissipative propagation of SPP wave packets at an Ag-vacuum interface recorded by the interferometric time-resolved PEEM. (e) The interference pattern in Ag film with different time delays. Reproduced with permission from Ref. [328], © 2007 ACS.

Fig. 8. (a)–(c) Three basic types of plasmonic phase modulation: (a) resonant phase, (b) geometric phase, (c) propagation phase. Reproduced with permission from Ref. [418], © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (d) V-shaped nanostructures that enable a resonant phase tuning range of up to $2\pi$. Reproduced with permission from Ref. [419], © 2012 ACS. (e) Schematic of the bipolar plasmonic metalens based on the PB phase. Reproduced with permission from Ref. [420], © 2012 Macmillan Publishers Limited. (f) Schematic diagram of dielectric meta-grating for broadband wavefront shaping. Reproduced with permission from Ref. [421], © 2015 ACS. (g) SPP vortex generation based on resonant and PB phases. The meta-atoms are a series of metal-insulator-metal nanostructures providing both PB and resonant phases by their orientation angles and geometric sizes, respectively. Reproduced with permission from Ref. [422], © 2023 OSA. (h) Demonstration of near-field plasmonic vortex generated by nanoslit array with geometric phase (slit orientation angle) and dynamic phase (radial position). Reproduced with permission from Ref. [423], © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (i) SEM (left) and SNOM (right) images of a spiral slit. SNOM images characterize amplitude (top) and phase (left) distributions of plasmonic vortex generated by the slit. Reproduced with permission from Ref. [424], © 2018 OSA.

Fig. 9. (a) Top: schematic of paired magnetic antennas (metallic-dielectric-metallic layers). Bottom: LRM images for different nanoantenna separation distances. The SPPs propagate in opposite directions for D = 300 or 600 nm. Reproduced with permission from Ref. [151], © 2012 ACS. (b) A defect aperture manufactured in a nanoscale waveguide for polarization-free SPP coupling. Top: the geometric structure. Bottom: The experimental results demonstrate the bidirectional SPP coupling from the p- and s-polarized laser beams. Reproduced with permission from Ref. [448], © 2015 by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (c) Schematic of phase gradient bifunctional metasurface. The MIM type unit cell is composed of an Ag nanobrick on top of a ${\mathrm{SiO}}_2$ spacer layer and a Ag substrate. $X$-polarization and $y$-polarization lights are respectively coupled into directional SPPs and an anomalously reflected wave. Reproduced with permission from Ref. [449], © 2018 the authors. (d) Demonstration of asymmetric SPP excitation via near coupling of single-slit and split-ring slit resonators. The field distributions (top) and spectra (bottom) of single-slit resonators, single-split-ring-shaped slit resonators, and paired slit resonators. The SPP amplitude is measured by a THz time-domain near-field spectroscopy system. Reproduced with permission from Ref. [450], © 2016 AAAS. (e) Geometric phase gradient metasurfaces for directional coupling. Experimental results demonstrate that the excited SPPs propagate along $-x$ ($+x$) direction under LCP (RCP) incidence. Reproduced with permission from Ref. [439], © 2013 the authors.

Fig. 10. (a) Schematic of the double-lined device for complex-field generation. Reproduced with permission from Ref. [464], © 2016 by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (b) Picture of the fabricated spin-modulated metasurface as well as the measured SPP field distributions. Reproduced with permission from Ref. [429], © 2023 ACS. (c) Schematic of double-lined Au nanoslits for plasmonic polarization-independent Airy beam generation. Reproduced with permission from Ref. [465], © 2020 by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (d) Schematic of the near- and far-field launchers based on an array of nanoslits, and the measured field distributions in the near and far fields. Reproduced with permission from Ref. [466], © 2017 by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (e) Calculated distributions of the corresponding electric field ($E_z$), magnetic field ($|H|$), and amplitude of the displacement current ($j_z$) for the nanostructure with toroidal dipolar response. Reproduced with permission from Ref. [467], © 2015 APS. (f) The characterization of anapole modes in a silicon nanodisk. Left: the top row shows SNOM images, while the middle and bottom rows show calculated transversal electric and magnetic near fields, respectively. As the wavelength approaches 620 nm, the central hotspot splits into two separate spots. A new hotspot appears in the middle of the disk, at 640 nm, which is close to the anapole mode wavelength. Right: experimental setup of SNOM measurement with the incident light coming through the substrate and collecting on the top. Reproduced with permission from Ref. ^[468], © 2015 Macmillan Publishers Limited.

Fig. 11. (a) Top: schematic view of the implementation of an arbitrary SPP profile derived from the modified matching rule. Bottom: measured SPP profiles for two incidences that show high consistency with theoretical prediction. Reproduced with permission from Ref. [493] (b) The metasurface composed of asymmetric crossed air-slits tailors near-field optical OAMs. The right-top panel demonstrates the calculated instantaneous $E_z$ field at 10 nm above the Au-air interface, which indicates that the single structure produces a vortex field. SNOM images prove that a plasmonic vortex with a topological charge of 2 and a subwavelength focusing spot carrying no OAM are generated under $-45°$ and 45° linearly polarized optical excitations, respectively, which is experimentally measured by SNOM. Reproduced with permission from Ref. [480], © 2015 ACS. (c) Schematic of a pin cushion structure for polarization beam splitting and SNOM measured patterns under two distinct linear polarizations. Two focal spots formed along a line parallel to the incident polarization direction. Reproduced with permission from Ref. [494], © 2013 ACS. (d) Experimental results of the tailorable polarization-dependent plasmonic directional coupler. Reproduced with permission from Ref. [15] © 2022 Wiley-VCH GmbH. (e) Left: schematic diagram and SEM image of the geometric phase metasurface. Right: measured SPP intensity distributions at wavelengths of 375, 450, 525, and 600 µm. Reproduced with permission from Ref. [435], © 2023 Wiley-VCH GmbH. (f) Schematics of multiple-wavelength SPP coupler and FDTD-simulated SPP field patterns under 820, 850, and 880 nm wavelengths. Reproduced with permission from Ref. [16], © 2011 ACS. (g) Schematic of valley topological photonic crystals for the realization of wavelength-dependent transmission in different edge channels. The topological photonic device is composed of TPC I and TPC II. A coaxial monopole antenna is placed at the center port S3 between TPC I and TPC II to excite the spoof SPP topological edge states. SPPs can transmit towards both sides when the incident frequency is within the overlap range of topological edge states of two TPCs; otherwise, they propagate along the side where the frequency is matched with a topological edge state. Reproduced with permission from Ref. [495], © 2024 the authors. (h) Design and experimental results of the switchable holographic metalens. A different source point for each wavelength (632, 670, 710, and 750 nm) is chosen such that light at each wavelength couples to SPPs via the nanoslits and is focused on the four corners. Reproduced with permission from Ref. [496], © 2015 ACS. (i) Schematic of plasmonic metasurface for spectral and polarimetric directional routing. High-performance frequency-dependent and spin-dependent unidirectional SPP excitation can be observed via a THz time-domain spectroscopy system. Reproduced with permission from Ref. [497], © 2025 ACS.

Fig. 12. (a) Left: SEM image of a truncated tetrahedral Au nanoparticle. Right: experimental CL spectra for 30 keV electron beam injection near the tip and the edge of the tetrahedron. The generation of distinct resonant modes is observed when an electron beam impacts at varying positions. Reproduced with permission from Ref. [522], © 2012 ACS. (b) Left: wavelength-selected CL images of the nonamer at 660 and 700 nm. Right: CL spectra for different excitation positions: the center particle (red) and a particle in the outer ring (blue). The inset shows an SEM image of a nonamer with blue and red squares to indicate the location of the beam for the blue and red spectra, respectively. Reproduced with permission from Ref. [523], © 2012 ACS. (c) Left: CL images of the Al-ZnO hybrid structure at 379 and 585 nm. Right: CL spectra for different electron beam excitation positions prove that the bandgap transition (379 nm) and the defect transition (585 nm) of ZnO are selectively enhanced. Reproduced with permission from Ref. [88], © 2020 ACS. (d) Left: schematic of free-electron-plasmon coupling through electron-energy-dependent CL spectroscopy. Right: measured (dots) and simulated (solid) CL emission probability for electrons passing through the center of Au nanospheres with a diameter of 100 (green) and 50 nm (purple). Reproduced with permission from Ref. [524], © 2024 ACS.

Fig. 13. (a) SPR polarization control with Babinet metasurfaces in the THz band. Reproduced with permission from Ref. [525], © 2016 APS. (b)  Schematic diagram of generating vortex Smith–Purcell radiation with free-electron bunches and holographic grating. The holographic grating is made of metal or dielectrics and has a fork structure with two teeth. Reproduced with permission from Ref. [526], © 2020 Chinese Laser Press. (c) The light-well structure can successfully generate circularly polarized SPR driven by the electron beam, and the chirality can be continuously controlled by moving the position of the electron beam. Reproduced with permission from Ref. [527], © 2023 Wiley-VCH GmbH. (d) Left: schematics of circular polarization resolved Al nanoantenna with electron beam excitation. Right: the averaged, LCP, and RCP CL intensities with different collection wavelengths in the left and right arm-end regions. When the electron beam excitation position is localized at the upper left corner of the structure, the coherent CL appears as LCP light. Reproduced with permission from Ref. [300], © 2018 ACS. (e) The nanodisk is positioned beneath a nanoantenna corner, which is a hot spot of chiral radiative LDOS. The chiral PL spectrum of the hybrid structure demonstrates that the chiral radiative properties of WSe2 are modified. Reproduced with permission from Ref. [69], © 2019 ACS.

Fig. 14. (a) Illustration of the method for the extraction of the SPP scattering phase through CL. TR from the metallic substrate is used as a reference wave, interfering in the far field (at the detector) without coupled light resulting from the SPP interaction with the sampled object. Reproduced with permission from Ref. [94], © 2020 ACS. (b) Numerically derived phase profile of p-polarized scattered field by the helical nanoaperture (indicated by a circle) showing phase singularity with topological charge of $-1$. Reproduced with permission from Ref. [95], © 2020 ACS. (c) Normalized angular CL emission patterns at 600 nm collected from a nanodisk with a diameter of 180 nm for different excitation positions. Reproduced with permission from Ref. [97], © 2014 NPG. (d) Top: schematic of the radiation system. Bottom: 3D representation of angular-resolved radiation pattern, together with a 2D projection. Reproduced with permission from Ref. [528] © 2011 ACS. (e) Schematic of experimental measurement of generating and focusing SPR with a chirped grating. SPR of different wavelengths converge at varying points. Reproduced with permission from Ref. [99], © 2022 ACS. (f) Experimental normalized angular CL S3 patterns obtained from the Au nanoantenna. The radiation patterns of LCP and RCP are inverted by changing the electron beam impact position (the midpoint of upper and bottom edges), and the nonsplitting pattern of spin states is detected when the electron beam impacts at the center of the Au nanoantenna. Reproduced with permission from Ref. [108], © 2021 AAAS.

Fig. 15. (a) Left: comparison of theoretical (top) and experimental (bottom) EELS spectra. Right: calculated and measured maps of distinct modes. Reproduced with permission from Ref. [539] © 2012 ACS. (b) Spectral response (top) and photon maps (bottom) of an Au nanoprism with a side length of 266 nm. Three in-plane modes have been experimentally revealed, including dipole (D), hexapolar (H), and quadrupolar (Q) modes. Which of them dominates the radiation depends on the excitation position. Reproduced with permission from Ref. [301] © 2018 ACS. (c) Left: the experimental setup including a focused electron beam and a co-propagating laser beam traversing the sample and reference. Right: the complex electromagnetic field response of a nanoprism, including both magnitude (left) and complex amplitude (right). (d) Temporal sequence of the out-of-plane electric field evolution within the optical cycle. Reproduced with permission from Ref. [392] © 2024 NPG. (e) Left: the nanoscale mode distribution and near-field spectra measured by PEEM. The yellow dashed box indicates the structure outline with a side length of approximately 200 nm. Right: interference curves for the dipole and quadrupole modes under the irradiation of a 7 fs broadband pulsed laser. By employing the plasmon oscillator model, the dephasing time of the dipole and quadrupole modes are extracted to be approximately 5 and 9 fs, respectively. Reproduced with permission from Ref. [540] © 2016 ACS. (f) Snapshot sequence of time-resolved PEEM from a plasmonic vortex cavity of order $m=5$, showing the revolution stages of the initial vortex (left, order $l=5+1$) and subsequent first (middle $t$, order $l=15+1$) and second (right $t$, order $l=25+1$) reflections of the wave packet at specific pump-probe time delays $\mathrm{\Delta }t$. Reproduced with permission from Ref. [342] © 2021, AAAS.

Fig. 16. (a)–(c) Schematic (a) and SEM (b) image of $\mathrm{h}\text{-}\mathrm{BN}/{\mathrm{WSe}}_2/\mathrm{h}\text{-}\mathrm{BN}$ and Au rectangle antenna hybrid nanostructure. (c) The principle of near-field manipulation of valley polarization. Reproduced with permission from Ref. [546], © 2021 the authors. (d), (e) Schematic illustration of chirality-selective far-field routing of valley photons in $\mathrm{Au}/{\mathrm{WS}}_2$ hybrid nanostructure. Distribution of Stokes parameter ${\mathrm{S}}_3$ of the radiation of the hybrid nanostructure under different excitation positions. Reproduced with permission from Ref. [530], © 2023 Wiley-VCH GmbH. (f), (g) The schematic drawing of valley plasmonic crystals and experimental visualization of valley-polarized plasmonic edge mode via STEM-CL. Reproduced with permission from Ref. [547], © 2021 ACS. (h), (i) Schematic overview of the $\mathrm{Au}/{\mathrm{WSe}}_2$ crystal with emerging hybridized bands and created plexciton quasiparticles via the strong coupling between plasmonic Bloch modes with excitons of the ${\mathrm{WSe}}_2$ material. Measured and simulated band structures of the plexcitons in the hybrid structure. Reproduced with permission from Ref. [548], © 2022 ACS.

Fig. 17. (a) CR generated by low-energy electrons in the graphene-based hyperbolic metamaterial. The metamaterial in the hyperbolic state produces the CR caused by low energy (left), and conventional CR (high threshold) corresponds to the elliptical state (right). Reproduced with permission from Ref. [559], © 2022 Chinese Laser Press. (b) The experimental setup for the 2D CR measurement. The schematic represents the emission of a single quanta of photonic quasiparticles (exemplified by the Feynman diagrams of one-photon emission), which is part of the joint electron-photon quantum state. Reproduced with permission from Ref. [560], © 2023 APS. (c) Schematic of the light well. The light well is a nanohole milled through a stack of alternating metal and dielectric layers. Reproduced with permission from Ref. [561], © 2009 APS. (d) Left: the SEM image of a nanosquare light well with seven excitation positions at the perpendicular line of the angular bisector, marked by colored symbols. Right: corresponding variation of chirality values of seven injection points at 740 nm wavelength. Reproduced with permission from Ref. [527], © 2023 Wiley-VCH GmbH. (e) 3D view (upper panel) and side view (lower panel) of an electron bunch moving atop a graphene layer on a 1D dielectric grating. Reproduced with permission from Ref. [562], © 2014 APS. (f) Collective and resonant effects in the interaction of free electrons with the disordered plasmonic structure. Left: SPR from the 2D plasmonic crystal with strong disorder. Right: SPR from the localized plasmonic resonance of a single Ag rod. Reproduced with permission from Ref. [87], © 2017 APS.

Fig. 18. (a) Left: schematic of an electron-driven light source based on a plasmonic metasurface. Right: comparison of CL emission and optical absorption of the plasmonic metasurface, with different metamolecule cell sizes. Reproduced with permission from Ref. [568], © 2012 APS. (b) Left: schematic of the micro-electron-driven source generating a vortex light beam with a spiral structure. The electron simultaneously produces SPPs propagating along the spiral chains and then generates a vortex field via scattering from nanoholes. Right: SEM images of spiral structures with four arms and corresponding angle-resolved CL distribution over a 380–900 nm bandwidth. Reproduced with permission from Ref. [101], © 2020 ACS. (c) Top: holographic electron-driven light source design. The electron beam excitation is approximately equivalent to an electric dipole source located in close proximity to the surface in the simulation. As the normal holographic design, the interference pattern of the field generated by the dipole and the required output field determines the geometry of the metallic metasurface. Bottom: angle-resolved CL intensity distribution maps for electron-driven generation of optical vortex beams with topological charges of 3, 6, and 9. Reproduced with permission from Ref. [569], © 2016 the authors. (d) The generated free electron radiation focuses at different locations when a free electron beam is normally incident on points A and B of the metallic photon sieve. Experimental measurements of far-field radiation show concentric rings, indicating the divergence of light from the focal plane to the far field. Reproduced with permission from Ref. [100], © 2019 NPG.