Surface Plasmon Resonance (SPR) Structures Containing Amorphous Chalcogenide (ChG) Films as Plasmonic Waveguides: A Review

The paper is a review of surface plasmon resonance (SPR) structures containing amorphous chalcogenide (ChG) films as plasmonic waveguides. The transfer matrix method was used for calculation of light intensity reflected from SPR structure. The method permits the calculation of reflectance for arbitrary number of layers. Specific characteristics of a four-layer SPR structures containing prism, metal, thin dielectric film and ambient medium were obtained by using a software realized in MATLAB. The results are presented for structures containing amorphous As2S3 and As2Se3 thin films characterized by a high refractive index and a metallic layer made of gold. The requirements to the metallic films are the same as in the case of widely known three-layer structures with SPR. The paper is mainly based on our previously obtained and published scattered results, to which a generalized point of view was applied.

The analysis shows that with the correct choice of film thickness and of refractive index, it is possible to control the resonance angles, the sharpness of the SPR resonance curve, the penetration depth, and the sensitivity to the refractive index of the analyte. Aspects regarding the choice of the coupling prism are studied. It was shown that in the case of GaP prisms with a refractive index greater than that of the waveguide, the entire spectrum of modes can be excited, which is generally understandable. However, it was found that at a certain waveguide thickness, the excitation of one selected mode is also possible with a prism having the refractive index lower than waveguide film material. This case is very important for practice, since it allows us to make a prism from ordinary materials, like of BK7 glass. The gold film thickness must be optimized in order to obtain near zero value of the resonant reflectance. It corresponds to 40 nm in the near IR spectrum and 50 nm in VIS spectrum for which R~0 can be obtained. The waveguide thin film thickness permits the adjustment of SPR sensor’s sensitivity.

Optical hysteresis was detected and explained in SPR resonance structure containing an amorphous As2S3 film, which have self-induced changes of the refractive index. Although such changes are small under normal conditions, the resonant conditions greatly enhance the effect, allowing for a transition to practical application. The four-layer structure of the SPR makes it possible to identify alcohols and hydrocarbons even with small differences in refractive index. The numerical model viability was experimentally demonstrated by identification of E. coli bacteria. The usefulness of other materials with high refractive index were discussed. However, the As-S-Se materials have the advantage of producing thin films with smooth surfaces due to their amorphous state.