Long-range surface plasmon polaritons with subwavelength mode expansion in an asymmetrical system

Long-range surface plasmon polariton (LRSPP) modes in anasymmetrical system, in which the thin metal film is sandwichedbetween a semi-infinite substrate and a high permittivity polymerfilm with a finite thickness, are theoretically calculated andanalyzed. Due to the high permittivity of the polymer film, atproper polymer film thicknesses, the index-matching condition of thedielectrics at both sides of the metal can be satisfied forsupporting LRSPP modes, and the electromagnetic field above themetal can be localized well. It is found that these LRSPP modes haveboth long propagation lengths and subwavelength mode expansion abovethe metal at the optimal polymer film thicknesses. Furthermore, therequirements on the refractive index and the thickness of thepolymer film to support LRSPP modes at the optimal thicknesses arefound to be not critical.