(color online) Some of the LEDs and SPEDs that have been realized. (a) An MIM structure with silicon nanocrystals placed between gold electrodes.^[45] (b) Another MIM waveguide, but with semiconductor emitters placed outside the waveguide. Plasmons are coupled into the waveguide via a slit.^[46] (c) A quantum well semiconductor emitter inside an MIM waveguide^[44] with the MIM layer orientation rotated 90 degrees with respect to those in (a) and (b). (d) Example of a metal-clad nanocavity LED coupled to a dielectric waveguide.^[48]