Ultrathin niobium nanofilms on fiber optical tapers--a new route towards low-loss hybrid plasmonic modes

Abstract

Due to the ongoing improvement in nanostructuring technology, ultrathin metallic nanofilms have recently gained substantial attention in plasmonics, e.g. as building blocks of metasurfaces. Typically, noble metals such as silver or gold are the materials of choice, due to their excellent optical properties, however they also possess some intrinsic disadvantages. Here, we introduce niobium nanofilms (~10 nm thickness) as an alternate plasmonic platform. We demonstrate functionality by depositing a niobium nanofilm on a plasmonic fiber taper, and observe a dielectric-loaded niobium surface-plasmon excitation for the first time, with a modal attenuation of only 3-4 dB/mm in aqueous environment and a refractive index sensitivity up to 15 μm/RIU if the analyte index exceeds 1.42. We show that the niobium nanofilm possesses bulk optical properties, is continuous, homogenous, and inert against any environmental influence, thus possessing several superior properties compared to noble metal nanofilms. These results demonstrate that ultrathin niobium nanofilms can serve as a new platform for biomedical diagnostics, superconducting photonics, ultrathin metasurfaces or new types of optoelectronic devices.

Figure 1. Comparison of confinement and loss properties of surface plasmon polaritons propagating on niobium (a, blue diagram), gold (b, green diagram) and silver (b, purple diagram), assuming that the metal films are sandwiched between two semi-infinite silica layers.

In the three left-handed performances maps (a–c), the hatched areas refer the regions of subwavelength confinement (δ < λ₀/2) of the SR-SPP mode. The filled colored regions indicate the domain of long-range propagation (L_abs > 1000 λ₀, long-range regime) of the LR-plasmon. The vertical dotted blue line in Fig. 1a indicates the thickness of the niobium film used in the experiments. The properties of the complex dielectric functions of the three discussed materials (Nb: blue, Au: green, Ag: purple) are shown in the two right-handed diagrams ((d): real parts of dielectric function (solid lines), (e) corresponding imaginary parts (dashed curves)).

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 n_a = 1.36.

Figure 3. Measured spectral distribution of the modal attenuation of the hybrid plasmonic mode of the ultrathin film based taper in the case of a predefined analyte (Cargille liquid, na = 1.36 at a wavelength of 589 nm).

The inset shows the measured transmission values (in logarithmic scale) for different stub lengths (length of the analyte column) at resonance (λ_R = 732 nm). The straight green line is a linear fit to the experimental data (in dB-scale), with the slope corresponding to the modal attenuation of the propagating hybrid plasmonic mode.

Figure 4. Refractive index properties of the niobium-based plasmonic taper.

(a) Resonance wavelength as a function of refractive index of the applied liquid. The blue dots refer to the measured resonance wavelength obtained from the dips in the transmission spectra (blue line is a guide-to-the-eye). The green curve shows the results from the planar multilayer model. The shaded green area refers to the potential change in resonance wavelength if a niobium thickness uncertainty of ±1 nm is considered. The inset shows examples of the spectral distribution of selected normalized transmission spectra (values are the RIs of the liquids at λ₀ = 589 nm according to the labeling of the used Cargille analytes). (b) Refractive index distribution of the sensitivity of the experimental data (blue) and of the planar multilayer waveguide model (green). The blue dot indicates the sensitivity value at the index of water.

Figure 5. Spectral distribution of the relevant optical quantities of the materials involved.

(a) Dielectric function of the niobium (measured using 300 nm thick film, solid (dashed) line: real (imaginary) part of ε_Nb). (b) Material dispersion of the three used dielectric materials (green: niobium oxide, purple: aluminum oxide, brown: fused silica). Since all three dielectrics have very low absorption in the spectral interval relevant here, the absorption coefficient has not been plotted.

Figure 6. Schematic of the transmission setup used to measure the optical response of the ultrathin film-based plasmonic taper: SC: supercontinuum source, F: water-based filter for removing the infrared light, P: input polarizer, A: analyzer, Obj: microscope objective, SMF: single mode fiber, OSA: optical spectrum analyzer.

Figure 7. Sequence of the materials used in the planar multilayer model employed to analyze the properties of the modes within the plasmonic taper section.

The various colors refer to the different materials involved (dark blue: silica (t_silica = 10 μm), magenta: ultrathin niobium film (t_Nb = 12.5 nm), dark green: Al₂O₃ (t_Al2O3 = 80 nm), light green: liquid analyte). The blue curve is a schematic representation of the magnetic field components (H_y) of the TM-polarized hybrid plasmonic mode (even TM₀₁-mode, mode amplitude and shape of the curve not to scale). The propagation direction is along the x-axis, and the waveguide is translational invariant along the y-axis. The dashed red line indicates the symmetry plane at which the boundary condition ∂H_y/∂z = 0 is applied.