Multipolar and bulk modes: fundamentals of single-particle plasmonics through the advances in electron and photon techniques

Abstract

Recent developments in the application of plasmonic nanoparticles have showcased the importance of understanding in detail their plasmonic resonances at the single-particle level. These resonances can be excited and probed through various methods, which can be grouped in four categories, depending on whether excitation and detection involve electrons (electron energy loss spectroscopy), photons (e.g., dark-field microscopy), or both (cathodoluminescence and photon-induced near-field electron microscopy). While both photon-based and electron-based methods have made great strides toward deepening our understanding of known plasmonic properties and discovering new ones, they have in general progressed in parallel, without much cross-pollination. This evolution can be primarily attributed to the different theoretical approaches driving these techniques, mainly dictated by the inherent different nature of electrons and photons. The discrepancies that still exist among them have hampered the development of a holistic approach to the characterization of plasmonic materials. In this review therefore, we aim to briefly present those electron-based and photon-based methods fundamental to the study of plasmonic properties at the single-particle level, with an eye to new behaviors involving multipolar, propagating, and bulk modes coexisting in colloidal nanostructures. By exploring the key fundamental discoveries in nanoparticle plasmonics achieved with these techniques, herein we assess how integrating this information could encourage the creation of a unified understanding of the various phenomena occurring in individual nanoparticles, which would benefit the plasmonics and electron microscopy communities alike.

Nanoscale Manipulation of Exciton–Trion Interconversion in a MoSe2 Monolayer via Tip-Enhanced Cavity-Spectroscopy

“…Before the TECS experiment, we performed far-field optical measurements of a MoSe 2 ML on a AuNW to characterize the SPP properties and the correlated excitonic emissions from the hybrid device. We excite this device with a linearly polarized light parallel with the axis of the AuNW, which is the optimal excitation for inducing standing SPP mode and effectively coupling to excitons of MoSe 2 ML with the enhanced radiative emission rate (see Figure S3 for angle-dependent scattering signals of SPP). , In addition, we expect that hot electrons generated from the local spots of negative charges in the multipole mode can induce excitonic conversion from X 0 to X- in a MoSe 2 …”

“…We excite this device with a linearly polarized light parallel with the axis of the AuNW, which is the optimal excitation for inducing standing SPP mode 45 and effectively coupling to excitons of MoSe 2 ML with the enhanced radiative emission rate (see Figure S3 for angle-dependent scattering signals of SPP). 3,46 In addition, we expect that hot electrons generated from the local spots of negative charges in the multipole mode 12 can induce excitonic conversion from X 0 to X-in a MoSe 2 . 6 As a first step, we measured PL spectra from the different positions on the hybrid device to understand the optical properties of AuNW and MoSe 2 , exhibiting spatial heterogeneity.…”

“…The coupling between plasmons and excitons in space crucially depends on the optical field distribution within plasmonic structures. To elucidate these coupling properties, previous studies showed the measurement results of surface plasmon polariton (SPP) scattering, exciton emission, and corresponding field distribution simulations. ,− In addition, notably, the spatial distribution of electric charges within plasmonic nanostructures plays a significant role in these coupling behaviors. − …”

Selective Probing of Longitudinal and Transverse Plasmon Modes with Electron Phase-Matching

“…The classical mechanism of electron energy loss in plasmonics is explained by the electric field induced in the sample which exerts a force on the incident electrons, making them to lose energy . The STEM-EELS technique has been used to probe the optical properties of metallic nanoparticles with unrivaled spatial resolution , and few meV spectral resolution. , STEM-EELS has also probed multipolar plasmon modes in metallic nanostructures such as nanorods, , nanotriangles, nanocrosses, nanosquares, nanostars, nanodisks, , among others. From a theoretical point of view, EELS is related to optical extinction as presented in − and more generally is also related to nanooptics through the electromagnetic local density of states (EMLDOS), , furthermore, in tomographic EELS experiments a complete EMLDOS reconstruction in 3D can be made .…”

“…The classical mechanism of electron energy loss in plasmonics is explained by the electric field induced in the sample which exerts a force on the incident electrons, making them to lose energy. 17 The STEM-EELS technique has been used to probe the optical properties of metallic nanoparticles with unrivaled spatial resolution 18,19 and few meV spectral resolution. 20,21 STEM-EELS has also probed multipolar plasmon modes in metallic nanostructures such as nanorods, 7,8 nanotriangles, 22 nanocrosses, 23 nanosquares, 24 nanostars, 25 nanodisks, 26,27 among others.…”