Principles of plasmonic sensing.
(A) A simple plasmonic sensing geometry, consisting of a gold nanosphere on a glass substrate. The system is excited by a linearly polarized electromagnetic wave with electric field vector E and wave vector k. (B) Calculated energy density of the electric field for the described sensing geometry. The dielectric functions of the sphere and the environment are given by εAu and εenv=(nenv)2, respectively. (C) Exemplary scattering spectra for the plasmonic system. The optical response follows a Lorentzian lineshape with clearly defined resonance wavelength λ and full width at half maximum (FWHM). Upon change of the refractive index in the environment by some (generally complex) value Δn, the resonance undergoes a wavelength shift Δλ. The sensitivity of the system is then given as S=Δλ/Δn, and can be normalized with the FWHM to obtain the figure of merit FOM=S/FWHM.