Engineering chiral plasmonic nanostructures for gain-assisted plasmon amplification and tunable enhancement of circular dichroism

Abstract

Circular dichroism (CD) from plasmonic nanostructures yields fascinating insights into their chiroptical properties; however, the weak signals make their investigations profoundly challenging. We have demonstrated a method for significantly improving the signal enhancement by introducing a gain medium on these single plasmonic nanoparticles and creating surface plasmon amplification of stimulated emission of radiation (SPASER). The chiral nanostructure, which consists of a single v-shaped gold nanorod dimer, showed a prominent plasmonic chiroptical response when excited with an obliquely incident beam of circularly polarized light. When the same gold nanorods were arranged in a twisted configuration, the response was intrinsically chiral irrespective of the angle of incidence. However, for both these classes of nanoparticle arrangement, the chiroptical response was enhanced by several orders of magnitude when a doped gain medium layer was introduced, and a significant enhancement in the anisotropy factor was observed when the gain coefficient reached a critical value of the lasing threshold. The chiroptical response could be efficiently tuned by changing the geometry of the nanoconstruct as well as the gain coefficient of the surrounding layer, thus demonstrating an alternative method for obtaining chiral plasmonic materials with remarkably high anisotropy factors.