Spaser Based on Dark Quadrupolar Mode of a Single Metallic Nanodisk

A spaser based on dark quadrupolar mode of a single metallic nanodisk coated with a layer of gain media is studied theoretically. The absorption efficiency of the metallic structure, the gain efficiency of the gain media, and the scattering efficiency of the whole nanosystem are calculated separately. It is found that the ratio of the absorption and the scattering intensities (RAS) of the dark quadrupolar mode depends strongly on the gain coefficient, which increases from 0.8 to 4.39 with gain coefficient reaches the threshold, by contrast the bright dipolar mode keeps its RAS unchanged at 0.16. This is attributed to that the gain media mainly amplifies quadrupolar eigenmode composition of the dark quadrupolar mode, thus the scattering loss caused by dipolar eigenmode composition is under effective control, which changes the RAS and leads to super low-threshold spaser. Our works may benefit the achievement of spaser system with low scattering loss.