Unveiling breathing plasmon modes in aluminum metal–insulator–metal cavities by cathodoluminescence

Abstract

Aluminum (Al) processes excellent plasmon response from the ultraviolet (UV) to visible spectrum. Understanding of the deep sub-wavelength plasmon response of Al nanostructures is essential for the Al-based plasmon device design, such as UV surface-enhanced resonance Raman scattering and emission control of emitters. In this work, by using cathodoluminescence, the plasmonic properties of Al metal–insulator–metal (MIM) disk nanocavities are investigated. The resonant breathing modes rather than edge modes are resolved by the CL spectra and real-space mode patterns, which are in good agreement with the electromagnetic calculations. Moreover, the dispersion behavior of plasmon modes of the MIM cavity can be traced back to the propagating plasmon modes in an Al MIM slab, which shows that the electromagnetic fields are strongly confined in the cavities. Furthermore, a mode volume as small as 1.1 × 105 nm3 is obtained for the 240 nm diameter cavity, demonstrating these MIM resonators to be ideal candidates for studies of strong plasmon-matter interactions.