Mode Mixing and Substrate Induced Effect on the Plasmonic Properties of an Isolated Decahedral Gold Nanoparticle

We report new results on the localized surface plasmon (LSP) assisted optical effects of a single noble metal nanoparticle (MNP) in nm level spectral and spatial domain which is related to the phase retardation of electromagnetic signal with larger particle size. Site selective electron beam excitation in a scanning electron microscope (SEM) show multiple resonance peaks in the cathodoluminescence (CL) spectra of an isolated gold decahedron of side edge length 230 nm sitting on a silicon (Si) substrate. Apart from a substrate induced LSP mode in the near-infrared (750 nm) region, finite-difference time-domain (FDTD) numerical analysis also identifies two prominent LSP modes in the visible region. While the shorter wavelength (560 nm) mode has a mixture of in-plane quadrupolar and out-of-plane quadrupolar charge distribution pattern, the longer wavelength (655 nm) mode has the dipolar charge pattern in both the direction. We also analyze numerically to show the critical size of the side edge length of the decahedron particle where mode mixing is initiated

Electron-driven photon sources for correlative electron-photon spectroscopy with electron microscopes

Electron beams in electron microscopes are efficient probes of optical near-fields, thanks to spectroscopy tools like electron energy-loss spectroscopy and cathodoluminescence spectroscopy. Nowadays, we can acquire multitudes of information about nanophotonic systems by applying space-resolved diffraction and time-resolved spectroscopy techniques. In addition, moving electrons interacting with metallic materials and optical gratings appear as coherent sources of radiation. A swift electron traversing metallic nanostructures induces polarization density waves in the form of electronic collective excitations, i.e., the so-called plasmon polariton. Propagating plasmon polariton waves normally do not contribute to the radiation; nevertheless, they diffract from natural and engineered defects and cause radiation. Additionally, electrons can emit coherent light waves due to transition radiation, diffraction radiation, and Smith-Purcell radiation. Some of the mechanisms of radiation from electron beams have so far been employed for designing tunable radiation sources, particularly in those energy ranges not easily accessible by the state-of-the-art laser technology, such as the THz regime. Here, we review various approaches for the design of coherent electron-driven photon sources. In particular, we introduce the theory and nanofabrication techniques and discuss the possibilities for designing and realizing electron-driven photon sources for on-demand radiation beam shaping in an ultrabroadband spectral range to be able to realize ultrafast few-photon sources. We also discuss our recent attempts for generating structured light from precisely fabricated nanostructures. Our outlook for the realization of a correlative electron-photon microscope/spectroscope, which utilizes the above-mentioned radiation sources, is also described

Synthesis of plasmonically active titanium nitride using a metallic alloy buffer layer strategy

Titanium nitride (TiN) has emerged as a highly promising alternative to traditional plasmonic materials. This study focuses on the inclusion of a Cr90Ru10 buffer layer between the substrate and the thin TiN film, which enables the use of cost-effective, amorphous technical substrates while preserving high film quality. We report best-in-class TiN thin films fabricated on fused silica wafers, achieving a maximum plasmonic figure of merit (FOM), -ε′/ε′′ of approximately 2.8, even at a modest wafer temperature of around 300°C. Furthermore, we delve into the characterization of TiN thin film quality and fabricated TiN triangular nanostructures, employing attenuated total reflectance and cathodoluminescence techniques to highlight their potential applications in surface plasmonics

Effect of Intertip Coupling on the Plasmonic Behavior of Individual Multitipped Gold Nanoflower

We report here, the first experimental realization on the selective excitation of two closely lying tips from the same spherical core of a multitipped gold nanoparticle with flower-like morphology. This gives strong multipeaked resonance in the near-infrared region of the far-field emission spectra showing a clear signature of tip to tip coupling. The cathodoluminescence (CL) technique in a scanning electron microscope (SEM) combined with finite-difference time-domain (FDTD) simulation has helped us to identify the coupled plasmon modes to be originated from the interaction between two closely spaced tips with a narrow angular separation. Our analysis further estimates a range of angular separation between the tips that triggers the onset of the intertip coupling

Efficient Excitation of Higher Order Modes in the Plasmonic Response of Individual Concave Gold Nanocubes

Recently, concave nanocube (CNC) shaped metal nanoparticles (MNPs) with high index facets have drawn special attention due to their high chemical activity and large electromagnetic (EM) field enhancements, making them good candidates for multifunctional platforms. However, most of the previously published works focused on the plasmonic properties of silver simple nanocubes of smaller dimension, i.e., within the quasi-static limit, hardly supporting efficient excitation of high-order plasmonic modes. Site-selective electron beam excitation of individual Au CNC particles gives rise to simultaneous excitation of edge and corner localized surface plasmon (LSP) modes. We show that spatial variation of the radiative modes is strongly localized at the corners and extended along the edges of the top surface of the CNCs. Extensive finite-difference time-domain (FDTD) numerical analysis reveals that the substrate-induced plasmon hybridization leads to the activation of corner octupolar and corner quadrupolar LSP modes, in agreement with the cathodoluminescence (CL) measurements. Remarkably, the strength of the hybridization is shown to depend on the CNC size. Furthermore, we show that the edge quadrupolar mode becomes prominent with increasing concaveness, thus opening up a new way of engineering the LSP modes

Synthesis of plasmonically active titanium nitride using a metallic alloy buffer layer strategy

Titanium nitride (TiN) has emerged as a highly promising alternative to traditional plasmonic materials. This study focuses on the inclusion of a Cr90Ru10 buffer layer between the substrate and thin TiN film, which enables the use of cost-effective, amorphous technical substrates while preserving high film quality. We report best-in-class TiN thin films fabricated on fused silica wafers, achieving a maximum plasmonic figure of merit, −ϵ′/ϵ″, of approximately 2.8, even at a modest wafer temperature of around 300 °C. Furthermore, we delve into the characterization of TiN thin film quality and fabricated TiN triangular nanostructures, employing attenuated total reflectance and cathodoluminescence techniques to highlight their potential applications in surface plasmonics.<br/

Probing Localized Surface Plasmons of Trisoctahedral Gold Nanocrystals for Surface Enhanced Raman Scattering

Trisoctahedral (TOH) shaped gold (Au) nanocrystals (NCs) have emerged as a new class of metal nanoparticles (MNPs) due to their superior catalytic and surface enhanced Raman scattering (SERS) activities caused by the presence of high density of atomic steps and dangling bonds on their high-index facets. We examine the radiative localized surface plasmon resonance (LSPR) modes of an isolated single TOH Au NC using cathodoluminescence (CL), with high resolution spatial information on the local density of optical states (LDOS) across the visible spectral range. Further, we show pronounced enhancement in the Raman scattering by performing Raman spectroscopic measurements on Rhodamine 6G (R6G)-covered TOH Au NPs aggregates on a Si substrate. We believe that the hot spots between two adjacent MNP surfaces (“nanogaps”) can be significantly stronger than single particle LSPRs. Such “nanogaps” hot spots may have crucial role on the substantial SERS enhancement observed in this report. Consequently, the present study indicates that MNPs aggregates are highly desirable than individual plasmonic nanoparticles for possible applications in SERS based biosensing