Finite-Difference Time-Domain simulations. We used commercial Lumerical FDTD software to calculate the optical properties of both single and array of Pd nanodisks. We modeled the nanodisks as cylinders with a taper angle (the angle between the base and the side wall) of 65° to be close with the fabricated samples. The permittivity values of Pd and Pd hydride (PdH 0.12 ) were taken from the literature. The nanodiks were placed directly on top of a fused silica substrate (n = 1.46). On top of the substrate and embedding the particles, a PMMA layer was added, whose permittivity was obtained from an ellipsometry measurement. Finally, on top of this layer, there was air (n = 1). The simulations of the scattering efficiencies were done using a total-field scattered-field (TFSF) source with a broadband (400-1100 nm) beam incident from air and along the normal direction. The TFSF source divides the simulation region into two concentric volumes: one centered around the particle with the total fields, and another external where only the scattered fields propagate. Power transmission monitors were positioned around the TFSF source to calculate the scattering cross sections. The efficiency was calculated by dividing the former quantities by the geometrical cross section, i.e., the area of the cylinder perpendicular to the propagation vector k of the incident field. Perfectly matched layer (PML) boundaries were implemented in every direction. The simulations of the periodic arrays were performed using periodic boundary conditions in the x-and y-directions, and PML boundaries in the z-direction. The illumination consisted of a broadband (400-1100 nm) beam, approximated by a plane wave, which was incident normal to the array plane (the xy-plane) from air. To extract the transmission, an xy monitor was placed at the substrate side. Another xy monitor was placed at the center of the particles to extract the fields. The polarization of the incident electric field was set along the y-axis. To extract the extinction (i.e. 1 -transmission) dispersion data, several simulations with different incident angles were performed.