Enhancement of Optical Coupling Efficiency of Surface Plasmon Resonance Based Sensors

The optical coupling efficiency of surface plasmon resonance (SPR) based sensors is theoretically investigated from the first principle. Analytical expressions for the instantaneous electric flux lines associated with the TM modes of a three-layer dielectric-metal-dielectric (DMD) waveguide structure are calculated. It is shown that these electric flux lines appear in the form of dis-continuous loops at the metal-dielectric interfaces. A careful investigation of the electric flux loops reveals that the transverse component of the electric flux dominates at the interface, and away from the interface the longitudinal electric flux component takes over. It is hence shown that in the incident p-polarized optical light which is used to excite the surface plasmon (SP) modes in SPR sensors if the transverse component of the electric field is increased then a significant increase in the input optical coupling efficiency is observed. The increased input optical coupling increases the real part of the effective index of the SP mode of the sensor yielding an increased sensitivity of the SPR sensor. Calculations are done to optimize various design parameters of an SPR sensor in a typical prism coupling Kretschmann configuration in an angle interrogation setup. It is shown that there is an increase of 64.33% per RIU in optical coupling efficiency with a decrease in the refractive index of the high index input coupling prism and an increase of 58.787% per RIU with an increase in the refractive index of the affinity layer. Enhancement of optical coupling efficiency due to the thickness of the metal film as well as the wavelength of the input optical light has also been evaluated. The corresponding improvement in the sensitivity and changes in full width at half maxima (FWHM), the figure of merit (FOM), and signal to noise ratio (SNR) of the SPR sensor are also reported.