Resonance amplification of left-handed transmission at optical   frequencies by stimulated emission of radiation in active metamaterials
  We demonstrate that left-handed resonance transmission from metallic metamaterial, composed of periodically arranged double rings, can be extended to visible spectrum by introducing an active medium layer as the substrate. The severe ohmic loss inside metals at optical frequencies is compensated by stimulated emission of radiation in this active system. Due to the resonance amplification mechanism of recently proposed lasing spaser, the left-handed transmission band can be restored up to 610 nm wavelength, in dependence on the gain coefficient of the active layer. Additionally, threshold gains for different scaling levels of the double-ring unit are investigated to evaluate the gain requirement of left-handed transmission restoration at different frequency ranges. 
Zheng-Gao Dong, Hui Liu, Tao Li, Zhi-Hong Zhu, Shu-Ming Wang,   Jing-Xiao Cao, Shi-Ning Zhu, and X. Zhang