Geometries and materials for subwavelength surface plasmon modes

TLDR

Plasmonic waveguides guide light along metal‑dielectric interfaces with wave vectors larger than free‑space values, enabling subwavelength confinement, though this condition alone is not sufficient for subwavelength optical modes. The reflection pole method is employed to solve the two‑dimensional modal solutions for both single planar waveguides and adjacent waveguide systems. Achieving subwavelength pitches requires a metal‑insulator‑metal geometry, which provides higher confinement and smaller spatial extent than conventional insulator‑metal‑insulator structures, and the trade‑off between propagation and confinement can be optimized through material selection.

Abstract

Plasmonic waveguides can guide light along metal-dielectric interfaces with propagating wave vectors of greater magnitude than are available in free space and hence with propagating wavelengths shorter than those in vacuum. This is a necessary, rather than sufficient, condition for subwavelength confinement of the optical mode. By use of the reflection pole method, the two-dimensional modal solutions for single planar waveguides as well as adjacent waveguide systems are solved. We demonstrate that, to achieve subwavelength pitches, a metal-insulator-metal geometry is required with higher confinement factors and smaller spatial extent than conventional insulator-metal-insulator structures. The resulting trade-off between propagation and confinement for surface plasmons is discussed, and optimization by materials selection is described.