The paper presents a numerical analysis of surface plasmon waveguides that exhibit both long-range propagation and spatial confinement of light with lateral dimensions less than 10% of the free-space wavelength. The study focuses on characterizing the dispersion relations, wavelength-dependent propagation, and energy density decay in two-dimensional Ag/SiO2/Ag structures with waveguide thicknesses ranging from 12 nm to 250 nm. Key findings include: 1. **Dispersion Relations**: Analytic dispersion results show a splitting of plasmon modes—symmetric and antisymmetric electric field distributions—as the SiO2 core thickness is decreased below 100 nm. Unlike conventional planar insulator-metal-insulator (IMI) surface plasmon waveguides, the surface plasmon momentum of the symmetric mode does not always exceed the photon momentum, with thicker films achieving effective indices as low as n=0.15. 2. **Energy Density Decay**: From visible to near-infrared wavelengths, 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, while localization remains constant. 3. **Mode Propagation and Skin Depth**: The interdependence of skin depth and propagation is discussed, showing that propagation is high in regimes of near-linear dispersion where high signal velocities overcome internal loss mechanisms. For MIM structures, SP penetration into the cladding is limited by the skin depth of optical fields in the metal. 4. **Transverse Electric MIM Modes**: The existence of transverse electric (TE) waves in MIM guides is explored, revealing that TE waves can propagate several microns for certain oxide core thicknesses and excitation wavelengths. 5. **Mode Energy Density**: The energy density profiles of the waveguides are analyzed, showing that the long-ranging modes generally have high densities at the metal-dielectric interface, while intensities within the waveguide can be comparable to those observed in nanoparticle array gaps. The results suggest that MIM waveguides can achieve micron-scale propagation with nanometer-scale confinement, making them promising for subwavelength plasmonic interconnects and potential applications in molecular biosensing, surface-enhanced Raman spectroscopy, and nonlinear optical devices.The paper presents a numerical analysis of surface plasmon waveguides that exhibit both long-range propagation and spatial confinement of light with lateral dimensions less than 10% of the free-space wavelength. The study focuses on characterizing the dispersion relations, wavelength-dependent propagation, and energy density decay in two-dimensional Ag/SiO2/Ag structures with waveguide thicknesses ranging from 12 nm to 250 nm. Key findings include: 1. **Dispersion Relations**: Analytic dispersion results show a splitting of plasmon modes—symmetric and antisymmetric electric field distributions—as the SiO2 core thickness is decreased below 100 nm. Unlike conventional planar insulator-metal-insulator (IMI) surface plasmon waveguides, the surface plasmon momentum of the symmetric mode does not always exceed the photon momentum, with thicker films achieving effective indices as low as n=0.15. 2. **Energy Density Decay**: From visible to near-infrared wavelengths, 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, while localization remains constant. 3. **Mode Propagation and Skin Depth**: The interdependence of skin depth and propagation is discussed, showing that propagation is high in regimes of near-linear dispersion where high signal velocities overcome internal loss mechanisms. For MIM structures, SP penetration into the cladding is limited by the skin depth of optical fields in the metal. 4. **Transverse Electric MIM Modes**: The existence of transverse electric (TE) waves in MIM guides is explored, revealing that TE waves can propagate several microns for certain oxide core thicknesses and excitation wavelengths. 5. **Mode Energy Density**: The energy density profiles of the waveguides are analyzed, showing that the long-ranging modes generally have high densities at the metal-dielectric interface, while intensities within the waveguide can be comparable to those observed in nanoparticle array gaps. The results suggest that MIM waveguides can achieve micron-scale propagation with nanometer-scale confinement, making them promising for subwavelength plasmonic interconnects and potential applications in molecular biosensing, surface-enhanced Raman spectroscopy, and nonlinear optical devices.