Collective modes in multipolar plasmonic lattices: control of interparticle-gap upward/downward energy streams

We numerically study the collective resonances and field profiles of two-dimensional arrays of large metallic nanodisks supporting multipolar plasmonic modes in nearly homogeneous environments. We show that, depending on the wavelength, these arrays can support two distinct collective modes with orthogonal field intensity distributions. In one of these modes (scattering mode), the optical scattering between the nanodisks forms a sharp optical feature with significant sensitivity to the refractive index of the environment. In the second mode (optical-band mode), the nanodisks are mainly coupled via an optical band with a field intensity profile perpendicular to the incident light polarization. This mode is associated with a subtle optical peak with much less sensitivity to the environment. We demonstrate that around this peak, the nanodisks support a unique asymmetrical field dispersion and energy flow paths. For a given wavelength range, the plasmon fields in this mode deeply penetrate into the substrate while energy moves up from the substrate between the gaps of the nanodisks, forming energy whirlpools in the superstrate. With a slight wavelength change, these fields switch to the superstrate, and the direction of the gap energy path is inverted. We investigate how such features can be tuned by adding a thin layer of silicon between the substrate and superstrate and discuss the applicability of the scattering mode for sensitive sensors.