Plasmonic Slot Waveguide: Subwavelength Mode Confinement

A metal-insulator-metal (MIM) plasmonic waveguide guides light by trapping a "gap plasmon" mode inside a dielectric slot only tens of nanometres wide — far below the diffraction limit of the light itself. This simulator solves the real transcendental dispersion relation for that mode with complex Newton-Raphson root-finding every time you move a slider, using a Drude free-electron model for the metal's permittivity. Narrow the gap and the effective mode index climbs while the transverse field profile squeezes tighter against the metal walls; widen it and the mode relaxes toward an ordinary single-interface surface plasmon. The live propagation-length readout and the visibly faster decay of the field bars along the waveguide make the core plasmonic trade-off concrete: every nanometre of extra confinement is paid for in propagation distance.