Simultaneous existence of amplified and attenuated surface-plasmon-polariton waves

The propagation of surface-plasmon-polariton (SPP) waves guided by the planar interface of (1) an isotropic metal and (2) an active uniaxial dielectric material was theoretically investigated, under the assumption that the optic axis of the uniaxial partnering material lies wholly in the interface plane. The uniaxial partnering material was conceptualized as a homogenized composite material (HCM) arising from both non-dissipative and active component materials. For a certain range of the volume fraction of the active component material, the SPP waves propagating in certain directions in the interface plane are amplified, but the SPP waves propagating in other directions are attenuated. In a critical propagation direction, the SPP wave is neither amplified nor attenuated. This critical propagation direction was found to be acutely sensitive to the composition of the HCM.