Within the emerging framework of “lab-on-fiber” technologies, we demonstrated the integration of metasurfaces on the tip of an optical fiber. The resulting optical-fiber “meta-tips” represent a major breakthrough in the lab-on-fiber technology roadmap and promise to empower the typical fiber-optics application scenarios with the advanced light-manipulation capabilities endowed by metasurfaces. Here, we explore the possibility to exploit this platform in label-free biological or chemical sensing applications. Specifically, we carry out a parametric study of the surface sensitivity, and show that the phase-gradient can be effectively exploited as an additional degree of freedom in the design of high-sensitivity fiber optic devices. We considered phase-gradient optical fiber meta-tips (see Figure 1a) based on Babinet-inverted plasmonic nanoantennas, i.e., rectangular aperture antennas in a gold film, rotated by 45° in the *x*-*y* plane. The geometry of the macrocell is shown in Figure 1b, where the two antennas have the same sidelengths, and are rotated by 90°. This configuration admits a simple gradient-free benchmark, which preserves the properties inherent of the single antenna geometry, but removes the phase-gradient effects (Figure 1c). We carry out a parametric study of the surface sensitivity, by varying the intensity of the phase gradient and the thickness of the gold layer, *t*. All numerical results are obtained by means of a free 2-D implementation of the Rigorous Coupled-Wave Analysis. For d=530, 700, 1000 nm and t=15, 30, 50, 80 nm, we compute numerically the MS and BC sensitivities, defined as the shift of the resonance wavelength upon the addition of a thin overlay. In Table 1, the values of the sensitivity gain of the MS with respect to the BC are shown for the different configuration analyzed. In Figure 2, the spectra computed for the MS and the BC for t=15 nm and d=530, 700, and 1000nm are shown.

The results from this study confirm that the phase-gradient can be effectively exploited as an additional degree of freedom in the design of high-performance sensors based on plasmonic arrays. Indeed, we show that the simple introduction of a constant phase-gradient always yields a gain with respect to the corresponding zero-gradient benchmark.