(color online) (a) Schematic of spaser structure demonstrated by Noginov et al. Organic dye provides the optical gain for plasmons generated with Au and stimulates coherent emissions.^[17] (b) The spasing model proposed by Stockman.^[18] Energy from external excitation generates e–h pairs, which then relax to the exciton level. Excitons with large momentum will transfer their energy to the dark plasmon mode and start the spasing process. The plasmon mode can also couple to the photon mode if the environmental dielectric condition allows. The corresponding photon will carry the same characteristics from the plasmon, resulting in a coherent emission.