Figure 2
Excitation of propagating plasmons on ultrathin niobium films using a multilayer on a fiber optical taper.
(a) Sketch of the plasmonic fiber taper with the section of the multilayer structure. (b) Schematic of the cross section of the taper within the plasmonic section. Different colors indicate the materials used (blue: silica, magenta: ultrathin niobium film, dark green: Al2O3 layer, light green: liquid analyte). The dashed black vertical line refers to the cross section which is used for the planar multilayer simulations. (c) Normalized effective index dispersion of the different modes of the systems, calculated using the planar symmetric multilayer model and considering TM-polarization. The blue curve refers to the hybrid plasmonic mode (even TM01-mode) and the dark yellow curve is the long-range SPP of the asymmetrically embedded film (no mirror symmetry in the silica assumed, SPP is evanescent in silica and analyte). The inset is a close-up view of the section at which the resonance occurs (to emphasize the domain of the resonance, the effective mode indices have been normalized to the dispersion of the fundamental taper mode which has no multilayer). (d) Attenuation of the modes plotted in (c). The purple curve indicates the modal attenuation of the HPM of the realistic full structure (realistic structure shown in (b), assuming maximum and minimum NB film thicknesses of 12.5 nm and 3 nm, respectively) calculated using Finite-Element modeling. For completeness, this plot also includes the attenuation of the SR-SPP (dashed dark yellow line). The right-handed images show the Poynting vector distributions of the dielectric (e) and SPP mode (f) (both at 600 nm) and the HPM at resonance ((g), 690 nm). The dashed green circles indicate the surface of the taper. In all simulations, we assume niobium and Al2O3 films thicknesses of 12.5 nm and 80 nm, respectively. The analyte is assumed to have negligible loss and dispersion in this spectral domain with a constant index of na = 1.36.
