Effective Modal Volume in Nanoscale Photonic and Plasmonic Near-Infrared Resonant Cavities

We survey expressions of the effective modal volume, Veff, commonly used in the literature for nanoscale photonic and plasmonic cavities. We apply different expressions of V_{eff} to several canonical cavities designed for nanoscale near-infrared light sources, including metallo-dielectric and coaxial geometries. We develop a metric for quantifying the robustness of different V_{eff} expressions to the different cavities and materials studied. We conclude that no single expression for V_{eff} is universally applicable. Several expressions yield nearly identical results for cavities with well-confined photonic-type modes. For cavities with poor confinement and a low quality factor, however, expressions using the proper normalization method need to be implemented to adequately describe the diverging behavior of their effective modal volume. The results serve as a practical guideline for mode analysis of nanoscale optical cavities, which show promise for future sensing, communication, and computing platforms.