Manipulating EIT and EIA Effects by Tuning the Gain Assisted Dark Mode

We investigate the scattering properties of an array of plasmonic structure consisting of a bright atom and a gain assisted dark atom. In light of the Lorentz model analysis, we demonstrate perfect electromagnetically induced transparency (EIT) like resonance with high Q factor when proper amount of gain is introduced. The non-Hermitian physics associated with the proposed structure can greatly enrich the electromagnetic property of it. Ultra-high transmission and reflection of a finite bandwidth are reported at the EIT resonance frequency when the gain level is increased to a critical point. Furthermore, we find that by introducing extra loss into one strip and equal amount of gain into another strip in the dark atom simultaneously, enhanced absorption of the incident wave is achieved. In addition to incorporating material gain to balance the loss, we propose that coherent pump waves can introduce effective gain in the dark mode. Compared to utilizing gain medium, such a route is much easier to implement. The demonstrated high-Q EIT resonance, lasing and enhanced absorption effects may find their applications in sensing, switching, absorbing, nonlinear optics and spaser areas.