Fig. 1. Schematic cross section of the studied plasmonic GNG-based sensors.

Fig. 2. Plasmonic spectra obtained with water (n=1.332) as surrounding medium for three different period P values: 500 nm, 1 μm, and 1.5 μm.

Fig. 3. Numerical results for three configurations with different P values (500 nm, 1 μm, and 1.5 μm) at a fixed W=400  nm and T=40  nm. (a) Absolute value of the variations in resonance wavelength (|Δλ|) calculated with respect to the water (n=1.332) versus the refractive index along with the linear fitting of the simulated data. (b) Full width at half maximum (FWHM) versus refractive index.

Fig. 4. Numerical results for three configurations with different W values (W=200  nm, W=400  nm, and W=600  nm) at a fixed P=1  μm and T=40  nm. Absolute value of the variations in resonance wavelength (|Δλ|) calculated with respect to the water (n=1.332) versus the refractive index along with the linear fitting of the simulated data.

Fig. 5. Numerical results for three configurations with different W values (W=200  nm, W=400  nm, and W=600  nm) at a fixed S = 600 nm and T=40  nm. Absolute value of the variations in resonance wavelength (|Δλ|) calculated with respect to the water (n=1.332) versus the refractive index along with the linear fitting of the simulated data.

Fig. 6. Simulated absolute value of the resonance wavelength variation in the refractive index range between 1.332 and 1.363, for different values of gold thickness (ranging from 30 to 60 nm), deposited on the optimized nanograting structure.

Fig. 7. Outline of the plasmonic sensor fabrication.

Fig. 8. (a) Outline of the specially designed transmission-based experimental setup. (b) Zoom on the specially designed 3D-printed metallic (AISI 316 steel) holder with the PMMA chips and POFs.

Fig. 9. SEM image of the fabricated gold nanograting.

Fig. 10. (a) Plasmonic spectra obtained at different refractive indices. (b) Absolute value of the resonance wavelength shift (|Δλ|) with respect to water (n=1.332) and linear fitting of the experimental values with error bars.

Fig. 11. (a) SEM image and (b) normalized plasmonic spectra at varying external refractive index relative to non-periodic configuration 1.

Fig. 12. (a) SEM image and (b) normalized plasmonic spectra at varying external refractive index relative to non-periodic configuration 2.

Fig. 13. (a) SEM image and (b) normalized plasmonic spectra at varying of the external refractive index relative to periodic configuration 1.

Fig. 14. (a) SEM image and (b) normalized plasmonic spectra at varying external refractive index relative to periodic configuration 2.

Fig. 15. Absolute value of the variation in resonance wavelength (|Δλ|) calculated with respect to water (n=1.332) for the optimized and non-optimized configurations.

Fig. 16. (a) Outline of the functionalized surface and (b) plasmonic spectra obtained at different BSA protein concentrations. Inset: zoom-in of the resonance region.

Fig. 17. Absolute value of resonance wavelength variation (|Δλ|), with respect to the blank, versus the concentration of BSA protein, with the Langmuir fitting of the experimental values and error bars, in semi-log scale.