Significant field enhancements in an individual silver nanoparticle near a substrate covered with a thin gain film

In this paper, we propose a method to significantly enhance the local-field of a gap plasmonic system by placing a metallic nanoparticle in close proximity to a substrate covered with a thin film using a gain material (∼100 nm thickness). Compared with a conventional dielectric substrate, the thin gain film can contribute to several, or dozens, of times more intense local electric fields in the gap between the particle and the substrate. We use the finite difference time domain method to numerically analyze the influences of the gain coefficient of the film and of the other parameters on the field enhancement. The numerical results show that there is an optimal refractive index of the gain film that enables us to achieve a maximal field enhancement for a given NP radius. Moreover, the optimal refractive index of the gain film can be incorporated into any available materials using metal nanoparticles with an appropriate radius.