Optically Coupled Plasmonic Nanopores Observed by Cathodoluminescence Scanning Transmission Electronmicroscopy

Abstract

Control of the optical properties of nano‐plasmonic structures is essential for next‐generation optical circuits and high‐throughput biosensing platforms. Realization of such nano‐optical devices requires optical couplings of various nanostructured elements and field confinement at the nanoscale. In particular, symmetric coupling modes, also referred to as “dark modes”, have recently received considerable attention because these modes can confine light energy to small spaces. Although the coupling behavior of plasmonic nanoparticles has been relatively well‐studied, couplings of inverse structures, i.e., holes and pores, remain partially unexplored. Even for the most fundamental coupling system of two dipolar holes, comparison of the symmetric and anti‐symmetric coupling modes has not been performed. Here, we present a systematic study of the symmetric and anti‐symmetric coupling of nanopore pairs using cathodoluminescence by scanning transmission electron microscopy (CL‐STEM) and electromagnetic simulation. The nanopore samples were fabricated by colloidal lithography and film transfer by wet etching of the sacrificial layer. To achieve very close separation of nanopore pairs and to obtain high spatial resolution in STEM, we chose ultra‐thin, free‐standing film structures. For the measurement of single and coupled pairs of nanopores, 135 nm nanopores in an AlN(8 nm)/Au(16 nm)/AlN(8 nm) trilayer membrane were used (Figure 1b). With this sandwich layer structure, it is possible to obtain very thin and stable metal layers, even at high temperatures. For CL‐STEM measurement, 80 kV acceleration was used to avoid possible damage due to relatively high beam current. (5nA) Depending on the electron beam position it is possible to distinguish the symmetric and anti‐symmetric dipolar coupling modes.(Fig. 1) The observed symmetric coupling mode, approximated as a pair of facing dipoles, appeared at a lower energy than that of the anti‐symmetric coupling mode, indicating that the facing dipoles attract each other. The anti‐symmetric coupling mode splits into the inner‐ and outer‐edge localized modes as the coupling distance decreases. These coupling behaviors cannot be fully explained as simple inverses of coupled disks. Electromagnetic simulation by finite difference time domain (FDTD) also showed consistent coupling behaviors. Models of FDTD simulation showed that the inner‐ and outer‐edge anti‐symmetric modes become fully localized with minimal influence of the opposite edges as the coupling distance decreases. Symmetric and anti‐symmetric coupling modes are also observed in a short‐range ordered pore array (Fig. 2), where one pore supports multiple local resonance modes, depending on the distance to the neighboring pores.