The effect of gain-assisted silica separate layer on the plasmonic local field enhancement of Ag-Si-Ag coaxial-cable type silver nanotube

Abstract

The plasmonic local electric field enhancement characters of gain-assisted coaxial-cable type Ag-Si-Ag nanotube have been investigated computationally. The calculation results based on quasi-static analysis indicate that the three surface plasmon resonance (SPR) hybridization modes depend on the gain medium of the silica separate layer in different ways. The SPR band from antisymmetric coupling between bonding tube plasmon mode and the wire plasmon mode (denoted as peak3) is mostly sensitive to the gain assist, and the greatest local field enhancement factor of 104 could be achieved with a critical gain coefficient. The physical mechanism has been attributed to the appearance of the local field hot spot in silica separate layer, in which the gain medium has been dropt. The critical gain coefficient could be tuned by altering the geometry factors of the nanostructure. It has been found that the thickness of the middle silica nanolayer with gain medium greatly affect the critical gain coefficient. For peak3, the maximum plasmonic field enhancement could be easily achieved with a small gain coefficient and within a slight change region. Whereas for SPR band from the symmetric coupling between the bonding tube plasmon mode and the wire plasmon mode (denoted as peak2), the local field hot spot appears in the inner Ag nanowire without gain medium. Thus the maximum plasmonic field enhancement could only be achieved with a large gain coefficient when the middle nanolayer with gain medium has a thin thickness.