Spaser in plasmonic nano-antenna evaluated by an analytical theory

Surface plasmon amplification by the stimulated emission of radiation (spaser) in plasmonic nanocavities as a novel concept has quickly advanced in recent years. Understanding the nature and mechanism of the spaser system is important for both fundamental studies and the development of new applications. We theoretically investigate the spaser made from a plasmonic nano-antenna embedded with active gain media by using an analytical semiclassical theory. It incorporates the four-level atomic rate equations in association with the classical oscillator model for active materials and Maxwell’s equations for fields. The nano-antenna cavity has a large Purcell factor and low absorption loss which is beneficial for the realization of low-threshold spaser. We use the theory to uncover all the characteristics of this nanocavity spaser system, including the enhancement of the local electric field, gain, saturation phenomenon and lasing threshold. It is found that an important quantity named the cavity loss coupling strength coefficient can be explored to provide a new way to design the nanocavity precisely to reduce the absorption power density and enhance the spaser output power density simultaneously. The theory can be commonly used in understanding and designing various micro/nanolaser and spaser systems.