Plasmon slot waveguides: Towards chip-scale propagation with subwavelength-scale localization

We present a numerical analysis of surface plasmon waveguides exhibiting both long-range propagation and spatial confinement of light with lateral dimensions of less than 10% of the free-space wavelength. Attention is given to characterizing the dispersion relations, wavelength-dependent propagation, and energy density decay in two-dimensional $\mathrm{Ag}∕\mathrm{Si}{\mathrm{O}}_2∕\mathrm{Ag}$ structures with waveguide thicknesses ranging from $12\mathrm{nm}\text{to}250\mathrm{nm}$. As in conventional planar insulator-metal-insulator (IMI) surface plasmon waveguides, analytic dispersion results indicate a splitting of plasmon modes—corresponding to symmetric and antisymmetric electric field distributions—as $\mathrm{Si}{\mathrm{O}}_2$ core thickness is decreased below $100\mathrm{nm}$. However, unlike IMI structures, surface plasmon momentum of the symmetric mode does not always exceed photon momentum, with thicker films $(d\sim 50\mathrm{nm})$ achieving effective indices as low as $n=0.15$. In addition, antisymmetric mode dispersion exhibits a cutoff for films thinner than $d=20\mathrm{nm}$, terminating at least $0.25\mathrm{eV}$ below resonance. From visible to near infrared wavelengths, plasmon propagation exceeds tens of microns with fields confined to within $20\mathrm{nm}$ of the structure. As the $\mathrm{Si}{\mathrm{O}}_2$ core thickness is increased, propagation distances also increase with localization remaining constant. Conventional waveguiding modes of the structure are not observed until the core thickness approaches $100\mathrm{nm}$. At such thicknesses, both transverse magnetic and transverse electric modes can be observed. Interestingly, for nonpropagating modes (i.e., modes where propagation does not exceed the micron scale), considerable field enhancement in the waveguide core is observed, rivaling the intensities reported in resonantly excited metallic nanoparticle waveguides.