knitr::read_demo("kretschmann_sensing", package="planar")
The Kretschmann configuration is commonly used for sensing applications: the sharp resonance associated with the excitation of SPPs at the metal/dielectric interface is strongly sensitive about the refractive index of the dielectric medium. Furthermore, because SPPs are associated with a exponential decay of the electric field in the surrounding medium, the technique provides a surface-sensitive, sub-wavelength, probe.
We define a function to model our system with the parameters of interest: a semi-infinite incident medium (glass), a 50nm gold film, a thin layer of varying thickness and refractive index, and a semi-infinite dielectric medium of varying refractive index.
We now look at the effect of changing the refractive index of a thin layer, of varying thickness.
For an optical thickness approaching $\lambda / 2$, a new reflectivity dip appears, that is associated with the excitation of a guided mode. It is sharper than the standard SPP mode, because the modal field is mostly located in the dielectric layer.
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