Auxiliary Top-Illumination in the Kretschmann-Raether Configuration: A Theoretical Study on the Surface Plasmon Resonance Response

A new variation of surface plasmon resonance (SPR) sensors design with Kretschmann-Raether (KR)-type configuration is presented. Using a rotational incident angle scanning platform, the conductor nanolayer is normally irradiated with an auxiliary unpolarized light beam from the dielectric side. Both illumination sources should have a coherence length much larger than their source/Au layer optical path. Beginning with a Lorentzian-type oscillator model, a general dispersion equation for the metal nanolayer is developed that incorporates interband transitions and polychromaticity of the top-illumination beam. SPR curves are then predicted based on the transfer-matrix method. Simulation results show that the top-illumination frequency and its relative amplitude with respect to the bottom laser beam may impart significant changes to the SPR curve. It is shown that the implementation of short-wave ultraviolet light (180 nm) may considerably improve the performance of SPR sensors in terms of the figure-of-merit. Larger wavelengths generally lead to performance deterioration. Based on the transverse magnetic solutions of the wave equation, a theoretical interpretation of the results is presented that confirms the strong effect of top illumination on the magnitude of the decay lengths in the conductor/dielectric and the propagation constant.