Fig. 1. (a) Schematic of the Pd-based metasurface considered in this study. Pd nanodisks are separated from an Au mirror by a dielectric spacer layer (magnesium fluoride, MgF₂). Upon H₂ absorption, the dielectric function and the size of the Pd nanodisks change, leading to a change in the reflectance spectrum of the structure. (b) The H₂ sensor is composed by a metasurface and a fiber flange plate that serves as a connector to the optical fiber. Inset: cross section of the fiber flange plate and the detail of the metasurface next to the fiber tip.

Fig. 2. Calculated reflectance difference spectra for varying Pd disk diameters and spacer thicknesses, for a disk thickness of 20 nm and period of 360 nm.

Fig. 3. Calculation of the reflectance of the Pd (red dot line) and Pd hydride (black double-dot dash line) as a function of incident wavelength.

Fig. 4. Calculation of the reflectance with the varying thicknesses of the spacer layer and incident wavelength for (a) Pd disk and (b) Pd hydride disk. (c) Calculation of the reflectance difference with the varying thicknesses of the spacer layer and incident wavelength when the phase transition has happened.