STEM-EELS: A Proper Tool for Observation of Strong Coupling Effects between Surface Excitations with High Spatial and Energy Resolution
Year: 2024
Type: article
Abstract: In recent years, strong coupling between different types of surface excitations, for instance, surface plasmon polaritons with excitons or phonon polaritons, has become a subject of growing interest. It has been caused by both fundamental and application research reasons covering new quantum nano-optics effects, electromagnetically induced transparency (EMT), chemical dynamics and reactivity, and other novel issues. Such an interesting physical phenomenon associated with the Rabi splitting and creation of new hybrid modes has been mostly studied by conventional reflection/transmission optical spectroscopic methods. However, using these techniques not all modes can be generated and observed. Therefore, there are attempts to study these coupling effects by application of electron beams capable of generating sub-radiant dark modes and being suitable for their detection by electron energy loss spectroscopy (EELS). To do it both at sufficient spatial (< 10 nm) and energy (≤ 100 eV) resolutions, STEM-EELS systems with monochromatized probe electron beams have to be used. In the presentation, the basic principles of STEM-EELS physics and technique will be explained and relevant examples shown. First, strong coupling between localized surface plasmons (LSPs) and excitons in the visible optical range will be presented. It will be demonstrated that a dark mode of an individual plasmonic bowtie antenna can interact with a small number of quantum emitters, as evidenced by Rabi-split spectra [1]. Second, strong coupling effects between LSPs and vibration modes of SiO2 membranes will be discussed. As the frequencies of these modes fall into the mid-IR regions, such experiments represent a challenging experimental task. Therefore, a state-of-the-art STEM-EELS system providing energy resolution below 10 meV (Nion) located in the Oak Ridge National Laboratory was used. More specifically, the study covers an electromagnetic coupling between MIR phonon polaritons (PhPs) in a silicon dioxide membrane and low-energy LSP modes formed by the confinement of plasmon polaritons in micrometer-long gold antennas. It will be shown that by precisely positioning the electron beam, the coupling between the polaritonic excitations can selectively trigger either uncoupled PhPs or coupled LSPs/PhPs (Fig. 1), [2]. Further, the results of comparative optical spectroscopy experiments showing that far-field IR spectra can be substantially different from the EEL spectra will be presented as well [3]. Comparison of extinction optical (a) and EELS (b, c) spectra for an approximately optimally coupled antenna-substrate system. Color dots in the inset in (b) approximately correspond to the electron impact positions. The primary energy of electrons was 60 keV.
Source: Microscopy and Microanalysis
Language: English
Subfield: Atomic and Molecular Physics, and Optics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: bronze
OpenAlex ID: W4401005444