To further characterize the nature of the extinction peaks, we mapped the field distribution and optical dispersion relation of the array with a = 550 nm and d = 180 nm, which pronouncedly features three extinction peaks (Fig. , see also a similar analysis for a = 300 and 400 nm in Supplementary Figure ). Fig. depicts the FDTD-calculated field distribution map of the array at three different y-polarized excitation wavelengths corresponding to each of the peak wavelengths (λ 1 -λ 3 ). At the two shorter wavelengths (λ 1 and λ 2 ), complex field distributions are found, with maxima lying in-between the nanodisks. These features suggest a strong contribution from the RA to the resonance, consistent with the discussion above. The excitation at the longest wavelength (λ 3 ) gives rise to field maxima in the vicinity of the nanodisks, suggesting a strong contribution of the LSPR to the resonance. Given such field distribution, these relatively localized but narrow peaks will emerge as the sensing peaks for direct plasmonic sensing of hydrogen in our Pd nanodisk arrays.