Despite their sensing potential, plasmonic SLRs have so far only been extensively studied on prototypical plasmonic metals, such as Au and Ag, with sensing applications limited to refractive index changes outside or at the surface of the metal. The use of SLRs for direct plasmonic sensing of phenomena occurring inside the metals requires the utilization of active plasmonic metals such as Y, Mg and Pd. Hence, as a crucial step towards our optimized plasmonic sensor, we first demonstrate the existence of SLRs in periodically-arranged Pd nanoparticles and characterize their optical spectra and field distributions. To this end, we fabricated an extensive set of square arrays of 45 nm high Pd nanodisks with varying diameters (d = 70-180 nm, steps of 20/30 nm) and pitch distances (a = 300-600 nm, steps of 50 nm) on fused silica (n sub = 1.46). To allow efficient radiative coupling between the nanodisks by the in-plane diffraction orders, an index-matching medium is essential. We thus coated the arrays with a 200 nm thick poly(methylmethacrylate) (PMMA) film (n PMMA = 1.48). Besides having a suitable refractive index, PMMA is also serendipitously beneficial for Pd hydrogen sensors because it accelerates sorption kinetics by lowering the H 2 absorption energy into the Pd lattice and rebuffs other interfering and deactivating gases, such as O 2 , CO, NO 2 , and volatile hydrocarbons. Figure shows the experimental extinction spectra of 42 Pd nanodisk square arrays (scanning electron microscopy, SEM, images in Supplementary Figure ) alongside finite-difference timedomain (FDTD) calculations (see Methods) that accurately reproduce all spectral features in the measured data. In particular, we observe extinction spectra with one, two, or three peaks, depending on the nanodisk diameter and array pitch. The Figure also includes the calculated extinction spectra of the corresponding single-particles, highlighting how arranging the nanodisks in a periodic array results in distinct optical properties compared to their isolated counterparts. Particularly, as the pitch of the array, a, increases, the -main‖ peaks (as referenced to the array with a = 300 nm) universally redshift and narrow (Supplementary Figures ). For example, for the d = 180 nm sample, its FWHM reduces by one order of magnitude from ~650 to ~65 nm. Furthermore, we observe the appearance of additional redshifting peak(s) at lower wavelength(s) when a reaches 350 and 500 nm. Last, scrutinizing closely these different peaks as a function of diameter reveals contrasting behaviors. While -main‖ peaks redshift as d increases, the peaks at lower wavelengths are relatively immobile (Supplementary Figure ). This observation hints that the -main‖ peaks are SLRs that are dominated by contributions from the LSPR, and that the peaks at lower wavelengths are dominated by RAs, since their position depends solely on the particle-to-particle distance and not on the nanodisk diameter. The pitch of the array, a, increases from left to right from 300 nm to 600 nm. The nanodisk diameter increases from top to bottom from 70 nm to 180 nm. For comparison, the leftmost panels plot the extinction cross-sections (in m 2 ) of the corresponding isolated single particles (a = ∞). Arranging the nanodisks in arrays results in distinct optical spectra compared to their isolated single particle counterparts. The spectra comprise peaks originating from hybrid RA-