Bulk and surface sensitivities of surface plasmon waveguides

The potential of surface plasmon waveguides for bulk and surface (bio)chemical sensing was assessed theoretically, anticipating their use in an integrated optics sensor such as a Mach–Zehnder interferometer (MZI). The performance of a generic MZI implemented with attenuating waveguides was assessed initially, revealing that attenuating waveguides constrain the sensing length to an optimal length equal to the propagation length of the mode used. The MZI sensitivities for bulk and surface sensing were found to be proportional to the ratio of the waveguide sensitivity to its normalized attenuation: H=(∂n_eff/∂n_c)/k_eff for bulk sensing and G=(∂n_eff/∂a)/k_eff for surface sensing. Maximizing H or G maximizes the corresponding MZI sensitivity and minimizes its detection limit, leading to preferred waveguide designs and operating wavelengths. The propagation constant, the sensitivities, and the H and G parameters were then determined for the surface plasmon in the single interface, the s_b mode in the metal–insulator–metal (MIM) waveguide and the s_b mode in three variants of the insulator–metal–insulator (IMI) waveguide, as a function of dimensions, for wavelengths spanning 600⩽λ₀⩽1600 nm, assuming Au and H₂O as the materials and adlayers representative of biochemical matter. The principal findings are: (i) the surface sensitivity in the thin MIM can be 100× larger than in the single interface, whereas that in the thin IMI is up to 5× smaller; (ii) the bulk sensitivity in the thin MIM can be 3× larger than in the single interface, whereas that in the IMI is slightly smaller; (iii) G in the thin MIM can be 3× larger than in the single interface, whereas G in the IMI is about 10× larger; and (iv) H in the thin MIM can be 10× smaller than in the single interface, whereas H in the thin IMI is about 10× larger. The IMI and the MIM both offer an improvement in sensitivity and detection limit for surface sensing over the single interface in an integrated MZI (or Kretschmann–Raether) configuration, despite the fact that they are at opposite ends of the confinement–attenuation trade-off. Preferred wavelengths for surface sensing were found to be near the short wavelength edge of the Drude region, where detection limits of about 0.1 pg mm⁻² are predicted. With regard to bulk sensing, only the IMI offers an improvement over the single interface. The results are collected in a form that should be useful for investigating other sensor architectures implemented with these waveguides or variants thereof.