A diagram showing (i) the composition of a spaser, a nanoparticle with “surface plasmon amplification by stimulated emission of radiation” composed of a gold sphere in the core and a silica shell containing the fluorescent dye uranine, and (ii) the activation of the spaser inside a cell. From Galanzha et al. Spaser as a biological probe. Nat. Commun. 8, 15528 doi: 10.1038/ncomms15528 (2017).