Fig. 4

a Spectral reflectance for single-metal 30-nm-thick layer made of gold (black) or silver (red), and for bi-metallic layer for two thicknesses combinations (blue and green). The yellow arrow selects the response for the optimum arrangement (25 nm-Ag / 5 nm-Au). b Spectral reflectivities of the optimum device that use a GaP buffer layer. The peaks show a similar sharpness for three different values of the index of refraction. c Sensitivity (left axis and black dashed line) and FOM (right axis and blue solid line) of the optimized sensor for an extended range of refractive index. The vertical line denotes the limit analyzed in the previous design where the buffer layer was made of AZO and the metallic layer was made of gold