Numerical investigation of finite thickness metal-insulator-metal structure for waveguide-based surface plasmon resonance biosensing

As a solution to enhance the coupling between the surface plasmon-polariton (SPP) and optical waves propagating along dielectric waveguides, we present the finite thickness metal-insulator-metal (f-MIM) structure. Achieving such opto-plasmonic coupling has been difficult due to the inherent difference in the two waves’ dispersion characteristics. The f-MIM provides an effective way to tune the dispersion characteristics of its SPP mode to match those of the waveguide modes. The resultant enhancement in mode coupling enables waveguide-based excitation of SPPs, which, in turn, leads to the realization of integrated optic surface plasmon resonance (SPR) sensors. Our simulations showed that the effective index of the f-MIMʹs SPP mode can be increased to match those of typical dielectric waveguide modes even when the f-MIM itself was surrounded by low-index material such as water (n = 1.33). Moreover, the SPPʹs mode profile can also be controlled simultaneously to exhibit its peak at the interface with the analyte. All these characteristics render the f-MIM an ideal element for waveguide-based SPR sensing in aqueous environment. By integrating the f-MIM with a polymer waveguide, we obtained SPR sensing resolutions of 1.53 × 10−5 in RIU and 0.016 nm in thickness in refractometric and affinity sensing modes, respectively.