Epsilon‐Negative Active Composites: Loss‐Free and Amplifying Plasmonic Materials

The main issue restricting the applications of plasmonics is the intrinsic losses in metals. In order to mitigate the effects of losses, plasmonic systems embedded with gain layers have extensively been investigated over the years. However, the ultimate solution is to use loss‐free or amplifying epsilon‐negative materials. This work reveals that the goal can be achieved by using gain composite materials containing metal nanoparticles as building blocks for plasmonics. By controlling the gain coefficient of the host medium and the filling factor of metal nanoparticles, one can significantly change the magnitude of the negative effective dielectric function while maintaining gain. Their superior performances are evidenced by the evaluations of quality factors for localized and propagating plasmons, and also the detailed studies of spasing behaviors of the composite nanoparticles and hyperbolic responses of their multilayered structures. The findings presented in this work introduce a new paradigm of plasmonics.