Metal-insulator-metal plasmon nanocavities: Analysis of optical properties

We present detailed analysis of the optical properties of a structure called a nanosheet plasmon cavity. It is a metal-insulator-metal (MIM) waveguide with a finite length. First, propagation of the lowest-energy surface-plasmon mode of a MIM waveguide, which is the fundamental structure of our cavity, is analytically investigated. In addition to the dispersion relation, localization and dissipation of the electromagnetic energy are discussed. Next, the optical properties of the nanosheet plasmon cavity are numerically examined with the boundary element method. The nanosheet plasmon cavity is found to inherit various natures of the original MIM waveguide. The resonance in this cavity can be understood as a Fabry-Perot-like resonance caused by the reflection of the guided mode at the entrance and the exit surfaces. This enables easy design of a cavity on the basis of the analytical dispersion relation of a MIM waveguide. The fields in a MIM waveguide are localized around the dielectric core, and the wavelength of the surface plasmon becomes shorter with decreasing the thickness of the core. Therefore the electromagnetic energy can be confined in a volume as small as $0.001\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}{\mathrm{m}}^{3}$ in a nanosheet plasmon cavity. Another feature of this cavity is that the electric field is maximized at the core entrance. By only letting molecules be adsorbed on the surface of the core, the molecules are exposed to the maximum field. This exhibits great potential of the nanosheet plasmon cavity for enhanced Raman spectroscopy.