Near-field properties of a shell nanocylinder pair with gain materials

We study the near-field response of a shell nanocylinder pair, with its core filled by gain materials, using a two-dimensional finite-difference time-domain method. It is shown that the gain materials in the core of the cylinder can compensate the intrinsic absorption of the metal shell, leading to local-field enhancement in the gap of the active pair. A linear dependence is found between the field enhancement and the gain coefficient at resonance. The detailed physics is studied by calculating the electrical-field distribution of the shell pair filled with different gain materials. The influence of the gap width and the shell thickness on the interaction of two adjacent active shell cylinders is also investigated.