Abnormal Cutoff Thickness of Long-Range Surface Plasmon Polariton Modes Guided by Thin Metal Films

Long-range surface plasmon polariton (LRSPP) modes guided by a thin metal film surrounded by semi-infinite dielectrics with different refractive indices are studied. Our calculation results show that the cutoff thickness of the metal film does not monotonically increase with refractive index difference △n between the substrate and superstrate. Just because of this abnormal behaviour of cutoff thickness, the existence of LRSPP illustrates complicated situations in asymmetric configurations. For a certain metal film thickness, LRSPP may exist in one, two or three refractive index difference △n regions.