Systematic analysis of plasmon excitations and coupling by lithographic structure patterning and fast, monochromated STEM ‐ EELS mapping

Abstract

High energy resolution electron energy‐loss spectroscopy (EELS) is now a recognised technique for mapping localized surface plasmon resonances and measuring their resonant energies with a nanometric spatial resolution [1–3]. In contrast to light‐based techniques it can further excite and map high order harmonics that are optically forbidden. Here we move on from the study of isolated or tandem plasmonic structures randomly deposited on TEM grids, or more complex structures painstakingly patterned by FIB, to a fast, systematic EELS mapping of precisely patterned Au and Ag films on Si 3 N 4 membranes. Following an electron lithography‐based preparation, the spatial distributions and mode energies of plasmon resonances and coupling are studied in function of well‐controlled variations in structure dimensions. The measurements are made in STEM‐EELS mode using a FEI Titan Themis 60‐300 with Gatan Digiscan and GIF Quantum ERS spectrometer. The combination of the X‐FEG gun and monochromator gives a sub‐nm incident beam with 100–110 meV FWHM of the zero‐loss peak and a current of up to 240–250 pA. A fundamental need of the work is that the plasmon excitation can be measured both in the pure Si 3 N 4 membrane regions and in the metal film regions. To this end, a high tension of 300 kV is used because, by reducing the relative intensity of bulk plasmon scattering from the metal films, it improves surface plasmon excitation signal to noise. EELS data are normalized by the zero‐loss intensity to give the true projected plasmon distribution, without “shadowing” by the metal film [4]. With the fast spectrum imaging mode and high beam current, dwell times are only 0.2–0.25 ms per pixel, allowing us to acquire maps with >10 5 pixels (e.g. 600 x 600 px) in < 10 minutes per map. Applying this fast mapping with high spatial sampling to the lithographically‐based structures of known layout and dimension gives a highly time‐efficient method for studying plasmonic excitations in nanophotonic structures. Figure 1 shows example plasmon resonance low‐loss EELS spectra and intensity maps. Parts (a–d) & (e–f) treat the well known plasmonic structures of a silver wire and nano‐triangle. Owing to the energy resolution and good signal to noise ratio, there is no need to perform systematic deconvolution of the data to reveal plasmon excitations, even for modes at < 0.5 eV energy loss. High order multipoles are additionally well resolved for both non‐penetrating and penetrating trajectories, such as the 2.8 eV breathing mode of the nano‐triangle. Figure 1 (g–h) shows data from more complex coupled gold heptamer apertures. We probe the effect of a nanoscale defect: a small 20 nm size gap in one of the heptamer arms. This feature induces the appearance of an additional low energy mode at 0.84 eV, and affects both the symmetry and intensity of higher order modes. This demonstrates how systematic study of varying geometries allows for in‐depth analysis of plasmon resonances. These experimental studies are combined with modeling of the excitations using a novel “in house” method which aids interpretation of multi‐body interactions by simulating EELS spectra using the properties of plasmonic structures, and not the work done on electrons [5]. The simulations and experiments have a symbiotic relationship, with the modeling used to interpret experimental data, and the experimental data used to guide the modeling (e.g. for peak resonance values). Nevertheless, our experimental approach is significantly quicker than the modeling. While so far it is primarily applied to the systematic study of particles and apertures, in the future it will be used to explore the optical excitations of novel nanophotonic structures and materials.