Examining Substrate-Induced Plasmon Mode Splitting and Localization in Truncated Silver Nanospheres with Electron Energy Loss Spectroscopy

Plasmonic Characterization of 3D Printable Metal-Polymer Nanocomposites

Plasmonic polymer nanocomposites (i.e., polymer matrices containing plasmonic nanostructures) are appealing candidates for the development of manifold technological devices relying on light-matter interactions, provided that they have inherent properties and processing capabilities. The smart development of plasmonic nanocomposites requires in-depth optical analyses proving the material performance, along with correlative studies guiding the synthesis of tailored materials. Importantly, plasmon resonances emerging from metal nanoparticles affect the macroscopic optical response of the nanocomposite, leading to far- and near-field perturbations useful to address the optical activity of the material. We analyze the plasmonic behavior of two nanocomposites suitable for 3D printing, based on acrylic resin matrices loaded with Au or Ag nanoparticles. We compare experimental and computed UV-vis macroscopic spectra (far-field) with single-particle electron energy loss spectroscopy (EELS) analyses (near-field). We extended the calculations of Au and Ag plasmon-related resonances over different environments and nanoparticle sizes. Discrepancies between UV-vis and EELS are dependent on the interplay between the metal considered, the surrounding media, and the size of the nanoparticles. The study allows comparing in detail the plasmonic performance of Au- and Ag-polymer nanocomposites, whose plasmonic response is better addressed, accounting for their intended applications (i.e., whether they rely on far- or near-field interactions).

Theoretical and computational methods for tip- and surface-enhanced Raman scattering

Raman spectroscopy is a versatile tool for acquiring molecular structure information. The incorporation of plasmonic fields has significantly enhanced the sensitivity and resolution of surface-enhanced Raman scattering (SERS) and tip-enhanced Raman spectroscopy (TERS). The strong spatial confinement effect of plasmonic fields has challenged the conventional Raman theory, in which a plane wave approximation for the light has been adopted. In this review, we comprehensively survey the progress of a generalized theory for SERS and TERS in the framework of effective field Hamiltonian (EFH). With this approach, all characteristics of localized plasmonic fields can be well taken into account. By employing EFH, quantitative simulations at the first-principles level for state-of-the-art experimental observations have been achieved, revealing the underlying intrinsic physics in the measurements. The predictive power of EFH is demonstrated by several new phenomena generated from the intrinsic spatial, momentum, time, and energy structures of the localized plasmonic field. The corresponding experimental verifications are also carried out briefly. A comprehensive computational package for modeling of SERS and TERS at the first-principles level is introduced. Finally, we provide an outlook on the future developments of theory and experiments for SERS and TERS.

Practical Considerations for Simulating the Plasmonic Properties of Metal Nanoparticles

Simulating the plasmonic properties of colloidally derived metal nanoparticles with accuracy to their experimentally observed measurements is challenging due to the many structural and compositional parameters that influence their scattering and absorption properties. Correlation between single nanoparticle scattering measurements and simulated spectra emphasize these strong structural and compositional relationships, providing insight into the design of plasmonic nanoparticles. This Perspective builds from this history to highlight how the structural features of models used in simulation methods such as those based on the Finite-Difference Time-Domain (FDTD) method and Discrete Dipole Approximation (DDA) are of critical consideration for correlation with experiment and ultimately prediction of new nanoparticle properties. High-level characterizations such as electron tomography are discussed as ways to advance the accuracy of models used in such simulations, allowing the plasmonic properties of structurally complex nanoparticles to be better understood. However, we also note that the field is far from bringing experiment and simulation into agreement for plasmonic nanoparticles with complex compositions, reflecting analytical challenges that inhibit accurate model generation. Potential directions for addressing these challenges are also presented.

Chemical Engineering of Cu-Sn Disordered Network Metamaterials

The design and fabrication of large-area metamaterials is an ongoing challenge. In the present work, we propose a scalable design route and low-footprint strategy for the production of large-area, frequency-selective Cu-Sn disordered network metamaterials with quasi-perfect absorption. The nanoscale networks combine the robustness of disordered systems with the broad-band optical response known from connected wire-mesh metamaterials. Using experiments and simulations, we show how frequency-selective absorption in the networks can be designed and controlled. We observe a linear dependence of the optical response as a function of Sn content ranging from the near-infrared to the visible region. The absorbing state exhibits strong sensitivity to both changes in the global network topology and the chemistry of the network. We probe the plasmonic response of these nanometric networks by electron energy loss spectroscopy (EELS), where we resolve extremely confined gap surface-plasmon (GSP) modes.

Ultrafast Photoemission Electron Microscopy: Imaging Plasmons in Space and Time

Plasmonics is a rapidly growing field spanning research and applications across chemistry, physics, optics, energy harvesting, and medicine. Ultrafast photoemission electron microscopy (PEEM) has demonstrated unprecedented power in the characterization of surface plasmons and other electronic excitations, as it uniquely combines the requisite spatial and temporal resolution, making it ideally suited for 3D space and time coherent imaging of the dynamical plasmonic phenomena on the nanofemto scale. The ability to visualize plasmonic fields evolving at the local speed of light on subwavelength scale with optical phase resolution illuminates old phenomena and opens new directions for growth of plasmonics research. In this review, we guide the reader thorough experimental description of PEEM as a characterization tool for both surface plasmon polaritons and localized plasmons and summarize the exciting progress it has opened by the ultrafast imaging of plasmonic phenomena on the nanofemto scale.

Effects of Plasmon Modes on Resonant Raman Images of a Single Molecule

Localized surface plasmons (LSPs) are excellent light sources at the nanoscale. How to precisely describe the interaction between LSPs and molecules has become an important issue. We present here a comprehensive study on the dependence of resonant Raman images on LSP modes generated by two typical nanostructures. Theoretical calculations demonstrate that the Raman images are sensitive to not only the spatial distribution but also the phase of the localized field, which should be attributed to the quantum nature of the interaction between LSP modes and molecules. We also find that the rotation of noncylindrical symmetry modes could affect the details of images, which offers an extra means to extract molecular information. Our findings extend the understanding of the LSP-matter interaction, which would be useful for the rational design of nanostructures and thus further applications of LSPs.

High Spatial Resolution Mapping of Localized Surface Plasmon Resonances in Single Gallium Nanoparticles

Plasmonics has emerged as an attractive field driving the development of optical systems in order to control and exploit light-matter interactions. The increasing interest around plasmonic systems is pushing the research of alternative plasmonic materials, spreading the operability range from IR to UV. Within this context, gallium appears as an ideal candidate, potentially active within a broad spectral range (UV-VIS-IR), whose optical properties are scarcely reported. Importantly, the smart design of active plasmonic materials requires their characterization at high spatial and spectral resolution to access the optical fingerprint of individual nanostructures, attainable by transmission electron microscopy techniques (i.e., by means of electron energy-loss spectroscopy, EELS). Therefore, the optical response of individual Ga nanoparticles (NPs) by means of EELS measurements is analyzed, in order to spread the understanding of the plasmonic response of Ga NPs. The results show that single Ga NPs may support several plasmon modes, whose nature is extensively discussed.

Multipolar Nanocube Plasmon Mode-Mixing in Finite Substrates

Facile control of the radiative and nonradiative properties of plasmonic nanostructures is of practical importance to a wide range of applications in the biological, chemical, optical, information, and energy sciences. For example, the ability to easily tune not only the plasmon spectrum but also the degree of coupling to light and/or heat, quality factor, and optical mode volume would aid the performance and function of nanophotonic devices and molecular sensors that rely upon plasmonic elements to confine and manipulate light at nanoscopic dimensions. While many routes exist to tune these properties, identifying new approaches-especially when they are simple to apply experimentally-is an important task. Here, we demonstrate the significant and underappreciated effects that substrate thickness and dielectric composition can have upon plasmon hybridization as well as downstream properties that depend upon this hybridization. We find that even substrates as thin as ∼10 nm can nontrivially mix free-space plasmon modes, imparting bright character to those that are dark (and vice versa) and, thereby, modifying the plasmonic density of states as well as the system's near- and far-field optical properties. A combination of electron energy-loss spectroscopy (EELS) experiment, numerical simulation, and analytical modeling is used to elucidate this behavior in the finite substrate-induced mixing of dipole, quadrupole, and octupole corner-localized plasmon resonances of individual silver nanocubes.

Studying substrate effects on localized surface plasmons in an individual silver nanoparticle using electron energy-loss spectroscopy

In this study, electron energy-loss spectroscopy (EELS) in conjunction with scanning transmission electron microscopy (STEM) was used to investigate surface plasmons in a single silver nanoparticle (NP) on a magnesium oxide substrate, employing an incident electron trajectory parallel to the substrate surface. This parallel irradiation allowed a direct exploration of the substrate effects on localized surface plasmon (LSP) excitations as a function of the distance from the substrate. The presence of the substrate was found to lower the symmetry of the system, such that the resonance energies of LSPs were dependent on the polarization direction relative to the substrate surface. The resulting mode splitting could be detected by applying different electron trajectories, providing results similar to those previously obtained from optical studies using polarized light. However, the LSP maps obtained by STEM-EELS analysis show an asymmetric intensity distribution with the highest intensity at the top surface of the NP (that is, far from the substrate), a result that is not predicted by optical simulations. We show that modifications of the applied electric field by the substrate cause this asymmetric intensity distribution in the LSP maps.

Super-Resolution Imaging and Plasmonics

This review describes the growing partnership between super-resolution imaging and plasmonics, by describing the various ways in which the two topics mutually benefit one another to enhance our understanding of the nanoscale world. First, localization-based super-resolution imaging strategies, where molecules are modulated between emissive and nonemissive states and their emission localized, are applied to plasmonic nanoparticle substrates, revealing the hidden shape of the nanoparticles while also mapping local electromagnetic field enhancements and reactivity patterns on their surface. However, these results must be interpreted carefully due to localization errors induced by the interaction between metallic substrates and single fluorophores. Second, plasmonic nanoparticles are explored as image contrast agents for both superlocalization and super-resolution imaging, offering benefits such as high photostability, large signal-to-noise, and distance-dependent spectral features but presenting challenges for localizing individual nanoparticles within a diffraction-limited spot. Finally, the use of plasmon-tailored excitation fields to achieve subdiffraction-limited spatial resolution is discussed, using localized surface plasmons and surface plasmon polaritons to create confined excitation volumes or image magnification to enhance spatial resolution.

Probing plasmons in three dimensions by combining complementary spectroscopies in a scanning transmission electron microscope

The nanoscale optical response of surface plasmons in three-dimensional metallic nanostructures plays an important role in many nanotechnology applications, where precise spatial and spectral characteristics of plasmonic elements control device performance. Electron energy loss spectroscopy (EELS) and cathodoluminescence (CL) within a scanning transmission electron microscope have proven to be valuable tools for studying plasmonics at the nanoscale. Each technique has been used separately, producing three-dimensional reconstructions through tomography, often aided by simulations for complete characterization. Here we demonstrate that the complementary nature of the two techniques, namely that EELS probes beam-induced electronic excitations while CL probes radiative decay, allows us to directly obtain a spatially- and spectrally-resolved picture of the plasmonic characteristics of nanostructures in three dimensions. The approach enables nanoparticle-by-nanoparticle plasmonic analysis in three dimensions to aid in the design of diverse nanoplasmonic applications.

Asymmetric optical properties of photocatalytically deposited plasmonic silver nanoparticles

Plasmonic Ag nanoparticles deposited and grown photocatalytically for a sufficient period of time on a TiO₂ thin film scatter blue or reddish light for the front or back incidence, respectively.