How small can “Nano” be in a “Nanolaser”?

We show that the lasing threshold of the single mode metal-semiconductor nano-laser (spaser) is determined only by the photon absorption rate in the metal and exhibits very weak dependence on the composition, shape, size (as long as it is less than half-wavelength) and temperature of the gain medium. This threshold current is on the order of a few tens of micro-amperes for most semiconductor-metal combinations which leads to unattainably high threshold current densities for a substantially subwavelength laser (spaser). Therefore, in our view, surface plasmon emitting diodes, (SPEDs), operating far below “spasing” threshold may be a more viable option for the chip scale integrated nanophotonics.