Refractive index sensing with localized plasmonic resonances – Theoretical description and experimental verification

Abstract

In this thesis the sensing properties of plasmonic resonators for changes in the surrounding refractive index are investigated. A self-consistent and general sensing theory is developed. This theory connects the electrodynamic properties of plasmonic resonators like resonance wavelength and electric field distribution to the sensitivity for refractive index changes. A figure of merit (FOM) is derived which includes the effects of noise and in its general form directly states if a certain change in refractive index will be measurable or not. For the FOM in the quasi-static limit absolute bounds and scalings are derived. These bounds are based on the localization of electromagnetic energy for which analytic expressions were known before. The important result of the quasi-static considerations is that the sensitivity is determined completely by the choice of material and resonance wavelength for refractive index changes that cover the whole sensing volume while for smaller analytes the energy confinement to the analyte volume is important. To confirm the developed theory numerical calculations and an experiment with crescent shaped plasmonic resonators is carried out and good agreement is found. In this experimental verification, local refractive index changes were introduced close to the crescent shaped particles and their resonance wavelength change was measured. As a model analyte polystyrene colloids were used and manipulated with an atomic force microscope. This approach leads to a very defined and controllable model system which allowed the theoretical predictions to be verified without parasitic effects. The proposed theoretical model predicts the measured wavelength changes with high accuracy and allows to extrapolate the result to the response of the resonator to the binding of a single molecule to its surface. From the theory together with the experiment it follows, that single molecule sensitivity will be possible by increasing the signal to noise ratio of the measurement