Long-range surface plasmons supported by a bilayer metallic structure for sensing applications.

We show, both theoretically and experimentally, that long-range surface plasmons (LRSPs) are supported by asymmetric structure, consisting of a thin silver/gold bilayer metallic film sandwiched between a magnesium fluoride (i.e., MgF₂) buffer layer and a sensing medium (water). The geometrical parameters of the structure are optimized to yield efficient excitation of LRSPs by using transfer matrix method based on Fresnel reflection. The excitation of LRSPs was performed by using a custom-made automated optical setup based on angular interrogation with the precision of 0.01°. We demonstrate that the bimetallic asymmetric structure achieves better minimum reflectivity resolution than monometallic (gold) asymmetric structure. Finally, figures of merit are compared for bimetallic, monometallic, and conventional SPR structures, and we found that the bimetallic asymmetric structure provides a higher figure of merit; e.g., more than double for monometallic LRSP configuration and 8 times as compared to the conventional surface plasmon resonance sensor.