Theoretical analysis of slab waveguides supporting SPP modes toward their sensitivity characteristics

Abstract

This work is devoted to studying the dielectric/metal multilayer planar waveguide structure that is composed of a highrefractive index, single mode, silica-titania film deposited on a BK7 glass substrate. The waveguide film is loaded with a thin gold film, through a thin silica film. It is assumed that a semi-infinitive cover have the same refractive index as the water. There are presented and analyzed spectral characteristics of effective indexes of TM-polarized: dielectric and surface plasmon polariton (SPP) modes. Analysis of these characteristics is aimed at finding conditions for exciting the dielectric TM 0 mode and two SPP modes: antisymmetric and symmetric. Assuming that excitation is done with a field distribution of the unloaded TM 0 mode carried in this structure if the gold film is not present. The analysis is carried for the gold film having thicknesses d m =50 nm. It is shown, that conditions for matching effective indexes of the unloaded TM 0 mode and SPP modes are met in a broad range of silica film thickness. The presence of the silica film is indispensable for reduction of a refractive index difference between loaded and unloaded TM 0 modes. It is shown that a sensitivity of this structure toward a small change of the cover refractive index is inconsiderably dependent on silica film thickness for the antisymmetric SPP mode. On the contrary, a sensitivity of the symmetric SPP mode, whose refractive index undergoes a discontinuous change with respect to wavelength changes its sign in function of the buffer film thickness.