(color online) Different forms of small electrically pumped metallic and plasmonic lasers. (a) A semiconductor heterostructure encapsulated in silver.^[8] This structure has been used in many electrically pumped metal lasers. With the silver removed, the structure is similar to many dielectric waveguide heterostructure lasers. (b) Plot of the magnitude of the electric field for a transverse magnetic 0 (gap plasmon) mode that propagates down the metal waveguide of (a). Red indicates the highest intensity and blue indicates the lowest intensity.^[8] (c) The semiconductor core encapsulated in silver in (a) can have many shapes, round pillars, straight waveguides, or, as shown here, perturbations to form distributed feedback lasers;^[36] the scale bar is 100 nm. (d) Another form of semiconductor heterostructure laser, but now having the mode travelling up and down the pillar, which is equipped with mirrors at each end.^[39] (e) The structure of a far infrared laser that employs an MIM structure and circuit concepts to reduce the laser size well below the wavelength of the laser emission.^[43]