(color online) Size comparison of metallic and plasmonic nanolasers to dielectric nanolasers. The electron microscopy pictures are scaled to the free space emission wavelength λ of each laser. (a) A photonic crystal laser, a type of small dielectric cavity laser;^[66] (b) metallic nonplasmon mode laser;^[7] (c) metallic propagating plasmon mode laser;^[10] (d) localized plasmon mode laser;^[9] and (e) another metallic nonplasmon mode laser.^[12] The metal cavity lasers developed recently exhibit a dramatic reduction in size compared to dielectric cavity lasers.