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. 2026 Jul 29;6(4):549–557. doi: 10.1021/acsnanoscienceau.6c00058

Plasmonic Hybridization and Near-Field Localization in Gold “Dog Bone” Nanoparticles

Ibrahim Tanriover †, Koustav Kundu ‡, Amit Jaiswal ‡,*, Haihua Liu †,*
PMCID: PMC13495461  PMID: 42630785

Abstract

Gold nanoparticles with complex anisotropic geometries offer unique opportunities for tailoring plasmonic near-fields beyond the limits of conventional nanorods. Here we combine photon-induced near-field electron microscopy (PINEM) and full-wave simulations to resolve plasmon hybridization and near-field localization in gold dog-bone nanoparticles and their dimers with and without conformal SiO2 shells. We demonstrate that the dog-bone geometry drives unconventional polarization-dependent field confinement, including pronounced tip-localized near-fields under both longitudinal and transverse excitation. Conformal SiO2 shells red-shift the plasmon resonances through dielectric screening and prevent charge-transfer coupling in adjacent dimers. Depending on the dimer orientation and excitation polarization, coupled structures exhibit distinct bonding- and antibonding-like hybridized modes as well as interparticle near-field channels. However, weakly interacting dimers retain largely independent particle responses with geometry-governed asymmetries. The close agreement between PINEM measurements and simulations establishes a direct picture of how shape, dielectric environment, and nanoscale coupling together define plasmonic modes in complex anisotropic nanoparticles. These findings provide design principles for engineering localized optical fields in plasmonic nanoscale structures.

Keywords: plasmonics, hybridization, near-fields, ultrafast electron microscopy, photon-induced near-field electron microscopy

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Introduction

Coherent collective oscillations of the conduction band electrons in noble metal nanoparticles (NPs), well-known as localized surface plasmon resonances (LSPR), have widely been used to confine and manipulate electromagnetic fields at the nanoscale. − Gold (Au) and silver (Ag) NPs have been utilized to generate strong electric fields around the NP in the visible and near-infrared (NIR) regions of the electromagnetic spectrum due to the tunability of their resonance frequencies based on size and shape. − Au NPs have also been in focus for biological applications due to high chemical stability and biocompatibility. Electromagnetic field enhancement by Au NPs has enabled numerous applications, including surface-enhanced spectroscopies, photothermal therapy, biosensing, and bioimaging. − Spatial distribution and the magnitude of the electromagnetic field enhancement strongly depend on the geometry of the nanostructures. For a spherical NP, the maximum field intensity is determined by the dipolar mode, , while geometries with sharp features and high curvatures, such as tips and protrusions can further increase the near-field intensities due to lightning-rod effects. , Individual NP also serve as building blocks for NP dimers, oligomers, and aggregates, which can provide an additional boost to electromagnetic field enhancement by near-field confinement in the gap between NPs. −

Au nanorods (NRs) represent one of the most widely studied anisotropic plasmonic nanostructures due to their well-defined longitudinal and transverse dipolar modes. Conventional Au NRs possess hemispherical ends, while introduction of sharp features and protrusions at the ends might significantly increase the local curvature, hence enhancing the localized electric field intensity. Compared to the hemispherical ends, protruding ends concentrate electric field into smaller spatial volume and generate stronger near-fields. This property leads to applications that rely on strong near-field interactions, such as surface-enhanced Raman scattering (SERS). −

While gold nanoparticles are widely regarded as promising materials for biomedical applications, their behavior in biological environments depends strongly on surface chemistry. For anisotropic Au nanostructures, residual surfactants such as CTAB can cause undesirable biological effects. Further, studies have shown that bare gold nanoparticles induce protein corona formation in physiological conditions leading to the aggregation of particles. , Compromise in the colloidal stability of nanoparticles or pathogenesis due to protein aggregation is not desirable in any biological environment. Thus, silica coating is often introduced to improve stability, reduce surface-related toxicity, and provide a versatile interface for further functionalization. A thin shell around NPs is unlikely to significantly alter the field distribution pattern produced by the metal core. Beyond preserving optical properties, silica coating also provides several advantages for biomedical applications. It has been shown to improve biocompatibility and enhance cellular uptake. The surface of the silica has been utilized to covalently conjugate biomolecules, fluorescent dyes, or targeting ligands enabling development of multifunctional platforms for bioimaging, sensing, and targeted therapy. , Porous silica shell can also be used for loading small molecules such as therapeutic agents for achieving controlled delivery. − Silica-coated NPs has also been shown to avoid the formation of protein corona when exposed to biological fluids. Additionally, presence of a silica layer has been shown to reduce the reactive oxygen species (ROS) generation by Au NPs under femtosecond laser irradiation making them more suitable for biological applications involving ultrafast light.

In this work, we explored the intricate plasmonic dynamics and the localization of near-fields of a Au dog bone (DB) nanostructure with sharp protruding ends. Using photon-induced near-field electron microscopy (PINEM) in an ultrafast electron microscope (UEM) and full-wave electromagnetic simulations, we examined the plasmonic hybridization and near-field localization of these nanoparticles and their dimers, analyzing both bare and silica-coated configurations. The nanostructures have a concave body with protrusions emerging from all four corners at both ends. Due to the presence of such protrusions, higher field localization is expected not only at the protrusions (due to lightning rod effects) but also within the crevasse regions between the protrusions where geometric confinement can play a role. Such anisotropy in geometry also leads to a plasmonic response that has strong dependency over the polarization of incident light.

Results and Discussion

The synthesis process for the Au “dog-bone” nanorods (DBs) is illustrated in Figure a. The procedure begins with the synthesis of Au NRs, after which protrusions are grown at the NR tips to create the dog-bone geometry. Subsequently, the Au dog-bone nanorods are coated with a thin silica layer using tetraethyl orthosilicate (TEOS). For PINEM measurements, the resulting Au DB nanoparticles were drop-cast onto a silicon nitride TEM grid. The length and width of the drop-cast dog bone nanoparticles are measured (at the center of the particles) to be 107 ± 13 nm and 44 ± 10 nm, respectively (the size distribution is provided in Figure S1). The detailed synthesis protocol is provided in Methods.

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Schematics of Au “dog bone” nanorod synthesis and PINEM setup. (a) Synthesis process of Au “dog bone” nanorod and subsequent silica coating. (b) Illustration of the PINEM setup integrated within a UEM. The insets show an exemplary PINEM spectrum and a representative PINEM image.

PINEM in UEM is an optical pump-electron probe technique, as illustrated in the schematic of the experimental setup shown in Figure b. − In this setup, both the pump and the probe beams originate from the same femtosecond laser, with precise control of the time delay achieved using a motorized delay stage. In PINEM experiments, the ultrafast optical pulse excites the sample creating evanescent plasmonic near-fields. When the electron pulses spatiotemporally overlap with the excited plasmonic field, the probe electrons interact with the photons, which cause the electrons to lose or gain quantized energy of nℏω by absorbing or emitting n photons, where n is an integer, ℏ is the reduced Planck constant, and ω is the angular frequency of pump photons. The PINEM spectrum (Figure b inset), exhibits multiple sideband peaks symmetrically distributed around the zero-loss peak, with adjacent peaks separated by the photon energy ℏω. To image the near-field of the plasmonic field around the sample, the PINEM image (Figure b inset) is recorded by selecting those electrons that gain energy from electron–photon interactions with an energy slit. The pump light polarization is controlled by a half-wave plate placed after a linear polarizer in pump path. A detailed description of the system is provided in Methods.

We first investigated the far-field and near-field optical characteristics of both bare and SiO2-coated individual nanoparticles using PINEM and full-field electromagnetic simulations. The excitation wavelength is selected as 800 nm based on the measured extinction spectra of colloidal dog bone nanoparticles in aqueous solution (Figure S2). Bright-field TEM images (Figure a,b) confirm the similar size and morphology of the uncoated and coated NPs, with the latter exhibiting a slightly increased overall dimension due to the conformal dielectric layer. The far-field optical response, as revealed by simulated scattering, absorption, and extinction cross sections (Figure c,d), is dominated by dipolar modes under both parallel and perpendicular polarizations relative to the NP long axis. Parallel polarization excites a pronounced longitudinal resonance, while perpendicular polarization induces a weaker transverse mode. For both coated and uncoated NPs, the longitudinal mode exhibits significantly higher intensity than the transverse, with absorption dominating in the transverse case and minimal scattering, consistent with a nonresonant, material-absorption-driven response. As shown in Figure c,d, both the longitudinal and transverse resonances of the SiO2-coated NPs exhibit an approximately 40 nm red shift (from 670 to 710 nm and 535 to 575 nm, respectively) compared to uncoated NPs. We attributed this spectral shift to a combination of increased NP size and dielectric screening by the SiO2 coating. Dielectric screening also affects the relative contributions of the scattering and absorption. While absorption dominates the longitudinal mode in uncoated NPs, the coated NPs display nearly equal contributions from scattering and absorption, with scattering slightly prevailing. The effect of size variation on the extinction spectrum is further investigated by systematically varying the size of the nanoparticles shown in Figure , with the corresponding simulation results presented in Figure S3.

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Far-field and near-field optical properties of individual gold dog bone nanoparticles (NPs) with and without SiO2 coating. (a, b) Bright-field TEM images of (a) uncoated and (b) ∼10 nm SiO2-coated gold NPs, showing similar morphology and size. (c, d) Simulated scattering (blue), absorption (red), and extinction (black) cross sections for (c) uncoated and (d) coated NPs under parallel (solid lines) and perpendicular (dashed lines) polarizations relative to the NP long axis. (e–h) PINEM images of (e, f) uncoated and (g, h) coated NPs under (e, g) parallel and (f, h) perpendicular polarizations at a delay time of 0 ps, respectively. Each sample is normalized independently (e, f share normalization; g, h share normalization). (i–l) Simulated PINEM near-field distributions for (i, j) uncoated and (k, l) coated NPs under (i, k) parallel and (j, l) perpendicular polarizations, normalized to the same input intensity. All PINEM experiments and PINEM simulations were performed at 800 nm illumination, corresponding to the off-resonance region of the extinction spectra. The double-ended red arrows in (e–l) show the pump light polarization orientation. Scale bars: 100 nm.

PINEM images in Figure e–h and corresponding full-field simulations in Figure i–l at 800 nm illumination, which corresponds to tail of the longitudinal mode, probe the near-field distributions in the off-resonance regime. The “+” and “–” signs indicate the positive and negative poles of the plasmonic modes, corresponding to regions of field accumulation with opposite phases according to the simulated E z field (see Figure S4). , Under parallel polarization, both the experiments (Figure e,g) and the simulations (Figure i,k) reveal strong field localization at the NP tips as characteristic of the longitudinal dipolar mode. The protrusions induce asymmetric field localization around the nanoparticles compared to regular nanorods of similar size (see Figure S7). This effect is especially pronounced under transverse polarization (Figure j,l). Unlike conventional cylindrical nanorods, where the field typically localizes at the sides under perpendicular polarization (Figure S7b,d), the distinctive dog bone geometry causes the field to concentrate at the tips, which in turn increases peak intensity of the localized fields (Figure f,h). Despite this tip localization, the mode remains transverse in nature, as confirmed by the simulated PINEM fields (Figure j,l). Note that characteristic localization patterns, such as longitudinal and transverse modes, are maintained in the off-resonance regime (Figure i–l), though with reduced intensity compared to resonance wavelengths (Figure S9). Nevertheless, PINEM effectively reveals these patterns even at off-resonance wavelengths. As expected from the NP aspect ratio and the extinction spectra, the longitudinal mode exhibits stronger field localization than the transverse mode in both coated and uncoated NPs. Additionally, quantitative comparison of simulated PINEM field intensities indicates enhanced field localization for coated NP under parallel polarization when compared to uncoated NP (∼2.5 times in the integrated PINEM fields), while the uncoated NP shows slightly stronger localization (∼1.4 times in the integrated fields) under perpendicular polarization, in agreement with the relative extinction cross sections at 800 nm. Additional polarization-dependent PINEM images for bare and coated NPs shown in Figure a,b are provided in Movies S1 and S2.

We investigated plasmonic hybridization and coupling in Au DB NP dimers, focusing on both uncoated and SiO2-coated configurations arranged in side-by-side and end-to-end geometries. Coupling between NPs introduces hybridized modes and enables tunable light-matter interactions in the near- and far-field. Bright-field TEM images of uncoated side-by-side, coated side-by-side, and coated end-to-end dimers are shown in Figure a,e,i, respectively. For all of these dimers, parallel polarized light (relative to the dimer long axis) excites a longitudinal resonance, while perpendicular polarized light induces a transverse mode, as seen in the PINEM images (Figure b,c, Figure f,g, and Figure j,k) and simulated PINEM fields (see Figure S8).

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Hybridization of plasmonic modes in gold dog bone nanoparticle (NP) dimers under parallel and perpendicular polarized light excitation. (a, e, i) Bright-field TEM images of (a) uncoated side-by-side dimer, (e) SiO2-coated side-by-side dimer, and (i) SiO2-coated end-to-end dimer, respectively. (b, c; f, g; j, k) PINEM images of the dimers shown in (a), (e), and (i) under (b, f, j) parallel and (c, g, k) perpendicular polarizations at a delay time of 0 ps, respectively. Each sample is normalized independently (b, c share normalization; f, g share normalization; j, k share normalization). (d, h, l) Simulated scattering (blue), absorption (red), and extinction (black) cross sections for the dimers shown in (a), (e), and (i) under parallel (solid line) and perpendicular (dashed lines) polarizations relative to the dimer long axis. All PINEM experiments and PINEM simulations were performed at 800 nm illumination. The double-ended red arrows in the PINEM images show the pump light polarization orientation. Scale bars: 100 nm.

In the uncoated side-by-side dimer configuration (Figure a–c), the nanoparticles are in direct contact, which enables charge transfer between them. Therefore, overlapping opposite poles cancel out, resulting in regions of zero net charge and enabling electron oscillation across the entire dimer as seen in Figure c. As a result, the dimer behaves as a single nanoparticle with an effective width approximately twice that of an individual NP. This increased width red shifts both the longitudinal and transverse dipole resonance peaks (Figure d) compared to the single NP (Figure c). Additionally, with the increased effective size, the transverse mode strength increases, and the higher-order modes emerge under both parallel (∼600 nm) and perpendicular (∼590 nm) polarized illuminations, as seen in simulated extinction spectra (Figure d) and simulated PINEM fields at resonance wavelengths (Figure S10).

In contrast, for the coated side-by-side dimer (Figure e), the presence of the SiO2 shell prevents carrier transfer between the nanoparticles. As a result, the optical response of the system is governed by plasmonic coupling. Under parallel polarized illumination, longitudinal dipole modes are excited along the constituent nanoparticles as seen in the PINEM image (Figure f). These modes exhibit antibonding coupling, which is evidenced by a blue-shifted resonance peak (Figure h) relative to that of a single nanoparticle. The inherent asymmetry of the dimer further broadens the line width of the longitudinal resonance. Moreover, as seen in Figure f, an entangled channel emerges between the nanoparticles at the zero-charge points. It is a void-field channel connecting coupled nanoparticles, where coherent particle–particle interactions lead to constructive or destructive interference patterns indicating quantum-like entanglement through their electromagnetic fields, as previously demonstrated for spherical nanoparticles by Yurtsever et al. When the coated side-by-side dimer is subjected to perpendicular polarization, the coupling between nanoparticles results in a bonding hybridized mode, as illustrated in Figure g. The bonding nature of the coupling is also evident from the red-shifted extinction peak (Figure h). The opposite charge regions at the NP-to-NP interface do not cancel out due SiO2 coating; instead, they form a weak field localization at the dimer interface, which is more pronounced in the simulated PINEM field distributions (see Figure S8). Analysis of the extinction spectra reveals that the transverse mode is significantly strengthened, becoming comparable in intensity to the longitudinal mode, and exhibits a red shift. This increased mode strength is attributed to the reduced aspect ratio (tip-to-tip versus side-to-side) and increased side-to-side separation inherent to the coated dimer configuration. Unlike the uncoated dimer, higher-order peaks do not appear in this case, as the observed red-shift and increased resonance strength arise from plasmonic coupling rather than an increase in overall nanoparticle size.

For the end-to-end dimer of SiO2-coated NPs (Figure i–k), parallel polarized light excites a bonding coupling mode, characterized by charge accumulation at the dimer tips and a red-shifted longitudinal resonance in the extinction spectrum (Figure l). In contrast, perpendicular polarization induces an antibonding mode, with charge separation across the dimer interface and a blue-shifted transverse resonance. Although the transverse mode remains weaker than the longitudinal mode, the merging of poles at the dimer junction under perpendicular polarization creates a pronounced field localization hotspot in Figure k. Overall, the plasmonic hybridization and coupling behavior observed in close-packed dog-bone dimers is consistent with our theoretical understanding of anisotropic nanoparticle pairs. Additional polarization-dependent PINEM images for the dimers shown in Figure a,e,i are provided in Movies S3–S5.

We further explored dimers in both side-by-side and end-to-end configurations for uncoated and SiO2-coated samples within the weak-interaction regime. As shown in the bright-field TEM images of these dimers (Figure a,e,i), the NPs are separated by approximately 100 nm, which limits plasmonic interactions. PINEM images acquired under parallel and perpendicular polarized illumination relative to the long axis of NP (Figure b,c, Figure f,g, and Figure j,k) demonstrate that individual dipoles are excited in each nanoparticle. Even in the side-by-side configuration (Figures b,c), the transverse mode (excited by perpendicular polarization) exhibits weaker field coupling compared to the longitudinal mode. This is due to field localization at the nanoparticle tips, a characteristic feature of the dog bone geometry. In contrast, the longitudinal mode (parallel polarization) shows strong field localization at the tips, resulting in relatively stronger, though still weak, coupling between the nanoparticles via overlapping evanescent electric fields. Additionally, similar to the adjacent side-by-side SiO2-coated dimer (Figure e), an entangled channel forms between the zero-charge regions of the nanoparticles in Figure c, despite their separation of 90 nm. For the end-to-end dimer (Figure e–g) with separation of 100 nm, PINEM imaging under parallel polarization reveals two distinct dipoles with asymmetric field localization at the tips of each nanoparticle. The field localization at the left tip of the right nanoparticle is weaker and narrower compared with its right tip. To further elucidate the role of coupling in field localizations within the dimer, we compared incoherent and coherent sums of simulated PINEM fields: the incoherent sum represents the superposition of squared field intensities from isolated nanoparticles, while the coherent sum corresponds to simulations of the dimer, accounting for interference and coupling effects. Simulated fields (Figures S16–S18) confirm that the observed asymmetry is primarily dictated by nanoparticle geometry rather than a coupling effect. In the SiO2-coated end-to-end dimer (Figure i–l), similar effects are observed; however, the increased effective separation due to the dielectric shell further suppresses coupling, eliminating any distinct coupled mode resonance in the extinction spectrum. Movies S6–S8 provides additional polarization-dependent PINEM images for the dimers shown in Figure a,e,i.

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Weak interactions in gold dog bone nanoparticle (NP) dimers: side-by-side and end-to-end configurations. (a, e, i) Bright-field TEM images of (a) uncoated side-by-side dimer, (e) uncoated end-to-end dimer, and (i) SiO2-coated end-to-end dimer showing significant interparticle separation of around 90, 100, and 150 nm, respectively. (b, c; f, g; j, k) PINEM images of the dimers shown in (a), (e), and (i) under (b, f, j) parallel and (c, g, k) perpendicular polarizations at a delay time of 0 ps, respectively. Each sample is normalized independently (b, c share normalization; f, g share normalization; j, k share normalization). (d, h, l) Simulated scattering (blue), absorption (red), and extinction (black) cross sections for the dimers shown in (a), (e), and (i) under parallel (solid lines) and perpendicular (dashed lines) polarizations relative to the dimer long axis. All PINEM experiments and PINEM simulations were performed at 800 nm illumination. The double-ended red arrows in the PINEM images show the pump light polarization orientation. Scale bars: 100 nm.

The far-field optical response, as depicted in the extinction spectra (Figure d,h,l), is characterized by multiple peaks under parallel polarization. Simulations of individual nanoparticles and comparison with dimer systems (see Figure S19) indicate that these peaks mainly originate from the independent contributions of each nanoparticle, with each resonance corresponding to one of the constituent NPs. In the end-to-end dimer (Figure h), the extinction spectrum displays a longitudinal mode peak with two shoulders: the main peak and the higher energy shoulder correspond to the resonances of the individual nanoparticles, while the lower energy shoulder is attributed to the hybridized bonding mode of the coupled system, as shown in Figure S19. Under perpendicular polarization, field localization remains weak, consistent with the transverse mode being dominated by material absorption rather than structural resonance. In summary, for weakly interacting dimers, the spatial distribution and asymmetry of the near-field localization are strongly influenced by the dog-bone geometry, resulting in geometry-driven effects that are not present in simpler anisotropic nanoparticle pairs.

Conclusion

We investigated the complex plasmonic behavior and near-field localization in gold dog-bone nanoparticles, both bare and SiO2-coated, across single-particle and dimer configurations. By integrating photon-induced near-field electron microscopy (PINEM) with full-field simulations, we confirmed several plasmonic phenomena, including dielectric screening and red shifts induced by SiO2 shells, plasmonic hybridization and mode splitting in dimers, charge transfer effects, and the expected weakening of coupling at larger interparticle separations. Importantly, our study also reveals distinctive effects arising from the unique dogbone geometry. At the single-particle level, the protrusions induce asymmetric field localization, particularly under transverse polarization, resulting in tip-localized modes that are not observed in conventional nanorods. In weakly interacting dimer configurations, the spatial distribution and strength of near-field coupling are strongly influenced by the dog-bone shape, leading to geometry-driven asymmetries and localization patterns that differ from those of regular nanorods. The strong agreement between the experimental and simulated results highlights the robustness of our approach and advances the understanding of plasmonic hybridization in anisotropic nanostructures. These insights provide guidelines for engineering plasmonic architectures with tailored optical properties for emerging applications in nanophotonics, sensing, and quantum technologies.

Methods

Synthesis of Dog Bone Nanoparticles

Synthesis of the Au DBs was done following a protocol reported by Wang and co-workers with modifications. First Au NRs were synthesized following an established protocol by Murray and co-workers with slight modifications. Briefly, a seed solution was prepared by adding 0.25 mL of 10 mM HAuCl4·3H2O to 9.75 mL of 0.1 M CTAB under mild stirring. Next, 0.6 mL of ice-cold solution of 10 mM NaBH4 was added to it under vigorous stirring. The solution was stirred at 1200 rpm for 2 min, followed by incubation for 30 min at 30 °C.

A growth solution was prepared by adding 0.481 mL of 10 mM AgNO3 to an aqueous solution of 24 mL containing 0.0078 M NaOL and 0.037 M CTAB. The solution was incubated for 15 min in the dark. Next, 1.25 mL of 10 mM HAuCl4·3H2O was added, and the solution was stirred at 700 rpm for 90 min. The color of the solution changed from yellow to colorless during that. After 90 min, 0.12 mL of 37% HCl and 41.6 μL of 0.1 M ascorbic acid were added under gentle stirring and the solution was stirred for 10 min. The stirring was then set at 1200 rpm, and 12 μL of seed solution was added to it. After vigorous mixing of 30 s, the solution was incubated overnight at 30 °C. Synthesized AuNRs were washed with nanopure water by centrifugation, first at 5500 rcf for 10 min followed by 2000 rcf for 30 min, before redispersing in 5 mM CTAB.

For the synthesis of Au DBs, a 10 mL growth solution consisting of 60 mM CTAB, 200 μM HAuCl4·3H2O, 9 μM AgNO3, and 10 mM ascorbic acid was prepared. Next, 100 μL of Au NR solution was added, and the solution was incubated at 30 °C for 1 h. The Au DBs were washed twice with nanopure water by centrifugation at 3500 rcf for 10 min.

Silica coating of Au DBs was done following an established protocol with slight modifications. Desired amount of Au DB solution was added to 1 mL CTAB solution of 1 mM to get an extinction of 3.5. Next, the solution was equilibrated for 20 h at 30 °C, and then 12 μL of 10% (v/v) TEOS in ethanol was added, followed by incubation for 1 h. Next, 4 μL of 0.1 M NaOH was added to the solution, and the silica shell was grown for 18 h at 30 °C. Silica-coated Au DBs were washed five times at 1500 rpm for 10 min with ethanol.

UEM Experiments

The PINEM experiments were performed on an ultrafast electron microscopy (UEM) platform at the Center for Nanoscale Materials (CNM) at Argonne National Laboratory. The UEM, based on a modified (by IDES) JEOL JEM-2100 Plus transmission electron microscope, was operated at an accelerating voltage of 200 kV. A femtosecond laser from Light Conversion supplied laser pulses to both the cathode and the sample within the UEM. This laser emitted pulses with a duration of 280 fs and a central wavelength of 1030 nm. The laser output was split into two beams: one was converted to a fourth-harmonic wavelength of 257 nm to generate photoelectrons at the cathode, serving as the probe beam, while the other beam at 1030 nm was processed through a collinear optical parametric amplifier to produce an 800 nm pump wavelength. The laser operated at a repetition rate of 250 kHz with the delay between pump and probe pulses precisely controlled via a motorized linear delay stage. The laser fluence under an 800 nm pump is 0.7 mJ/cm2. PINEM images were acquired using the Gatan image filtering system with a 10 eV slit width to select those electrons that gain energy. The silicon nitride TEM grid was from Ted Pella.

Electromagnetic Simulations

Electromagnetic simulations of far-field spectra and the near-field distributions were performed using the finite-difference time-domain (FDTD) method using commercial software Ansys Lumerical FDTD. Johnson and Christy’s data set and Palik’s data set were used to model gold (Au) and silica (SiO2), respectively. The geometries, including the SiO2 coatings, were imported from the bright-field TEM images using the software’s built-in import tool. The particle thicknesses were set to the central width of the NPs. A total-field scattered-field (TSFS) source was positioned above the structures on the +z side. Perfectly matched layer (PML) boundary conditions were applied along all directions (±z, ±x, ±y). A 3D near-field field and power monitor were placed to encompass the structures and capture the fields. A 3D area of constant mesh size of 2.5 nm was placed to encompass the samples, and conformal variant mesh is used elsewhere. PINEM fields are calculated as F(x, y) = ∫E z (x, y, z)­e–i(ω/v)z dz through numerical integration of the simulated E z field, where v, the speed of the electron, is assumed to be 0.7c for our system. , To record the absorption and scattering cross sections, additional box monitors are positioned around the structure.

Supplementary Material

ng6c00058_si_001.zip (6.2MB, zip)
ng6c00058_si_002.pdf (1.8MB, pdf)

Acknowledgments

Work performed at the Center for Nanoscale Materials, a U.S. Department of Energy Office of Science User Facility, was supported by the U.S. DOE, Office of Basic Energy Sciences, under Contract DE-AC02-06CH11357. A.J. acknowledges the research and infrastructure support provided by the Advanced Materials Research Centre, IIT Mandi and Advanced Research Grant - Anusandhan National Research Foundation, Government of India (file no. ANRF/ARG/2025/004299/LS).

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsnanoscienceau.6c00058.

  • Movies S1–S8 (ZIP)

  • Size distribution data for Au dog bone samples; measured extinction spectra of colloidal dog bone nanoparticles in aqueous solution; simulated extinction spectra of single nanoparticles with size variations; simulated Re­{E z } fields of dog bone nanoparticles and dimers; simulated PINEM fields of regular nanorods; simulated PINEM fields of strongly coupled dimers shown in Figure ; simulated PINEM fields at resonance wavelengths; coherent and incoherent simulations of weakly coupled dimers shown in Figure ; captions for Movies S1–S8 (PDF)

The authors declare no competing financial interest.

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