This paper presents a numerical analysis of surface plasmon waveguides with subwavelength-scale localization and long-range propagation. The study focuses on Ag/SiO₂/Ag structures with waveguide thicknesses ranging from 12 nm to 250 nm. The dispersion relations, wavelength-dependent propagation, and energy density decay are characterized. The results show that as the SiO₂ core thickness decreases below 100 nm, plasmon modes split into symmetric and antisymmetric electric field distributions. Unlike conventional planar insulator-metal-insulator (IMI) structures, the symmetric mode's plasmon momentum does not always exceed photon momentum, with thicker films achieving effective indices as low as n=0.15. Antisymmetric mode dispersion exhibits a cutoff for films thinner than d=20 nm, terminating at least 0.25 eV below resonance. Plasmon propagation exceeds tens of microns with fields confined to within 20 nm of the structure. As the SiO₂ core thickness increases, propagation distances also increase with localization remaining constant. Conventional waveguiding modes are not observed until the core thickness approaches 100 nm. At such thicknesses, both transverse magnetic and transverse electric modes can be observed. Nonpropagating modes exhibit considerable field enhancement in the waveguide core, rivaling intensities in resonantly excited metallic nanoparticle waveguides. The study also explores the dispersion and propagation of transverse magnetic (TM) and transverse electric (TE) modes in MIM structures, showing that TE modes can propagate for certain oxide core thicknesses and excitation wavelengths. The results suggest that MIM structures can support propagation over many microns across a wide spectral range as core thickness is reduced to subwavelength scales. The energy density profiles show that MIM waveguides have high densities at the metal-dielectric interface, similar to surface electromagnetic waves. The study concludes that MIM structures can support both conventional and plasmonic modes, with wide tunability of energy density throughout the electromagnetic spectrum. The results suggest potential for both waveguiding and field-sensitive applications, including biological sensing.This paper presents a numerical analysis of surface plasmon waveguides with subwavelength-scale localization and long-range propagation. The study focuses on Ag/SiO₂/Ag structures with waveguide thicknesses ranging from 12 nm to 250 nm. The dispersion relations, wavelength-dependent propagation, and energy density decay are characterized. The results show that as the SiO₂ core thickness decreases below 100 nm, plasmon modes split into symmetric and antisymmetric electric field distributions. Unlike conventional planar insulator-metal-insulator (IMI) structures, the symmetric mode's plasmon momentum does not always exceed photon momentum, with thicker films achieving effective indices as low as n=0.15. Antisymmetric mode dispersion exhibits a cutoff for films thinner than d=20 nm, terminating at least 0.25 eV below resonance. Plasmon propagation exceeds tens of microns with fields confined to within 20 nm of the structure. As the SiO₂ core thickness increases, propagation distances also increase with localization remaining constant. Conventional waveguiding modes are not observed until the core thickness approaches 100 nm. At such thicknesses, both transverse magnetic and transverse electric modes can be observed. Nonpropagating modes exhibit considerable field enhancement in the waveguide core, rivaling intensities in resonantly excited metallic nanoparticle waveguides. The study also explores the dispersion and propagation of transverse magnetic (TM) and transverse electric (TE) modes in MIM structures, showing that TE modes can propagate for certain oxide core thicknesses and excitation wavelengths. The results suggest that MIM structures can support propagation over many microns across a wide spectral range as core thickness is reduced to subwavelength scales. The energy density profiles show that MIM waveguides have high densities at the metal-dielectric interface, similar to surface electromagnetic waves. The study concludes that MIM structures can support both conventional and plasmonic modes, with wide tunability of energy density throughout the electromagnetic spectrum. The results suggest potential for both waveguiding and field-sensitive applications, including biological sensing.